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The Offgas System

The ThorCon Pot
Schematic of Offgas System

A critically important advantage of a liquid fuel reactor is that the noble gas fission products, xenon and krypton, are continuously removed rather than building up in the fuel elements destroying cladding and consuming neutrons. This is essential to the ThorCon’s ability to burn up transuranics, to convert thorium to U-233, to go for long periods without reprocessing the salt, and to dramatically reduce the amount of radioactive gases released in a containment breach. But it also means we must continuously process this small but initially, extremely radioactive gas stream. This is the job of the offgas system, shown in orange in this sketch.

Each ThorCon module will produce about 0.12 kg/day of Xe and Kr, initially generating a remarkable 1600 kW of decay heat. The ThorCon offgas system comprises six stages:

  • Header Tank

    Extraction and 25 min holdup in the primary loop pump header tank. This step employs the same combination of helium cover gas and sprayer system successfully used by the MSRE to extract noble gases from the fuelsalt.

  • Offgas Recuperators

    The offgas vented from the top of the sprayer system is directed to two Offgas Recuperators (OGR’s). The OGR’s are in-Can, one hour holdup, heat exchangers filled with high void metal packing. They transfer the early stage offgas decay heat to the secondary salt. They also separate the entrained salt from the offgas and return this salt to the primary loop. A nice bonus of this design is that the bulk of the offgas “waste” heat is recovered.

    More importantly, most of the offgas radiation is confined within the Can, where there are four barriers between the offgas and the biosphere. In particular, virtually all the Xe-137 and Kr-90 will have decayed to Cs-137 and Sr-90 before the offgas leaves the OGR’s. These two particularly troublesome fission products will not show up in the offgas stream. Rather they will oxidize to fluorides and dissolve into the fuelsalt. In the event of a breach of all barriers, they will stay in the salt.

    The OGR’s also keep almost all I-131 in the fuelsalt. Iodine, like fluorine, is a halogen. At the redox levels at which ThorCon operates, the ratio of iodine gas to iodine dissolved is less than 1.0e-8. The only way for I-131 to escape the Can is via its precursor Te-131 which has a half-life of 25 minutes. Even if all the Te-131 made it into the offgas which seems quite unlikely, 7/8ths would decay in the header tank and OGR’s. And we expect tellurium plate out on the OGR packing as well.

  • Low Pressure Holdup Tank

    12 hour hold up in the Low Pressure Holdup (LPHUP) tank. Each power module is fitted with two offgas hold up tanks, shown in orange in the drawings. Both tanks are contained in membrane wall silos and are cooled by radiation to the membrane wall and natural circulation in the same manner as the Cans. The two tanks operate in series. The first tank operates at near ambient pressure and has a residence time of 12 hours. It extracts 90% of the remaining decay heat and cools the offgas to less than 200C.

  • High Pressure Holdup Tank

    Compression and 115 hour hold up in the High Pressure Holdup (HPHUP) tank. This tank is essentially identical to the LPHUP tank but operates at a higher pressure and a lower temperature. Almost all the remaining decay heat is removed during this period. This makes cryogenic cooling at the next stage possible and allows almost all the neutron poison Xe-135 to decay to the innocuous Cs-135.

  • Cryogenic Separation Unit 1

    Helium separation is accomplished in two stages. The offgas leaving each module’s HPHUP tank is piped to the module’s Cryogenic Separation Unit (CSU) shown in orange in the SW corner of the grid. Liquid nitrogen freezes the Xe and Kr but allows the helium to be returned to the Cans. The Xe and Kr are regasified and stored in high pressure bottles for at least 130 days which allows essentially all the Xe-133 to decay to stable Cs-133.

  • Cryogenic Separation Unit 2

    The first stage separation reduces the helium mass flow by about a factor of 400. But helium is so much lighter than Xe and Kr the residual helium still takes up most of the volume in the Stage 1 bottles. Therefore we need two stages of separation. After 130 days plus of holdup, the Xe and Kr and residual helium from each module is fed to a single CSU unit in the Transfer Module at the control room end of the silo hall. The second stage cooler reduces the helium content by another factor of 200. This allows four years of Xe and Kr from a 1 GWe ThorCon to fit into ten 100 liter bottles. At this point, the only radionuclide remaining is Kr-85 and the decay heat is less than 2 kW.

    Every four years the Canship returns these ten bottles to the Centralized Recycling Facility where the valuable xenon and krypton can be separated and sold.