Resolution of Generic Safety Issues: Item B-10: Behavior of BWR MARK III Containments (Rev. 1) ( NUREG-0933, Main Report with Supplements 1–35 )
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DESCRIPTION
Historical Background
The description of this item given in NUREG-04713 is as follows:
"This is an ACRS generic concern. Evaluation and approval is required of various aspects of the MARK III containment design which differs from the previously reviewed MARK I and MARK II designs. The task involves the completion of the staff evaluation of the MARK III containment and documentation of the method used to validate the analytical models and assumptions needed to predict the containment pressures in the event of a LOCA."
The MARK III suppression pool dynamic loads were reviewed by the NRC at the CP stage for Grand Gulf Nuclear Station, Units 1 and 2, and at the preliminary design analysis (PDA) stage for GESSAR-238NI. It was concluded at the time that the information available was sufficient to adequately define the pool dynamic loads for those nuclear plants under review for CPs. Since the issuance of the GESSAR-238NI SER in December 1975, GE has conducted further tests and analyses to confirm and refine the original load definitions. To keep the NRC and MARK III applicants apprised of the current status of these tests, GE issued an Interim Containment Loads Report (22A4365) in April 1978 and revised this report several times before GESSAR-II was provided to the NRC staff in March 1980. GESSAR-II is GE's FDA submittal for their standard BOP design and is to be referenced by MARK III OL applicants. Appendix 3B of GESSAR-II provides the finalized pool dynamic load definition for MARK III containments and is the basic document used for review by the NRC staff and its consultants.
The NRC staff is currently reviewing GE's pool dynamic load definitions to arrive at a finalized hydrodynamic load definition that can be utilized by MARK III containment applicants for operating licenses. The pool dynamic loads were being reviewed under USI A-39, "Determination of Safety Relief Valve (SRV) Pool Dynamic Loads and Temperature Limits for BWR Containment." The end product of these two generic programs will be applicable to Grand Gulf.
Safety Significance
Following a postulated LOCA, escaping steam forces the suppression pool out of the drywell into the wet well. This action results in pool swell and loads from vent clearing, jets, chugging, impact of water, impact from froth impingement, pool fallback, condensation, and containment pressure.
The concern is that these loadings may damage structures and components located within the wet well. Although many of these structures (e.g., walkways) are by themselves not related to safety, the various ECCS systems take suction from the wet well and, therefore, damage in the wet well may affect the performance of the ECCS.
Possible Solution
The MARK III plants affected by this issue will be reviewed to determine if their structures meet the NRC Acceptance Criteria for MARK III LOCA-related pool dynamic loads. Structural fixes will then be implemented where necessary.
PRIORITY DETERMINATION
The assessment of this issue and its proposed resolution is based in part on an investigation performed by PNL.64
Frequency Estimate
The proposed resolution of the issue is the implementation of the structural fixes mentioned above. The applicable plants include all GE BWR/6 MARK III containments beginning with Grand Gulf 1 and 2. There are a total of 17 such plants listed in GESSAR II, Table 1.4-1, of which 4 have been cancelled, leaving a total of 13 applicable plants for forward-fit (i.e., fixes to be made before plant start-up). The Grand Gulf plant is selected as the representative plant. It is a typical GE BWR/6 with a MARK III Containment.
The parameters in the plant risk equations assumed to be affected by the BWR MARK III containment modifications are those related to the emergency core cooling system. The LOCA is taken to have already occurred, i.e., the dynamic loads are a result of the LOCA. The parameters are then selected from those which tend to mitigate the LOCA effects. The assumptions for the base case are:
(1) A LOCA occurs
(2) Some piping, equipment, or walkways are dislodged and fall onto the suction piping for the ECCS located in the suppression pool, plugging it in some manner. The plugging scenario described above is judged to have a probability of 10% for each suction. It affects elements L, LA2, LB2, LC, SA, and SB each of which becomes augmented by the amount 0.1. (The L sequences correspond to loss of suction for the various ECCS trains; SA and SB correspond to the suppression pool makeup system.)
The assumption for the adjusted case is that, after the postulated LOCA, no damage occurs to the ECCS piping because of structural fixes and, therefore, the originally calculated dominant accident sequences and frequencies prevail.87
Consequence Estimate
The affected sequences lead to a spectrum of consequences. The summed probability for Release Category BWR-2 is affected most, but the probabilities associated with the other three core-melt release categories are also affected. When the release category consequences are multiplied by their changes in frequency and then summed, the resultant risk reduction is 1,930 man-rem/RY. Based on an average remaining life of 30 years for the 13 affected plants, the total risk reduction associated with this issue is 7.5 x 105 man-rem.
Cost Estimate
Industry Cost: The structural fixes required to resist the LOCA-related pool dynamic loads at Grand Gulf 1 and 2 were selected as typical fixes for the generic issue and evaluated for cost. These included:
(1) Deleting solid circumferential concrete floor and adding a steel grating catwalk at the same elevation due to pool swell, plus relocating equipment to a higher elevation
(2) Relocating and strengthening main steam tunnel floor above pool swell zone
(3) Adding suppression pool makeup system
(4) Projecting TIP station floor down into suppression pool to eliminate pool swell loads
(5) Relocating piping to the region above bulk pool swell
(6) Changing piping submerged in pool to smaller sizes and heavier wall to accommodate submerged structure loads.
It is estimated that the modifications will cost $2.6M plus $10,000/year in extra maintenance for 30 years, giving a total cost of $2.9M/plant for 13 plants. Therefore, the total industry cost is $37.7M.
NRC Cost: NRC costs for review and inspection are estimated to be $1.4M for the 13 plants.
Value/Impact Assessment
Based on a total risk reduction of 7.5 x 105 man-rem and a cost of $39.1M, the value/impact score given by:
CONCLUSION
This issue addresses the design adequacy and, therefore, the availability of containment, one level of the "defense-in-depth." Based on the value/impact score, the issue was identified as high priority. However, since the prioritization was completed, a value/impact analysis was published in NUREG-0978600 and resulted in Revision 6 to SRP11 Section 6.2.1.1.C. Thus, this item has been RESOLVED and new requirements were issued.601
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