ISP-46 analysis with RELAP5/SCDAPSIM computer code

The thermal-hydraulic and severe accidents analysis code RELAP5/SCDAPSIM was used in the calculation of the Phebus FPT1 in-pile experiment. This experiment, carried out on 26 July 1996 in the Phebus facility, Cadarache, France, was chosen as the basis for the OECD International Standard Problem (ISP-46) exercise. Investigation of severe accidents phenomena like fuel degradation and hydrogen production was the objective of the ISP and of the presented analysis. The ISP was an open exercise, that is, all the relevant experimental results were available to the participants from the start. The FPT1 test bundle included 18 PWR fuel rods previously irradiated to a mean burnup of 23.4 GWd/tU, two instrumented fresh fuel rods and one silver-indium-cadmium control rod. The bundle was housed in an insulating shroud and introduced into the Phebus driver core which supplied the nuclear power. The fuel degradation phase of the test lasted about 5 hours during which the bundle was cooled by steam at pressure of about 2 bar with the mass flow rate varying between 0.5 g/s and 2.2 g/s, while the bundle nuclear power was being progressively increased from zero up to 36.5 kW. RELAP5/SCDAPSIM modelling of the Phebus facility and the main results, such as the temperature response of all rods and shroud, the oxidation and resulting hydrogen production, will be discussed and presented in this paper. The analysis of fuel rods degradation and problems related to SCDAPSIM underprediction of the amount of relocated fuel and cladding will also be covered. (author)