New fuels for CANDU and SMRs, and burning surplus weapons material.

Uranium nitride in CANDU

Uranium nitride packs more uranium atoms into the same volume than the uranium dioxide used in today’s CANDU fuel, and it conducts heat better. Natural nitrogen, however, is mostly nitrogen-14, which absorbs neutrons; nitrogen enriched in nitrogen-15 avoids most of that loss.

With Janne Wallenius of KTH Royal Institute of Technology, he studied natural uranium nitride made with nitrogen-15 in a CANDU core (PHYSOR 2018, Cancún). Compared with UO₂, the average residence time of a fuel bundle rose from 210 to 360 days. The coolant void worth rose by 20 % and the total power coefficient was reduced by half: longer-lasting fuel, with reactivity effects that a design would need to account for.

Thorium fuel and thorium reactors

Thorium-232 does not fission with slow neutrons, but it is fertile: when it captures a neutron it becomes thorium-233, which decays through protactinium-233 to uranium-233, a fissile isotope. A thorium reactor therefore needs a starter supply of fissile material (enriched uranium or plutonium) and then breeds part of its own fuel. Thorium is more abundant in the Earth’s crust than uranium, thorium dioxide is chemically stable with a high melting point, and a thorium cycle produces less plutonium and fewer heavy transuranic elements than a uranium cycle. Against that, uranium-233 comes with traces of uranium-232, whose decay chain (through thallium-208) emits hard gamma rays, which complicates fuel handling and recycling, and no commercial thorium fuel cycle is yet in operation. Heavy-water reactors such as CANDU, which use neutrons efficiently, and several small modular reactor designs are among the candidates studied for it.

As a Senior Research Fellow at the Royal Military College of Canada from October to December 2023, he worked on thorium-based fuel for small modular reactors, aimed at fuel sustainability and waste management through better fuel utilisation. A paper is in preparation.

Burning weapons material in a hybrid core

His doctoral thesis designed the Multispectrum CANDU Reactor: a small helium-cooled fast-neutron core inside a CANDU 6 thermal core, each with 37-element bundles. The fast core is meant to burn fissile material left over from disarmament; the thermal core runs on natural uranium. Neither core is critical on its own, so disturbing either one shuts the reactor down.

Six models were built in Serpent, fuelled with 19.9 % enriched uranium or with plutonium MOX. Uranium Model II destroyed 10.7 % of its fissile isotopes in 1000 days without refuelling; plutonium Model IV destroyed 15.3 % over 2600 days; Model VI had the highest consumption rate, reaching subcriticality after 420 days. The full design study is on the MSCR page.