Resolution of Generic Safety Issues: Issue 36: Loss of Service Water (Rev. 3) ( NUREG-0933, Main Report with Supplements 1–34 )
On June 19, 1981, AEOD issued a preliminary report557 on the incident at Calvert Cliffs Unit 1 in which the plant lost both redundant trains of service water when the service water system became air-bound as a result of the failure of a non-safety-related instrument air compressor aftercooler. On August 5, 1981, NRR provided comments494 on this report, but indicated that their review was limited because of the preliminary nature of the AEOD report which did not contain recommendations and conclusions. Subsequently, AEOD issued the final version558 of this case study on December 17, 1981, in which three generic recommendations were presented and NRR was requested to comment on this version of the AEOD work.
NRR provided another review559 of the case study on September 23, 1982, which was more detailed and specifically addressed the AEOD generic recommendations. However, in an AEOD memorandum560 to NRR on May 2, 1983, additional clarification to the case study was provided. In response to this memorandum, DL requested461 DSI to review the latest AEOD memorandum. DSI performed this review and provided specific and detailed responses562 to all concerns identified by AEOD. Based on the content of the DSI memorandum,562 the AEOD concerns were addressed and solutions were identified in an NRR memorandum563 to AEOD on September 15, 1983. Also, in an AEOD memorandum564 to OIE on August 18, 1983, detailed information was provided so that an appropriate Information Notice could be issued by OIE. As a result, IE Information Notice No. 83-77565 was issued on November 14, 1983.
The significance of this event lies in the fact that it involved two fundamental aspects in the design of safety-related systems: (1) interaction between safety and non-safety-related systems and components; and (2) common cause failure of redundant safety systems.
A summary of the AEOD recommendations and the NRR responses are as follows:
I. AEOD Recommendations of Section 8, Part (a) of the AEOD Report 558
1. AEOD Recommendation 1
It is recommended that butterfly valves SW-4 and SW-5 have valve operators added (pneumatic or electric motor) and that these valves either close automatically, as do the valves on the turbine supply header, or as a minimum have the capability to be remote manually-operated from the control room.
Inasmuch as the service water system at Calvert Cliffs is a "dual purpose" system, as defined in SRP11 3.6.1, a single failure in one redundant safety-related train is not postulated concurrently with a moderate-energy line break in the other train. Therefore, it is always assumed that the check valve will function as designed to provide the isolation of both trains. Consequently, this configuration is acceptable under the current staff criteria. However, during the review of operating reactor compliance with the moderate-energy pipe break criteria, the staff considered the effect of leak rate from postulated cracks on the system operability and the time required to isolate the break before loss of system safety function. Based on these reviews, NRR believed that sufficient instrumentation (service water head tank level alarms) and time (30 minutes) were available for the Calvert Cliffs operators to locally close the manual isolation valve in such an event. Further, makeup to the heat tank provides additional time for taking this action.
2. AEOD Recommendation 2
It was recommended that the four check valves and the four solenoid operated three-way valves in the instrument air lines that provide control air for the four diesel-generator 12 service water supply and return valves be added to the IST program.
NRR Response 559
A number of plant-specific recommendations were made for the Calvert Cliffs plant. Although NRR agreed that implementation of the AEOD plant-specific recommendations may be beneficial to the operation of the facility, NRR did not believe that ordering such changes would be accompanied by an appropriate increase to the health and safety of the public. Considering that the licensee participated in the peer review of the AEOD case study, they were aware of the AEOD plant-specific recommendations. Therefore, NRR did not believe that further regulatory actions were necessary.
NRR did, however, have the following comment on this plant-specific recommendation. The NRR/OIE working group on instrument and service air system, which was formed after growing concerns of air system degradation, was considering generic recommendations regarding the isolation and boundary between safety and non-safety-related air systems. By copy of the NRR memorandum,559 the AEOD recommendation to include system boundary valve in the IST program was forwarded to the working group for their information. This working group was disbanded before they could review this recommendation. Therefore, a memorandum566 was issued to DL for their review and action on this item.
3. AEOD Recommendation 3
If there is a loss of offsite power and the service water system supplying diesel generator 12 becomes unavailable, the diesel will transfer to an inactive service water loop. Since operator action is necessary to realign the diesel generator such that it energizes the bus which powers the service water pump in the loop to which it was transferred or, alternatively, to start a third service water pump, it is recommended that the human factors of these actions be evaluated against the length of time the diesel can run without service water before it trips.
NRR Response 563
In general, NRR believed that implementation of AEOD's plant-specific recommendations could be beneficial to the operation of the plant. Therefore, NRR forwarded a copy of the AEOD final report to the licensee with a statement that the recommendations were considered to be valid and that their implementation should be considered.567
4. AEOD Recommendations 4 and 6
For operating plants and plants currently in the licensing process that have service water systems that contain both safety and non-safety-related portions, it was recommended that the system isolation provisions be reviewed to identify any procedural or hardware changes necessary to protect the safety-related portion of the service water system from a failure portion of the service water system from a failure in the non-safety-related portion during normal operation and accident conditions.
It was recommended that the guidance in the SRP11 be clarified to emphasize automatic isolation of the non-safety-related portion of the service water system when it degrades the operability of the safety-related portion of the system.
NRR agreed that the guidelines in SRP11 Section 9.2.2 could more clearly state when automatic isolation of safety and non-safety-related portions of the system is necessary, such as indication of low pressure in the non-safety-related portion as would occur in a failure of the non-seismic Category I piping due to an SSE. The staff was to propose revisions to the review procedures (Paragraph III.3.a) of SRP11 Section 9.2.2 to more clearly indicate the types of isolation signals required and when isolation was necessary.
5. AEOD Recommendation 5
It was recommended that an IE Circular on common cause failure of service water systems be issued.
An IE Information Notice565 was issued on November 14, 1983.
II. AEOD Recommendations of Section 8, Part (b) of the AEOD Report 558
All of the recommendations (1-3) in this section addressed the concern of SGTR events. Upon approval by CRGR and the Commission, the action on these recommendations was carried forward under Steam Generator Staff Actions (Issue 67), stated in References 559 and 563, as the generic studies performed by the staff.
III. Additional AEOD Recommendation 560
The accessibility of the steam generator dump valves (ADVs) during accident conditions should be reviewed to determine the acceptability of the assumption that the affected steam generator can be isolated in 30 minutes with manual operation of the ADVs.
NRR Response 563
This matter was addressed in Issue 67, "Steam Generators Staff Actions," subsequent to approval. However, there were no plans for the backfitting of RSB BTP 5-1. A cost/benefit analysis concerning backfit of RSB BTP 5-1 was considered in the resolution of Issue A-45 which addressed decay heat removal system improvements, including consideration of the ADV. The staff requirements for the successful completion of this effort were outlined in a DST memorandum.568
All but one generic concern and one plant-specific matter raised in the AEOD case study558 were addressed as part of Issues A-45 or 67.5.2. The generic concern was resolved with the issuance of SRP11 Sections 9.2.1, Rev. 4, and 9.2.2, Rev. 3, in June 1986.901 These revisions did not incorporate any new guidelines or requirements.902 The remaining plant-specific matter concerning Calvert Cliffs was brought to the attention of DL for appropriate action.566 Thus, this issue was RESOLVED and no new requirements were established.