Mitigation of End-Flux-Peaking in Fresh CANDU Fuel Bundles Using Neutron Absorbers

Abstract

End-flux-peaking (EFP) is a phenomenon where a region of elevated neutron flux occurs between two adjoining fuel bundles, leading to an increase in fission rate and therefore greater heat generation. It is known that the addition of neutron absorbers into fuel bundles can mitigate EFP, yet the implementation in Canada deuterium uranium (CANDU) type reactors using natural uranium fuel has not been pursued. The computer code Monte Carlo N-Particle code (MCNP) 6.1 was used to develop a three-dimensional CANDU bundle–bundle contact model and simulate the addition of neutron absorbers positioned strategically within various locations of the fuel bundle. The burnable absorbers of interest include Gd2O3 and Eu2O3. The locations investigated include within the end pellets of a fuel stack, within the CANDU lubricant (CANLUB) layer, within thin disks located at the ends of the fuel stack, and alloyed in the endplate. Concentrations of the absorbers were varied to gain better insight into their effect on the thermal neutron axial flux profile of the fuel bundle. The results of the study indicated that adding a combination of ∼4 mg/∼12 mg of Eu2O3 into the pellet adjacent to the end pellet and the end pellet, respectively, at each end of all six of the fuel elements in the inner fuel ring, as well as, ∼2 mg/∼6 mg of Eu2O3 into the same respective pellets, at each end of the 18 fuel elements in the outer fuel ring, provides the most effective mitigation of the EFP phenomena in fresh CANDU fuel bundles.