Speaker
Description
The fission reaction in the core of a Nuclear Research Reactor (NRR) produces free neutrons with energies extending from MeV to meV. In principle, neutrons of specific energy groups can be selected using configurable beam-conditioning filters. The feasibility of such a concept depends on the extent to which the mixed neutron and gamma source external of the core can be transformed into beams that satisfy the spectral and background requirements of applications. Materials applicable as filters utilise energy-dependent attenuation to preferentially transmit neutrons of the required energy.
A filter response matrix methodology was developed to enable rapid evaluation and optimisation of multilayer sequential filter stacks for this purpose. In this approach, each candidate material is represented by a response matrix that maps an incident neutron and gamma spectrum to the transmitted spectrum as a function of thickness. These matrices were generated with OpenMC for simplified slab geometries and then combined through forward multiplication to predict the spectral effect of complete filter stacks. This provides an algorithmically efficient alternative to elaborate direct full-transport optimisation, allowing large numbers of material combinations, layer orderings and thicknesses to be considered.
The method was applied to a set of fast- and epithermal-neutron reference applications requiring set target flux and energy specifications. Application-specific beam-quality metrics were defined in terms of in-band flux, out-of-band contamination and gamma background. Filter thicknesses were then optimised subject to practical constraints such as total stack length and layer bounds, while preserving a modular filter-bank concept suitable for beam-line implementation.
The results show that the response matrix framework is effective as a design and decision-support tool for early-stage beam-line development. It rapidly identifies promising material combinations, quantifies trade-offs between flux and spectral purity, and highlights which application requirements are compatible with the assumed NRR source term. Several application cases could be matched through suitable optimisation of filter composition and thickness. Comparison with detailed OpenMC models confirmed that the method reproduces the main spectral trends, although it tends to overestimate transmitted flux because the forward multiplication scheme does not account for particle return to preceding layers due to back-scattering.
The study demonstrates that the filter response matrix approach provides a practical framework for assessing the feasibility of fast and epithermal beam implementation on a NRR beam line and for guiding subsequent high-fidelity design studies.
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