MYRRHA is a fast neutron spectrum irradiation facility cooled by liquid lead-bismuth eutectic (LBE) currently under development at SCK-CEN (for more information, see http://myrrha.sckcen.be). MYRRHA will be able to operate in subcritical mode (when the facility is coupled to a proton accelerator) and in critical mode. A main safety objective for the design of the MYRRHA reactor is to practically eliminate all scenarios that lead to damage of the core. Despite this objective, present regulations require to postulate core degradation conditions as an extra level of defence in depth, and to develop a mitigation strategy to limit the radiological consequences of these postulated severe accident conditions.
Severe accident studies are an important aspect of the safety evaluation of a nuclear reactor and a lot of research and engineering work is being devoted to this subject for light water reactors and sodium fast reactors, the more so after the Fukushima accident of March 2011. Particular properties of a fast reactor cooled by heavy metal probably lead to a very different behavior of severe accident scenarios and therefore such reactors might require very different mitigation strategies for severe accident conditions. The subject of this postdoc position is to contribute to the understanding of the in vessel behavior of the fuel after postulated core desintegration events in the MYRRHA reactor. This understanding is required to design the necesssary engineering features to control reactivity and decay heat removal in degraded severe accident conditions.
In LWRs and SFRs the main force that determines fuel relocation after the core has desintegrated, is gravitation and the relocation of fuel is relatively easy to predict. Fuel relocation in the primary system after a core degradation in an HLM reactor is a much more difficult problem to solve. In an HLM cooled reactor the density diffrence between coolant and fuel is so small that fuel relocation is determined by coolant flow. Flow in the reactor after a core degradation is determined by natural circulation which in turn is determined by the fuel particle distribution because they constitue the mean heat source in the system.It is therefore important to evaluate whether the fuel relocation after a severe accident is compatible with the present design of the decay heat removal systems and whether severe criticality events are unlikely or how they can be prevented.
A large part of the work will be dedicated to analyse the origen of fuel particles released and to study their flow path and accumulation points. For this, CFD seems to be the most appropriate tool. The second part of the work will be to analyze whether the heat of the fuel particles can be adequately removed and no hot spots occur on the structures. Special attention will be needed on examining the interaction of heat producing fuel particles on the vessel wall and the risk for recriticality in case of fuel accumulation. An important outcome of this study is an indication whether supplementary devices like a so-called core catcher would have to be integrated in the current design.