Discussion on Common Bolt Scuffing Factors and Control Measures in the Installation and Maintenance Stage of Nuclear Power Plant

Abstract

Because of its high control precision, high efficiency, easy operation and safe to threads, bolt tensioning is widely used in many mainstream reactor types of nuclear power plants at home and abroad, such as M310, EPR, VVER, AP/CAP and HPR 1000. It plays an indispensable role in the installation and removal of foundation fixing bolts and flange connecting bolts of dynamic and static mechanical equipment in the construction and maintenance stage of domestic nuclear power plants. With the development and upgrading of bolt tensioner, the technology of bolt tensioning has become more and more mature. At present, it has become an irreplaceable means of bolt pre tightening.

However, in the installation and maintenance stage of nuclear power plant, the fixing bolts and flange connecting bolts of mechanical equipment will still be jammed, damaged or even be scuffed. This situation may cause damage to bolts and bolt holes, which requires additional repair and rework costs and the construction period would delayed, or it may cause scrapping of bolts and bolt holes, and even affect the main functions of equipment and flanges, cause more serious economic losses and even nuclear safety accidents.

This paper analyzes the common bolt scuffing factors in the installation and maintenance stage of nuclear power plant through the experience of actual disposal means on site of bolt tensioning and scuffing, and defines the corresponding control means pertinently, so as to reduce the bolt scuffing probability, and then reduce the additional cost in the installation and maintenance stage and even the probability of nuclear safety accident.

Mainly analyze from the aspects of thread material, working environment, manufacturing tolerance, thread cleaning, bolt assembly, bolt tensioning and bolt fastening, and formulate standard and effective work plans for thread material inspection, working environment isolation, manufacturing tolerance inspection, thread cleaning, bolt assembly, bolt tensioning and bolt fastening in combination with the actual situation of the construction site, so as to ensure the reduction or even elimination of nuclear power plant installation The probability of bolt scuffing in the maintenance stage can enhance the quality of nuclear power installation and maintenance at the construction site of the nuclear power plant and improve the operation safety probability of the nuclear power plant.

1 Introduction

1.1 Research Background

1.1.1 Application Situation of Bolt Tension in Nuclear Power Plant During Installation and Maintenance at Home and Abroad

The pre tightening method of bolt tension is widely used in the installation and maintenance stages of many mainstream pressurized water reactor types of nuclear power plants in operation and under construction, such as M310/EPR, VVER, AP/CAP, Hualong 1, and other fourth-generation mainstream nuclear power plants in the future, such as high temperature gas cooled reactor, molten salt cooled reactor, sodium cooled reactor, etc., which are in the stage of experimental construction.

The pre tightening method of bolt tensioning is easy to operate, has high tensioning precision, does not damage the thread, and is not affected by the operation space and environment. Bolt tensioners can be used individually or in groups. Multiple tensioners can be used in parallel, which can not only improve work efficiency, but also ensure the uniformity of force on multiple bolts, which is particularly important in the flange connection of high-pressure seal. For example, the main bolts connecting the pressurized water reactor pressure vessel and the head cover, the primary and secondary side manholes of the steam generator of the pressurized water reactor cooling system, and the flange connecting bolts of the primary circuit helium pipe of the high temperature gas cooled reactor all need to be tensioned and locked with a paralleling bolt tensioner.

1.1.2 Hazard of “Scuffing” of Bolt Tensioner, Bolt and Nut

Due to its simple structure, easy to carry and connect, convenient operation and simple introduction for operators, the hidden safety hazards of the bolt tensioner are easy to be ignored. On the one hand, during the bolt tensioning process, the bolt tensioner is blocked with the flange, nut and bolt, which is not easy to be found, especially during the use of the parallel bolt tensioner. Once the bolt tensioner is jammed with a bolt, nut or flange surface, due to its ultra-high pressure and ultra-high tensioning force, it is very easy to cause oil pipe cracking, thread fracture or even bolt fracture, and then cause the bolt tensioner to slip off. The operators and supervisors are easy to be hit by objects and mechanical injuries because they can’t avoid them; On the other hand, due to the influence of the material, cleanliness, operating environment, manufacturing tolerance and many other factors of bolt, nut and bolt hole, after one or more times of tensioning, the thread is damaged or even glued and broken, resulting in the bolt being stuck in the bolt hole or the nut being stuck on the bolt, which is often a disastrous accident in the flange connection of high-pressure seal. The bolt and threaded hole thread will be damaged and need to be reworked and repaired, which will affect the construction period; In serious cases, the bolts, nuts and bolt holes will be scrapped, which will seriously affect the construction period and leave serious potential safety hazards.

For example, in the maintenance stage of a nuclear power plant, it was found that the main bolt connecting the reactor pressure vessel and the top cover was stuck in the main bolt hole of the reactor pressure vessel, the thread was glued and broken, and the main bolt was removed after the bolt was broken. It involves the breaking and re manufacturing of the main bolts and the repair of the main bolt holes of the reactor pressure vessel, resulting in great economic losses; In the installation stage of a nuclear power plant, due to improper operation by the construction personnel, the thread of the steam generator anchor bolt of the reactor cooling system was jammed with the edge of the bolt hole after tensioning, which was not found and handled in time, resulting in the fracture of the anchor bolt at the thread, springing up and impacting the steel structure beam between the steam generators. Fortunately, no personal injury was caused.

1.2 Research Purpose and Method

1.2.1 Research Purpose

By discussing the standard operation and precautions of bolt tensioning in the installation and maintenance stage of nuclear power plant, this paper provides normative reference for the bolt tensioning operation in the installation and maintenance stage of nuclear power plant, ensures the safety and reliability of the bolt tensioning operation in the installation and maintenance stage of nuclear power plant, ensures the personal safety of construction personnel, improves the construction quality, and builds a strong nuclear safety defense line.

1.2.2 Research Methods

• Step 1: case analysis of bolt tension during installation and maintenance of nuclear power plant;

• Step 2: standardized operation process of bolt tension during installation and maintenance of nuclear power plant.

2 Detailed Description

2.1 Case Analysis of Bolt Tension During Installation and Maintenance of Nuclear Power Plant

2.1.1 The “Scuffing” of Anchor Bolts and Nuts for Lateral Support of Reactor Coolant Pump in a Nuclear Power Plant

During the installation of M310 pressurized water reactor unit in a nuclear power plant, the anchor bolts of the lateral support 3# hydraulic damper anchor seat of the reactor coolant pump were tensioned. It was found that after the fractional tensioning (three times of tensioning locking at 20%, 60% and 100% of the maximum tensioning torque + one-time tensioning verification at 100% of the maximum tensioning torque, and the tensioning torque was converted by the reading of the hydraulic pressure gauge), after the maximum tensioning torque reached 60%, the tightening resistance of some anchor nuts became larger, and when the maximum tensioning torque reached about 69%, the nuts could not rotate. Immediately, tensile verification was carried out on all the anchor bolts of the hydraulic damper anchor seats supporting the lateral support of the three loop reactor coolant pumps on site, and it was found that the anchor nuts of most of the anchor bolts “bite” after reaching about 69% of the maximum tensile torque.

Through analysis and demonstration, the cause of “scuffing” of the anchor bolt and nut is analyzed on site and verified step by step.

Firstly, it is assumed that the stroke of the bolt tensioner does not meet the installation of the anchor bolt on site, the anchor nut interferes with the inner wall of the bolt tensioner, and the friction is too large to rotate the nut. On site verification was carried out immediately. After the anchor nut was coated with grease, it was still “locked” after being tensioned again, and after the bolt tensioner was removed, the inner wall was not adhered with grease. Therefore, the conjecture that the anchor nut interfered with the inner wall of the bolt tensioner was denied, and the problem of the bolt and nut itself was considered.

Then, consult the manufacturer’s completion documents, and arrange the manufacturer’s personnel to recheck the go no go gauges on the anchor bolts and nuts. The manufacturer’s personnel used ring thread go no go gauges with a tolerance code of 6h for anchor bolts and cylindrical thread go no go gauges with a tolerance code of 6h for anchor nuts. Due to the phosphate coating on the surface of anchor bolts and nuts, some of the go no go gauge inspection results were unqualified. At the same time, by comparing with the design documents, it is found that the tolerance zone used by anchor bolts and nuts does not meet the matching requirements of anchor bolts 6h and nuts 7h in the design documents.

Therefore, it is determined that the main cause of the “scuffing” of the anchor bolts and nuts of the lateral support of the reactor coolant pump this time is the manufacturer’s wrong use of the anchor bolts and nut tolerance zone. At the same time, the manufacturer’s inspection time of the go no go gauge is before the phosphating of the anchor bolts and nuts, and the go no go gauge of the anchor bolts and nuts is not rechecked after the phosphating. Afterwards, the anchor bolts and nuts were returned to the factory according to the process, and finally the tensile work of the anchor bolts of the lateral support of the reactor coolant pump was successfully completed.

2.1.2 Other Common Bolt and Nut “Bite” Cases in the Installation and Maintenance Stage of a Nuclear Power Plant

In the process of anchor bolt tensioning of steam generator primary and secondary side inspection holes and pressurizer inspection holes during the installation and maintenance of M310 pressurized water reactor unit in a nuclear power plant, it is summarized that the main aspects affecting the tension safety of anchor bolts are as follows:

First, cleanliness control. If there are burrs, iron filings, rust or small slag inclusions on the internal thread of the anchor bolt hole on the flange surface, the external thread of the anchor bolt or the internal thread surface of the nut, they will be involved in the thread connection pair during the thread tightening process, and then squeezed into the internal and external thread connection thread, increasing the relative friction between the local internal and external threads, and causing local thread jamming under the influence of the force between the threads, high temperature, vibration and other conditions, Even glued, resulting in “bite” of anchor bolts and nuts.

Second, the control of relative cooperation relationship. The anchor bolt holes, anchor bolts and nuts used for the inspection holes on the primary and secondary sides of the steam generator and the inspection holes of the pressurizer are sealed with steel seal inscriptions to ensure that the anchor bolt holes, anchor bolts and nuts are used together during the disassembly and assembly process during the installation and commissioning stages. After many times of connection, coordination and tension, and under the high temperature conditions during the test and operation stages, the anchor bolt holes The internal and external threads of the anchor bolt and nut squeeze and run in with each other, and the connection pair is stable, avoiding the “scuffing” of the anchor bolt and nut caused by the matching relationship between the internal and external threads.

Third, prevent the misuse of “bite dead” agents. In order to prevent the anchor bolts and nuts from seizing, anti “seizing” agents such as molybdenum disulfide and N5000 will be used during the installation and commissioning of the nuclear power plant. The correct use of anti seize agent reduces the risk of “seizing” of anchor bolts and nuts to a certain extent. However, due to the characteristics of high temperature and high pressure of internal medium during the experiment and operation of steam generator and pressurizer, the wrong use of anti seize agent that is not resistant to high temperature will cause “seizing” of anchor bolts and nuts.

Fourth, improper operation methods. The correct installation method of anchor bolts and nuts is to manually install and tighten the anchor bolts and nuts. If manual tightening fails, immediately stop tightening and remove the anchor bolts and nuts for inspection. During the installation and commissioning of nuclear power plants, in order to pursue progress, many construction personnel will tighten the anchor bolts and nuts by increasing the torque, especially when the anchor bolts and nuts are tightened and jammed, increase the tightening force and lengthen the force arm, which will greatly increase the risk of “bite” of the anchor bolts and nuts.

Fifth, improper selection of tensioning tools. Bolt tensioning tools are divided into single type and parallel type, which should be paid special attention when using parallel bolt tensioning machine. The deviation of the relative position of the bolt hole on the flange surface and the manufacturing error of the parallel bolt tensioner will cause the parallel bolt tensioner to jam with the anchor bolt and nut, and then cause the anchor bolt to tilt in the bolt hole, and the anchor bolt to jam with the inspection hole on the primary and secondary sides of the steam generator or the sealing cover plate of the inspection hole of the pressurizer. If the operator fails to check in place, continue to complete the tensioning, the anchor bolt The risk of nut “seizing” is high.

2.2 Specification and Operation Process of Bolt Tensioning During Installation and Maintenance of Nuclear Power Plant

First of all, in order to prevent the design deviation of bolt tensioning in the installation and maintenance stage of nuclear power plant, it is necessary to carefully identify the upstream documents when technicians prepare the scheme. For the anchor bolts and nuts that need to be tensioned, especially the bolts and nuts with large diameter, it is necessary to check the tolerance zone selected by the designer to avoid the “bite” event of anchor bolts and nuts caused by design deviation.

Secondly, during the installation and maintenance stage of the nuclear power plant, the compliance acceptance of threaded holes, anchor bolts and nuts should be carried out before the on-site construction. If serious manufacturing deviations of threaded holes, anchor bolts and nuts are found, they should be reported in time, and repaired, reworked or scrapped according to the relevant procedures. At the same time, the material of threaded holes, anchor bolts and nuts shall be inspected. If there is a risk of “seizing”, the corresponding process shall be opened in time for treatment.

Finally, it is mainly necessary to control the operation specifications of construction personnel:

First, control the cleanliness, carefully clean the threaded holes, anchor bolts and nuts, remove foreign matters, and protect against foreign matters;

Second, control the relative cooperation relationship, code and record the threaded holes, anchor bolts and nuts, and check them many times after installation to prevent misuse;

Third, before the installation of anchor bolts and nuts, apply an appropriate amount of anti seize agent according to the bolt material, control the dosage and use model, and ensure that the use of anti seize agent conforms to the corresponding working environment of anchor bolts and nuts;

Fourth, the selection of tensioning tools and operation methods are standardized. Single or parallel bolt tensioners are correctly selected and used in strict accordance with the operation requirements of bolt tensioners. The following is a detailed description of the tensioning of anchor bolts and nuts at the primary and secondary inspection holes of steam generators during the installation and maintenance of M310 PWR units in a nuclear power plant:

• 1. During the installation of anchor bolts and nuts, install and tighten them manually, check whether there is obvious jamming during the installation of anchor bolts and nuts, and return half a circle to one circle after tightening;

• 2. Install the bolt tensioner. During the installation process, check whether there is jam between the bolt tensioner and the anchor bolt and nut, and tighten the fixing nut of the bolt tensioner evenly. Check that the bolt tensioner can move relatively and all anchor bolts are tightened evenly;

• 3. Install the dial gauge base and dial gauge, ensure that the dial gauge base is tightened evenly, there is no jam with the bolt tensioner, and the dial gauge reading is normal;

• 4. Establish a control area and set up a warning line. Non operators are not allowed to enter the bolt tensioner operation area;

• 5. Start the bolt tensioner, tighten and tighten the nuts according to the three-time tension locking of 20%, 60% and 100% of the maximum tension torque + one-time tension verification of 100% of the maximum tension torque, and ensure that all nuts are tightened evenly;

• 6. Check whether the maximum elongation and residual elongation of the bolt meet the operating requirements of the anchor bolt and nut; Remove the bolt tensioner, remove the fixing nut of the bolt tensioner evenly, remove the bolt tensioner, and adjust the position of the bolt tensioner and nut in case of jamming;

• 7. Clean and protect anchor bolts and nuts.

3 Conclusions

After eliminating the two uncontrollable “bite” factors of design deviation and manufacturing deviation at the site during the installation and maintenance of the nuclear power plant, we should mainly start with the standardized operation of the construction personnel to ensure that the probability of “bite” of anchor bolts and nuts caused by human factors during the installation and maintenance of the nuclear power plant is reduced.

It is necessary to carry out necessary pre job training and pre job disclosure for the construction personnel who carry out the tensioning operation of anchor bolts and nuts, and carry out necessary inspections during the tensioning process of anchor bolts and nuts, so as to reduce the “bite” of anchor bolts and nuts caused by human factors during the installation and maintenance of nuclear power plants.