ML20210U813
| ML20210U813 | |
| Person / Time | |
|---|---|
| Issue date: | 05/22/1986 |
| From: | Stello V NRC OFFICE OF THE EXECUTIVE DIRECTOR FOR OPERATIONS (EDO) |
| To: | |
| Shared Package | |
| ML20209H803 | List: |
| References | |
| FOIA-86-864, TASK-PII, TASK-SE SECY-86-162, NUDOCS 8606060268 | |
| Download: ML20210U813 (7) | |
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tiay 22, 1986 POLICY ISSUE SECY-86-162 (Information)
For: The Comissioners From: Victor Stello, Jr.
Executive Director for Operations
Subject:
TREATMENT OF EXTERNAL EVENTS IN THE IMPLEMENTATION OF THE SEVERE ACCIDENT POLICY STATEMENT
Purpose:
To infom the Comission of the staff's plans on how external initiators of accidents are to be handled in the context of the Severe Accident Policy.
Summary: The Severe Accident Policy Statement sets goals and a schedule for addressing issues related to severe accidents in the licensing of future plants and for the systematic examination of existing plants. The Policy Statement does not differentiate between events initiating within the plant and events caused by external initiators, such as earthquakes, floods, and high winds. Current risk assessments indicate that risk from external events could be a major contributor.
However, there is an increasing belief that risk from external events, for example seismic risk, has been over-estimated because of the conservatisms used in its evaluation.
! It is the staff's intent to proceed with the implementation of
- the severe accident policy outlined in SECY-86-76 for internally initiated events. The assessment of external events will require more research and development, hence, a longer schedule than for internal events. However, based on data obtained from NRC research programs, actual earthquake experience, and PRAs treating external and internal events, the evaluation of severe accidents for internal and external events can be treated separately without jeopardizing the results and conclusions. The staff plans to proceed with the evaluation of severe accidents initiated by external events in two phases. The first. phase, an assessment of the margin that past and ongoing programs provide relative to external events beyond the design basis and an identi-CONTACT
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2 fication of areas where an examination for external vulnerabili-ties is needed, will be completed in approximately a year. During this period, development of a simplified seismic review procedure which includes a trial plant review will be completed under the RES sponsored NRC Seismic Design Margins Program. The second phase will consist of a program for plant specific evaluation,
- if needed. Infomation developed in phase I will be used as guidance for phase 2.
Discussion: On August 8,1985, the Comission issued a Policy Statement on severe accidents. The Policy Statement provides criteria i and procedural requirements for the licensing of new plants, and sets goals and a schedule for systematic examination of existing plants with respect to severe accident vulnerabilities.
The Comission emphasized that it intends to take all reasonable steps to reduce the chances of a severe accident occurrence and to mitigate the consequences of such an accident, should it occur. The Policy Statement does not specifically address the issue of accidents initiated by external events, nor does it differentiate between internal and external events.
. To implement the Comission's Policy Statement, the staff has developed a task plan documented in SECY-86-76. As
, part of the plan, the staff will prepare guidance on the role of probabilistic risk assessments (PRAs) in future plant applications and will issue guidance and criteria for the systematic examination of existing plants for severe accident vulnerabilities.
I Parallel to NRC's effort, the Industry Degraded Core Rule-making Program (IDCOR), on behalf of the nuclear industry, is developing a simplified method for plant examination. This i
method will be reviewed by NRC.
Completion of NRC's efforts, including the review and approval l of the IDCOR methodology, is scheduled for early 1987.
! Both NRC's and IDCOR's current efforts on severe accidents
) are limited to internal events; and IDCOR also covers internally initiated fires and floods. External events (earthquakes, floods, etc.) are not addressed. These facts prompted the Advisory Comittee on Reactor Safeguards (ACRS), in a letter to Mr. Dircks, dated August 13, 1985, to state "In view of the fact that a number of current PRAs have concluded that external initiators make a significant contribution to risk, we recommend
- that whatever method is applied for the systematic analysis of l operating plants should treat these initiators insofar as is
- feasible."
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External Events There are a number of recent PRAs that have treated external events in addition to internal events. Many of these indicate a core-melt frequency associated with external events that is comparable to that for internal events. Based on a review of these PRAs, it is believed that the main contribution to core melt from external events comes from seismic events and fires; the contribution from floods and winds is low and varies from site to site.
There are, however, an increasing number of experts who -
believe that the contribution of seismic events to core-melt frequency and risk is overestimated in current PRAs.
For example, seismic design conservatism has been demonstrated through actual earthquake experience. Comparable heavy industrial structures and components that have been subjected to large earthquakes have shown that structures, piping and equipment, designed with far less care than nuclear, exhibit capacities far beyond their design levels provided they are adequately supported. In addition, the methods and data used in seismic evaluations are believed to be more conservative than those used in the evaluation of internal events. Examples of these seismic conservatisms include the broad band design response spectra specified by Regulatory Guide 1.60; conservative combination of responses and loads; and conservative assumption on structural properties of concrete, steel, and reinforced concrete used in the design and analysis.
An extensive discussion on seismic design conservatism is documented in a Lawrence Livermore National Laboratory report UCID-20572, "LLNL/ DOR SEISMIC CONSERVATISM PROGRAM: Investi-gations of the Conservatism in the Seismic Design of Nuclear Power Plants," dated October 1985.
There are a number of ongoing hRC programs that have examined or have some potential to assess seismic risk or seismic margins.
The Office of Nuclear Regulatory Research (RES) sponsored the Seismic Safety Margins Research Program (SSMRP), which developed a methodology to assess seismic risk using a PRA approach. A demonstration analysis of the Zion Plant provided some insights on system vulnerability. This study predicted that core melt frequency and risk are due predominately from earthquakes in the range of from 2 to 4 times the SSE.
The complex calculational methods used to estimate seismic margins or seismic risk have not been fully validated. RES is currently participating in cooperative experimental programs to get hard data to validate these calculational methods. This effort will be completed in a time frame compatible with the implementation schedule described below.
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RES is also developing the NRC Seismic Design Margins Program to estimate available seismic margins in operating plants at a predetermined seismic level above the current SSE. The pro-cedures draw on the insights gained from completed seismic PRAs (SSMRP and industry sponsored) and other studies to focus on those plant functions that contribute most to seismic risk. Screening guidelines have been established for earth-quakes up to several times above the SSE level. The seismic margin reviews will identify the components and systems most vulnerable to a seismic event and establish the earthquake level (above the SSE) where there is high confidence of low probability of failure (i.e., a seismic level where safety is not compromised).
Other programs contributing to the understanding of seismic design conservatisms involve Unresolved Safety Issue (USI)
. A-46, " Seismic Qualification of Equipment in Operating Plants",
USI A-45, " Decay Heat Removal Requirements", and USI A-40, " Seismic Design Criteria, A Short-Tem Program." The program associated with USI A-46 is developing a methodology to verify the seismic adequacy safe shutdown of operating) earthquake (SSE . plant equipment for the currentAltho seismic margins beyond the SSE, this effort is expected to provide some insight on equipment capability beyond the SSE level. The program related to USI A-45 has examined the of the decay heat removal system of four operating ability plants (Quad Cities, Cooper, Point Beach, and Turkey Point) to perfom its function during and after seismic events up to and beyond the SSE. USI A-40 investigated selected areas of the seismic design process, and is proposing alternate approaches to parts of the design sequences, and to modify the NRC criteria in the Standard Review Plan to reflect current state of technology and industry practice.
Although some of the programs referenced have been limited to analysis of design basis events, they have usually provided some insight on available margins and have indicated capabili-ties beyond the seismic design basis. In addition, the design procedures (i.e., codes, standards, and industrial practices) used in the design of structures and piping generally result in inherent capacity substantially beyond the design level.
Therefore, it is expected that substantial seismic margin exists.
In addition to the programs previously mentioned, the RES and NRR sponsored Seismic Hazard Characterization of the Eastern United States Project has developed a methodology to assess the seismic hazard for the region east of the Rocky Mountains.
While this methodology will not directly provide an assessment of available seismic margins, it will provide a better means of assessing the seismic hazard used in the plant design.
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5 Similar to NRC's ongoing programs, the nuclear industry has a number of programs that have, or will have examined seismic risk or seismic margins. For example, EPRI is sponsoring a seismic margins program and an eastern United States seismic hazard characterization program complementary to the staff programs.
Fires have been analyzed in seismic PRAs, and the. analyses indicate that fires could represent a significant portion of the total risk from nuclear power plants. Currently, RES and NRR are assessing this issue to detemine what, if any, research needs to be done in this area. For example, to what extent would the full implementation of Appendix R requirements reduce this residual risk. The question of how and when fire will be included in the external events program will be deter-mined later.
Other external phenomena specific to a given site have been routinely reviewed during the licensing process for each plant and the potential frequency and severity of such events have been used in detemining the plant design bases. These events include not only natural phenomena such as floods and high winds, but also potential man-related events as well, such as aircraft impacts, explosions, and releases of toxic and flamable chemicals.
External events other than seismic will only be reviewed in
.l terms of Severe Accident Policy as warranted by site specific conditions.
Based on insights from PRAs that have treated both external and internal events it is evident that the internal events analysis would not be invalidated if the seismic event intro-duced an initiator not considered in the internal events analysis.
Therefore, based on the known seismic conservatisms, NRC programs described earlier, actual earthquake experience data, and data from PRAs treating external and internal events, evaluations
! necessary for detemining the vulnerabilities for severe acci-
, dents from internal and external events can be treated separately without jeopardizing the results and conclusions.
l Implementation: To achieve the main goal of the Severe Accident Policy Statement (namely, to bring stability to licensing and regulation with regard to all severe accident issues),
it is important that all initiators of severe accidents receive appropriate consideration. This does not mean that the extent and timing of the review and examination for both internal and external events need be identical.
The Comission's policy states that the goal is to identify vulnerabilities and to take corrective actions, where appro-priate. There are two basic issues with regard to external events: (1) the inherent margin in the design for external
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i events and (2) the identification of plant-specific vulnera-bilities. A better understanding of the apportionment of risk between internal and external events would be helpful in accomplishing this goal. However, the magnitude of the uncer-tainties is such that a meaningful comparison between inter-nal and external risk can not be reasonably made at this time.
The implementation program will concentrate, therefore, on finding plant vulnerabilities.
The program will be conducted in two phases. The first phase, an assessment of the margin that past and ongoing programs provide relative to external events, includes the following steps:
(1) Establish the extent to which plants have been reviewed for external event vulnerabilities in the past, or will be reviewed under ongoing programs, such as USI A-46.
(2) Estimate the degree of protection these reviews afford relative to external events beyond the design basis; for example, seismic events beyond the SSE.
! (3) Specify external events that should be included in the vulnerability search.
I (4) Identify those areas in which the existing requirements and reviews have not ensured sufficient prote-tion for the external events specified above.
(5) Complete development of a simplified seismic review procedure under the RES sponsored NRC Seismic Design Margins Program. Conduct a trial plant review.
On completion of the first phase, the staff will evaluate the need for plant-specific examinations of existing plants with respect to external events. If the need clearly exists, a second phase will be developed to address plant-specific examinations consistent with the Severe Accident Policy.
As mentioned earlier, there are many ongoing NRC and industry sponsored programs that contribute to our understanding of plant capabilities beyond the design basis. Most of these programs will be completed or be nearing completion by FY 1988.
l It is our intent to take advantage of all available infoma-tion and existing knowledge'in the execution of the program.
We are now arranging industry participation in the first phase of the program to share the burden of collecting all relevant '
infomation and to ensure that important aspects of external event considerations will not be overlooked.
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Scheduling: The tentative schedule for completion of the first phase is approximately one year. The scope and schedule of the second phase will be established later. ,
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