High-Utilization Lattices for Thorium-Based Fuels in Heavy Water Reactors

• Published in: Nuclear technology Vol. 186; no. 1; pp. 17 - 32
• Main Author: Bromley, Blair P.
• Format: Journal Article
• Language: English
• Published: Taylor & Francis 01.04.2014
• ISSN: 0029-5450; 1943-7471
• DOI: 10.13182/NT13-86

New fuel bundle and lattice concepts to implement thorium-based fuel cycles in pressure tube heavy water reactors (PT-HWRs) have been explored to achieve maximum resource utilization. As an existing, operational technology, PT-HWRs are highly advantageous for implementing the use of thorium-based fuel cycles because of their high neutron economy and online refueling capability. A PT-HWR is flexible in that it can use one, two, or more different types of fuels in either homogeneous or heterogeneous cores to optimize power production, fuel burnup, and new fissile fuel production. In a heterogeneous PT-HWR core, higher fissile content seed fuel will be optimized for power and excess neutron production, and lower fissile content blanket fuel will be optimized for production of 233 U. Five different lattice concepts were investigated for potential use in a once-through thorium cycle in a PT-HWR. The lattices involved 43-, 35-, and 21-element bundles with a central cluster of ThO 2 pins, or a Zircaloy-4 (Zr-4) central displacer tube containing either stagnant D 2 O coolant or solid ZrO 2 , to help reduce coolant void reactivity (CVR). The fuel in the outer pins is a homogeneous mixture of Th and low-enriched uranium (LEU) (~5 wt% 235 U/U) or reactor-grade Pu (~67 wt% fissile). The content of the LEU or Pu was varied to achieve different levels of burnup, and it is presumed that low-reactivity fuel would be used as blanket bundles. It was found that the various lattice concepts could achieve burnups ranging from ~10 to 80 MWd/kg and that the fissile utilization could be up to 60% to 100% higher than what is currently achieved in a PT-HWR using natural uranium fuel. Burnup-averaged CVR ranges from approximately +1 to +16 mk (1 mk = 100 pcm = 0.001Δk/k), depending on lattice type and fuel composition. Assuming a maximum linear element rating of ~50 kW/m, the maximum permissible bundle power ranges from ~520 to 800 kW.