LA PRISE EN COMPTE DES RISQUES D'ACCIDENTS GRAVES DANS LES CENTRALES NUCLEAIRES D'EDF EQUIPEES DE REACTEURS A EAU SOUS PRESSION

摘要

ASSESSMENT OF THE RISKS OF SEVERE ACCIDENTS IN EDF's PWR PLANTS. It is of primary importance to ensure that an incident or accident analysed during the design stage does not degenerate into a severe accident during operation. Accordingly, measures are taken to ensure effective control of accidents taken into account in the design: automatic safety devices which give the operator a minimum period of about twenty minutes in which to act; computer aids to help operators monitor and diagnose events; unambiguous procedures validated on simulators; training of staff and teams on simulators which represent the whole system or certain functions. It is necessary to ensure that preventive measures are fully consistent and to compare the level reached with the objectives established by the safety authorities. Probabilistic studies carried out in France have indicated the necessity of including cases of redundant safety systems in the design conditions. Steps have been taken and the corresponding procedures (known as 'H procedures') have been introduced at power plants. The last stage of core meltdown prevention involves establishing an ultimate procedure, the 'U1 procedure', the aim of which is to maintain or restore adequate cooling of the core in situations which are not foreseen in the normal procedures. In the hypothetical situation of a core meltdown accident, the objective is to maintain a level of containment of radioactive products that ensures acceptable protection of the public and the environment. The behaviour of the large volume 'dry-type' containment vessels of French PWRs has therefore been analysed in terms of possible hazards that might endanger them. An early failure of integrity is considered sufficiently unlikely for it not to be taken into account in the safety approach, making it unnecessary to take specific measures to trigger the combustion of hydrogen, for example. Studies of hydrogen behaviour and, in particular, of deflagration-detonation transition characteristics are nevertheless continuing. Design provisions and a number of ultimate procedures (procedures U2, U3 and U5) have been introduced to cope with credible risks of loss of integrity of the containment after core meltdown. The last of these, procedure U5, which provides for deliberate venting of the containment to the atmosphere through a simple filter system at least 36 or 48 hours after an accident has begun, will be fully operational at all units at the beginning of 1989.