SEVERE ACCIDENTS

16.6.1 Integral Codes

Integral computer codes are being developed to provide a LWR accident analysis capability for modelling the course of a severe accident through its various stages.

Table 16.5. Severe accidents

Code type

Computer code/model

Integral

Mechanistic

ASTEC, ECART, MELCOR, MAAP SCDAP/RELAP5, VICTORIA, CONTAIN

Jacq and Allelein (2000), Allelein et al. (2000, 2001), NUREG/CR-6119 (1998), IAEA-TECDOC-752 (1994), Allison et al., NUREG/CR-5545 (1992) and NUREG/CR-6533 (1997).

They provide a phenomena coupling capability from degradation of the fuel rods through to formation of a molten pool and if the accident progresses unchecked, to the containment loading and release to the environment. They include modelling for the release of fission products and aerosols (e. g. from control rod materials and core-concrete interactions). They include models for fission product transport through the reactor coolant circuit to the containment, including deposition, re-suspension of aerosols and also the fission product source to the environment, should the containment fail or be vented by operator action.

Examples of such codes include ASTEC (Jacq and Allelein, 2000; Allelein et al., 2000; Allelein et al., 2001), ECART, MELCOR (NUREG/CR-6119, 1998) and MAAP (IAEA-TECDOC-752, 1994; Table 16.5). These have been validated against various severe fuel damage and fission product release experiments during the course of their development. Further data are now becoming available from the integral PHEBUS FP experiments. The first objective of PHEBUS is specifically to provide high-quality data on the strongly coupled processes that occur in severe accidents, as described above. The second objective is to validate the codes against these data and to define the envelope of validation of the codes. The PHEBUS programme is ongoing currently. In addition to integral analysis interpretation, it is supported by additional analysis from detailed CFD codes.

The Accident Source Term Evaluation Code (ASTEC) aims to model all stages of a severe accident sequence from the initiating event through to fission product release from the containment. It is a European code developed by GRS (Germany) and IRSN (France). The code adopts a best estimate approach and aims to include all the major phenomena and their interactions and also the main plant systems. The other requirements are that it should be fast running, flexible for performing sensitivity analyses and with appropriate validation. The code has been made available to the EC European Validation of the Integral Code ASTEC (EVITA) 5th Framework Project for further validation activities. The applications of the code are for determination of source terms, support to level 2 PSA and to promote better understanding of the physical phenomena.

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