Detailed Core Design and Flow Coolant Conditions for Neutron Flux Maximization in Research Reactors

Following the design of the German research reactor, FRM-II, which delivers high thermal neutron fluxes, we have already developed an asymmetric compact cylindrical core with an inner and outer reflector. The goal was to maximize the maximum thermal flux to power ratio, which is a desirable characteristic of a modern research reactor. This design, for a 10 MW power, was analyzed using MCNP, ORIGEN2 and MONTEBURNS codes, considering a homogeneous mixture for the core material. Promising results showed that with standard fuel material of low enrichment uranium, high thermal fluxes are delivered by this core in the outer reflector. In addition, the life cycle, which is a limiting parameter regarding the compactness of the core, was calculated to be 41 days. In this paper, we report the results of recent developments in the design of this core. A detailed modeling of a suitable fuel element is performed with MCNP. Neutron fluxes are recalculated to assure that the same levels are achieved with this more detailed description. In this regard, three different zones with different flux levels were identified: a high neutron flux zone (4.0E14 n·cm⁻²s⁻¹), a moderate thermal neutron flux zone (2.5E14 n·cm⁻²s⁻¹), and a low thermal flux zone (1.0E14 n·cm⁻²s⁻¹). Heat deposition in the cladding, coolant and fuel material is also calculated to determine coolant flow rate, coolant outlet temperature and maximum fuel temperature for safe operating conditions.