Severe Accident Analysis

The SARDB consists ofhundreds ofcalculationresults for significant core damage scenarios by using integrated severe accident analysis codes, for example, MAAP (FAl, 1994) or MELCOR (USNRC, 1990). It includes most of the high frequency accident sequences which are selected from the plant damage state event tree of a level 2 probabilistic safety analysis... 

The database information consists of the thermal-hydraulic/radiological variables of the core, reactor coolant system, containment system, and safety systems which are generated from integrated severe accident analysis code, such as MAAP or MELCOR program. 

Outside the EU, IPPE/Russia and India are developing their own computer tools for severe accident analysis. 

As a result of a common interest for developing severe accidents analysis codes, a first common benchmark exercise about BN-8(X) in its non-zero void reactivity version has been proposed jointly by EU and IPPE at the December 1992 meeting of the WAC Group. Li December 1994, a second common follow-up benchmark exercise about BN-800 in its nearly-zero void reactivity version has been proposed jointly by EU and lAEA/IWGFR with the aim of including also India and Japan besides IPPE/Russia. 

The severe accident analysis should address a set of representative sequences in which the safety systems have malfunctioned and some of the barriers... 

The severe accident analysis should model the wide range of physical processes that could occur following core damage and that could lead to a release of radioactive material to the environment. These should include, where appropriate ... 

The severe accident analysis should also provide input to civil authorities for off-site emergency planning and response. 

The results of the severe accident analysis should be used to identify source terms which could be used as a basis for off-site emergency planning. 

Severe accident analysis codes which model the progression of an accident sequence from core damage through to containment failure,... 

The paper describes the multi-Venturi scrubber system developed in Sweden for filtered venting of reactor containments. Additional details can be found in E. Soderman, editor. Severe Accident Analysis in Sweden - Methods and Results, RAMA III Final Report, RAMA-III 89-02, ISBN 91-7010-116-7, December 1989. 

MELCOR (Integral Severe Accident Analysis Code) Fast-Running, parametric models. 

For severe accident analysis, a reactivity insertion of 0.3 %5K/K was assumed, coupled with the failure of the B4C based control rod scram system. As shown in Fig. XXX-6, the maximum fuel temperature is 1100°C, which is also below I500°C temperature limit for fuel... 

The results of severe accident analysis performed for the FUJI MSR indicate that the reactor has a sufficient integrity margin in the situations considered. 

The consequences of hypothetical system failures and combinations of failures, which have not been taken into account explicitly when designing the plant, were and are also being investigated within the scope of safety studies, reactor research and risk studies. (See also AA 4, Need for severe accident analysis and MA 10, Adequacy of emergency operating procedures). 

ISSUE TITLE Need for severe accidents analysis (AA 5)... 

The fault identification process is detailed in Chapter 5 of this PCSR. Fault identification is based on the use of fault checklists, which are augmented by operating experience and consideration of the APIOOO design. These checklists inform the design basis assessment, the probabilistic risk assessment and the severe accident analysis. 

Following the discussion of the DBA, the probabilistie risk assessment (PRA) and severe accident analysis are described in subsections 5.4 and 5.5. The PRA and severe aceident analysis seek to demonstrate that the numerical risk to operators and members of the public are acceptably low, and serve as an input into the ALARP assessment in Chapter 8. 

The application ofthe N18.2 checklist has been reviewed against the APIOOO design and PRA, and been appropriately updated to reflect the plant s specific design features. On this basis, while potential Anticipated Transient Without Scram (ATWS) faults are included in ANSI N 18.2, for the APIOOO no causes for these have been identified within the Design Basis (i.e. the initiating event frequency is less than 10-5 per reactor year). These faults are addressed via PRA and Severe Accident Analysis (as described in Section 5.4 and 5.5 of this chapter). This issue is discussed in sub-section 4.4.1.3 ofthe APIOOO Fault Schedule (Reference 5.1). 

Analyses are performed with the MAAP4 code to study the progression of severe accident sequences and to define the radionuclide source terms. The severe accident analysis is discussed in subsection 5.5. 

Current activities in the severe accident domain are addressed by translating these understandings of phenomenology into their resultant safety implications and capturing this phenomenology in disruption models for the SAS4A severe accident analysis code. 

For severe accidents, and for all PSA applications, true best-estimate assumptions, methods, data, etc., are used throughout, because of the distortions to the outcomes that result from imbedding conservatism inside the analysis. Depending on the intended use of the PSA results or deterministic severe accident analysis results, some degree of conservatism may be added at the end to account for overall uncertainty. 

MELCOR was the first of the severe accident analysis codes to undergo a formal peer review process. One of the major conclusions of the recent MELCOR Peer Review [2] was the need for a more comprehensive and more systematic program of MELCOR assessment. A systematic program of code assessment provides a number of benefits, including ... 

Because some assessment is available for MELCOR in the areas of containment ther-mal/hydraulics and core damage assessment, calculations done at Sandia since the Peer Review concentrate on primary system thermal/hydraulic response, on fission product and aerosol release, transport and deposition, and on integral severe-accident analysis, areas where little or no MELCOR assessment was previously available. Completed and ongoing MELCOR assessment analyses at Sandia, whose results are summarized in this paper, include ... 

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