It is possible to almost double the estimated operating time of a small modular fast reactor on a single fuel load from about 9.3 years to 17.5 years through the use of thorium-based nitride fuel. Such was the conclusion made by Moustafa Aziz, a researcher with the Egyptian Atomic Energy Authority, who simulated the operation of a sodium-cooled fast reactor with a thermal output of 125 MW. During the use of two types of thorium fuel, burnup reached 85.5 GWd/t, compared to 37.5 GWd/t for conventional uranium-plutonium mixed oxide (MOX) fuel.
Although thorium-232 is not a fissile material, it can be converted into nuclear fuel inside the reactor. After absorbing a neutron, thorium-232 becomes thorium-233, which undergoes two successive beta decays to transform first into protactinium-233, and then into fissile uranium-233. As a result, thorium turns into a new fuel during reactor operation, partially offsetting the consumption of the initially loaded fissile material.
For his study, Aziz created a computer model of a reactor that used a Monte Carlo method to simulate neutron movement and interactions within the core. The model was based on the design of a small modular fast reactor with 121 hexagonal assemblies that had been developed at Argonne National Laboratory in the United States. The scientist compared three fuel loading options: standard MOX fuel and two types of nitride fuel in which thorium was combined with either plutonium or uranium-233. The model made it possible to track changes in fuel composition, burnup, reactivity and power distribution across the entire estimated cycle.
Calculations showed that both types of thorium fuel could sustain a chain reaction for 6,400 days versus 3,400 days for MOX fuel. The advantages of the thorium cycle were especially evident for the thorium-plutonium fuel. At the start of operation, the fuel contained no uranium-233, but as the plutonium burned up, this isotope emerged from the thorium. By the end of the cycle, some 481 kg of uranium-233 had accumulated in the reactor core, accounting for about 38% of all fissile materials. The mass of fissile plutonium isotopes decreased from about 1,420 kg to 813 kg. This means that the generation of uranium-233 partially offset the consumption of the initial plutonium, sustaining the chain reaction for a longer period.
Another advantage of thorium-based fuel was a lower concentration of xenon-135, a fission product that actively absorbs neutrons, hindering the chain reaction. Xenon-135 production was lowest in the thorium/uranium-233 fuel and highest in the MOX fuel. Reducing the concentration of this isotope helps minimize neutron losses and contributes to a longer fuel life.
The researcher also evaluated the behavior of the three fuel options after a reduction in the amount of sodium coolant in the reactor core. For the thorium/uranium-233 fuel, reactivity decreased, with an increase for MOX fuel and the thorium-plutonium mixture. The first option is considered better from a safety standpoint, since the intensity of the chain reaction tends to decrease after the loss of some coolant. However, the uranium-233 fuel exhibited high initial reactivity, necessitating an increase in the amount of neutron-absorbing boron-10 in the control rods from 19.9% to 40% in the model.
It should be noted that the estimated 17.5-year period cannot be considered a verified period of continuous operation for an actual reactor without refueling. The author’s calculations concerned largely the neutron-physical characteristics of the core; he notes that the cycle duration in practice will also depend on radiation damage to fuel cladding, as well as thermal-hydraulic and metallurgical constraints. This is why the study primarily demonstrates the potential for nearly doubling the fuel cycle of a fast reactor through the use of thorium and the generation of uranium-233 during its operation.
