Industrial Safety at Particle Accelerators

Abstract

The construction and operation of particle accelerators implies the use of numerous technologies and trades which are well-known from the manufacturing and construction industries. Consequently, their safety hazards are described in the literature and standard best practice solutions exist for controlling the risks emerging from these activities. In this section, the occupational hazards of electricity, mechanical equipment and pressure vessels are illustrated with examples from particle accelerator facilities. Further sections are dedicated to accelerator-specific protection against fire, occupational noise, and environmental damage.

4.1 Electrical Safety

The fundamental principle of accelerator operation is the movement of charged particles by electro-magnetic forces. Electrical current generates magnetic fields and high frequency electrical power drives RF cavities, the two technologies at the heart of every accelerator. Electrical hazards from accelerator equipment are described in the relevant sections (Safety Aspects of Magnets, 2.2.3 and Hazards from RF systems, 2.4.3).

Electrical energy is also often the most convenient form of transmitting and using energy. It is found wherever objects must be moved, or matter converted. This justifies highlighting electrical safety in this section from a general point of view (Fig. 4.1).

Fig. 4.1

Warning sign against electrical risk, after [1]. (Image source: https://publicdomainvectors.org)

4.1.1 Electrical Hazards

If electrical energy is released in an uncontrolled way, different types of harm may result. One or more of the following electrical hazards may occur:

4.1.1.1 Electric Shock and Burns

Electric shock may arise from direct contact with current-carrying parts, for example when a person touches a live conductor that has become exposed because of damage to the insulation of an electric cable. Alternatively, it may result from indirect contact if, for example, an internal fault results in the exposed metalwork of an electrical appliance, or even other metalwork such as a sink or plumbing system, becoming live.

In either case there is a risk of an electric current flowing to earth through the body of a person who touches the live conductor or live metalwork. The severity of the electrical shock is determined by electrical voltage and current. The resistance of the body depends on the applied voltage and on environmental factors, foremost humidity. As a rule of thumb, dry skin isolates against voltages below 50 V. Higher voltages can overcome higher electrical resistance, for example of insulating tool handles, gloves, and at very high voltages even the resistance of air. Once the body connects electrical current to ground, the magnitude of electrical current flowing through the body determines the extent of the damage (Table 4.1).

Table 4.1 Physiological effects of electrical current traversing the body

For voltages above a few hundred volts, the resistance across the body between hand and foot or between both hands amounts to about 1 kΩ. Connecting the standard voltage of 230 V to ground could therefore lead to a current of 230 mA traversing the body. Accidental contacts with the 230 V mains are not always deadly because the accident victims may have a higher-than-average skin resistance, carry shoes with insulating soles, or their unconscious reflex of pulling back the hand is fast and strong enough to safe their live.

As a consequence of the muscular contractions of a non-lethal electrical shock, the victim may be thrown off balance and fall, for example from a ladder, with severe consequences. Non-lethal electrical shocks can also lead to potentially deadly heart arrhythmia.

4.1.1.2 Fire

Electrical current leads to heating of ohmic resistances or may cause sparks at badly executed electrical connections. Both phenomena can ignite surrounding material and lead to a fire. Approximately 25% of domestic and industrial fires have their origin in electrical malfunctions, for example faulty equipment or overloaded circuits. An additional hazard exists in environments where flammable liquids or gases are stored and where excessive heat or sparks can lead to an explosion. Finally, an electrical arc (see next paragraph) may ignite a fire.

4.1.1.3 Electrical Arc

Multiple causes can provoke an electrical arc (arc flash):

• At high voltage, an electrical spark can jump over an air gap. The distance it can traverse increases with the applied voltage. The initial spark ionises air molecules along its path which become conducting for a short amount of time.

• A short circuit between two conductors, for example by connecting them accidentally by a metallic object, suddenly lowers the electrical resistance to a very small value and allows a high current to flow, which can melt the object causing the short-circuit.

• Condensation or water projection can cause the breakdown of electrical insulations, making a path for electricity to connect two conductors or to ground.

In an electrical arc, a large amount of energy is released in a short time interval. The temperature in the arc may attain several thousand degrees, leading to the emission of a broad spectrum of electromagnetic radiation between ultraviolet and heat radiation, and a sudden expansion of the surrounding air, causing a shock wave.

The consequences for a person struck by an electrical arc are similar to being struck by a lightning (a naturally occurring electrical arc) and potentially lethal: electrical shock, burns by the electrical current and radiant heat, exposure to UV radiation and noise, impact of projected high-speed hot or molten fragments.

4.1.2 Electrical Safety

4.1.2.1 Electrical Conformity

One aspect of electrical safety is the sound design and construction of electrical devices. Correctly dimensioned electrical installations minimise the risk of electrical faults during standard operation and provide protection to users if faults should nevertheless occur. In the European Union, the Low Voltage Directive 2014/35/EU [3] sets the frame for conformity of electrical equipment with operating voltages of up to 1000 V (AC) or 1500 V (DC), but, in contrast to other directives, the “Principal safety objectives for electrical equipment” in Annex I are not detailed prescriptions but only a declaration of broad goals. One of the declared aims of the European directive is to protect persons of harm from electrical hazards if the electrical equipment is used according to its intended purpose.

Detailed technical requirements for the construction and testing of electrical equipment are found in the harmonised standards to the directive, listed by the European Commission [2]. This document is regularly updated and contains references to approximately 700 standards published by CEN and CENELEC (see Annex B) regulating all aspects of design and construction of low-voltage electrical equipment for use in the European Union. Should no harmonised standard exist for a specific safety aspect, then the Directive states that the relevant standards of the International Electrotechnical Commission shall be used. An important prerequisite for obtaining a conformity certificate is a complete and accessible documentation of the equipment, not only for its use but also for repair and maintenance.

The declaration of conformity with the Directive and its harmonised standards is a self-declaration by the manufacturer, who engages his full responsibility. Certification by an independent testing laboratory is not required for the declaration, but it is advisable to consult specialists from electrical testing laboratories on the relevance and application of the various standards. This applies also to electrical devices which are purpose-built in one or a few exemplars for specific needs in a particle accelerator, and which cannot be found by a commercial supplier.

High voltage equipment is not covered by a European directive but standards for such equipment are published by IEC and CENELEC.

Standards for electrical equipment with a global scope are published by the International Electrotechnical Commission IEC.

4.1.2.2 Practical Electrical Safety

The consequences of an electrical accident can be severe, even lethal, and suitable means for protection against the electrical hazard must be taken. Electrical equipment and assemblies meeting the requirements of conformity do not present electrical hazards when they are used in accordance with manufacturers directives. Critical phases in the life cycle of electrical equipment are maintenance, repair, and modification. In these phases, protections are removed from the equipment and electrical conductors become accessible for direct contact.

Workers engaged in maintenance, repair and modification of electrical equipment must have a professional competence in electricity or electronics and specific knowledge of the equipment that they are asked to work on. The professional competence is obtained during vocational training, higher education, and courses in the frame of continuous education. In France it must be periodically complemented by specific, electrical safety-oriented training courses as required by law [4], which singles out the French approach within the European Union. Equipment-specific knowledge is gained from user- and maintenance manuals, complemented by electrical circuit diagrams. This underlines the importance of proper documentation in the process for obtaining conformity.

Before engaging work on electrical equipment, workers must make certain that it is no longer connected to the supply voltage, and that it cannot be reconnected during the work. For small equipment this is simply achieved by pulling the power cord. For installed equipment supplied by fixed cables coming directly from a transformer, a process called lockout is applied. This process is an example of a safe system of work (Sect. 5.2.2).

The lock-out process follows five steps which must be executed in order, and without omission, to create a safe working environment:

• 1. Identification of the connection: the worker must identify the output of the transformer or a switchboard which powers the equipment under question. This is very important in large installations where multiple equipment is connected via switchboards to the same source of electrical power.

• 2. Separation from the source: the identified connection is separated from the power source. This separation is secured by the worker, for example with a personal padlock. This step gives the whole procedure its name, lockout.

• 3. The operator leaves a signed note that (s)he has performed the lockout, this is called tag-out.

• 4. Then, the operator verifies on the equipment that it is indeed disconnected from the electrical power source and that all capacitors and impedances, able to store energy, are short-circuited. He uses standard electrical measurement instruments for this.

• 5. Finally, the operator connects the equipment safely to ground, making all attempts of repowering impossible.

In its simplest realisation a single worker is in charge of the whole lockout process. It is unrealistic to assume this to be possible in all but the smallest accelerator facilities. Usually, the responsibility for the components of the accelerator chain is shared between different entities, and the power sources may be geographically far from the equipment fed by them. These facts complicate the simple lock-out process with padlocks, as in the following example:

Before a normal-conducting magnet is maintained, it must be brought in a safe state by disconnecting it from its power source. The magnet expert requests from the electrical power group the separation of the specific magnet from the current source. A member of the electrical power group will separate the connection feeding the indicated magnet from the current source. He may secure the separation with his personal lock. He will inform the magnet expert of the separation. Multiple errors may occur at this stage:

• the electrical expert misreads the identification of the requested magnet or he accidentally locks out a different magnet.

• the electrical expert reconnects the magnet before the work on it has terminated

Note that in a process with distributed responsibilities the magnet expert cannot apply a personal padlock to secure the separation of the power source. In general, he lacks the competence to identify the power sources.

This makes the independent verification of the absence of electrical power and the connection the magnet to ground essential and virtually life-saving for the magnet expert.

The evolution of the lock-out process to complex scenarios with distributed responsibilities is treated in a general way in Sect. 5.2.2 under the headline “Safe Systems of Work”.

The best performing system of electrical lockout cannot prevent accidents in the cases where workers must approach electrical equipment while it is powered. These workers must be equipped with personal protective equipment (PPE) protecting against electrocution and arc flash (Fig. 4.2):

• Insulating gloves, with appropriate voltage rating. These gloves may have a short shelf-life and must be exchanged regularly.

• Safety shoes with an isolating sole

• A helmet with a face-shield, both resistant to heat and to projected fragments during an arc flash

• A jacket or shirt from non-ignitable fabric

Fig. 4.2

Electrician equipped with PPE: Helmet, arc-flash resistant visor, insulating gloves, fire-resistant jacket. Not visible: shoes with isolating soles. (Image credit: SUVA)

4.2 Mechanical Safety

Mechanical hazards occur when masses are moved or altered. The functional elements of an accelerator, often with a mass of many tons, are produced and maintained in mechanical workshops, must be moved into place by cranes and transport vehicles and connected with each other electrically and mechanically. This section starts with a view on machines and then focusses on transport hazards.

4.2.1 Machines at Particle Accelerators

Machines, defined as mechanisms animated by other than human forces [5], are ubiquitous in a particle accelerator during all phases of the life cycle:

• Production: an accelerator centre does not have the capacity to produce the elements for kilometre-long facilities, and the components for large particle accelerators are built in industry. The centre will still have workshops to produce prototypes or small series of specialised items. These workshops are equipped with standard machine tools (milling-, drilling- and turning machines). Special machines have functions like winding magnet coils, polymerising them and assembling whole magnets in mechanical presses. These machines are tailor-made for purpose.

• Operation: some elements of a particle accelerator are moved by motors or actuators and constitute “machines” according to the strict definition above: vacuum valves, beam intercepting devices and elements of beam instrumentation. For the next generation of particle accelerators with very small beam sizes, remotely operated, motorised systems for aligning the accelerator components are under discussion.

• Maintenance: the maintenance of accelerator elements is sometimes performed in-situ with portable machines, more extensive revisions are done in workshops equipped to disassemble and re-assemble magnets, cryostats, and other equipment.

• Decommissioning: accelerator components which have failed or have reached the end of their lifetime are disposed of. The separation of the radioactive components must be sorted by activity level and by nature of the material, requiring mechanical tools. Certain waste categories can be compacted with help of mechanical presses.

Warning signs symbolize machine hazards, such as hand injury, entrainment and crushing (Fig 4.3). 

Fig. 4.3

Warning signs against hazards from machines: hand injury, entrainment, crushing, after [1]. (Image source: https://publicdomainvectors.org)

4.2.2 Machine Safety

Most of the time, the function of a machine can be reduced to the movement of masses: a rotating drill, the cutting tool of a milling machine, the movement of the ram in a mechanical press require forces which may cause injury when directed against a person. This is expressed in the mechanical hazard triangle (Fig. 4.4).

Fig. 4.4

Mechanical hazard triangle. The adverse interaction of a mechanical element its energy and the operator may lead to an accident. After [11]

Extensive guidance exists for the control of mechanical hazards in the manufacturing industry, covering the use of all types of machine tools and of hand-held power tools [8, 11].

4.2.2.1 The European Machinery Directive

In the European Union, safety aspects of machinery are regulated in the Directive 2006/42/EC of 17 May 2006 on machinery [5, 6]. Under EU regulations, the content of directives must be adopted by the member states into national law within a reasonable delay. The purpose of the directives with focus on safety is twofold: on one hand, all EU members states shall apply the same safety standards on products, so that manufacturers can sell them unhindered in the territory of the EU. On the other hand, the directives are an instrument to guarantee minimal safety standards for the workers and the public for products imported from countries outside the EU.

The directive lists in its Annex 1 the essential health and safety requirements that machinery must meet before it can be introduced to the European Market. The 2006 edition of the directive also counts the construction of machines for one’s own use as introduction to the market, so that these devices must meet the same standards as goods for sale. This is obviously in the interest of protecting health and safety of the workers and operators. A machine conforming to the standard may bear the CE-marking, described in detail in Annex III of the directive and reproduced in Fig. 4.5.

Fig. 4.5

CE-Marking, after [5]. (Image source: https://publicdomainvectors.org)

A voluminous application guide to the Machinery Directive is available from the European Commission [6] as well as numerous titles of secondary literature, for example [12].

4.2.2.2 Conformity with the EU Directive

The straightforward way to obtain conformity with the European Directive is to construct a machine following a harmonised standard . Machinery constructed under respect of the technical requirements in a harmonised standard is presumed to conform to the legally binding European Directive. The list of harmonised standards is published by the European Commission [7].

In the European Union, it is advisable to buy standard machines from established manufacturers who will deliver a CE-certified machine, either from a EU member state or from countries with a long-standing commercial relationship with the EU.

The demonstration of conformity with the European Directive is more time-consuming and possibly costly for modified, entirely self-built, or imported machines. In international research centres, such as particle accelerator facilities, it is common that collaborators make in-kind contributions. These may come in form of a machine in the broad definition of the EU, and then conformity with the European Directive must be sought. A starting point for a self-built machine would be to follow a harmonised standard for conception and construction. Foreign machines or modified, old machines one must be document in a technical file, as described in Annex VII of the Directive. This process is called “assessment with internal checks” and is generally sufficient to attest the conformity of a “machine” installed in a particle accelerator, such as movable collimators or elements of beam instrumentation.

The prescriptions of the directive are not mandatory for machinery specially designed and constructed for research purposes for temporary use in laboratories (Article 1.2(h) of the Directive). This leaves a certain margin for very specific, self-built machinery in particle accelerators. In case this provision shall be used, it is advisable that a complete technical documentation, user manual and a risk assessment of the equipment is drawn up be able to mitigate any obvious safety risks. The risk assessment can take account of workers’ training in the use of a specific machine.

These remarks are valid, in slightly modified form, for obtaining the conformity for other categories of regulated products in the European Union (Annex B).

4.2.3 Transport at Particle Accelerators

Depending on the size of a particle accelerator, transport of persons and goods is an activity consuming time and resources. Warning signs for transport hazards are illustrated in (Fig. 4.6). The presently largest particle accelerator in the world, CERN’s Large Hadron Collider, has a circumference of 27 km. It touches the main site of the organisation in Meyrin (Canton of Geneva, Switzerland) at one point, from where the farthest point is at 8.6 km as the bird flies. Depending on traffic density, the transfer by car may take between 20 and 30 min between the access points on the perimeter. Personnel and material are transported on public roads with conventional vehicles. Accelerator magnets, 15 m long and with a mass of 15 tons, are lowered by overhead cranes into a tunnel connecting to the LHC and are moved on a special magnet transporter (Fig. 4.7). The largest distance to be covered undereground is 30 km. The transporter advances automatically along the tunnel with an optical guidance system at a speed of 3 km/h, an operator is on board for emergency action.

Overhead cranes play an important role in transport for a particle accelerator: heavy, bulky items are lowered into the underground areas with cranes, and they serve to move equipment in the accelerators (Fig. 4.8), in workshops and in testing areas.

Fig. 4.6

Warning signs against transport hazards: overhead load, industrial vehicle, after [1]. (Image source: https://publicdomainvectors.org)

4.2.4 Safety of Transport and Handling

As in machines, mechanical forces are employed to transport objects. These forces and the inertia of the loads are increasing with the mass and the speed of the moved objects. Figures 4.7 and 4.8 show typical transport situations in a large accelerator, they have a high potential for accidents if not handled correctly.

Fig. 4.7

Special vehicle for the transport of superconducting magnets in CERN’s LHC tunnel. The vehicle draws electrical energy by the orange catenary from an overhead rail. The vehicle advances by an optical guidance system, following the white line painted on the floor. The white structure to the left is the cryogenic supply line for the superconducting magnets. Copyright CERN, reused with permission

Fig. 4.8

Handling of an LHC dipole magnet with an overhead crane in the LHC tunnel. Copyright CERN, reused with permission

For road transport, one can rely on vans and lorries driven by professional, certified drivers. National regulations define minimal safety standards for road vehicles, they must be checked periodically by certified bodies. All lifting gear (cranes, chains, ropes, slings, hooks) must be regularly inspected for signs of ageing.

In the European Union, special transport vehicles, such as those operating in accelerator tunnels, fall under the Machinery directive [5], with additional requirements in Chap. 3 of Annex I.

The employer is responsible to supply conforming and tested transport equipment to his employees. To prevent transport accidents, a few core rules should be applied:

• Transport operators and vehicle drivers must be trained in the use of their equipment, including the safety aspects. Under certain national legislations, professional certificates are required to use mechanical transport and lifting equipment, elsewhere an instruction by the employer is sufficient.

• Operators and drivers must be well rested and concentrated before their shift.

• Transport and lifting operations shall be scheduled at moments where no other activities take place in the area. This avoids putting bystanders in danger and gives the operators the liberty to choose the optimal path of transport. For example, the magnet transport through CERN’s LHC tunnel (Fig. 4.7) takes place from 17:00 h on, after normal working hours.

• The transport/lifting equipment must be suitable for the task. Questions to be asked before every use are: is the capacity sufficient for the masses to be moved? Is it in good shape, no deformation or excessive traces of wear and tear?

Lifting and transport operators shall wear personal protective equipment:

• Safety shoes, to prevent slipping and to provide a limited protection of the toes against heavy loads.

• A safety helmet, in Europe according to standard EN 397, to prevent head injury from small pieces falling off from the load.

• Handling gloves, to protect the hands from bruises, cuts and abrasions while guiding and directing the load.

• High-visibility clothing, to provide optimal visibility by the other operators.

• If necessary, this equipment must be complemented by hearing protection or respiratory protection.

Guidelines for safe transport at the workplace can be found for example in [9].

4.2.4.1 Manual Handling

Despite being at the forefront of science and technology, and of having a parc of sophisticated transport and handling devices, manual handling is sometimes unavoidable at particle accelerators. On the last metre of a transport path, where an equipment must be hauled into its position, manual handling is often the last resort, for lack of adapted mechanical means or for lack of space for applying them. Manual handling bears the risk of injury from sharp edges, rough surfaces and from heavy loads getting out of control from the handlers. The risk of occupational illness such as back pain and other musculoskeletal disorders (MSD) is high, and these illnesses can become permanently disabling. Manual handling of heavy loads more than 25 kg shall be avoided wherever possible, mechanical handling aids shall be used, and only when this is impossible, the load must be shared among several workers.

Manual handling guidelines, especially with respect to avoiding occupational illness are given in [10]. In addition to authoritative information, this website provides tools for handling risk assessment.

4.3 Pressure Vessels

Recipients containing fluids (liquids or gases) at a pressure higher than atmospheric pressure are termed pressure vessels . The mechanical energy stored in the pressurised fluid is equal to the product of pressure and volume:

$$ W=p\cdot V\left[\mathrm{Pa}{\mathrm{m}}^3\left]=\right[\mathrm{J}\right] $$

If this energy is released instantaneously, severe damage to personnel and equipment may result. In addition, the hazards of the pressurised fluids must be considered, for example cryogenic, flammable, or toxic fluids (Fig. 4.9).

Fig. 4.9

Warning sign: Bottle under pressure, after [1]. (Image source: https://publicdomainvectors.org)

4.3.1 Pressure Vessels at Accelerators

A typical field of application for pressure vessels at particle accelerators is as a part of cryogenic systems (see Sect. 2.3). They are used for containing the cooling medium of superconducting devices, and to contain cryogenic liquid particle detectors (Argon, Krypton and Xenon calorimeters). Gases and fluids under pressure are used for cooling detectors at temperatures close to 0 °C. Conventional applications of pressurised fluids include pressurised air, sometimes used in workshops as a power-transmitter for driving tools without electricity. Pressurised air is also used to move actuators, for example in vacuum valves.

The components for conventional applications can be bought from manufacturers who build the equipment according to legal prescription, for example to the European Directive and harmonised standards in the EU. The institutions developing superconducting or particle detector systems with pressure vessels are responsible for meeting the applicable legislation and standards.

4.3.2 Pressure Vessel Safety

Pressure vessels are ubiquitous in the processing industry and they may present a considerable accident risk with severe consequences [39]. The rupture of a pressure vessel results in the following accident hazards:

• Release of the fluid at high pressure by the leakage, leading to mechanical injury.

• Other than the risk from a pressure jet, the released fluid may be flammable or have chemical or cryogenic hazards, exposing workers and bystanders to additional risks after the release has terminated.

• Disintegration of the whole vessel by explosion, followed by projection of fragments and pressurised fluid.

Badly designed pressure vessels can be the source of severe accidents leading to loss of property and possibly of life.

Two principles in pressure vessel design protect against accidental rupture and release of the contents:

• First, a solid construction of the vessel following current engineering practice and described in national or international standards. The mechanical resistance of the construction materials (usually steel, sometimes other metals for special applications) and their assembly balances the expected pressure variations of the contents up to a maximal operational pressure or service pressure P_s.

• Second, for the case that pressure in the vessel rises accidentally to values higher than P_s, pressure relief devices are foreseen to release some fluid and thus lower the pressure. These valves or burst-discs open at a predefined set-pressure and vent the contents of the vessel in a controlled way. Pressure relief devices are mounted in pairs, a relief valve which opens at a pressure P_v1, is combined with a burst-disc with set-pressure P_v2 > P_v1. The burst-disc opens in two failure cases:

  • the relief-valve fails mechanically, or

  • the sudden mass flow to be evacuated surpasses the valve’s capacity.

  It is a single-use device and must be replaced after opening. The pressure relief devices must have a sufficiently large opening surface to release the overpressure. The calculation of safety devices is treated in international standards for ordinary [14] and for cryogenic pressure vessels [13]. For cryogenic pressure relief devices, the calculations consider the heat influx to the cryostat, the temperature-dependent thermodynamic parameters of the fluid and phase changes from liquid to gaseous. In the regime where the operator of the pressure vessels works with a notified body, the existence and performance of safety devices becomes an important part of the safety assessment.

4.3.3 The European Directive on Pressure Vessels

Legislation in all countries takes account of the dangers of pressure vessels by prescribing minimal safety standards. In member states of the European Union, the Pressure Vessel Directive [40] regulates the manufacturing and use of pressure vessels. The deadline for transferring its principles into national law in the member states was mid-2016. With this directive, the European Commission aims to harmonize safety standards in the Union, so that goods containing pressure vessels can be freely exchanged on the internal market while at the same time minimal safety standards for workers and consumers are respected.

Annex 1 of the directive lists the essential safety requirements (ESR) which must be met by pressure vessels which are placed on the market, i.e. made available to customers. For inert gases, as helium, the ESR are mandatory if the service pressure PS of the vessel exceeds 1000 bar (the directive still quotes pressures in bar, as in engineering practice) or the volume V exceeds 1 litre (L) and the product PS V is larger than 50 bar L.

Then, vessels are subdivided in those carrying dangerous fluids (for example flammable, oxidising, or acutely toxic gases and liquids), generally not of concern at accelerators, and those for other fluids. They are further classified in 4 categories by the product of service pressure PS and volume V. The categories determine the procedure according to which the manufacturer must demonstrate conformity with the ESR. For vessels in the lowest category I (PS V < 200 bar L), the mechanism of “internal production control” is deemed sufficient to prove that the ESR are fulfilled. This requires the drawing up of detailed technical documentation and a risk assessment. During production, internal controls shall be made to ascertain that the technical documentation is followed.

From category III onwards (PS V > 1000 bar L), a notified body must be consulted during the conformity assessment process. Notified bodies are entities accredited by a Member State of the EU to assess whether a product meets certain standards. Familiar examples are the associations or companies entitled to periodically check the traffic-worthiness of a car. As for machines (see Sect 4.2.2), the construction of pressure vessels for proper use falls under the application of the directive [40]. Consequently, the consultation of a notified body must be considered in cost- and schedule estimates for the construction of pressure vessels of categories III or IV for an accelerator facility.

4.4 Fire Safety

Fire hazard is found at workplaces and in homes likewise. Accelerator facilities and their associated workshops, test areas and offices make no difference, and fire prevention is an important point in a safety prevention programme. In this section, the conditions for a fire to break out are enumerated, the potentially devastating effects of a fire described, and prevention measures derived (Fig. 4.10).

Fig. 4.10

Warning signs against fire hazards: flammable substance, explosive substance, oxidant substance, after [1]. (Image source: https://publicdomainvectors.org)

4.4.1 The Fire Triangle

Three ingredients are necessary to start a fire and make it grow, they are often represented in form of the “Fire Triangle” (Fig. 4.11):

Fig. 4.11

The fire triangle: The combination of fuel, oxygen (or another oxidizing substance) and heat, or more general an ignition source may lead to the outbreak of a fire

• 1. Fuel: burnable substances are fuel for a fire. They can be solids (for example wood, paper, cardboard, packing materials, plastics), liquids (for example solvents, paint and varnish, petrol) or gases (for example butane, propane, acetylene, hydrogen).

• 2. Oxygen: from the chemical point of view, at the heart of a fire are exothermal oxidation reactions, therefore no fire without oxygen. The oxygen content in air is often enough for keeping a fire going, it may be enhanced by a ventilation system, carrying fresh air in a burning area, or by oxygen in cylinders for welding or for experimental purposes.

• 3. Ignition source: the presence of fuel and oxygen is not enough to ignite a fire; an ignition source is necessary to provide the energy to start the exothermic reaction. Ignition sources may be naked flames, or smouldering ashes (from smoking materials, in laboratories), sparks (from grinding, welding and faulty electrical equipment or static electricity), hot surfaces (from insufficiently ventilated or cooled electrical and mechanical equipment, lighting, heating).

A fire may start when an ignition source comes close to fuel in a well-ventilated atmosphere. Easy flammable fuels (characterised by a low temperature of ignition) often start the fire, which can spread to other, less flammable materials. Once started, a fire will continue to burn and spread as long as fuel and oxygen are available.

4.4.2 Fire Hazards at Accelerators

A fire hazard exists at those locations, where the three elements of the Fire Triangle (Sect. 4.4.1) come together. Oxygen in air is ubiquitous, and it is enough to consider ignition sources and flammable materials (fuel).

The availability of ignition sources for a fire depends on the operational state of the accelerator:

• During accelerator operation, all electrical systems are powered. Electrical resistances, short circuits or sparks can generate enough heat to set neighbouring material at fire.

• During periods where the accelerator is stopped, many of the accidental ignition sources from electricity are turned off. In these phases the focus is on work activities using open flames, hot points or producing sparks, like welding, brazing, and grinding. When fire prevention measures during this work are neglected, flammable materials close to the work site may be ignited.

Various types of flammable material can be found in an accelerator facility:

• Cable insulations and other organic materials may burn readily once lit up. An electrical fire can start by igniting a small quantity of insulating organic material and then spread over to the contents of an electrical rack. Cable insulations are often rated for fire resistance, but with enough heat energy from a fire close by, they will eventually start to burn.

• Recently, metal-ion electrical storage batteries are gaining importance, for example Li-ion batteries. They consist of Lithium- and carbon electrodes, separated by an electrolyte made from an organic, flammable substance. Short circuits in these batteries may ignite the electrolyte and lead to a runaway reaction with very high flame temperatures from the oxidation of the alkali metal. Li-ion battery fires in electrical vehicles have been reported to destroy the battery and igniting flammable material in its vicinity [19].

• Another source of fuel is packing material, like carton and wood. Packing often contains fillers made from polystyrene, which is easy to ignite. These materials are brought to the accelerator area with new equipment, and they may accumulate if no regular housekeeping is performed.

• Some detectors for particle physics use flammable gases in ionising radiation detectors, or flammable solvents for certain types of scintillation detectors.

• Flammable solvents are also found in laboratories with frequency-tuneable dye lasers, and in workshops, where they are used for many applications.

4.4.3 Tunnel Fires

Tunnel fires came into the light of public attention with the tragic events in the Mont-Blanc road tunnel between France and Italy in 1999, the Kitzsteinhorn funicular, Austria in 2000, and the underground metro station fire in Daegu, South Korea in 2003. The enclosed nature of tunnels gives rise to special fire phenomena in conjunction with the ventilation. Escape of personnel and access for emergency forces is more perilous and can become impossible if temperatures rise too high or smoke obstructs the pathways. A comprehensive reference for tunnel fires is [15].

The potential causes of fire in accelerator tunnels are different from road or rail tunnels, but the dynamics of fire development and fire spread, and its consequences are similar.

4.4.3.1 Fire Dynamics in Tunnels

Fires in open air radiate heat in all directions and receive a nearly unlimited oxygen supply. In compartments (closed environments), the walls reflect a part of the fires heat energy and lead to more intensive burning. Also, the supply of oxygen is limited. One commonly used parameter to describe the size of a fire is the Heat Release Rate (HRR).

One speaks of a fuel-controlled or oxygen-rich fire when the rate of combustion and the HRR are dominated by the availability of combustible materials. Oxygen is available in sufficient quantity. In fuel-controlled conditions, the HRR of the fire grows until all fuel is consumed.

In a ventilation-controlled or oxygen-starved fire, there is not enough oxygen to burn all the fuel available. Oxygen supply is limited by the enclosure of the fire zone. The heat of the fire releases flammable gases from the fuel, but these are incompletely combusted. In ventilation-controlled conditions, the HRR of the fire stagnates. The air flowing out of the fire zone is entirely depleted of oxygen and loaded with toxic carbon monoxide and partially combusted fire gases. If a ventilation-controlled fire is suddenly supplied with oxygen (by opening a door or window in a compartment, or by activating mechanical ventilation in a tunnel), the hot, unburned fire gases ignite spontaneously and lead to the phenomenon of “backdraft”, a sudden development of fire engulfing a large area, radiating a lot of heat, and dangerous for evacuating persons and fire fighters.

Ventilation-controlled fires are a concern in compartments such as electrical substations or chemical storages. Generally, in tunnels, the oxygen supply is high enough for a full combustion of the fuel. The onset of a ventilation-controlled fire can be estimated by putting the available combustible material in relation to the amount of oxygen required to consume them entirely, by evaluating the chemical combustion reactions in the fire. Methods for this are explained in detail in [15, 20].

4.4.3.2 Smoke Control by Ventilation

A fire releases heat, toxic gases, and smoke. Smoke, microscopic particles suspended in air, is carried by hot air and combustion gases and will raise by buoyancy to the tunnel ceiling in a fuel-controlled fire. At the location of the fire, turbulence mixes air, gases and smoke, and in some distance, from the fire, a layered structure is established (Fig. 4.12). With increasing distance from the fire, the smoke and gases will eventually cool and fill the whole tunnel cross section. Tunnel ventilation with a speed equal or higher than a specific “critical velocity” can keep the part of the tunnel which is upstream from the fire essentially smoke free and allow safe evacuation and access by fire fighters from this direction.

Fig. 4.12

Sketches of smoke stratification during a tunnel fire. Top: low ventilation velocity (0–0.5 m/s). Bottom ventilation speed close to the critical value. From [21]

The critical velocity depends on the tunnel geometry (cross sectional area) and on the characteristics of the fire, principally its heat release rate, which can be evaluated from the amount of combustible material and a fire development scenario.

The design of the tunnel ventilation can be considered as an important part of the fire prevention strategy in an accelerator tunnel. Nowadays the engineering process is supported by numerical fire simulation programs. In simple cases, a so-called compartment model may be sufficient, for more complicated situations a computational fluid dynamics (CFD) code is necessary. The “industry standard” of fire and smoke propagation CFD codes is the Fire Dynamics Simulator from the National Institute for Standards and Technology NIST [23].

4.4.4 Fire Prevention

Going back to the fire triangle, fire prevention focusses on limiting the amount of combustible material and on controlling ignition sources. Only exceptionally, the oxygen content of air can be reduced to prevent fires, for example in archive- or computer buildings. Fire prevention consists of evaluating the fire risk, taking mitigating measures, detecting a fire, and securing the evacuation of personnel.

4.4.4.1 Fire Risk Assessment

The prevention of fires starts with a fire risk assessment. During this exercise the assessment team, which ideally consists of safety experts, personnel familiar with the facility and line managers, the presence of flammable materials and of ignition sources is determined. This is a specialised from of the hazard register for general risk assessments (Sect. 5.1.2).

In a second step, the probability of fire scenarios from the conjunction of ignition sources and fuel are evaluated. While the probability of electrical failures leading to fires can be taken from industry statistics, the estimation of a probability for human failure is more approximate.

In the last step, the consequences of the most probable fires must be evaluated. Heat release curves for the identified flammable materials inform about energy released in during the fire, this must be put into relation with the fire resistance classes of the building materials. An elaborate consequence analysis can make use of fire and smoke propagation CFD programs [23].

Fire risk analysis is the topic of a series of international standards [22], its third part consists a worked example for an industrial facility. This is the closest one comes in the literature to an accelerator facility.

4.4.4.2 Fire Risk Mitigation

The fire risk assessment informs about the most probable fire scenarios, in relation with the presence of ignition sources and flammable materials. The logical steps of mitigation are

• Elimination of ignition sources

• Removal of flammable material

It has been outlined (Sect. 4.4.2) that the principal ignition sources in accelerators are of electrical and human nature.

Electrical ignition sources are overheating, sparking, and arcing of electrical components. These can be eliminated already at the design stage of an equipment by a sound design of the electrical circuits, and by enclosing the equipment in spark-proof cabinets. The latter step is mandatory in locations where an explosive atmosphere may potentially form, for example where large quantities of liquid fuels or solvents are stored or used. The technical term is ATEX area (from the French ATmosphère EXplosive). In the European Union, these situations are regulated in the ATEX Workplace directive [17] and the ATEX Equipment directive [18].

During operation, defective components are identified either by a malfunctioning device, or can be detected with infrared cameras which show excessive heating.

Human error as an ignition source is combatted with a system of administrative controls, i.e. regulations, procedures and permits. After a local fire risk assessment, concentrating on the absence of flammable material and the availability of extinguishing means, a fire permit is given to a worker to perform hot work (welding, brazing, grinding). Workers in these trades must be made aware of the fire risk they may provoke and trained in methods to extinguish a small fire before it spreads out of control. The discipline of the fire permit process is important, only if it is regularly reminded and enforced one can effectively reduce the number of cases of fire caused by human error.

Control of the characteristics and the amount of flammable materials is the second path to minimisation of fire risk. Easy flammable materials shall be replaced with materials which are difficult to ignite. This measure can be applied for example to electrical cables, where ignition resistant insulations exist today. Old cables should be replaced with such newer types when they arrive at the end of their useful lifetime. The same measure applies to a range of other organic materials. Another example are insulating fluids in high-voltage switchgear. They are mineral oils with a low flashpoint, defined as the temperature where the vapour layer over the flammable liquid spontaneously ignites, and should be replaced with synthetic ester-based fluids with higher flashpoint.

The amount of flammable materials in premises, especially where ignition sources are present, shall be regularly controlled. Only materials necessary for the operation of the facility must remain, everything else, for example packaging materials, goods stored in operational area, and waste, must be cleared out to reduce the probability of a fire.

4.4.4.3 Fire Detection and Evacuation

If a fire emerges it is important to detect it as early as possible, to secure the evacuation of personnel, and to start firefighting before the fire is fully developed. Fire detection systems work by two principles, either smoke detection by measuring the opacity by light absorption, or heat detection with infrared detectors. Smoke detectors with ²⁴¹Am radioactive α-emitters are no longer considered good practice, because of the long half-life of the radionuclide (t_1/2 = 432 years), which made them hazardous waste once the smoke detectors had exceeded their useful lifetime.

In large facilities, multiple fire detectors are combined in a network with a computer-based monitoring application and with local alarm panels, giving an indication of the fire location based on the detector(s) which have been triggered. When a fire alarm is confirmed, usually by the triggering of two independent detectors, the fire brigade is alerted, and an evacuation of the affected building or area started.

The provision of evacuation pathways is prescribed in local regulations, defining the maximum length, minimal width and standardised signposting of paths leading to emergency exits. These regulations are integrated in the contemporary design of surface buildings (offices, laboratories, workshops), but they are often impossible to implement in large accelerator areas, especially when built underground.

Since construction of any building depends on the obtention of a construction permit by the local authorities, the application process is the right moment to develop exceptional measures for the evacuation of personnel from facilities which cannot be built according to the standards.

The exceptional measures will be based on the fire risk assessment of the facility, which must be completed by an evacuation study . The studies must demonstrate the possibility for personnel to evacuate safely from any accessible part of the facility in case of fire. Technical measures to ensure evacuation are:

• Construction of the facility with structural elements resisting the fire heat for a minimal amount of time. Construction materials are classified in fire resistance classes between 30 min and 2 h for normalised design fires.

• Installation of fire and smoke resistant doors along the evacuation pathway, to bring workers already in relative safety before reaching free air. Like the construction materials, these doors are rated for their resistance capabilities.

• Installation of a fire detection system for early warning of personnel and alerting of emergency forces.

• Clearly visible signage of the emergency pathway with standardised signs. In the European Union, signage of hazards and of emergency paths is regulated in a council directive [1] (Fig. 4.13). Entirely based on pictograms, this signage is understood by personnel of any native language.

Simulation codes for fire and smoke propagation codes can help to design an efficient fire detection system and to demonstrate that the evacuation of personnel is possible without exposing them to excessive heat or smoke.

Fig. 4.13

Examples of emergency path signage (left) and assembly point (right), after [1]. (Image source: https://publicdomainvectors.org)

4.5 Occupational Noise

Exposure to high levels of noise has been recognized as an adverse factor for occupational health and personnel must be protected from it (Fig. 4.14). Short-term effects are acute acoustic trauma, affecting the eardrum or the bones in the middle-ear, temporary threshold shift (a reduced sensitivity to certain frequencies of the audible spectrum) and temporary tinnitus (a ringing in the ears which may continue for extended periods after noise exposure). After long-term exposure, threshold shift and tinnitus may become chronic affections. A long-term exposure to noise may also cause stress with the accompanying symptoms of high blood pressure and bad sleep. Noise is also a concern for the public living in the vicinity of research establishments (Sect. 4.6.1).

Fig. 4.14

Obligation sign to use ear protection, after [1], (Image source: https://publicdomainvectors.org)

4.5.1 Noise Measurement

Noise, or sound in general is transmitted by an acoustic pressure wave. The ear is sensitive to sound intensity I, related to the root mean square pressure p_rms of the acoustic wave by

$$ I=\frac{p_{rms}^2}{\varrho c} $$

ρ is the density of air and c the speed of sound. Young healthy adults can distinguish minute pressure changes of p₀ = 20 μPa, just above the level of thermal noise in air. The pain threshold of hearing is situated at about 60 Pa. Like other human senses, hearing has a logarithmic sensitivity scale.

To characterise noise at the workplace or in the environment, sound pressure level (L_p) is measured. L_p is a logarithmic measure of the sound intensity and is the quantity indicated by sound level meters:

$$ {L}_p=10{\log}_{10}\left(\frac{p_{rms}^2}{p_0^2}\right)=10{\log}_{10}\left({p}_{rms}^2\right)+94\left[\mathrm{dB}\right] $$

Sound level meters are weighting the frequency spectrum with a sensitivity function mimicking the human ear, the standard weighting function is designated “A” in the reference document IEC 60651 and thus sound level is expressed in units of dB(A). Sound pressure levels of a few sources are indicated in Table 4.2.

Table 4.2 Selected sound pressure levels. From [28], [31]

From the definition of L_p follows that an increase of 10 dB corresponds to a ten-fold sound pressure level, and 3 dB are approximately doubling sound intensity. Care must be taken when noise from different sources is added, the correct way is to add the respective values of ${p}_{rms}^2$. The addition of two noise sources with equal sound pressure level at the location of measurement increases the total L_p by 3 dB(A).

A simple method to obtain a rough estimate of instantaneous sound pressure level is described in Table 4.3. Its purpose is to trigger precise measurements with a calibrated sound pressure meter if one of the thresholds is likely to be crossed.

Table 4.3 Simple test to judge if a noise risk assessment is needed [26]

At the workplace, not only the instantaneous noise level, but also the time-integral of sound pressure level is of interest. The total noise exposure is normalized to an 8-h working day L_EX,8h. [30]. A worker exposed to 85 dB(A) during an activity lasting 4 h and then working in a silent environment for the rest of the day has an 8-h exposure level of 82 dB(A). The evaluation of L_EX,8h for a specific worker requires a detailed analysis of his activities, each of which must be accompanied by a measurement of sound pressure level L_p. Finally, the contributions of the different activities must be added and normalized to 8 h. The necessary remedial action depends on the calculated 8 h-sound pressure level (Table 4.4).

Table 4.4 Required Mitigation for Noise in the European Union [24]

Further information about occupational noise, its sources, effects, and prevention can be found in [30]. The calculations described in ISO 9612:2009 are facilitated by a spreadsheet which can be downloaded from UK Health and Safety Executive [27].

4.5.2 Protection Measures Against Noise

From an 8 h-exposure level of L_EX,8h = 80 dB(A) on, protection measures are recommended to prevent adverse effects of noise exposition for the worker. They become obligatory at L_EX,8h = 85 dB(A). One distinguishes technical measures and personal protection. Technical measures are preferable, they protect all workers indiscriminately:

• Replace the equipment at the origin of noise with a newer, less noisy type.

• Isolate the noise source by encasing it with noise-absorbing materials.

• Isolate the vibrations of the source transmitted to other structures by placing it on isolating floor mats or by using absorbers.

Only where technical measures are impossible or too costly, or where access to the noisy environment is infrequent, workers must be protected with personal protective equipment and clear instructions. The use of this equipment depends on the level of sound pressure that must be reduced to meet the legal requirements. The attenuation of the protector should reduce the sound pressure level perceived by the employee to about 72 dB(A). Overprotection is counterproductive because the worker can no longer communicate or perceive warning signals. Different options are shown in Fig. 4.15.

Fig. 4.15

Personal protection equipment against noise. From left: earplugs, individually moulded earplugs, earmuffs. From [26] with permission 

In environments where sound pressure level exceeds 110 dB(A), more than one protection can be worn to reduce the sound pressure level at the ear to below 85 dB(A). In the EU, all hearing protection should carry the CE mark as an indication that it meets the essential requirements of the Personal Protective Equipment Regulations [25]. The reference [26] contains a detailed guide for the selection of personal hearing protection.

4.6 Environmental Impact

Today, technical projects cannot be realised without a strong consideration of environmental protection. Both the public opinion and the legislator demand accelerator facilities where damaging effects on the environment are minimised or compensated for (Fig. 4.16).

Fig. 4.16

Warning sign against environmental hazard. (Image source: https://publicdomainvectors.org)

Climate change and the fact that the globe has only limited resources must be considered in the design and development of new accelerator facilities. Initiatives have started to reduce the energetic footprint of particle accelerators.

4.6.1 Releases to the Environment

Accelerator centres, as other technical plant, release various substances to the environment, either planned and within authorised quantities, or accidentally, with potentially damaging consequences. An accelerator facility is not a closed system, but it is open to its environment. Effluents can be either liquid and solid, released with water, or gaseous and aerosols released with air. Noise can be a nuisance for the public living in the vicinity of the accelerator’s plants.

4.6.1.1 Liquid Effluents

The operation of CERN’s LHC for example produces 160 MW heat in the cryogenic plants, normal conducting magnets, and the air-conditioning system. This heat must be removed continuously. A primary cooling water circuit transports the heat energy by heat exchangers from the secondary, equipment specific circuits. The primary cooling water is then circulated to cooling towers where the heat is shed to the environment.

The secondary cooling circuits are in contact with the accelerator equipment. In radiation areas, cooling water can become slightly activated. The activation products of distilled water are short lived with exception of tritium (³H), with a half-life of 12 years but a very low radiological effect on living organisms. Cooling water occasionally transports activated metal corrosion products, with longer half-lives and higher activity concentrations. Before cooling water circuits are drained for maintenance a sample from the circuit is taken and measured for residual radioactivity. It can only be release to the public network when the activity concentration lies below the legally fixed release limit.

Run-off from building roofs, streets and places can be collected in a separate drainage network connecting to natural surface waters (streams and rivers). Accidental pollution of the drainage network is possible by the accidental rupture of tanks or pipes for polluting liquids, often chemical products. A second source for surface water pollution is the inattention of personnel, cleaning polluted tools or vessels and shedding the cleaning water simply outside of the building.

4.6.1.2 Air-Borne Effluents

Environmentally polluting products can be released by air from a particle accelerator centre.

Under certain weather conditions, the steam plumes from cooling towers can be clearly distinguished even from a distant vantage point, and while this may not please the spectator, it does not represent a danger in itself. However, the cooling towers must be regularly treated against legionella. Bacteria from this family thrive in stagnating, lukewarm and represent a hazard for the causation of serious respiratory illness.

The air from accelerator tunnels is activated by secondary particle cascades (Sect. 3.2) and the products from the spallation process are transported to the environment. Most spallation products of pure air are short-lived, with the exception of ⁷Be with a half-life of 57 days and ³H with 12.3 years. If activated molecules are attached to aerosols, then they can be retained by filters, but gaseous activity cannot be retained. The environmental impact of radioactive releases is estimated with environmental screening models [35], making conservative assumptions about the migration of radionuclides in plants and livestock and the eating habits of the population. In such models the impact CERN’s accelerators on persons living in its vicinity has been calculated to be a factor of 10 below the legal limit of 300μSv/year.

Some technical gases have a high potential to either damage the atmosphere’s ozone layer (e.g. fluorinated gases) or they have a high climate potential (e.g. methane). Ozone–layer damaging gases are controlled under the Montreal protocol [37] and they are used under exceptional permission, with the aim to remove them entirely. The use of climate-active gases should be reduced with the efforts to make the industrialised economies CO₂-neutral in a time-span of 20 to 30 years.

4.6.1.3 Environmental Noise

Environmental noise is a by-product of industrial processes which may cause distress and even illness to persons exposed to it [38]. The World Health Organisation recommends an environmental noise level of less than 35 dB in the night to guarantee a reposing sleep. Legally binding limits for environmental noise are decided on national level.

At accelerator facilities, sources of environmental noise are cooling and ventilation plants (fans, blowers), cryogenic plants (compressors), transformers and other power electrical equipment (transformer noise). When these plants are properly designed and protected, environmental noise should not cause problems. As for occupational noise, sources must be isolated from the environment so that vibration and sound cannot propagate. Transformer parks, which cannot be placed in a building, must be surrounded by sound-proof walls.

Many accelerator facilities in Europe were originally built as green-field projects, reasonably far from towns and villages. The growth of urban areas had the consequence that densely inhabited quarters were developed in the vicinity of the accelerator sites. Their inhabitants are exposed to environmental noise from the facility. In these cases, compromises between the local governments, the inhabitants and the accelerator plant’s management must be negotiated to achieve a solution which satisfies all parties.

4.6.1.4 Environmental Monitoring

Environmental monitoring serves to demonstrate that the unavoidable release of pollutants from a technical plant is well below the legal limits.

Environmental monitoring takes place at two locations: at the source, where the effluents leave the plant (emission measurement), and in the environment, where one tries to measure the concentration of the substance which has been deposited in the environment (immission measurement).

At the source,

• the activity of radioactive isotopes in released air is measured with special ionisation chambers for gases, and with gamma- or beta-spectrometric methods for the deposits of aerosols on filters in the ventilation release;

• radioactive effluents in water are registered with scintillation detectors immersed in the outflowing water;

• the pollution of water with hydrocarbons can be assessed on-line by catalytic oxidisation or by optical measurements.

In the environment,

• the level of stray radiation escaping the accelerator shielding is measured by ionisation chambers and passive thermo-luminescence dosimeters;

• a potentially radioactive pollution of the biosphere is excluded by spectrometric measurements of media like soil and water and produce like fodder and grain;

• aerosol samplers are placed in some distance to the monitored plant, they aspire large quantities of air which passes a filter, which is measured for radioactive substances after a fixed collection time;

• environmental noise is assessed with sound detectors which are ruggedized for environmental, all-weather use.

4.6.2 Reducing the Energetic Footprint

In the last decade, society became increasingly aware of the negative impact of fossil energy consumption on the environment and the climate. High-energy accelerators and colliders built for applied and fundamental research consume large amounts of electrical energy and it becomes imperative to minimise their energetic and climatic impact.

In Europe, major particle accelerator centres have pooled their resources and collaborate in the framework of EU research programmes on the development of strategies to reduce the energetic footprint of accelerator facilities. A series of workshops summarises the results of this research and development efforts. [33, 34], in the proceedings of these workshops more details are found to the following examples.

4.6.2.1 Energy Supply from Sustainable Sources

The European Spallation Source ESS in Lund (SE) plans to use only energy from renewable resources (water, wind, solar) bought on the Swedish electricity market, on which more than half of the offer is sustainable (mostly water-powered electricity).

4.6.2.2 Energy Recovery

Accelerator facilities use large amounts of energy for cooling (cryogenic plants, electromagnets, data centres, air conditioning). Conventionally, the excess heat from these processes is shed to the environment by heat exchangers and cooling towers. The excess heat from cooling applications is available at a low temperature level. Heat pumps can convert it to high-temperature heat energy with little additional energy input. The overall efficiency of such systems is positive. ESS collaborates with an international energy supplier to build a heat network, supplying parts of the city of Lund with excess heat from the accelerator and target. CERN will supply excess heat from one of LHC’s cryogenic plants to an Eco-quarter in the nearby town of Ferney-Voltaire. Part of the energy generated through summer will be injected in a geological storage so that it is available in the heating period.

4.6.2.3 Accelerator Technology

Elements of the particle accelerator can be optimised to a lower energy consumption.

Under certain conditions, electromagnets can be replaced by permanent magnets. Owing to their fixed magnetic flux density, they are useful in portions of accelerators or beamlines where the energy does not change. In a new linear proton accelerator at CERN (LINAC 4), permanent magnet blocks are used to build a quadrupole magnet in a reduced available space [36]. Adjustable tuning blocks from magnetic steel permit an adjustment of its field gradient within 20% of the nominal value. While space-saving was the technology driver, this application of permanent magnets demonstrates their potential in accelerator technology.

FASER, a forward spectrometer in the LHC tunnel in the for the search for exotic particles [32] employs dipole magnets assembled from permanent magnets to separate charged and neutral particles.

RF cavities can make large gains in energy efficiency when their walls are made from superconducting material for their walls. In a comparison of a hypothetical linear accelerator for a terminal energy of 10 GeV with conventional Cu cavities or s.c. cavities from Nb, R. Porter [in 34] finds that the superconducting solution would use more than 100 times less power than the conventional, and it would be 20 times shorter. Most of the power in the s.c. accelerator goes in the cooling of the cavities to 2 K. Switching to the novel s.c. material of Nb₃Sn, which can work at 4.2 K, would allow a power gain of a factor 400.

As a conclusion to this section of energy efficiency one may remark that particle accelerator technology is a field at the forefront of research and development. It is expected that energy-efficient technologies developed for accelerators will rapidly make their way to applications in other fields of society, as has happened previously with accelerator-related technologies for industry and medicine.