To begin with, it is necessary to measure the transuranic content of large volumes of rather heterogeneous wastes. Basically there are three ways to do so, namely, by making use of gamma — and x-ray spectra accompanying the alpha decay, of radiation produced by spontaneous fission, and of radiation produced by induced fission.

Recovery of transuranium elements is mainly of interest for solid waste from the refabrication of mixed-oxide fuel. Plutonium is the major element to be recovered, and 241 Am may be recovered as a by-product. Other transuranium elements are usually present in minor quantities. The treated wastes are seldom decontaminated to levels of plutonium that would permit unrestricted release.

The most rigorous recovery technique is burning of plutonium-impregnated material in a plutonium scrap-recovery incinerator followed by grinding and leaching the ash with a mixture of hot nitric and hydrofluoric acids. Undissolved plutonium in the ashes may be recovered by fusion with a suitable salt, such as a 10:1 K^SO^-NaF melt, to get a product soluble in nitric acid. Nonbumable solids can be leached directly with a HN03-HF mixture.

Once the plutonium is in solution, it can be recovered and purified for recycle by well-established solvent extraction and ion-exchange techniques. Aluminum nitrate is added to the feed to complex the fluoride and thus decrease its interference with plutonium recovery.

Figure 11.22 presents a scheme of typical plutonium recovery operations. The Plutonium Reclamation Facility (PRF) [Rl] at Hanford incorporates many of these options. Geo­metrically favorable process equipment and storage tanks are used to ensure criticality safety.

The PRF also recovers 241 Am from the raffinate of the TBP-solvent extraction plutonium — purification process. The process employs 30 v/o dibutyl butylphosphonate in ССЦ as the solvent to extract both americium and residual plutonium from the high-nitrate feed solution, adjusted to about pH 1 by the addition of NaOH. Americium is selectively stripped from the solvent and purified by a cation-exchange procedure.

3.1 Immobilization

Volume reduction as described above usually leads to a product that still contains considerable quantities of water or that is quite easily leached or dissolved by water. The policy as to the degree of immobilization required for final disposal varies in different countries. As yet, there is no official regulation in the United States requiring that non-high-level waste be immobilized before disposal. It is, however, practiced in many places. In West Germany, by regulation, any non-high-level waste has to be immobilized before disposal in such a way that low leachability is warranted over a sufficient period of time.

There is no doubt that immobilization at least of alpha-bearing waste must generally be required and will be in the future. It has been mentioned before that the total transuranic inventory of alpha-bearing non-high-level waste will be in the same order of magnitude as that of HLW.

The range of suitable immobilization products for non-high-level waste is broader than that for HLW because there will be no significant heat generation. It includes glasses as well as cement, bitumen, and polymers.

Hydraulic cement. Immobilization of radioactive waste by incorporation into hydraulic cements, as typified by portland cement, has been practiced for many years. The optimum proportions of cement and waste vary with the type of waste to be immobilized.

Several additives have been used to improve the setting properties, fission-product

retention, or the volumetric efficiency of cement. A useful mixture is the Portland cement sodium silicate system developed by United Nuclear Industries, Inc. The sodium silicate additive produces a quick set with no free water, readily solidifies pressurized-water-reactor boric acid solutions, which set poorly with cement alone, and provides a significant reduction in the solidified volume [H3].

Another way to improve cement products is polymer impregnation. The process being developed in Italy consists of preparation of the cement product, thermal dehydration of the cement (165°C), impregnation with a catalyzed organic monomer, such as styrene, and polymerization by heating at 75 to 85°C [D3].

In spite of experience, solidification with cement is still an art. Each new waste application must be considered individually because of possible interactions between cement and the waste constituents.

Bitumen. Bituminization systems for immobilizing liquid and solid wastes are used in several countries. Bitumen, or asphalt, has certain advantages for immobilizing LLW and MLW. It is highly leach-resistant, it has good coating properties, and it possesses a certain degree of plasticity. Perhaps the greatest advantage is that at the operating temperature of 150 to 250°C, 99 percent of the water evaporates, resulting in a volume reduction of up to fivefold compared with conventional cementing techniques for products made with evaporator concentrates. Typical bitumen products contain 40 to 60 w/o waste solids.

One of the major drawbacks of bitumen is its potential fire hazard, particularly if used to encapsulate oxidants such as nitrates. The combustion problem is minimized by using bitumen grades with high flash points (>290°C). Improved safety can also be obtained by substituting more expensive polyethylene for bitumen. Fires have occurred in bituminization facilities, but they have been readily controlled.

Another problem to be observed is the radiation resistance of bitumen. There may be some radiolysis resulting in the release of hydrogen, methane, carbon dioxide, and ethylene. In the order of 0.5 cm3 H2/g product has been found to be generated per 108 rad. This is of significance primarily for alpha-bearing products.

The bituminization process is performed basically by adding a concentrated waste slurry or even a predried waste mixture to the molten bitumen. Residual water is evaporated and the solids are evenly distributed in the bitumen. After solidification a homogeneous product is obtained. Figure 11.23 shows a flow sheet of a screw extruder plant for bituminization; Figure

11.24 is a photograph of the screw extruder evaporator.

Glass. For liquid non-high-level alpha-bearing wastes with sufficiently high activity concentra­tion, glass may be a suitable fixation product as it is for high-level waste.

In terms of radiation stability, glass is superior to cement and particularly to bitumen. The leach rates, however, are about the same for glass and for bitumen, both being smaller than that of cement by up to three orders of magnitude depending on the type of cement. The fire hazard is a disadvantage of bitumen compared with both glass and cement. The costs of immobilization decrease in the order glass, bitumen, cement.