REACTOR CORE ANALYSIS Objectives

All the features listed above had to be represented in the nuclear design analysis. It was clear at the outset that particular attention would have to be devoted to the problem of core representation If the overall objectives of predicting reactivity levels and void coefficients accurately, as already achieved in simple lattices, were still to be attained in the actual reactor core. Moreover, there was the problem of predicting the power distribution to an accuracy of about 5 of the peak value In order to satisfy the requirements of thermal hydraulic design. 

On 7 April 1989, a fire broke out in the stem section of the Komsomolets nuclear submarine. The submarine sank to a depth of 1685 m at 73°43T6"N, 13°15 52"E, near the south-west of Bear Island. The site is about 300 nautical miles from the Norwegian coast. The wreck contains one nuclear reactor and two nuclear warheads, one of which was fractured. The radionuclide inventory includes 1.5 PBq Sr, 2 PBq Cs, about 16 TBq " Pu in the two warheads and 5 TBq of actinides in the reactor s core. During June/July 1994, an international expedition to the Komsomolets site at the request of the Russian Federation was organised. The objectives of the scientific cmise on board the R/V Mstislav Keldysh were to close nine door holes, including torpedo tubes, by capping them with titanium metal cover caps, and to sample and monitor for ambient radioactivity. A series of 280-600 1 sea-water samples collected in profile, a suite of surface sediments and cores and various biota samples were returned to lAEA-MEL for analysis. The results showed that a very limited leakage of caesium and tritium had occurred from the submarine. 

The deterministic approach to the design of nuclear reactors was rapidly supplemented by the development of probabilistic studies, referred to as PSAs and also as PRA. The first study of this kind carried out in the United States was published in 1975 (Rasmussen report—USNRC 1975) and provided the first assessment of the potential risk of core damage for two power reactors. The accident in 1979 at the Three Mile Island plant generated renewed interest in this type of study. One of the recommendations made after the accident was that probabilistic analysis techniques should be used to supplement conventional safety assessment procedures for NPPs, and that probabilistic objectives should be developed in order to facilitate the determination of acceptable safety levels for nuclear facilities. 

As a brief conclusion to this section, let us recall that the global safety objectives are fully transposable to the MSFR reactor. The difficulty lies, among other things, in the identification of severe accidents for this type of reactor. Thus a core melt in the case of solid-fueled reactors is central to present safety studies and has no immediate equivalent in a Uquid-fiieled reactor. A safety analysis for the MSFR must then proceed from the fundamentals of nuclear safety. 

---