Pre-dimensioning the catalytic recombiners

Severe accident calculations for Belgian PWRs have confirmed that the production of hydrogen and carbon monoxide is a concern for the containment integrity. Therefore several preventive solutions were examined, among which the catalytic recombiners offer decisive advantages. This paper briefly describes the calculation method developed to determine the required surface area. The release rates are provided by STCP/MARCH3 runs and are used as input data of a post processor developed in Belgatom. This program computes the modified atmosphere composition for a given catalyst surface area. In addition to this, it calculates the adiabatic combustion pressure with and without recombiners, which allows to assess the risk of containment failure in both situations. Calculations were performed for two 900 MW plants with different concrete compositions. One of them is of the siliceous type which releases limited quantities of carbon monoxide, while the second is made of limestone common sand and produces large quantities of that gas during the decomposition process. Because the decomposition process is governed by a time constant, the pre-dimensioning was based on accident scenarios with the highest computed instantaneous release rates of flammable gases. Hence, attention was focused on scenarios initiated by a small break LOCA because they lead to high release rates of hydrogen at vessel breach. The calculation results show that a catalyst surface area of 250 m2 prevents the deflagration pressure to exceed the ultimate containment capability