STL's HTMR100 reactor operates on a much longer burn-up fuel cycle of pebble fuel when compared to conventional nuclear reactors. No thorium fuel cycle involving the separation and recycle of 233U would approach the proliferation resistance of unprocessed spent fuel.
HTMR100 has been designed to circulate pebbles through the core just once, therefore eliminating all the complexities of recirculating pebbles. This fuelling concept is called Once-Through-Then-Out, or OTTO cycle. An important non-proliferation characteristic of the OTTO cycle is the long residence time of pebbles inside the core, making it impossible to divert partially burnt fuel for recycling of weapon grade uranium/plutonium. By the time the pebbles are extracted from the core, the remaining fissile material (mostly 233U in the case of thorium based fuel) is contaminated with a considerable amount of absorbent material (namely 234U). Another strong deterrent with233U as a fissile material for proliferation purposes is that the spent fuel contains an admixture of 232U, whose decay products produce penetrating gamma rays. This spent fuel containing 232U is undesirable as weapons material by virtue due to the fact that their gamma emissions bring with them the potential for significant radiation doses and shielding requirements during weapons proliferation.
STL's commitment of using a proliferation resistant technology naturally extends to a philosophy not supportive of recycling. The HTR technology, characterized by relatively high burn ups, allows using the converted fissile material in situ, at the reactor itself, before extracting the fuel as nuclear waste. In addition, the silicon carbide coating of the fuel kernels can be considered a first step in immobilization (vitrification) of nuclear waste, making HTMR100 fuel less suitable for reprocessing.