Research and Analysis on the Optimization of the Preventive Disassembly Inspection Intervals of Important Wedge Gate Valves in the Primary Cooling System of CPR1000 Nuclear Power Plant

At present, the important wedge gate valve of the primary coolant system of the CPR1000 nuclear power plant performs preventive disassembly inspection according to the intervals of 5 years, the average dose of a single disassembly is about 10mSv, the average time of each overhaul occupies the critical path is 1 hour, and the valve has been disassembly inspection in good condition. Based on the historical maintenance data, operating environment, failure mode, failure mechanism, operation and maintenance experience feedback of the same type of valve, and failure risk control of the important wedge gate valve of the primary coolant system, this paper demonstrates and analyzes the feasibility of extending the disassembly inspection intervals of the important wedge gate valve of the primary coolant system from 5 years to 10 years. This work plays an important role in improving the economic efficiency of nuclear power plant maintenance and reducing the radiation dose of maintenance.


Introduction
Wedge gate valves are widely used in the nuclear island system of nuclear power plants (abbreviated as NPP).Some important wedge gate valves in the primary coolant system (normally called RCP system) are closed for a long time during the daily operation of the NPP, serving as isolation boundaries to prevent medium leakage in the primary or auxiliary systems.At the same time, they are opened as needed to carry out reactor safe shutdown or reactor refueling drainage work.Due to the high working pressure and temperature, as well as the serious consequences of the malfunction, preventive disassembly inspections are carried out every 5 years (3 cycles) to avoid the occurrence of valve internal

The Basic Situation and Maintenance Items of Wedge Gate Valves
The wedge gate valve of the RCP system is driven by an electric actuator, and the valve disc is a wedgeshaped elastic gate plate.The valve seat and disc sealing surface are welded with Stellite alloy which is very durable and wear-resistant [1][2] except for the reason of unqualified production process [3][4].The valve is composed of a valve body, valve stem, bearings, yoke, valve cover, wedge-shaped gate disc, coupling components, etc.The valve body is generally cast, and the valve cover and body are connected by bolts.The packing is used graphite material, and its structure is shown in figure 1.The normal operating environment for the important wedge gate valve of the RCP system is a low concentration boric acid environment of 2000 ppm, with an operating pressure of 155 ± 1bar abs and a temperature of 291-310℃.The primary circuit medium is mainly composed of boric acid and lithium hydroxide, with a pH value of approximately 7.0.The preventive maintenance tasks currently being carried out for the important wedge gate valve of the RCP system are shown in table 1.The wedge valve has been in good condition during daily use and previous disassembly inspections for many years.

Statistical Analysis of Failure History Data of Wedge Gate Valve
The data of 130 preventive disassembly inspections of the important wedge gate valves of the RCP system from 2007 to 2022, a total of 24 similar nuclear power units, were collected and statistically analyzed.Among them, 56 disassembly inspections showed local minor defects, as shown in table 2. However, none of the disassembly inspections affected the availability of the valves and the valve function is in normal during daily operation.
As can be seen from table 2, the internal defects of the valve mainly focus on the deformation, wear and corrosion of the sealing surface of the valve disc and seat.Other minor defects are caused by the deviation of installation quality, abnormal manufacturing or welding quality of the components, not caused by the long-term operation of the equipment.Some components failures don't affect the availability of the valve, such as the failure of the limit indicating.During the operation of the unit, the deformation of some wedge gate valve seats and valve discs can be divided into two types.The first type is due to the incomplete heat treatment of the valve sealing surface material, resulting in thermal stress in the welding area [5].After the valve is put into use, the thermal stress is released, leading to local deformation and internal leakage of the sealing surface.According to the statistics of 230 sealing tests, 90% of the faults that caused internal leakage exceeding the allowable value due to sealing surface deformation occurred during the first and second round overhaul of the unit in the initial stage of commercial operation date (the internal leakage rate of the sealing test was less than the threshold value of the internal leakage rate of the valve function failure), and rarely occurred during the subsequent operation.
After repairing the valve sealing surface, no similar failures have occurred during the subsequent operation of the unit.
The second possible reason is that the actual stress of the valve sealing surface exceeds the design stress, resulting in local deformation of the sealing surface and internal leakage.According to the mechanical check calculation, the sealing surface stress under the closed state of the valve disc is far from the allowable stress value, and the margin is large, so the sealing surface will not be deformed under the closed state. 2

) Evaluation of the influence of valve sealing surface deformation on the extension of valve disassembly intervals
The slight deformation of the sealing surface of some wedge gate valves is related to the manufacturing quality of the valve, because the thermal stress is not effectively released.After 1~2 unit overhaul, the thermal stress of the valve sealing surface is released, and after re-grinding and aligning the sealing surface, the defect does not appear again in the equipment, which is not caused by long-term operation of the equipment.In addition, the actual stress intensity of the valve sealing surface is less than the allowable value, so the failure mode does not affect the valve disassembly inspection frequency from 5 years to 10 years, and does not reduce the reliability of the valve.

Analysis of the Causes of Valve Sealing Surface Wear and Its Influence on the Extension of Valve Disassembly Intervals 1) Failure mechanism analysis of valve sealing surface wear
The surface of the valve seat and valve disc is flat and smooth on the macro level, and convex and concave on the micro level, and it can be found that there are both peaks and troughs on the surface of the micro-magnification, as shown in figure 2. The sealing surfaces of RCP wedge gate valves are all 6# Stellite alloy.During the opening and closing process, the sealing surfaces will squeeze and slide, and the small uneven bumps between the sealing surfaces friction with each other under the action of shear force, and the grinding chips will be included in the sealing surfaces, further aggravating the wear of the sealing surfaces and directly affecting the leakage of the sealing surfaces.According to table 2, the failure mode of wear defects were found on the sealing surface of the valve for 25 times, accounting for 45%.The miscibility of the same metal is large, and adhesive wear is easy to occur [6].The relative movement of the valve disc and the seat leads to slight scratches or uniform wear of the alloy sealing surface, which is the result of normal use [7].
2) Analysis on the influence of prolonged disassembly intervals of valve The RCP wedge gate valve relies on the thrust provided by the electric actuator to push the gate valve into the seat.The seat and valve disc are subjected to extrusion pressure, and wear will occur during the extrusion process.The wear degree of the valve sealing surface is related to the number of valve opening and closing.The opening and closing numbers of RCP important wedge gate valve needed in each overhaul refueling process of the unit were counted, with an average of 4 times for per overhaul.
In order to verify whether the valve disassembly intervals can be extended from 3 overhaul to 6 overhaul to meet the reliability requirements of valve use, it is necessary to carry out valve operating number and valve availability tests, that is, to verify whether the valve internal leakage rate exceeds the standard, whether the valve external leakage exceeds the standard, and whether the valve is operated smoothly after the valve reaches the corresponding operating number of 6 overhaul.Combined with the valve identification test data of the manufacturer's prototype, the test process and result are as follows: Test conditions: valve opening and closing reaches 1000 times, operating ambient temperature 180℃, valve opening pressure difference 15Bar.
Test conclusion: The valve seal leakage measured at room temperature 20℃ and pressure 250bar is 0.0032l/h, no visible leakage and no damage occurred during the test.Penetrant inspection was performed on the valve and no defects were found on the hard alloy sealing surface.Comparing the usage data of important wedge gate valves in the RCP system with the manufacturer's prototype valve qualification test data, the results are shown in table 3.As can be seen from table 3, the important wedge gate valve of the RCP system has undergone six rounds of overhaul only 24 times, and the service life of 60 years (40 rounds of overhaul) has been opening and closing 160 times, which is far less than the 1000 times of the prototype valve.Moreover, the internal leakage rate of the prototype valve is also far less than that of the important wedge gate valve of the RCP system.Therefore, it can be inferred that after the valve is used for 6 refueling cycles, the valve can still be reliably used.The internal leakage rate caused by wear of the valve sealing surface will still not exceed the allowable standard of the valve, and the wear of the valve sealing surface will not affect the extension of the inspection intervals of the valve disintegration to 6C.

Cause Analysis of Valve Sealing Surface Corrosion and Influence Analysis on Valve Disassembly Intervals Extension 1) Analysis of corrosion mechanism of valve sealing surface
Wedge gate valve metal sealing surface corrosion exists in two forms, the first is that the valve metal sealing surface is exposed to corrosive media, when the passivation film on the metal surface of the valve disc and seat is broken or the erosion in a specific area is much larger than the surrounding surface, pitting phenomenon will occur, and it appears in the shape of visible holes or gaps on the metal surface [8].The second is uniform corrosion, a chemical or electrochemical reaction occurs on the surface of the metal to turn the metal into an oxidation state, which will reduce the strength of the metal and increase the wear.
2) Analysis on the influence of prolonged disassembly intervals of valve Wedge gate valve sealing surface material is 6# STELLITE alloy, which is a kind of cemented carbide with Co, Cr and W as the main elements, resistant to various types of corrosion and high temperature oxidation [9], and the environment is low concentration boric acid (boric acid concentration decreased from 2000ppm to 5ppm from unit startup to overhaul).It is not enough to produce boric acid corrosion hazards on the valve metal sealing surface, and the Cr element in the valve metal sealing surface has a large affinity for oxygen.When the content is more than 12%, the valve sealing surface will be oxidized to form a thin layer of dense blunt oxide protective film.The Cr content in the wedge gate valve sealing surface material is 25%~33%, so the oxidation performance and corrosion resistance are better.The pictures of the sealing surface of the disassembled valve seat and disc are showed in figure 3, and we can see the surface is smooth and clean without any signs of corrosion.At the same time, according to the feedback of the 10-year disintegration and overhaul experience of the valve in the low-concentration boric acid environment of the same nuclear power unit for many years, there is no corrosion of the low-concentration boric acid in the primary circuit, so the failure mode does not affect the valve disassembly intervals from 5 years to 10 years, and will not reduce the reliability of the valve.

Reliability Calculation After Extended the Maintenance Intervals
Based on years of operation and maintenance experience of RCP important wedge gate valves, there have been no large-scale failures or unavailability of wedge gate valves within 10 years.Therefore, it is believed that the valve failure pattern within 10 years follows an exponential distribution.Referring to the exponential distribution pattern, the reliability calculation of valve disassembly inspection intervals is extended from 5 years to 10 years.According to [10], the calculation process is as follows.The meaning of Letters and Symbols are in table 4.
The disassembly and inspection cycle of RCP important wedge gate valves has been extended from 5 years to 10 years, with a failure rate increase of 3.58%, which is relatively small.The data used this time is the failure data of the sealing experiment, and the criterion for determining the failure leakage rate is much smaller than the criterion for valve functional failure leakage rate.Therefore, the actual increase in failure probability will be smaller after the inspection intervals is extended.

Risk Control Measures for Wedge Gate Valve Disassembly Intervals Optimization
The disassembly intervals of the important wedge gate valve in the RCP system is extended from 5 years to 6 years, and other failure modes that lead to internal leakage, external leakage and jammed can be discovered and dealt with in advance by the following ways, so that the failure risk during the extended disassembly intervals of the valve can be controlled.

Valve Internal Leakage Random Failure Risk Control Measures
During the normal use of the RCP important valve, foreign bodies in the system may cause damage to the valve sealing surface and internal leakage, resulting in excessive leakage rate of the system.This failure mode has no direct relationship with whether the valve disassembly intervals is prolonged or not.However, in order to ensure the safe and reliable operation of the NPP, the RCP important valve sealing test of each overhaul can be detected in advance, and the next round of overhaul can be repaired and restored in time.The standard of internal leakage rate of valve sealing test and the standard of internal leakage rate of functional failure are shown in table 5. Valve leakage is mainly caused by the relaxation of the packing fastening force [11][12], the valve packing is made of graphite material, and the seal is a contact seal.The graphite packing is filled into the cavity between the valve stem and the packing box through the packing gland.The axial compression deformation generated by the gland causes the packing to expand radially inside and outside, so that the packing is tight on the inner surface of the valve stem and the packing box.This prevents the leakage of fluid from the internal valve chamber to the valve stem.If the valve packing compression force is insufficient and leakage occurs, it can be found through the appearance inspection and tightening torque treatment during overhaul, and does not affect the subsequent normal use of the valve.

Valve Failure to Open and Close Risk Control Measures
The main reason why the RCP important wedge gate valve cannot open and close smoothly is the lack of lubricating grease on the valve stem, valve stem nut, and bearing.Currently, there is a two cycles lubrication task which can achieve sufficient lubrication of the valve's moving parts.At the same time, there is a three year's regular electric actuator inspection of the valve.After the inspection of the electric actuator, the valve opening and closing operation tests will be carried out to verify the smoothness of the valve opening and closing.If problems are found, repair and recovery works can be arranged timely during the overhaul period to ensure the reliability of the valve.

Conclusion
Through the analysis of the main failure modes, failure mechanism, reliability impact and risk control that affect the disassembly intervals of the important wedge gate valve in the RCP system from 5 years to 10 years, it can be inferred that the important wedge gate valve in the RCP system in the new disassembly intervals can still meet the reliability standard.The valve disassembly intervals is extended to 10 years with theoretical and data support.At the same time, the reduction of valve disassembly frequency can effectively reduce the preventive maintenance workload of the NPP overhaul, shorten the overhaul critical path time, and reduce the overhaul maintenance dose, which has a great contribution to improving the comprehensive strength of the unit's operation and maintenance.

Figure 1 .
Figure 1.Wedge gate valve structure.Figure 2. Micro morphology of valve seat and wedge.

Figure 2 .
Figure 1.Wedge gate valve structure.Figure 2. Micro morphology of valve seat and wedge.

Figure 3 .
Figure 3. Macro morphology of valve seat and wedge.

Table 1 .
List of Preventive Maintenance Tasks for Wedge Gate Valve.

Table 2 .
Statistics of local defects during disassembly inspection of wedge gate valve.

Table 3 .
Analogy analysis of RCP important gate valve usage data and the prototype valve qualification test data.

Table 4 .
The Meaning of Letters and Symbols.

Table 5 .
The Meaning of Letters and Symbols.