Neutronic characterization of a Molybdenum-99-producing CANDU fuel bundle and implications for reactivity-device worth and refuelling strategies
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Operating CANDU reactors can potentially produce significant quantities of Molybdenum-99 due to their ability to be refuelled online, high thermal neutron flux and fuel-design flexibility. A Molybdenum-producing bundle (MPB) has previously been designed for that purpose and shown to be equivalent to the standard CANDU bundle (SCB) under normal, steady-state, conditions. However, before an MPB can be employed inside a CANDU reactor, steady-state characterization of MPB needs to be supplemented by additional neutronic evaluations. This study therefore evaluates the neutronic characteristics of the MPB relevant to transient behaviour and refuelling, and compares the results to those of the SCB. This includes derivation of reactivity coefficients, incremental macroscopic cross sections for reactivity devices and refuelling strategies for the MPB. The evaluations are made using well-established analysis methods and models where applicable, and new or improved methods and models are developed and used where necessary. In particular, a perturbation-theory approach is employed for evaluating the differences between the reactivity coefficients of the MPB and the SCB, and high-resolution 2D and 3D supercell lattice models are developed in the neutron transport code DRAGON. The high-resolution lattice models incorporate a large number of spatial and spectral subdivisions and account for the radial variation of fuel temperature. The study of the reactivity feedback effects shows that the MPB and the SCB have almost identical (within 1.5 mk) reactivity feedback when key reactor parameters are perturbed over wide ranges. The study of the reactivity device incremental cross sections for CANDU reactivity devices shows that these cross sections are very similar for MPB and SCB with a maximum difference of less than 2% for any given device. At the same time, this study finds that the 3D supercell model currently used in the industry underestimates the reactivity worth of adjuster and shutoff rods by 7%-11%. Finally, a full-core 3D model is constructed in the diffusion code DONJON and a fuelling strategy for achieving the desired weekly yield of Molybdenum-99 is developed. The adequacy of the proposed refuelling scheme is evaluated using a series of time-average calculations, which show that a small increase in the core reactivity (< 0.4 mk) results from employing a set of 4 MPBs in three different fuel channels in the inner region of the core. The small increase in the core reactivity can be managed by slightly increasing the discharge burnup in the non-MPB-bearing fuel channels, thus also improving slightly the fuel utilization in the reactor.