ANALYTICAL INVESTIGATION OF FUEL COOLING CONDITIONS IN WWER-440 SPENT FUEL POOL DURING TRANSIENT CAUSED BY FAILURES IN THE COOLING SYSTEM
17th Symposium of AER on VVER Reactor Physics and Reactor Safety (2007, Yalta, Crimea (Ukraine, back then))
REACTOR DYNAMICS, THERMAL HYDRAULICS AND SAFETY ANALYSIS
Nowadays some Ukrainian NPPs replace their fuel storage system in the spent fuel pool with densely storage system of new design (so-called SUHT). This allows to increase significantly the number of stored fuel assemblies, that in turn results in increase of total decay power in the spent fuel pool. In this case, according to the requirements of national regulatory body and IAEA recommendations additional safety substantiations are needed. One of the main parts of such substantiation is an analysis of initiating events which lead to loss of cooling of the spent fuel pool.
One example of such a work is a safety substantiation of densely fuel storage for RNPP Units 1,2 provided by Skoda. ?Analytical Research Bureau? of SSTC NRS was taking a part in the regulatory review of this substantiation. The model of spent fuel pool for ATHLET computer code was developed to make benchmark calculations. This model allows simulating two possible cases of fuel location: (1) fuel is placed only in the densely fuel storage and (2) fuel is placed both in the upper removable and in the lower densely storages. Last feature allows checking aspects of spent fuel pool operation, which have not been estimated by Skoda.
ATHLET computer code has been selected for this task as a system thermal hydraulic code, which has a modular structure and is world wide known as an appropriate tool for safety analyses of different reactor types. Also, ATHLET allows to simulate natural convection and thermal siphon phenomena in the low parameter range (minimum temperature is 20?? and minimum pressure is 10 kPa), that is necessary for correct simulating spent fuel pool physics.
The benchmark analysis has demonstrated good agreement between results of justification analyses made by Skoda and results of comprehensive ATHLET calculations. In both cases analyses were conducted using conservative approach to the selection of initial and boundary conditions and demonstrated the acceptable level of safety.