This spring, the NRC Special Inspection Team (SIT) conducted an inspection of the LaSalle Nuclear Power Plant to investigate the cause of valve failure and evaluate the effectiveness of corrective measures taken.
The two units of Exelon Generation Company’s LaSalle County Nuclear Power Plant, about 11 miles southeast of Ottawa, Illinois, are boiling water reactors (BWR) that began operation in the early 1980s. Although most BWRs operating in the United States are BWR/4 with the Mark I containment design, the “newer” LaSalle devices use the BWR/5 with the Mark II containment design. The main difference in this review is that although the BWR/4 uses a steam-driven high-pressure coolant injection (HPCI) system to provide supplemental cooling water to the reactor core when the small pipe connecting the reactor vessel ruptures, the BWR/5 uses a motor-driven High Pressure Core Spray (HPCS) system to achieve this safety role.
On February 11, 2017, after system maintenance and testing, workers tried to refill the No. 2 high-pressure core injection (HPCS) system. At that time, the reactor of Unit 2 was shut down due to the interruption of refueling, and the downtime was used to check emergency systems, such as the HPCS system.
The HPCS system is usually in standby mode during reactor operation. The system is equipped with a motor-driven pump that can provide a designed supplemental flow of 7,000 gallons per minute for the reactor vessel. The HPCS pump draws water from the containment tank in the containment. If the small-diameter pipe connected to the reactor vessel breaks, the cooling water will leak, but the pressure inside the reactor vessel is operated by a series of low-pressure emergency systems (ie, waste heat discharge and low-pressure core spray pump). The water flowing out from the broken pipe end is discharged to the suppression tank for reuse. The motor-driven HPCS pump can be powered from the off-site grid when it is available, or from an on-site emergency diesel generator when the grid is unavailable.
Workers were unable to fill the pipe between the HPCS injection valve (1E22-F004) and the reactor vessel. They discovered that the disc was separated from the stem of the dual-clapper gate valve made by Anchor Darling, blocking the flow path of the filling pipe. The HPCS injection valve is a normally closed electric valve that opens when the HPCS system is started to provide a channel for make-up water to reach the reactor vessel. The motor applies torque to rotate the spiral valve stem to raise (open) or lower (close) the disc in the valve. When fully lowered, the disc will block the flow through the valve. When the valve flap is fully raised, the water flowing through the valve flows unimpeded. Since the disc is separated from the valve stem in a fully lowered position, the motor may rotate the valve stem as if to raise the disc, but the disc will not move.
Workers took pictures of the separated double discs after removing the valve cover (sleeve) of the valve (Figure 3). The bottom edge of the stem appears in the top center of the picture. You can see the two discs and the guide rails along them (when connected to the valve stem).
The workers replaced the internal parts of the HPCS injection valve with parts redesigned by the supplier, and reiterated the No. 2 unit.
The Tennessee River Basin Authority submitted a report to the NRC in January 2013 under 10 CFR Part 21 concerning defects in the Anchor Darling double disc gate valve in the high-pressure coolant injection system of the Browns Ferry Nuclear Power Plant. The following month, the valve supplier submitted a 10 CFR Part 21 report to the NRC concerning the design problem of the Anchor Darling double disc gate valve, which may cause the valve stem to separate from the disc.
In April 2013, the Boiling Water Reactor Owners Group issued a report on the Part 21 report to its members and recommended methods for monitoring the operability of the affected valves. Recommendations include diagnostic tests and monitoring the rotation of the stem. In 2015, workers performed recommended diagnostic tests on HPCS injection valve 2E22-F004 in LaSalle, but no performance problems were found. On February 8, 2017, workers used the stem rotation monitoring guide to maintain and test the HPCS injection valve 2E22-F004.
In April 2016, the boiling water reactor owner group revised their report based on information provided by a power plant owner. Workers disassembled 26 Anchor Darling double disc gate valves that may be vulnerable and found that 24 of them had problems.
In April 2017, Exelon notified the NRC that the HPCS injection valve 2E22-F004 had failed due to the separation of the valve stem and the disc. Within two weeks, a special inspection team (SIT) authorized by the NRC arrived in LaSalle to investigate the cause of the valve failure and evaluate the effectiveness of the corrective measures taken.
SIT reviewed Exelon’s assessment of the failure mode of the Unit 2 HPCS injection valve. SIT agreed that a component inside the valve ruptured due to excessive force. The broken part causes the connection between the valve stem and the intervertebral disc to become increasingly misaligned, until the intervertebral disc finally separates from the valve stem. The supplier redesigned the internal structure of the valve to solve the problem.
Exelon notified the NRC on June 2, 2017 that it plans to correct 16 other safety-related and safety-important Anchor Darling double disc gate valves that may be vulnerable to this failure during the next refueling interruption of the two LaSalle units The impact of the mechanism.
SIT reviewed Exelon’s reasons for waiting to repair these 16 valves. SIT believes that the reason is reasonable, with one exception-the HCPS injection valve on Unit 1. Exelon estimated the number of cycles of the HPCS injection valve for Unit 1 and Unit 2. The Unit 2 valve was the original equipment installed in the early 1980s, while the Unit 1 valve was replaced in 1987 after it was damaged for other reasons. Exelon argued that the greater number of valve strokes for unit 2 explained its failure and there was reason to wait until the next refueling interruption to resolve the valve problem for unit 1.
SIT cited factors such as unknown pre-operation test differences between units, slight design differences with unknown consequences, uncertain material strength characteristics, and uncertain differences in valve stem to wedge thread wear, and concluded that “this is a’what “Time’s problem” instead of “If” 1E22-F004 The valve will fail if there is no failure in the future. In other words, SIT did not purchase a delayed inspection of the Unit 1 valve.
Exelon closed LaSalle Unit 1 on June 22, 2017 to replace the internal parts of HPCS injection valve 1E22-F004.
SIT found that the torque values developed by Exelon for the motors of HPCS injection valves 1E22-F004 and 2E22-F004 violated 10 CFR Part 50, Appendix B, Standard III, Design Control. Exelon assumes that the valve stem is the weak link, and establishes a motor torque value that does not subject the valve stem to excessive pressure. But the weak link turned out to be another internal part. The motor torque value applied by Exelon put the part under excessive stress, causing it to break and the disc to separate from the valve stem.
The NRC determined the violation as a severe level III violation based on the valve failure that prevented the HPCS system from performing its safety functions (in the four-level system, level I is the most severe).
However, the NRC exercised its law enforcement discretion in accordance with its law enforcement policy and did not publish violations. The NRC determined that the valve design flaw was too subtle for Exelon to reasonably foresee and correct before the Unit 2 valve failure.
Exelon looked pretty good at this event. NRC’s SIT records indicate that Exelon is aware of the Part 21 report made by the Tennessee Valley Authority and the valve supplier in 2013. They were unable to use this awareness to identify and correct Unit 2 HPCS injection valve problems as a reflection of their poor performance. After all, they implemented the measures recommended by the Boiling Water Reactor Owner’s Group for the two Part 21 reports. The disadvantage lies in the guide, not Exelon’s application of it.
The only flaw in Exelon’s handling of this matter was that the reason for running Unit 1 was weak before checking whether its HPCS injection valve was damaged or damaged, until its next planned refueling was interrupted. However, NRC’s SIT helped Exelon decide to expedite the plan. As a result, Unit 1 was closed in June 2017 to replace the vulnerable Unit 1 valve.
NRC looked very good at this event. Not only did the NRC lead Exelon to a safer place for LaSalle Unit 1, but the NRC also urged the entire industry to resolve this issue without unreasonable delay. NRC issued the 2017-03 information notice to the factory owners on June 15, 2017, concerning the design defects of the Anchor Darling double disc gate valve and the limitations of the valve performance monitoring guidelines. NRC held a series of public meetings with industry and valve supplier representatives on the problem and its solutions. One of the results of these interactions is that the industry has listed a series of steps, a settlement plan with a target deadline no later than December 31, 2017, and an investigation into the use of Anchor Darling double disc gate valves in US nuclear power plants. Investigations show that approximately 700 Anchor Darling double disc gate valves (AD DDGV) are used in nuclear power plants in the United States, but only 9 valves have the characteristics of high/medium risk, multi-stroke valves. (Many valves are single-stroke, because their safety function is to close when opened, or open when closed. Multi-stroke valves can be called open and close, and may be opened and closed multiple times to achieve their safety function.)
The industry still has time to regain its failure from victory, but the NRC seems ready to see timely and effective results from this matter.
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Post time: Jun-23-2021
