On the Physics of Core Disruptive Accidents in Heavy Liquid Metal Fast Reactors- Case Study: MYRRHA -

MYRRHA is a fast neutron spectrum irradiation facility cooled by liquid lead-bismuth eutectic (LBE) currently under development at SCK-CEN. MYRRHA will be able to operate in subcritical mode (when the facility is coupled to a proton accelerator) and in critical mode. For the safety analysis, we study design basis accidents but also severe accidents. For the mitigation of severe accidents, we adopt an in-vessel retention strategy as we believe this is the most effective in limiting radiological consequences to the population. This implies that we must analyze whether the vessel is able to hold the possibly large mechanical load caused by a severe accident, for example after a large power excursion driven by a reactivity insertion. Since fast neutron reactor cores, in contrast to light-water reactor cores, are not designed to be in the most reactive configuration, a core degradation leading to fuel relocation or core compaction could increase reactivity and generate large power excursions that deposit large amounts of energy in the vessel challenging its integrity and hence its confinement function. In the past, these core disruptive accidents have been studied for sodium cooled fast reactors. Although this type of accident was always classified as 'highly unlikely', many designs of sodium fast reactors were strongly influenced by the outcomes of the studies on these so-called hypothetical core disruptive accidents (HCDAs). One family of methods to study these accidents is the Bethe-Tait model. Bethe and Tait developed in 1956 an analytical model to evaluate the energy release in a liquid metal cooled system after a prompt critical reactivity accident. The Bethe-Tait model required some important approximations that make the estimate for the power and energy release very conservative. For example, in the original model there was no Doppler feedback taken into account and the excursion is ended by complete core disassembly by fuel boiling. Later on, modified Bethe-Tait models were developed that would include more sophisticated models, both for the neutronics as for the hydrodynamics and the equations of state for the different materials. Later on, specialized computer codes like SIMMER were developed to analyze severe accident scenarios in liquid metal fast neutron reactors. SIMMER couples neutronics, thermo-hydraulic and fuel pin models to evaluate scenarios in a best estimate approach. Two versions of SIMMER are developed at this moment: SIMMER-III (two-dimensional geometries) and SIMMER-IV (three-dimensional geometries). Unfortunately, the computing time to study these scenarios is very large, certainly for the three-dimensional version. Therefore, these codes are not well suited for parametric analysis or sensitivity studies which are essential in a thorough safety analysis. The development of a modified Bethe-Tait model for MYRRHA that can be benchmarked for some reference cases to a SIMMER-III or SIMMER-IV calculation could bridge that gap. Starting from a postulated configuration of a binary system 'fuel-LBE', PhD student will investigate the possible core disassembly mechanisms in MYRRHA and build a modified Bethe-Tait model to estimate the power excursions and associated energy releases. PhD student will build relevant SIMMER-III models to understand the physics behind the accident sequence. PhD student will build a model to convert the energy release into a mechanical load on the vessel. Applying all these models, PhD student will perform a sensitivity study on the parameters that might have an impact on the hypothetical core disruptive accident sequence in MYRRHA. This will lead to recommendations to the design of the core and vessel to ensure the in-vessel retention policy.