Evaporators

Evaporators can obtain high decontamination factors, 104 to 106, if carryover is eliminated or if the evaporator is followed by ion exchange of condensate.

The evaporator at the National Reactor Testing Station is typical of evaporators used at research and production sites. The continuous evaporator is constructed of stainless steel (Type 347) and is a thermosyphon type with an external heat exchanger of 48 sq m area, rated at 800,000 kcal/hr heat duty. Vapor passes to an entrainment chamber with 4 bubble cap trays, where it is scrubbed with clean water. The evaporator is capable of processing 1800 l/hr. The tritium in the wastes is not concentrated at all, but the other radionuclides are concentrated by a factor of 50, and the condensate is decontaminated (Lohse, Rhodes and Wheeler 1970) by a factor of 2000.

Ground disposal of decontaminated liquid waste took place at many sites, but the practice lost favor because of the uncontrolled nature of the release and the irreversibil-ity of the process (IAEA 1967). The process takes advantage of the slow movement of groundwater (enabling shorter half-lived radioisotopes to decay), the ion exchange properties of soil, and if above the water table, the capacity of the unsaturated soil to store moisture. In addition, there is slow dispersion of wastes in the groundwater system.

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