Fuel-salt processing system

    Breeding with thermal neutrons is economically feasible with a molten-salt reactor because it is possible to process the fluid fuel rapidly enough to keep the neutron losses to protactinium and fission products to a very low level. The equipment used to strip gaseous fission products from the fuel salt was described in Sects. 2.1 and 2.5. The concentrations of protactinium, rare earths, and some other fission products are limited by continuously processing a small stream of the fuel salt in an on-site processing system, described below.

    There are several basic processes which could be incorporated in a molten-salt reactor "kidney." The effective cycle times for protactinium and fission product removal assumed in the calculations of breeding performance (Table 3.7) were based on the use of the system described in ref. 1. Recent developments have shown that it is possible to attain the same breeding performance by using a somewhat different processing plant having equipment that should be considerably simpler to develop and operate. 1 1 The newer, more attractive concept is described here.
    The flowsheet for the continuous salt-processing system is shown. In essence, the process consists of two parts: (1) removal of uranium and protactinium from salt leaving the reactor and reintroduction of uranium into salt returning to the reactor and (2) removal of rare-earth fission products from the salt. A small (0.88-gpm) stream of fuel salt, taken from the reactor drain tank, flows through a fluorinator, where about 95% of the uranium is removed as gaseous UF6 . The salt then flows to a reductive extraction column, where protactinium and the remaining uranium are chemically reduced and extracted into liquid bismuth flowing countercurrent to the salt. The reducing agent, lithium and thorium dissolved in hismuth , is intmdw:P.d at the top of the extraction column. The bismuth stream leaving the column contains the extracted uranium and protactinium as well as lithium, thorium, and fission product zirconium. The extracted materials are removed from the bismuth stream by contacting the stream with an HF-H2 mixture in the presence of a waste salt which is circulated through the hydrofluorinator from the protactinium decay tank. The salt stream leaving the hydrofluorinator, which contains UF4 and PaF4 , passes through a fluorinator, where about 95% of the uranium is removed . The resulting salt stream then flows through a tank having a volume of about 130 fe ' where most of the protactinium is held and where most of the protactinium decay heat is removed. Uranium produced in the tank by protactinium decay is removed by circulation of the salt through a fluorinator.
    Materials that do not form volatile fluorides during fluorination will also accumulate in the decay tank; these include fission product zirconium and corrosion product nickel. These materials are subsequently removed from the tank by periodic discard of salt at a rate equivalent to about 0.1 fe 1 day.
    In summary, in the protactinium isolation system, all the uranium that leaves the reactor, plus that produced by decay of the protactinium, appears as UF6 , whereas the effluent salt from the extraction column carries fission products but no uranium or protactinium.
    The rare earths are removed from the salt stream leaving the top of the protactinium extractor by contacting it with a stream of bismuth that is practically saturated with thorium metal. This bismuth stream, with the extracted rare earths, is contacted with an "acceptor salt," lithium chloride. Because the distribution coefficient (metal/salt) is several orders of magnitude higher for thorium than for the rare earths, a large fraction of the rare earths transfer to the LiCI in this contactor, while the thorium remains with the bismuth. Finally, the rare earths are removed from the recirculating LiCl by contacting it with bismuth streams containing high concentrations of lithium (5 and 50 mole %). These materials, containing the rare earths, are removed from the process.
    The fully processed salt, on its way back to the reactor, has uranium added at the rate required to maintain or adjust the uranium concentration in the reactor (and hence the reactivity) as desired. This is done by contacting the salt with UF6 and hydrogen to produce UF4 in the salt and HF gas.