PhD Defense | Đorđe Petrović | On the Physics of a Core Disruptive Accident in a Heavy Liquid Metal Fast Reactor | Case Study: MYRRHA

On the Physics of a Core Disruptive Accident in a Heavy Liquid Metal Fast Reactor | Case Study: MYRRHA

Unlike traditional nuclear reactor cores that sustain the fission chain reaction by thermalised neutrons, many advanced reactors are designed to sustain the fission chain reaction by fast neutrons. Despite the selection of advantages offered by the fast neutron spectrum, such reactor cores are susceptible to a specific accident sequence that may result in the release of a considerable amount of energy and endanger their confinement structures.
A fast-spectrum reactor core is not designed to operate in its most reactive configuration. As a consequence, the fission chain reaction sustained in such system is sensitive to changes in system geometry and/or the rearrangement of fuel material. It is therefore possible that a core degradation event leads to a runaway chain reaction, excessive power buildup and the disruption of the reactor core. This sequence, referred to as a Core Disruptive Accident (CDA), has traditionally been analysed for public consequence considerations in fast-spectrum reactor cores cooled by sodium, the so-called Sodium Fast Reactors (SFRs).
A fast-spectrum reactor core can also be cooled by Heavy Liquid Metal (HLM), such as lead or an alloy of lead and bismuth called Lead-Bismuth Eutectic (LBE). This fast-spectrum reactor core design, referred to as a Heavy Liquid Metal Fast Reactor (HLMFR), is studied due to several important safety advantages it offers in comparison to SFR. One of these advantages includes an apparent lack of mechanisms that would lead to a CDA, which leaves the scientific community with unanswered questions about the possibility and probability of such sequence taking place, as well as about its governing mechanisms and potential consequences. A CDA sequence taking place in an HLMFR has never been investigated in great detail. This (lack of) knowledge, nevertheless, represents an important component necessary for the safety demonstration of the HLMFR technology.
This research therefore aims to provide a fundamental understanding of the physics and phenomena that govern a CDA sequence in an HLMFR and to establish a robust foundation for the associated safety analyses in the Multipurpose hYbrid Research Reactor for High-tech Application - MYRRHA. In the absence of a tool suitable to provide a reliable assessment of the core degradation scenario and exclude the possibility of a CDA taking place, a CDA is postulated in order to envelop all the sequences that may occur following a core degradation event. The phenomenological aspects of different physical mechanisms that govern the considered transient are comprehensively investigated and quantitatively assessed by a carefully constructed set of mathematical models and a developed multiphysics tool.
An assessment of the mechanical impact of the enveloping CDA sequence on the primary system of MYRRHA yields a result that may not be covered by the current design requirements, but that is likely not to impair the structural integrity of the primary system. The outcome of this research hints at the possibility of successful confinement of the nuclear fuel following a core degradation event, thus providing a strong basis for a successful safety demonstration of MYRRHA and the HLMFR technology in general.

Promoters:

• William D'haeseleer (KU LEUVEN)
• Martine Baelmans (KU LEUVEN)

SCK CEN mentor:

• Guy Scheveneels
• Matteo Zanetti