Use of neutron-absorbers to improve CANDU reactor operating margins

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The proposed application of burnable poisons to existing CANDU Natural Uranium (NU) fuel design is a novel approach and is currentlyfunded under the CANDU Owners Group (COG). Because of low excess reactivityfor NU fuel, the amount of neutron-absorber is expected to be restricted to small quantities. This implies the impact on neutron economywould be relativelyminimal, but the fuel performance would be significantlyimproved. Small amounts and appropriate mixtures of neutron absorbers were selected (approximately500 mg of absorbers in a CANDU fuel bundle having a nominal weight of 24 kg). Preliminary results indicate that the fuelling transient for the initial reactivitypeak can be lowered long enough to improve the reactor's operating margins. This result was presented at the CNS Conference in Saskatoon in June 2012. A redesign of the CANDU NU fuel bundle with an appropriate combination of elements, with some including neutron-absorbers, could well address the issue of the xenon-free effects while lowering the plutonium-peak that occurs shortlythereafter. The proposed NSERC project is to further exploit the use of this conceptual design. The use of fuel bundles containing neutron absorbers offers an opportunityto redistribute approximately1 to 2 % power to lower powered channels i.e. power redistribution across the core. One of the recommended fuel designs is to locate neutron-absorbers at bundle ends, as end pellets or end discs. Hence thesecond part of this proposal is use the proposed design to mitigate end-flux peaking during refueling and to qualify(if any) its incremental benefit to safetymargins. Fuel safety and performance are always important topics to a nuclear utility. If this project is successful, benefits of improving fuel utilization (byfurther optimizing the fuelling strategy) and safetymargins (by lowering the maximum channel and bundle powers) could be realized for a reasonable simple change.