Characterization of aqueous wastes. Reprocessing plants generate many aqueous waste streams, which differ widely with respect to their content of radioactivity, solids, and nitric acid. Radioactivity is characterized as low-level, intermediate (or mediumyievel, or highrlevel, but with no generally accepted quantitative definition for each category. The terms low, medium, or high activity are also used. Until around 1975 low-level liquid wastes were regarded as those that could be discharged directly to groundwaters or the ocean because after natural dilution their radionuclide concentrations were below the maximum permissible values for general population exposure. More recently, the requirement that the concentration and amount of radioactive effluents be made as low as practicable has led to a preference for discharging no liquid wastes to ground or surface waters and disposing of excess water by evaporation into plant off-gases. About the only universally accepted usage is characterization of the aqueous waste stream from the first, codecontamination cycle as high-level, or high-activity waste. This waste contains many curies per liter and must be cooled to prevent self-boiling.

Liquid wastes are sometimes characterized as low-salt or high-salt wastes. Low-salt wastes are those that can be greatly reduced in volume by evaporation without precipitation of solids. High-salt wastes are those that can be only moderately reduced in volume.

Low-acid wastes are those whose nitric acid content is too low to justify fractionating the distillate for acid recovery. If necessary to remove the little acid present, this is better done by neutralization or ion exchange. High-acid wastes are those whose nitric acid can advantageously be recovered by fractional distillation.

Steps in aqueous waste processing. Because of the great variety of aqueous waste streams and differences in process flow arrangements in different plants, there is no standard flow sheet for processing aqueous wastes from the Purex process. Figure 10.10 shows the principal steps in one possible scheme for concentrating the wastes and recovering water and nitric acid from them.

Low-level, low-acid, low-salt wastes are neutralized if necessary and concentrated in a simple flash or vapor-compression evaporator to produce low-level waste concentrates and water sufficiently decontaminated for return to process. With simple wire-mesh entrainment separators, decontamination factors of several thousand are easily obtained.

The intermediate-level waste concentrator handles the low-level waste concentrate, con­taminated aqueous solutions from solvent washing, and many other streams with appreciable solids content. With more exhaustive entrainment removal, as by partial reflux of condensate through a bubble-plate or sieve-plate column, water sufficiently pure for return to process can be produced. If concentrator bottoms are concentrated to the point of incipient crystallization, they are routed to waste storage. If still unsaturated, they are routed to the high-level waste concentrator.

The principal feed to the high-level waste concentrator is the high-level waste stream (HAW) from the codecontamination solvent extraction cycle. This typically contains about 2.5 mol HN03, 3 to 9 g fission products, and 400 to 1200 Сі/liter and generates heat at the rate of 2 to 6 W/liter. Additional feed may be intermediate-level waste concentrate and nitric acid evaporator bottoms. The high-level waste concentrator is usually operated at subatmospheric pressure and made of a corrosion-resistant material such as titanium, to extend life and minimize maintenance. Wastes are concentrated as far as possible without appreciable solids formation. If solids other than fission products are absent, a concentration of about 90 g fission products per liter can be obtained. Products are contaminated nitric acid overhead, slightly under 2.5 M, and evaporator bottoms, about 7 Af in HN03. Because evaporator bottoms self-heat at a rate up to l°C/min, the evaporator and the bottoms storage tanks must be provided with reliable cooling.

Previous Page

Figure 10.10 Steps in Purex waste processing and acid recovery.

Denitration of high-level wastes. To reduce corrosion during subsequent storage of concentrated high-level wastes, it is desirable to reduce their nitric acid content from around 7 to between 2 and 4 M, a value high enough to prevent major precipitation of hydrolyzed fission-product nitrates. To avoid loading the wastes with additional nonvolatile solids, nitric acid concentration can be reduced either by steam distillation or by reduction to gaseous nitrogen oxides by an organic reducing agent such as sucrose or formaldehyde.

Steam distillation is used in British Magnox plants. There, when the HAW stream has been evaporated to the desired solids content, water is substituted for radioactive feed for about 2 days and evaporation continued at constant volume. In this way the acidity of the concentrate is reduced to about 4 M.

Some of the reactions that occur with organic reducing agents are

With formaldehyde: HCHO + 4HN03 ->• C02 + 3H20 + 4N02

With sucrose: C12H22Ou + 24HN03 -» 12CO + 23H20 + 24N02

Denitration with formaldehyde was first studied at Harwell [H5, H6]. Above 80°C the reaction proceeds smoothly, and the acidity can be reduced to 1 M in from 1 to 2 h. At lower temperature, if unreacted formaldehyde is allowed to accumulate, the reaction may become uncontrollable.

Denitration with sucrose was studied at Hanford [В16]. If the temperature is above 85°C the reaction proceeds smoothly through many intermediate stages, after an incubation period of several minutes. As with formaldehyde, sugar should not be added at lower temperature.

Treatment of process water. Water recovered in processing aqueous wastes usually contains so little radioactivity that it can be recycled to the reprocessing plant, but it must be treated
further if it is to be discharged. Some water must be discharged because more water is fed to the plant in nitric acid and aqueous solutions than leaves it in aqueous wastes. This excess water is evaporated into gases and ventilating air discharged through the plant stack. Before excess water vapor is discharged in this way, it is treated to remove radioiodine and filtered to remove suspended solids.