A scoping analysis of the neotronic design for a new South African research reactor

Abstract

Together with many other research reactors around the world, the SAFARI–1 reactor has been classified as an ageing research reactor. In order to continue the provision of the current irradiation services, the operator of the reactor, NECSA, needs to consider the replacement of SAFARI–1 with a new large neutron source, and therefore ultimately a new reactor. A replacement research reactor will have to provide irradiation services that primarily include: radio–isotope production, thermal– and cold neutron beamlines, NTD and material testing. With these specifications, a number of additional design parameters were specified which involved: the fuel design, core layout and beamline layout. The design of the reactor fuel was required to be equivalent to the current plate type MTR–type fuel primarily due to the existing infrastructure for this design. Additionally, the fuel material was specified as uranium–silicide dispersoid (U3Si2) in order to support the high uranium–loading required for LEU–fuel. The core layout was ranged from a small 4 by 4 core to a large 9 by 9 core with different amounts of in–core irradiation positions, reflector types and reflector regions (high leakage zones). The neutron beamline designs were varied to investigate the effects of radial orientation. The design aspects were investigated by utilizing the OSCAR–4 code collection and MCNP5. Two additional software applications, called KNERSIS and MAAS, were developed: one for the automation of the MGRAC code (part of OSCAR–4); and the other for the interface of data between MGRAC and MCNP5. With this collection of software, a number of design iterations could be performed in rapid succession which included elimination of power peaks, optimization of discharge burnup, optimal reload patterns, equilibrium cycle analysis and accurate isotope inventories (with correct burnup profiles) for use in MCNP5. The study of the fuel design parameters found that for an increased uranium loading per assembly, the reactive life was increased. This increased loading can be achieved by means of a thicker fuel “meat” section, more fuel plates per assembly or a higher uranium–loading fuel material such as uranium 2wt% molybdenum. The reactivity was shown to be weakly dependant on all three these parameters due to the effect of the moderator to fuel ratio. The study of the radial orientation of beamlines indicated that the epi–thermal– and fast neutron–, as well photon–, output currents from beamlines can effectively be reduced by orientating the beamlines tangentially, an aspect which can reduce beamline noise. With a fixed fuel design, the study of different core layouts principally shown that the ex–core thermal neutron flux per unit power is inversely proportional to the size of the core design while the total in–core irradiation capacity indicated the opposite. The investigated parameters allowed for the recommendation of a core design which, for purposes of providing the primary irradiation services, is a medium sized core with sufficient amount of in–core irradiation positions, a heavy water reflector and tangentially orientated thermal– and cold neutron beamlines.