Resolution of Generic Safety Issues: Task II.E.5: Design Sensitivity of B&W Reactors (Rev. 2) ( NUREG-0933, Main Report with Supplements 1–34 )
The objective of this task was to reduce the sensitivity of B&W plants to feedwater transients, with emphasis on the overcooling transients that had been observed at B&W operating plants.
ITEM II.E.5.1: DESIGN EVALUATION
The NRC staff concluded that B&W reactors exhibited unique sensitivity to secondary system transients (both undercooling and overcooling events). Therefore, B&W plants under construction were required to propose recommendations on hardware and procedure changes relative to the need for methods for damping primary system sensitivity to perturbations in the once-through steam generator (OTSG). This issue also considered the backfitting of the recommendations on operating plants.
The safety significance of this TMI Action Plan item48 was the same as that for Item II.E.5.2, i.e., the perception of what constitutes acceptable response to transients.
All B&W plants under construction were required [10 CFR 50.54(f)] to provide recommendations to reduce plant sensitivity.154 The recommendations (with proposed modifications) were submitted for NRC review. The staff also evaluated the modifications proposed by the applicants for possible backfit to operating plants.159 , 160, 443 The staff concluded that the portion of this issue that dealt with plants under construction was completed with the issuance of the Midland-1&2 SER which evaluated the modifications.159, 160, 443 The other B&W plants under construction were to be evaluated as part of the normal licensing review.
The portion of the issue which dealt with backfit considerations was also completed.159, 160, 443 Specifically, the staff concluded that the Midland modifications would be effective in reducing both the frequency and severity of overcooling transients and recommended that similar modifications be made at operating B&W plants. The staff also concluded that the following related activities were underway:
(1) Operating B&W plants were implementing upgrades to meet NUREG-0737.98
(2) Issue A-47, "Safety Implications of Control Systems," was addressing steam generator overcooling/overfilling as it related to control system failures.
(3) The staff was also pursuing resolution of overcooling events (steam bubble formation/natural circulation interruption) on a generic basis with the B&W Owners' Group [NUREG-0737,98 Item II.K.3(30)].
(4) Consideration of pressurized thermal shock (PTS) concerns relating to overcooling were being addressed by the staff as part of the resolution of Issue A-49, "Pressurized Thermal Shock."
Based on the above, the staff concluded that the B&W-designed operating reactors had responded to staff concerns regarding the frequency of overcooling and steam generator overfill events by implementing plant modifications. The adequacy of these modifications were to be confirmed by other ongoing programs. Thus, this item was RESOLVED and requirements were established.
ITEM II.E.5.2: B&W REACTOR TRANSIENT RESPONSE TASK FORCE
After TMI-2, the NRC staff investigated155 the response of B&W reactors to transients and determined that, in their opinion, they were overly responsive to certain transients. This responsiveness or sensitivity was attributed to a number of design and operational features including the small secondary water inventory in the steam generator, the small pressurizer volume, the pilot-operated relief valve (PORV) set-point, and the high pressure injection (HPI) set-points. As a result of the investigation, a number of recommendations were made for improving the plant response.155
The recommendations covered a number of design changes and operational considerations. DST/NRR provided a prioritization for the recommendations158 in August 1980. A number of these recommendations (referred to as Category A items) had already been implemented (or were being implemented) for the B&W operating plants.156, 157 The other recommendations (referred to as Category B items) had not been issued as requirements, although a number of them had been implemented by some licensees with B&W plants as part of their own investigations.
The safety significance of this TMI Action Plan48 item depended on the perception of what constitutes acceptable response to transients. NRC requirements were outlined in the SRP11 and all plants were required to meet these, as a minimum. It was suggested159 by DSI/NRR that additional performance criteria were necessary to more restrict the plants' response to transients and, as a result, limit the potential for plant damage.
The technical resolution to this issue was defined in NUREG-0667.155 It was suggested159 that implementing the resolution required additional specification of the staff's performance criteria for transient response. (Existing criteria were contained in the SRP.11) Therefore, DSI/NRR proposed159 that a uniform requirement in the form of criteria be issued by the NRC to ensure that adequate steps were taken by all B&W plants. Specifically, the recommended criteria were:
(1) ECCS actuation or loss of pressurizer level indication should not normally occur following a reactor trip or main feedwater control failure.
(2) Credit for operator action to mitigate overcooling events should be consistent with the guidelines of ANSI/ANS-58.8.45
(3) Steam generators should be protected from overfill from main or auxiliary feedwater flow to limit overcooling. This equipment should be safety grade if flooding of the steam lines is an unanalyzed event.
Based on a DST/NRR evaluation160 of the issue, it was recommended that implementation would be best accomplished by issuance of a statement of NRC's performance criteria for transients. It was also recommended that the first two criteria and accompanying value/impact statements be submitted to CRGR for review. The third criterion was included in Issues A-47 and A-49. Thus, the issue was RESOLVED and requirements were established.656, 657