Molten Salt Breeder Reactor Waste Management

The fuel reprocessing (recycling) system invoked by the developers of Oak Ridge National Laboratory's molten salt–based breeder (of ²³³U from ²³²Th) reactor (MSBR) would generate high-level reprocessing waste consisting of ∼3 mol % fission product fluoride salts in a matrix consisting primarily of sodium and potassium fluoride salts. This technical note discusses a management scenario for such waste that invokes the following steps: (a) mixing of the waste salt with dilute nitric acid with a pug mill; (b) volatilization/separation of the bulk of the fluoride as hydrofluoric acid (HF) with a wiped film evaporator; (c) vitrification of the thus “converted” (to nitrate) salt waste to an iron phosphate glass waste form with a stirred melter; (d) reduction of the nitric acid/NO_x in the combined off-gas to elemental nitrogen with hot charcoal; (e) condensation of the water and HF in the reduced off-gas; (f) neutralization of that solution with an alkali (sodium and/or lithium and/or potassium) hydroxide; (g) drying of that solution to produce the fluoride salts utilized by the process; and finally, (h) off-gas disposal after treatment implemented with a condenser, wet electrostatic precipitator, catalytic converter, and high-efficiency particulate air filters. This scenario's advantages relative to those that invoke the preparation of a synthetic fluoride mineral (cation-substituted fluorapatite) waste form include much higher effective waste loading, lower cost, and a product (glass) more consistent with stakeholder expectations.