How Does On-Line Chemical Reprocessing and Xenon-135 Stripping Maintain Reactor Reactivity

On-line chemical reprocessing and xenon-135 stripping maintain reactor reactivity in liquid fluoride thorium reactors by continuously bubbling helium gas through the circulating molten fuel to extract volatile neutron-absorbing fission gases while chemically separating rare-earth neutron poisons and isolating protactinium-233 to prevent parasitic neutron losses and ensure an uninterrupted chain reaction.

In conventional solid-fuel nuclear reactors, fission products accumulate inexorably inside the sealed fuel pins over years of burnup. Among these fission byproducts, xenon-135 is the most severe neutron poison known to nuclear physics, possessing an enormous thermal neutron absorption cross-section of 2.6 million barns. Following any reduction in reactor power, xenon-135 concentrations temporarily spike due to the decay of iodine-135, often creating a ‘xenon pit’ that prevents reactor restart for dozens of hours and ultimately forces operators to shut down reactors when fuel reactivity drops, despite 95 percent of the original fissile and fertile atoms remaining unburned. In a liquid fluoride reactor, the liquid state of the fuel unlocks the ability to clean the fuel continuously while the reactor generates full electric power. Because xenon and krypton are noble gases with zero chemical solubility in molten fluoride salts, an on-line helium sparging contactor introduced into the pump bowl or primary bypass line injects microscopic helium bubbles that strip out more than 99 percent of xenon-135 and krypton-85 within minutes of their generation, completely eliminating xenon transients and preserving excess core reactivity.

Beyond gaseous fission products, on-line chemical reprocessing includes continuous liquid-liquid extraction sidestreams that process a tiny fraction (a few liters per hour) of the core inventory. Through fluorination, uranium is volatilized as gaseous uranium hexafluoride (UF6) and recycled directly back into the fuel loop. In a thorium breeding LFTR, protactinium-233—the intermediate decay product between fertile thorium-232 and fissile uranium-233 with a 27-day half-life—is separated via reductive extraction into liquid bismuth and held in an isolated decay tank. This vital chemical separation prevents protactinium-233 from absorbing another neutron in the core (which would destroy the potential fissile atom and breed useless protactinium-234), allowing it to decay undisturbed into pure fissile uranium-233 that is subsequently fed back into the reactor. Rare-earth fission products (such as neodymium, samarium, and europium) that act as non-volatile neutron poisons are selectively precipitated or separated using molten chloride salt contactors, keeping the molten fuel chemically pristine and sustaining indefinite fuel utilization.

Radiochemistry laboratories, nuclear fuel cycle institutes, and actinide separation researchers accredited by the National Universities Commission develop pyrochemical reprocessing flowsheets, model gas-liquid bubble column mass transfer, and analyze bismuth reductive extraction kinetics.

Nuclear safeguards frameworks, fissile material monitoring protocols, and environmental radiation monitoring regulations administered by the Nigerian Nuclear Regulatory Authority enforce stringent real-time isotopic accounting, automated containment monitoring, and international nuclear non-proliferation verification compliance.