ML13322A375
| ML13322A375 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 04/02/1990 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML13322A374 | List: |
| References | |
| NUDOCS 9004110110 | |
| Download: ML13322A375 (3) | |
Text
CpjFtRE I4 -q UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO COMPONENT COOLING WATER SYSTEM PERFORMANCE FOLLOWING A SAFE SHUTDOWN EARTHQUAKE AND HIGH ENERGY LINE BREAK SAN ON0FRE NUCLEAR GENERATING STATION, UNIT NOS. 2 AND 3 DOCKET NOS. 50-361 AND 50-362 The following is the result of the staff evaluation of the San Onofre Units 2 and 3 component cooling water (CCW) system design and performance capability following a safe shutdown earthquake and high energy line break. This evalua tion was performed in response to the Region V request dated November 10, 1988, and is based primarily on the licensee's Component Cooling Water System Oper ability Assessment submitted by letter dated December 16, 1988, and supplemented by letters dated July 25, 1989 and January 9, 1990.
I. Component Cooling Water System Capability Following a Safe Shutdown Earthquake Because the San Onofre Units 2 and 3 CCW system serves safety-related equipment whose function is required in order to ensure a safe plant shutdown following a safe shutdown earthquake (SSE), its design must ensure proper system performance in the event of a SSE in accordance with the requirements of GDC 2 and 44, and the guidelines of Regulatory Guide 1.29. The specific concern identified in the CCW system design involved the assumption regarding CCW system leakage and its impact on system availability following a SSE.
The licensee provided an analysis of CCW system performance following a SSE in the Component.Cooling Water System Operability Assessment. In that analysis, the licensee assumed a moderate energy system critical crack in the nonseismic Category I noncritical loop rather than a full break. This postulation is contrary to staff guidelines in the Standard Review Plan which assumes complete rupture of the most limiting nonseismi cally qualified line during a SSE. However, it appears clear that the staff accepted this SRP deviation for San Onofre Units 2 and 3 during the initial licensing review. Therefore, to impose the staff position as indicated would be a backfit.
The above deviation is of concern because loss of the CCW.system can occur if isolation of the noncritical loop from the critical loopis not achieved in sufficient time before too great an inventory loss occurs through the pipe break..Loss of the CCW system results in an inability to cool the reactor coolant pump (RCP) seals which can cause a seal failure possibly resulting in a small break LOCA. In addition, because 9004110110 90040:2 PDR ADOCK 05000361 P -
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-2 the CCW system also cools the high pressure safety injection pumps, miti gation capability would also be unavailable. Because the CCW inventory system loss is based on the closure time of the noncritical/critical loop isolation valves from receipt of a low-low surge tank level signal assuming system water loss through a crack rather than a full break, this issue is significant to establishing the adequacy of CCW system performance following a SSE.
The staff has been studying the effect of loss of RCP seal cooling for several years under Generic Issue (GI) 23. Resolution of this issue will have a direct impact on the above concern for San Onofre Units 2 and 3 because the impact of loss of the CCW system on RCP seal cooling and the resulting safety of the plant is part of the GI 23 scope. Therefore, rather than pursue a plant specific backfit of San Onofre Units 2 and 3, the staff believes it is more appropriate to determine the need for further action by the licensee in accordance with the GI 23 regulatory analysis. Final resolution of GI 23 including newly imposed requirements is expected in the near future.
It should also be noted that the licensee previously provided the results of tests on the Byron Jackson.RCP seals which the licensee stated provide evidence of long-term seal integrity under loss of cooling water conditions.
The staff is considering these test results under the GI 23 resolution effort.
The licensees analysis of post-SSE response of the CCW system indicated that with the critical crack assumption in the noncritical loop, adequate CCW pump NPSH is available, and proper CCW heat removal is provided.
Adequate time is available following the initial system blowdown and isolation to refill the surge tank from the fire tankers and pumpers.
While some CCW system voiding may occur due to the rapid initial blowdown, the licensee's analysis concluded that this would not cause a loss in CCW system function. In response to a staff question on water hammer potential in the CCW system, the licensee indicated in their January 4, 1990 letter that modifications are underway to reduce the potential for void formation in the CCW system. These include a seismically qualified backup nitrogen system on the surge tank, relocation of the throttle valves downstream of the containment emergency coolers, and optimizing the surge tank level setpoints.
Based on the above, the staff concludes that the licensee's analysis of CCW system performance following a SSE is in accordance with staff guide lines with the exception of the assumption regarding a postulated critical crack rather than full break in the nonseismic portion of the system.
Resolution of this concern is being deferred to the generic resolution of GI 23 on loss of RCP seal cooling, and will be addressed by the licensee at that time. The staff, therefore, concludes that proper CCW system performance following a SSE has been demonstrated.
II. Component Cooling Water System Capability Following a High Energy Line Break Proper CCW system performance following a high energy line break is necessary in order to ensure a safe plant shutdown in accordance with the requirements of GDC 4 and 44. The licensee determined that the most conservative high energy line break interaction on the CCW system is a postulated break in a 42-inch steam line inside containment which could cause a full rupture of a 10-inch noncritical loop CCW line. However, this high energy line break also causes a containment isolation actuation signal (on high containment pressure) which closes the critical/noncritical loop isolation valves. Assuming the CCW surge tank is at the low level set point at the time of the high energy line break event, the licensee established the CCW system inventory loss. The licensee's analysis indi cated that while some CCW system voiding may occur as was the case for the SSE, adequate NPSH is maintained to the CCW pumps, and CCW heat removal is not impaired. Sufficient time is available for the operators to sub sequently hook-up the fire tankers and pumpers in order to provide makeup to the surge tank. As previously discussed, in response to a staff ques tior regarding water hammer in the CCW system as a result of voiding, the licensee indicated in the January 4, 1990 letter that engineering improve ments were being implemented in the CCW system to reduce the water hammer potential.
Based on the above, the staff concludes that the licensee's analysis of CCW system performance following a high energy line break is in accord ance with staff guidelines and is, therefore, acceptable. Thus, proper CCW system performance under such an event has been demonstrated.
Principal Contributor: J. Wermiel Date:
April 2, 1990