Application of the SERPENT2 code to neutronic analyses of the MYRRHA core: a sensitivity approach

In 80 years of nuclear reactor history two types of reactors were constructed, one leading to energy production and the other one for scientific research. At the moment, the operational nuclear reactors are represented by 440 nuclear power plants and 220 research reactors; therefore, it can be easily deducted the high relevance that the scientific community reserves for research reactors. In the early days of the nuclear history, research reactors were used to product fissile nuclides for weapon applications. Nowadays, they have several utilities in the medical field as the production of radio-nuclides for cancer treatments and in the industrial field for the recycling of nuclear waste and material testing applications. The Multipurpose hYbrid Research Reactor for High-tech Applications (MYRRHA) designed at SCK•CEN in Belgium is one of the promising projects in this area. This reactor is committed to fulfill the criteria of the future GEN-IV reactor type for what concerns safety, proliferation resistance and sustainability. The innovative features of such reactor are the reprocessed mixed oxide fuel (MOX) with high enrichment, the lead-bismuth eutectic (LBE) as coolant and spallation target, and the possibility to operate as critical reactor as well as a subcritical system driven by a spallation neutron source based on a proton accelerator; operational mode known as Accelerator Driven System (ADS). In this thesis, all the analyses were performed on the MYRRHA version 1.6 while operating under critical mode. Such neutronic analysis aims to address answers to some of the safety requirements of the reactor under critical operation. The Monte Carlo code chosen for the evaluation of such safety parameters corresponded to SERPENT2, a modern code with several interesting features. This code was applied in particular for several void injection analyses due to the hypothetical occurrence of different accidental scenarios leading to distinct reactivity effects. In addition, sensitivity calculations were conducted to prove the SERPENT2 capabilities and to assess which neutron induced reaction (and from which nuclide) has the greatest effect on different parameters as, for instance, the effective multiplication factor and the kinetic parameters of the core. The simulations highlight a reactivity enhancement due to the void presence, an increase affected either by the volume of the void and the spatial location of the injection, or both. In order to understand the physics behind the reactivity increase, local and global analyses were performed. Regarding local analyses such as flux and reaction rates, different tallies for different nuclides were performed. On the other hand, global analyses came as sensitivity calculations, in which the nominal condition and the void injection condition leading to the highest reactivity increase were also studied. Another feature of this work was to provide a comparison between the SERPENT2 results for several MYRRHA neutronic observables with respect to the well-established MCNP code. In the end, the nature of all the aforementioned calculations based on SERPENT2 can be used to compare previous safety studies carried out with other codes, as well as to form a basis for future ones.