Fission product transport during pipebreaks in a PBMR confinement

Abstract

The Pebble Bed Modular Reactor (PBMR) (Pty) Ltd Company intends to develop a demonstration power plant to be operated by the South African state owned utility Eskom. This demonstration plant will be a high temperature gas cooled reactor (HTGR) that will be graphite moderated and helium cooled. In the event of a pipe break within the helium pressure boundary (HPB) of a PBMR module, the circulating helium coolant is released into the confinement building and if the resulting pressures are large enough, is vented through high efficiency particulate air (HEPA) filters into the environment. In support of the design and safety analysis of the plant, pipe break scenarios are analysed to provide insight on the expected consequences of such an event. This study focused on quantifying the retention capability of the confinement building for graphite dust and fission products that follow pipe breaks in a modular HTGR. The high pressure and temperature gases that are released during the accident may result in intolerable pressures within the confinement therefore the structural integrity of the building was investigated by analysis. Iodine is a major contributor to the source term that could be released to the environment, the HEPA filter filtration efficiency of I2 is lower than that compared to aerosols while almost negligible for organic iodides thus the chemical form of iodine reaching the filters was analysed. Two separate cases were investigated, the first considered a single 65 mm double–ended guillotine break (DEGB) of the reactor outlet pipe near the inlet to the steam generator, while the second case considered a simultaneous 65 mm DEGB of the steam generator outlet pipe. The integral accident analysis code ASTEC (Accident Source Term Evaluation Code) was used to simulate these scenarios. ASTEC has been developed jointly by the French institute IRSN (Institut de Radioprotection et de Sûreté Nucléaire) and the German institute GRS (Gesellschaft Für Anlagen– Und Reaktorsicherheit Mbh) since 1994. It was developed for investigating scenarios of a hypothetical severe light water reactor (LWR) accident, from initiating event to possible radionuclide release outside the reactor containment. For this study, only the CPA and IODE modules were used from the ASTEC package since these modules compute the thermalhydraulic, aerosol and fission product, and iodine behaviour in the containment respectively. Analyses of the results have shown that the design pressure limit is exceeded for at most three compartments during the transients. It was also found that organic iodide production is possible during the initial release phase only as the compartmental temperatures are higher during this phase than during the delayed release phase. The higher temperatures increase reaction kinetics in favour of organic iodide production. An analysis of the results obtained from the metal fission products of the delayed release showed that ASTEC could not tolerate the small masses of fission products that were injected into the system, with the mass balance of the system not converging. This deficiency is attributed to the fact that ASTEC was developed specifically for LWR accident scenarios. LWR accident scenarios typically involve significant fission product release into the containment with the possibility of a core melt. This is in contrast to a PBMR accident scenario since the silicon carbide layer of the fuel kernel retains the majority of the fissions products with only little escaping into the reactor building. The analysis of the delayed release is an important aspect for the PBMR safety analysis therefore it is suggested for future work that a more suitable code which can tolerate small quantities of fission products be used.