ENS 46715
ENS Event | |
|---|---|
19:32 Mar 31, 2011 | |
| Title | Unanalyzed Condition Identified for Inoperable Rwst Level |
| Event Description | In response to a condition identified in late 2010 concerning the control and removal of hazard barriers in the plant, a review of the basis and analysis for high energy line breaks (HELBs) and the barriers for protecting against such events has been underway at Callaway in accordance with the plant's corrective action program. While following up on a question from the NRC Resident Inspector, and as a result of an additional question from the Nuclear Oversight organization at Callaway, it was identified that non-safety piping located in the valve room associated with the Refueling Water Storage Tank (RWST) could potentially [make] all four RWST low water level pressure transmitters inoperable in the event of a malfunction of the non-safety piping concurrent with a design-basis loss-of-coolant accident (LOCA) and/or following a seismic event. The RWST water level transmitters (which are located in the RWST valve room) perform a safety-related function for the emergency core cooling system (ECCS) by automatically swapping suction sources for the ECCS during a LOCA from the RWST to the containment sumps when a low water level condition is reached in the RWST. These instrument channels are required to be OPERABLE in Modes 1, 2, 3 and 4 per Callaway Technical Specification 3.3.2, 'Engineered Safety Feature Actuation System (ESFAS) Instrumentation.'
The subject non-safety piping delivers steam supplied by the Auxiliary Steam system to (and from) heaters surrounding the RWST for maintaining RWST contents above the minimum required temperature during winter conditions. The piping passes through the RWST valve room containing the noted RWST water level transmitters which were designed only for a mild environment. It has been identified, however, that the non-safety Auxiliary Steam piping constitutes a high energy line and that its failure could create harsh (hot and wet) conditions in the valve room to which the RWST water level instrumentation was not designed. Per the Callaway FSAR, where non-safety piping interfaces with safety-related piping or systems, the design must be such that failure of the non-safety piping does not adversely affect the safety function(s) of the interfacing safety-related piping or system (since non-safety piping may be assumed to malfunction in conjunction with a design-basis accident). In this case, and based on a conservative interpretation of the FSAR, if the non-safety piping in the RWST valve room is assumed to malfunction (i.e., break), a failure of the RWST instrumentation could occur, thereby preventing the ECCS suction swap over from occurring as required or assumed for LOCA mitigation. This condition required declaring all four RWST water level channels inoperable. In light of recognizing that the RWST water level instruments could be subject to a harsh environment when they were only designed for a mild environment, and could thus fail as a result, this condition represents an unanalyzed condition that significantly degrades plant safety. With regard to the impact on the required ECCS suction swap over function that requires the RWST water level channels to be operable, the inoperability of all four instrument channels is a condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to shutdown the reactor and maintain it in a safe condition, remove residual heat, control the release of radioactive material, and mitigate the consequences of an accident. Upon declaring the RWST water level instrument channels inoperable, TS Limiting Condition for Operation (LCO) 3.0.3 was entered at time 1432 CDT on 3/31/2011. At 1634 CDT, the Auxiliary Steam system was isolated and depressurized. This removed the energy that could be released from a break in the non-safety piping, thereby restoring Operability for the RWST water level instruments. The NRC Senior Resident Inspector was notified.
On March 31, 2011, event notification EN 46715 documented that a harsh environment from a postulated High Energy Line Break (HELB) in the Refueling Water Storage Tank (RWST) valve room could affect RWST level transmitters. These level transmitters provide RWST water level indication in the main control room, which is identified as a safe shutdown function in the Callaway FSAR. They also provide low RWST water level signals for effecting automatic swap over of suction sources for the Emergency Core Cooling System in the event of a loss-of-coolant accident (LOCA). This break may be postulated to occur on non-safety related auxiliary steam lines that run through the RWST valve room and on to the RWST heaters. This condition was initially reported both as an unanalyzed condition that significantly degraded plant safety and as a condition that could have prevented fulfillment of a safety function. When EN 46715 was reported, it was assumed that breaks were required to be postulated at any intermediate fitting, welded attachment, or valve on the subject auxiliary steam lines. Subsequent analysis shows that the sections of auxiliary steam piping in the RWST valve room are able to withstand safe shutdown earthquake (SSE) loadings and rupture loadings. For piping of this qualification, breaks at all intermediate fittings, welded attachments, and valves do not need to be postulated. Instead, line breaks are only required to be assumed at the terminal ends of the lines and at the locations specified for ASME Class 2 and 3 piping. None of these postulated break locations are located inside the RWST valve room, and a postulated auxiliary steam line break outside of the room would not adversely affect the RWST level transmitters. Since none of the postulated break locations are located inside the RWST valve room, there exists reasonable assurance that the RWST level transmitters would have remained capable of performing their safe shutdown function following a postulated break of the subject auxiliary steam lines. Further, there is no adverse effect on the assumed response to a postulated design basis LOCA since a hazard (such as a break in an auxiliary steam line) is not assumed to occur concurrently with the LOCA. Therefore, this condition does not meet the reporting requirements for an unanalyzed condition that significantly degraded plant safety or a condition that could have prevented fulfillment of a safety function. Event notification 46715 is hereby retracted. The NRC Senior Resident Inspector has been notified. Notified R4DO (Haire). |
| Where | |
|---|---|
Callaway  Missouri (NRC Region 4) | |
| Reporting | |
| 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident | |
| Time - Person (Reporting Time:+1.3 h0.0542 days <br />0.00774 weeks <br />0.00178 months <br />) | |
| Opened: | Micah Benningfield 20:50 Mar 31, 2011 |
| NRC Officer: | Dong Park |
| Last Updated: | May 26, 2011 |
| 46715 - NRC Website
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Unit 1 | |
|---|---|
| Reactor critical | Critical |
| Scram | No |
| Before | Power Operation (100 %) |
| After | Power Operation (100 %) |
Callaway with 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident, 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition | |
WEEKMONTHYEARENS 548062020-07-30T13:15:00030 July 2020 13:15:00
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