Thorium Utilization in Pressurized Water Reactors Using Bimetallic Thorium-Zirconium Alloy Cladding

A fuel rod design consisting of a bimetallic cladding tube of thorium metal bonded to a zirconium alloy and containing seed fuel in the interior space is proposed for thorium utilization in pressurized water reactors. The design mitigates the severe thermal penalty that arises in radial microheterogeneous designs when thorium is present as an oxide. The level of thorium loading has an important effect on the achievable discharge exposure as too high a loading results in a large reactivity penalty that is not compensated by rapid enough ²³³U breeding. In the bimetallic cladding design, the level of thorium loading could be adjusted by varying the thorium metal thickness, and analyses are presented to evaluate optimal levels of thorium loading. Results of cases for higher levels of initial seed loading are presented with a view to extending exposure and reducing the number of discharged assemblies. Liquid metal bonding the seed fuel–cladding gap is preferable as it reduces the seed fuel temperature and at the same time provides more room for fuel swelling. Helium bonding the gap is also possible with a seed fuel modified by an inert matrix. Both approaches need data for fuel thermal modeling, swelling, and fission gas release at high burnup not currently available.