Level-waste concentrator

Low- and intermediate-level waste Concentrations of radionuclides above exempt levels and thermal power density (decay heat) less than about 2 kW m 3 ... 

Low-level, low-acid, low-salt wastes are neutralized if necessary and concentrated in a simple flash or vaporwire-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 nitric acid evaporated from the high-level waste concentrator is too dilute and contains too much entrained radioactivity to be recycled without additional treatment. This acid, together with dilute acid waste streams from the uranium and plutonium purification solvent extraction systems, is decontaminated in the nitric acid evaporator. Entrainment can be suppressed by providing partial reflux through a few bubble-plate or perforated-plate trays, backed up by wire-mesh mist eliminators. 

At present a total of about 750 m of high-level waste concentrate, containing about 4 X 10 Ci (beta) and having a radioactive decay heat of about 2 MW, is contained in special storage tanks at Windscale. This is the cumulative total from about 25 years of reprocessing operations. 

Although it is considered that this storage system will safely contain the high-level waste concentrate for at least several decades, a process is now under development for the incorporation of this waste into a glassy solid which will be suitable for disposal into a deep geological formation on the bed of the deep ocean. 

The principal feed to the HLW concentrator is the HLW stream from the contamination solvent extraction cycle. This typically contains about 2.5 M HNO3, 3-9 g FPs L L and 400-1200 Ci L"k and generates heat at the rate of 2.6 W L"L Additional feed may be intermediate-level waste concentrate and nitric acid evaporator bottoms. The HLW concentrator is usually operated at subatmospheric pressure and made of corrosion-resistant material such as titanium to extend life and minimize maintenance. Wastes are concentrated as far as possible without appreciable solid formation. If solids other than FPs are absent, a concentration of about 90 g FPs per liter can be obtained. Products are contaminated nitric acid overhead, slightly under 2.5 M, and evaporator bottoms, about 7 M in HNO3. Because evaporator bottoms self-heat at a rate up to 1 °C mn", the evaporator and the bottoms storage tanks must be provided with reliable cooling (Benedict, Pigford, and Levi, 1981,489). 

Sulfur Polymer Cement. SPC has been proven effective in reducing leach rates of reactive heavy metals to the extent that some wastes can be managed solely as low level waste (LLW). When SPC is combined with mercury and lead oxides (both toxic metals), it interacts chemically to form mercury sulfide, HgS, and lead sulfide, PbS, both of which are insoluble in water. A dried sulfur residue from petroleum refining that contained 600-ppm vanadium (a carcinogen) was chemically modified using dicyclopentadiene and oligomer of cyclopentadiene and used to make SC (58). This material was examined by the California Department of Health Services (Cal EPA) and the leachable level of vanadium had been reduced to 8.3 ppm, well below the soluble threshold limit concentration of 24 ppm (59). 

Low level waste from commercial facilities is buried on site. The Nuclear Regulatory Commission (NRC) has projected the activities and volumes of low level radioactive waste from all sources buried at commercial sites to the year 2000 using information from the Idaho National Environmental and Engineering Laboratory (INEEL) waste retrieval project and assuming that the waste disposal practices then used would continue into the future. The 20-year decayed 241Am and 243Am concentrations were estimated to be 380 and 230 pCi/m3 (14 and 8.5 Bq/m3), respectively (Kennedy et al. 1985). 

Goldberg [44], surprisingly high concentrations were detected for the former 16, 000 Bq/kg, for the latter 43,000-46,000 Bq/kg. All these data indicate that technetium should be carefully recovered in reprocessing plants. The systematic recovery of technetium together with elements of the platinum group from high level wastes have been discussed by Kubota [45],... 

Total or partial ion suppression is a well-known LC-MS effect, which is induced by coeluting matrix components that can have a dramatic effect on the intensity of the analyte signal. As can be observed in Fig. 1, analyte suppression occurs as a consequence of the different matrix interferences present in waste-water samples, making the identification and/or quantification process difficult or unfeasible. Even when working under selection ion monitoring (SIM) conditions, these matrix effects can cause ion suppression in the detection of some analytes that are present at low levels of concentration, as seen in this figure. Several papers have reported this effect [30-32] and different alternatives to overcome these problems, such as the inclusion of a size-exclusion step [33] or sequential SPE [28], have been applied for the determination of pesticides in... 

Berner, U. 2002a. Project Opalinus Clay Radionuclide Concentration Limits in the Near-Field of a Repository for Spent Fuel and Vitrified High-Level Waste. Nagra Technical Report NTB 02-10, Nagra, Wettingen, Switzerland. Also issued as PSI Bericht Nr. 02-22, Paul Scherrer Institut, Villi-gen, Switzerland. 

Another important feature of the classification system for nuclear fuel-cycle wastes in the United States is the definition of low-level waste only by exclusion there is no definition of what low-level waste is, only a definition of what it is not. As a result, in contrast to the earliest descriptions of low-level waste prior to the establishment of definitions in law, this class is not restricted to waste that contains relatively low concentrations of radionuclides compared with high-level waste. Rather, low-level waste can range from virtually innocuous to highly hazardous over long time frames. 

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