05000354/LER-2007-004
| Docket Numbersequential Revmonth Day Year Year Month Day Year N/Anumber Na | |
| Event date: | 07-31-2007 |
|---|---|
| Report date: | 09-28-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat |
| Initial Reporting | |
| 3542007004R00 - NRC Website | |
Energy Industry Identification System {EIIS} codes and component function identifier codes appear as {SS/CCC}
IDENTIFICATION OF EVENT
Event Date/Time: May 29, 2007 / 0834 Discovery Date/Time: July 31, 2007 / 0051
CONDITIONS PRIOR TO EVENT
Hope Creek was in Operational Condition 1 with reactor power at 100%. No other structures, systems, or components were inoperable that contributed to the event.
DESCRIPTION OF EVENT
On May 29, 2007, at 0834 while operating with the reactor at 100% power and the main generator synchronized to the grid, a manual scram was initiated in anticipation of a low reactor water level condition.
After the scram, reactor water level lowered to Level 2 and a valid ECCS initiation signal caused HPCI {BJ} and Reactor Core Isolation Cooling (RCIC) to start and inject to the core. This HPCI injection was secured by control room operators 17 seconds after initiation in accordance with procedures. On July 31, 2007, at 0051 during performance of the quarterly In-service Test (IST) for HPCI system valves, the HPCI Feedwater injection valve (BJ-HV-8278) {BJ/INV} failed to stroke open. The valve tripped on electrical overloads 10 seconds after the open pushbutton was depressed. Failure of the valve to open resulted in HPCI inoperability and an unplanned 14-day hot shutdown LCO. The HPCI feedwater injection Valve is an Anchor / Darling 8 inch — 900 lb carbon steel flex wedge gate valve with a Limitorque SMB-0-25 actuator. The root cause is that the thermal binding was not anticipated during the design of the system.
CAUSE OF EVENT
The cause of the HPCI Feedwater injection valve not opening on July 31, 2007 was that it was thermally bound as a result of the short duration HPCI injection event on May 29, 2007. Previous to the HPCI injection the Feedwater injection valve was approximately 424°F due to its close proximity to the feedwater line. During the 17 second HPCI injection, cold water, at approximately 85°F, was injected through the HPCI Feedwater injection valve and cooling of the valve seat and the valve disc occurred at different rates.
When HPCI was secured, and the valve shut, the disc went further into the seat than normal. During the subsequent heatup of the valve, the disc and seat rings expanded, increasing the wedging interference and therefore, the required opening force. The increase in wedging interference resulted in the required opening force exceeding the actuator capacity and failure of the valve to open. The root cause is that the thermal binding was not anticipated during the design of the system. This type of thermal binding is different than the classical thermal binding experienced in the industry. Classical thermal binding involves a valve that is closed 'hot' and after cooling, the valve is thermally bound.
The only similar configuration as the HPCI Feedwater injection valve is the RCIC injection valve. RCIC also injects into the feedwater system and uses the same source of low temperature water as HPCI. The RCIC Feedwater injection Valve, BD-HV-F013, is a 6 inch 900# Flexwedge gate valve with a SMB-0 Limitorque Operator. The RCIC System injects 600 GPM upon initiation, which is much less than the HPCI system.
During a plant transient when reactor water level reaches Level 2, HPCI and RCIC inject. Due to the higher HPCI flow, HPCI typically is shutdown earlier than RCIC. Thus RCIC typically does not have short duration injections.
During the May 29, 2007 event, RCIC also injected into the reactor through the feedwater system. During HPCI system shutdown, its suction pressure oscillated. The oscillation caused a RCIC trip. The RCIC injection duration was a few seconds longer than the HPCI injection of 17 seconds. RCIC injection valve was stroked open and closed successfully during its quarterly IST on June 12, 2007 proving it was not thermally bound.
A review of the previous IST open stroke time test results for BD-HV-F013 revealed that there are no inconsistencies in the test-to-test times that might suggest the presence of minor classical thermal binding.
The RCIC Feedwater injection Valve has similarities to the HPCI Feedwater injection valve but has operational differences; therefore it is difficult to conclude whether the RCIC Feedwater injection Valve could have the same thermal binding issue. Based upon the data gathered, the RCIC Feedwater injection valve does not exhibit the same characteristics as the HPCI Feedwater injection valve. Therefore the investigation team expects the thermal binding issue does not exist for the RCIC Feedwater injection Valve.
To confirm this, an analysis will be performed which will determine if thermal binding is plausible for the RCIC valve.
HPCI also injects into the reactor vessel through the Core Spray system via the BJ-HV-F006 injection valve.
The Core Spray system is cool and Core Spray has a check valve between the HPCI connection and the hot reactor vessel. Therefore HPCI Core Spray injection valve is cool and not susceptible to thermal binding of this type.
PREVIOUS OCCURRENCES
A review of previous reportable events at Hope Creek was performed to determine if a similar event had occurred. No similar events were identified.
SAFETY CONSEQUENCES
The safety consequences of this event were minimal. This event resulted in no nuclear, radiological, or industrial safety consequences. Although HPCI was inoperable from May 29 to July 31, 2007 due to the binding of the feedwater injection valve, HPCI was still capable of starting and injecting some flow through the core spray injection valve. A PRA assessment was performed for the failure of the feedwater injection valve to open with a 64-day exposure time. This PRA assessment determined that the delta in core damage frequency (CDF) for this period was 2.27E-7/yr and the delta large early release frequency (LERF) was 1.0E-8/yr. In accordance with NRC Inspection Manual Chapter 0609 this event would not be considered risk-significant.
A review of this event determined that a Safety System Functional Failure (SSFF) occurred as defined in Nuclear Energy Institute (NEI) 99-02.
A compensatory action to re-open the HPCI Feedwater injection valve following a short duration HPCI injection has been added to the system operating procedures.
Longer-term solutions to prevent re-occurrence are currently being developed.
COMMITMENTS
This LER contains no commitments.