Waste from actinides, decontaminating

CEA Cadarache and seven European universities were involved in a research programme to synthesize and test new macrocyclic extractants (crown-ethers and calixarenes). The main aim of this study was to selectively remove caesium, strontium and actinides from medium level liquid waste (MLLW) to decontaminate them to the extent that they can be disposed of in a near surface site. A new calixarene has been synthesized by the University of Parma, which has a cesium/sodium selectivity 100 times higher than that of the best current extractant for cesium. The large selectivity of this molecule has been explained theoretically. The results obtained for cesium extraction from simulated MLLW have been confirmed with real HLLW. Finally, new functionalized calixarenes have been also synthesized, which are more selective to actinides and lanthanides than the best extractant available on the market. 

The yield and rate of the tantalothermic reduction of plutonium carbide at 1975 K are given in Fig. 3. Producing actinide metals by metallothermic reduction of their carbides has some interesting advantages. The process is applicable in principle to all of the actinide metals, without exception, and at an acceptable purity level, even if quite impure starting material (waste) is used. High decontamination factors result from the selectivities achieved at the different steps of the process. Volatile oxides and metals are eliminated hy vaporization during the carboreduction. Lanthanides, Y, Ti, Zr, Hf, V, Nb, Ta, Mo, and W form stable carbides, whereas Rh, Os, Ir, Pt, and Pd remain as nonvolatile metals in the actinide carbides. Thus, these latter elements... 

Sodium titanate has been found to be very effective in removing Sr from defense waste typified by a 6m NaNO - 0.6m NaOH solution also containing the sodium salts of aluminate, nitrite, phosphate, carbonate, sulfate, and chromate in the range of O.lU to 0.007 N and Sr in the analytical concentration range of O.OU to O.U ppm ( ). Sodium titanate columns have provided a Sr decontamination factor of greater than io3 for 2500 column volumes of the waste at flow rates of 2 to 6 column volumes per hour. The material has also been shown to remove residual actinide contamination from the same and similar waste streams ( ). 

However, due to the chemical similarities of the trivalent actinide and lanthanide elements, historically, it has been easier to develop step-by-step processes first, An(III) + Ln(III) coextraction processes, which also address the problem of waste alpha decontamination, and second, An(III)/Ln(III) separation processes, which can only be implemented on the solutions produced by the first-step processes. Today, however, a few processes are available that allow recovery of the trivalent actinides in a single step from highly active liquid waste. 

The efficiency of freshly precipitated iron hydroxide in scavenging plutonium and other actinides from aqueous waste solutions is well known. Processes based on two or more cycles of iron hydroxide precipitation have long been used successfully at the U. S. Department of Energy Rocky Flats and Los Alamos sites to effectively decontaminate actinide-containing waste solutions generated at those sites. (ll+,15.,16)... 

Zr-Al coprocess waste test, the feed, extractant, and scrub flows were 1, 0.5, and 0.1 mL/min, respectively. For the high sodium concentration waste, the feed, extractant, and scrub flows were 0.75, 1, and 0.25 mL/min, respectively. Samples of raffinate were drawn for analytical analysis approximately five hours after equilibrium had been reached. The resultant decontamination factors agreed reasonably well with our calculations. For the coprocess waste run, we expected an americium decontamination factor of 200. We purposely built in a large, overkillM in the sodium waste run by increasing the organic to aqueous flow rates. The sodium waste run produced a raffinate that, when calcined, would be well below the guideline for alpha-free waste with no allowance for decay. Analytical analysis of feeds and raffinates confirmed our batch results in that actinides were fractionated from major waste constituents such as aluminum, zirconium, sodium, and fluoride. 

During the decontamination steps, acid streams containing small amounts of actinides and fission products are produced. These streams are evaporated to concentrate the metal ions and recycle them. Nitric acid is recovered from the condensates and recycled. Excess HNO3 may be destroyed by formaldehyde. Fission product concentrates are routed to the aqueous raffinate of the first extractor of the partitioning cycle which contains > 99% of the FP. This constitutes the high level liquid waste (HLLW, or alternatively called HAW, high active waste). All other liquid wastes can be subdivided into intermediate level waste... 

Decontamination of minor actinides from low or medium-level reprocessing wastes of PUREX origin by LMs cannot be accomplished unless the system possesses adequate radiation stability. Therefore, it is important to examine the radiation... 

In the future, LMs are expected to play an important role in the decontamination of minor actinides from reprocessing wastes. 

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