Development of computer code for level 2 PSA of CANDU plant

The objective of this study is to develop a computer code, ISAAC (Integrated Severe Accident Analysis Code for CANDU Plants), which could handle the severe accident phenomena at Wolsong plants. The MAAP 4 computer code, which is a fast running code with state-of-the-art models, has been used as a basis. The new features in ISAAC are : 1) core module describing the configuration of core (horizontal fuel channel, calandria tubes, pressure tubes, fuel bundles, etc.) and core behavior such as channel heatup, oxidation, sagging and relocation of molten corium into the calandria, 2) the PHTS module modeling water/gas pressure and temperature, water level, break flow rates in the two separated figure-of-eight primary loop, 3) calandria transfer rates between calandria and water in the calandria bottom, 4) special safety systems including emergency core cooling system (ECCS), dousing sprays, moderator cooling system, shield cooling system, and over-pressure protection system which mitigate the severe accidents. The code has been tested for the various representative severe accidents. The results were reasonable from the view point of accident progression and its phenomena. Therefore, ISAAC can be said to have the basic capability to simulate the severe accidents in the Wolsong plants. (author). 28 refs., 7 tabs., 140 figs