Optimized Flux Trap Dimensions in a Research Reactor Core

Research reactors are used for many applications: material testing; radioisotope production; beam-line applications for material research; nuclear transmutation doping; neutron activation analysis; neutron radiography experiments; fuel waste management; and other neutron and nuclear material related quantities, features, and research areas of interest. Each application requires enhanced neutron fluxes in a specific section of the energy spectrum; therefore, appropriate irradiation positions in the core or an appropriate configuration of the beam line need to be chosen. In several cases the required flux exceeds the maximum value that can be obtained in the existing irradiation positions of the operating reactor core, but the desired neutron flux amplification through the reactor power upgrade would require large-scale transformations, high costs, and long shutdown periods. With the creation of a flux trap at a central core position in the open pool Greek Research Reactor (GRR-1), a noticeable local increase of the thermal neutron flux was achieved, compared to the irradiation channels at peripheral core positions. In the present technical note, calculational and measurement results concerning the original core modification are presented, while the possibility of larger sample irradiation at higher thermal neutron flux in the GRR-1 is investigated. The presented results are based on deterministic and stochastic neutronic calculations with numerical models validated using measurements conducted for the original flux trap. The work is completed with a thorough thermal-hydraulic analysis to evaluate the impact of the proposed modifications to reactor operation. The study showed that the flux trap enlargement with complete removal of a central control fuel assembly increases the maximum thermal neutron flux by ∼41%, while further removal of the neighboring fuel assembly leads to an average flux increase of ∼45%, thus offering capabilities for extended reactor utilization such as additional isotope production.