GENERAL

5.32. Safety of Nuclear Power Plants: Design [1] states in para. 5.31 that “Consideration shall be given to… severe accident sequences, using a combination of engineering judgement and probabilistic methods, to determine those sequences for which reasonably practicable preventive or mitigatory measures can be identified”. The occurrence of accidents with severe

environmental consequences should be made extremely unlikely by means of preventive and mitigatory measures.

5.33. Severe accidents should be evaluated by means of the best estimate approach[13]. In a best estimate approach, the combination of assumptions, computer codes and methods chosen for evaluating the consequences of a sequence should be such as to provide reasonable confidence that the results will reflect the probable occurrence of phenomena. In adopting best estimate approaches, special attention should be paid to ensuring that:

— Input parameters are in the range of what might be expected on the basis of present knowledge.

— Computer codes reflect an internationally accepted state of knowledge based on accepted research and development (in particular, the modelling of phenomena should not be controversial).

— All relevant aspects of the severe accident are considered (e. g. by the application of integral computer codes covering the hydraulics of the containment and the behaviour of fission products).

— The uncertainties in the values calculated are taken into consideration.

5.34. The validation domain of the computer codes used for evaluating all pertinent parameters should be verified to cover their expected range of variation adequately. Computer codes should not be used beyond their validation domain. As an exception, the use of computer codes beyond their range of validation might possibly be acceptable in areas for which it is widely recognized that there is a lack of coherent data. Such exceptions should be allowed only on the following conditions:

— The exception is clearly specified.

— A comprehensive sensitivity analysis is carried out to evaluate the effects of variations in the assumptions and in the modelling.

— An independent assessment is made of the credibility of the results.

— Appropriate margins are introduced if knowledge is limited.

5.35. For existing plants, the phenomena relating to possible severe accidents and their consequences should be carefully analysed to identify design margins and measures for accident management that can be carried out to prevent or mitigate the consequences of severe accidents. For these accident management measures, full use should be made of all available equipment, including alternative or diverse equipment, as well as of external equipment for the temporary replacement of design basis components. Furthermore, the intro­duction of complementary equipment should be considered in order to improve the capabilities of the containment systems for preventing or mitigating the consequences of severe accidents.

5.36. For new plants, possible severe accidents should be considered at the design stage of the containment systems. The consideration of severe accidents should be aimed at practically eliminating[14] the following conditions:

— Severe accident conditions that could damage the containment in an early phase as a result of direct containment heating, steam explosion or hydrogen detonation;

— Severe accident conditions that could damage the containment in a late phase as a result of basemat melt-through or containment overpressuri­zation;

— Severe accident conditions with an open containment — notably in shutdown states;

— Severe accident conditions with containment bypass, such as conditions relating to the rupture of a steam generator tube or an interfacing system LOCA.

5.37. For severe accidents that cannot be practically eliminated, the containment systems should be capable of contributing to the reduction of the radioactive releases to such a level that the extent of any necessary off-site emergency measures needed is minimal.

5.38. Severe accident conditions may pose a threat to the survivability of equipment inside the containment owing to the high pressures, high tempera­tures, high levels of radiation (the effects of deposition of aerosols should be taken into account in estimating the values of temperatures and levels of radiation) and hazardous concentrations of combustible gases. Furthermore, the larger uncertainties in relation to the conditions in the containment following severe accidents should be taken into account by using appropriate margins in the survivability demonstration or in specifying protective measures (such as shielding). These factors should be taken into account in verifying the necessary survivability of equipment and instrumentation.