ML031110470
| ML031110470 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 09/25/2002 |
| From: | Demoss G Office of Nuclear Regulatory Research |
| To: | Bennett Brady Office of Nuclear Regulatory Research |
| References | |
| FOIA/PA-2003-0018 | |
| Download: ML031110470 (3) | |
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-s Gary emoss -rough cdraft of mments attached Page 1 From:
Gary Demoss ee797-To:
Bennett Brady Date:
9/25/02 4:58PM
Subject:
rough draft of comments attached N
Gary Demoss - Comments on SDP.wpd Page 1 Comments on Davis-Besse SDP Per you request, OERAB has reviewed the draft Response to Request for Technical Assistance
- Risk Assessment of Davis-Besse Reactor Head Degradation. In general, we are very concerned that this TIA is, as you state "... unconstrained by the details of the significance determination procedures (SDP) for the new Reactor Oversight Process (ROP)." It is important to determine the significance of this condition within the normal SDP, and then address the reason why additional risk analysis is necessary.
We have comments in four areas, all of which can lead to significant differences in risk estimates between the Accident Sequence Precursors (ASP) and SDP. The first three areas are the accident sequences that can contribute to increased risk, and the fourth is the preliminary information from the existing PRA models.
- 1. Medium LOCA of the as-found exposed section of clad - We agree with your statement presented under specific request 5 that the use of the tails of a probability distribution centered on a flawless plate rupture pressure of 7600 psig is essentially meaningless. The NRC needs to conduct a better probabilistic analysis that explicitly uses the analytically determined characteristics of the clad to examine the probability of that clad failing in the 2000 to 2550 psig range. As with the SDP, the ASP analysis will use the best available information at the time the analysis is completed.
- 2. Large LOCA of the section of clad exposed by a larger wastage area - We do not feel that the TIA approach to estimate the risk of this issue is appropriate for an SDP, and it cannot be used for an ASP analysis. The approach uses two factors - plant availability and start-up date -
to calculate the chance of Davis-Besse having spent more time at high temperature.
We do not agree with the use of a distribution based on the increased availability factor at Davis-Besse because a different availability factor implies different management and operation practices. The possible effects of different management on the corrosion history are not quantifiable, and it is not correct to use availability factors from other B&W plants to quantify these effects. For example, improved management practices at Davis-Besse could have led to higher availability factors (more operation time) and early detection and correction of the leakage issues.
We do not agree with the use other possible start-up dates based on B&W plants. Davis-Besse was constructed and its management was authorized to operate on July 31, 1978. The times at which other B&W plants began to operate are easily quantifiable, but not relevant to the condition at Davis-Besse.
RES is looking into several promising approaches to create probability distributions of possible corrosion rates, corrosion start dates and failures following different amounts of corrosion.
- 3. Medium LOCA from CRDM ejection - RES is comfortable with the general structure of the approach, which determines the annual probability of CRDM ejection given an annulus wetted from an axial crack. NRC needs to realize that the reference analysis used to estimate these CRDM ejection probabilities has not been widely circulated or reviewed. OERAB is concerned about the acceptability of this analysis because of the large number of upcoming ASP events Rough Draft - Sept 26
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.omments.on.....wpd-Page 2 I that involve CRDM cracking.
- 4. Conditional Core Damage Probabilities - The SDP is using licensee-provided sequence Conditional Core Damage Probabilities (CCDPs) shown in the table below. The licensee's recalculated MLOCA CCDP is essentially the same as the current SPAR model. The SPAR model CCDP for LLOCA is about twice as high the licensee's estimate. We don't have enough detail to determine the reason.
'S O urc e VSLOCA" z MLOCA,. ch-LLOCA DB IPE not provided 6.87x1 0-3 1.08x1 0-2 DB Recalculated for size of the N/A 2.91x1 0-3 N/A exposed clad area SPAR 3i 3.47x10 4 2.54x10-3 2.11x1 0-2 (MLOCA range is 2" to 5")
Note: SLOCA is not in use, but provided for information.
The SPAR models used for the ASP analysis will include GSI-1 91 issues related to the probablity of the sump plugging with debris. We don't know what the effect will be, but we expect that the LLOCA CCDP is most likely to affect the outcome of the analysis.
Rough Draft - Sept. 26