00/02548 Global rupture of a nuclear reactor lower head with elevated temperature due to severe accidents

Abstract

The CONGA project concentrated on theoretical and experimental studies investigating the behaviour of advanced light water reactor containments containing passive containment heat removal systems and catalytic recombiners expected to be effectively operational during a hypothetical severe accident involving large quantities of aerosol particles and noncondensable gases. The central point of interest was the investigation of the effect of aerosol deposition on the condensation heat transfer of specially designed finned-type heat exchangers (HX) as well as the recombination efficiency of catalytic recombiners. A conceptual double-wall Italian PWR design and a SWR1000 design from Siemens were considered specifically as the reference Pressurized Water Reactor (PWR) and Boiling Water Reactor (BWR) designs. An assessment of selected accident scenarios was performed in order to define the range of boundary conditions necessary to perform the experimental studies of the other work packages. Experimental investigations indicated that aerosol deposition accounted for up to 37% loss in the heat removal capacity of the two-tube-layer BWR HX units. However, no significant heat transfer degradation could be observed for the PWR HX units. These results can be attributed to the important differences in the designs and operating conditions of the two units. The tests to study the effect of hydrogen (simulated by helium) on the heat transfer rate for heat exchanger units designed for BWR and PWR applications indicated a degradation less than 30% under various conditions. This was found to be acceptable within the over capacity designed for the heat exchangers or containment characteristics. The tests performed to study the long-term aerosol behaviour in the pressure suppression chamber of the current operating BWRs indicated that the water pool scrubs the aerosol particles effectively and reduces the ultimate aerosol load expected on the off-gas system. The efficiency of the catalytic recombiner system designed by Siemens for the off-gas system was found to be insensitive to the aerosol deposited in the recombiner. A computation code, HTCFOUL, was developed to predict the heat transfer rate of a finned-type heat exchanger subjected to a steam–noncondensable gas mixture containing airborne aerosol particles. The model predicts the non-aerosol part of two tests within a variation of 26% and the aerosol part within 32%.