05000334/LER-2010-003
| Docket Number | |
| Event date: | 11-15-2010 |
|---|---|
| Report date: | 02-18-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 3342010003R01 - NRC Website | |
BVPS Unit 1 License Condition 2.C.6 requires a program to reduce leakage from systems outside containment that would or could contain highly radioactive fluids during a serious transient or accident to as low as practical levels. BVPS Unit 1 License Condition 2.C.6 would apply to LHSI System flow after the 'Transfer to Recirculation' phase occurs post-LBLOCA since the piping would contain potentially highly contaminated sump fluid.
Although License Condition 2.C.6 does not specify an explicit limit for outside-containment leakage, the BVPS Unit 1 licensing basis safety analyses utilize a limit of 5700 cubic centimeters per hour (cc/hr). Thus, the identified RV-1SI-845B leakage would exceed the intent of BVPS Unit 1 License Condition 2.C.6 since the 20 gpm is well above the safety analyses limit of 5700 cc/hr.
Since BVPS Unit 1 and Unit 2 share a common control room, both BVPS Units entered TS 3.7.10, Action B for an inadequate control room envelope boundary to assure compliance with projected post-accident control room personnel close when the dose concern was initially discovered at 0001 hours1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> on 11/16/2010. The specification applied since the source term used to calculate the control room dose was significantly elevated due to the elevated outside-containment leakage, even though the control room envelope boundary was not physically challenged, and assumed control room in-leakage was unchanged.
In addition, TS Bases for 3.5.2, "Emergency Core Cooling System — Operating" which includes the LHSI System, does not address either BVPS Unit 1 License Condition 2.C.6 nor ESF leakage outside of containment, and only addresses decay heat removal considerations and other flow-related criteria. In this event, there was a reduction in the total LHSI flow of 20 gpm going to the Reactor Coolant System [AB]. However, this reduction was not significant as there was sufficient flow margin above this reduction for each train. Hence, the flow loss through the relief valve leakage would not have prevented the LHSI System from performing its flow-related safety functions.
Nevertheless, as with the precedent for non-compliance with "Programs" listed in TS Chapter 5.5, any non-compliance with a License Condition would similarly require invoking the applicable TS Chapter 3 Limiting Condition for Operation (LCO) for the subject system.
In this case, the BVPS Unit 1 License Condition 2.C.6 limit for outside-containment leakage was challenged by the lack of LHSI System integrity due to the inappropriate opening of RV-1S1-845B, This adversely affected both trains of LHSI since RV-1S1-845B is aligned to a common discharge pipe location. Therefore, both trains of LHSI were not operable per TS 3.5.2. Action A for one or more trains of ECCS inoperable was entered, even though two trains of sufficient LHSI flow capability remained functional.
Given the excessive projected post-accident EAB and control room doses, this was an unanalyzed condition that significantly degraded plant safety, and is reportable pursuant to 10CFR50.73(a)(2)(ii)(B). Given that both trains of LHISI trains were declared inoperable, this was a condition that could have prevented the fulfillment of the safety function of a system the relief valve lifting (i.e., relief valve discharge piping misalignment resulting in relief valve nozzle loading) was not confirmed through subsequent bench testing of this relief valve.
CAUSE OF EVENT
Relief valve RV-1SI-845B lifted during the LHSI full flow testing and did not reseat, resulting in a leakage rate outside containment that exceeded regulatory limits. Gas voiding on the inlet side of RV-1SI-845B and system piping was determined to be the most probable cause for this relief valve to lift and an incorrect guide ring setting caused the relief valve to not reseat after it opened. The gas voiding created a condition that exposed the relief valve to an increase in pressure when the LHSI pump was started. The increase in pressure from the LHSI pump starting resulted in the relief valve lifting. The significant contributor to the gas voiding on the inlet side of the relief valve is most probably the result of a height difference of approximately one and a half feet between the relief valve and the discharge of the LHSI pump. During filling and venting it is postulated that this height difference would be enough to keep a gas void at the inlet to the relief valve without a path to vent the gas.
An incorrect guide ring setting resulted in the relief valve not reseating following the relief valve opening. The guide ring setting for RV-1SI-845B was verified to be incorrectly set such that the valve had a significantly lower reset pressure than desired. An adjustable guide (or blowdown) ring determines when a relief valve will reseat after lifting; The incorrect guide ring setting was most probably caused by not having process or procedural controls in place to ensure that the relief valves with adjustable guide ring settings are set correctly. Although relief valve nozzle loading was confirmed to exist on RV-1SI-845B, the contribution of the nozzle loading to the occurrence of this event is not considered to be significant.
ANALYSIS OF EVENT
The plant risk associated with the BVPS Unit 1 relief valve RV-1SI-845B lifting on 11/15/2010 during the Safety Injection Pump test is considered to be very low. This is based on an Engineering technical assessment, which concluded that there is reasonable assurance that the loss of ECCS flow was small enough that there would not be a loss of safety function or significant impact on the BVPS Unit 1 ECCS, and the containment bypass would not lead to any large, unmitigated releases in a time frame prior to effective evacuation of the nearby population that have the potential to cause early health effects.
Based on the above, the safety significance of the relief valve RV-1SI-845B lifting event that occurred on November 15, 2010 was very low.
This event was previously reported as an unanalyzed condition that significantly degraded plant safety, pursuant to 10 CFR 50.72(b)(3)(ii)(B), and as an event that could have prevented the fulfillment of a safety function of systems needed to control the release of radioactive material, pursuant to 10 CFR 50.72(b)(3)(v)(C) at 05:48 hours on 11/16/2010 (Event Notification No. 46421).
CORRECTIVE ACTIONS
1. The discharge piping configuration for relief valve RV-1SI-845B and for sister valves RV-1S1-845A/C was adjusted to minimize nozzle loading as required.
2. Steps (barriers) were established in the maintenance procedures and work orders task list instructions on relief valves to check for improper piping configuration.
3. The engineering process documents were reviewed and revised as necessary to consider the effects of nozzle loading on relief valves. Additional relevant changes were also incorporated into plant procedures.
4. Plant operating experience reports were issued on this event (OE 32604 and OE 36020).
5. An analysis was performed to determine the maximum amount of air in the system which would result in relief valve lift. Limits were incorporated into the void monitoring program.
6. The LHSI pump discharge piping fill and vent procedures have been modified to require the use of a vacuum fill to minimize the potential for air trapped in the system.
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