Abstract
In the late phase of a severe loss-of-coolant accident (LOCA) in a light water reactor (LWR), carbon monoxide (CO) may be generated inside the containment due to molten corium concrete interaction (MCCI). As a component of the accident atmosphere, CO will interact with passive autocatalytic recombiners (PARs) which are installed inside LWR containments for hydrogen (H_2) removal. Depending on the boundary conditions, CO may either react with oxygen to carbon dioxide (CO_2) or act as catalyst poison, reducing the catalyst activity and hence the hydrogen conversion efficiency. An experimental test program performed in co-operation between Forschungszentrum Julich and RWTH Aachen University investigates these aspects aiming at providing data for model development for advanced severe accident analyses. In the first test series to be presented, the parallel catalytic reaction of H_2 and CO on the catalyst surface has been studied, i.e. the hydrogen recombination reaction was started before CO was injected. The test results show that under the given conditions the conversion of CO into CO_2 has no negative impact on the parallel hydrogen conversion. Additionally, the efficiency of the CO reaction in terms of molar rates is significantly smaller than the corresponding H_2 conversion efficiency. Due to the exothermal reaction, the parallel CO conversion may also have an impact on the possible ignition of the flammable gases at hot PAR surfaces. A second test series aimed at investigating the influence of low oxygen (O_2) concentrations on the parallel H_2/CO reaction. Basically the same experiments as in the first test series were performed, only with stepwise decreasing O_2 content until the break-up of the catalytic reaction. Main result of this test series was the observation of different break-up mechanisms dependent on the initial gas concentrations. A spontaneous break-up of the catalytic reaction took place at tests with low H_2/CO concentrations (i.e. 2 vol.%). In these tests, no loss of efficiency during O_2 reduction was observed. All tests with higher H_2/CO concentrations revealed a stepwise break-up of the catalytic reaction accompanied by significant loss of conversion efficiency for both species during O_2 reduction. This paper provides an overview of the results of both test series including first modelling approaches.
