Implementation of in-rod axially heterogeneous thorium-uranium fuel in a typical PWR

Abstract

Given the potential of the thorium fuel cycle and the predominance of PWRs in today’s commercial fleet of nuclear power plants, further investigations on the introduction of thorium-based fuel in open-cycle reactors such as PWRs are warranted. The benefits of microheterogeneous thoria-urania fuel (e.g. increased burnup / fuel cycle length, reduced radiotoxicity and increased stability of spent fuel, proliferation advantages due, feasibility of converting UO2-based fuel manufacturing plants to ThO2-UO2), are diminished by fuel manufacturing costs, the need to denature thorium sections of the fuel, thermo-hydraulic design requirements, higher fission gas release and cladding temperature gradients.This paper presents results of the neutronic performance of homogeneous and multiple heterogeneous fuel pin cell designs and explores the fundamental reasons and reaction balances between the different fuel models. The work utilises fuel models with variations in the length and composition of thorium and uranium sections. We compare the various urania-thoria fuel models with a reference PWR (North-Anna 1) with similar fissile content. We postulate a number of fuel design criteria, to appraise fuel models in trade-off studies, including improved burnup and cycle length, limitations on fuel enrichment grade and excess reactivity during burnup (typically at beginning of cycle), safety requirements such as limitations on total burnup, power peaking and feedback mechanisms. In addition, criteria are set for proliferation resistance and radiotoxicity.Verification studies were performed on fuel with homogeneous and axially heterogeneous fuel cell pins, followed by initial assembly and full-core analyses, to affirm confidence in the models employed. Our results confirm significant conversion of thorium to 233U, resulting in an improved burnup and fuel cycle length. Shut down margins and neutron economy are improved by significantly reducing beginning of cycle excess reactivity and hence reducing the need for burnable and soluble neutron absorbers. The conversion of thorium to 233U then leads to an increase in reactivity later in the cycle. To address proliferation concerns, fuel models with denatured thorium sections are introduced.We conclude that among the well-performing fuel models, fuel comprising heterogeneous pins repeating 3 cm thorium and 1 cm uranium sections, with the uranium region enriched to 20 at% 235U, and the thorium region denatured to a specific degree, performed best in terms of the fuel criteria considered. The promising results for the fuel pin cell models were confirmed with initial assembly and full core models: Burnup improvement rendered by the 3Th1U fuel models is 25.6% / 22.3% / 17.9% for the pin cell / assembly / full core models respectively, in spite of employing denaturing of thorium sections, while also fulfilling the other stated fuel criteria.