Fission product precipitation

The hold-hack carriers are those used to prevent adsorption on precipitates. For instance in the separation of Sr and La from fission products, precipitation of the Sr as sulphate takes place only when sufficient inert strontium... 

In nitric acid solution containing uranium, plutonium, zirconium, molybdenum and other fission products, precipitation of zirconium molybdate occurs preferentially. Solutions containing only zirconium and molybdenum yield the "same" precipitated mater-... 

Hydrolysis products of fission products precipitated when the feed was made acid-deficient. 

At higher fuel burnup, more and more fission product precipitations appear in the fuel matrix, i.e metallic and ceramic inclusions within the fuel grains and fission product compounds at the grain boundaries. These fission products are lacking in the lattice of the fuel grains. 

Kleykamp, H. (b) Post-irradiation examination and composition of the residues from nitric acid dissolution experiments of high-bumup fuel. J. Nucl. Materials 171, 181 — 188 (1990) Kleykamp, H., Paschoal, J. O., Pejsa, R., Thiimmler, F. Composition and structure of fission product precipitates in irradiated oxide fuels Correlation with phase studies in the Mo-Ru-Rh-Pd and BaO-U02-Zr02-Mo02 systems. J. Nucl. Materials 130, 426-433 (1985) Kopp, D., Munzel, H. Release of volatile carbon-14 containing products from Zircaloy. J. Nucl. Materials 173, 1-6 (1990)... 

MWd/tU, rating 3 MW/tU (time-average), centre temperature < 1500 0. Structure near pellet centre, showing discrete pores (A) most of which contain solid fission product precipitates, and Interllnkied pores (B)... 

Pig. 11 Same specimen as Fig. 10, showing extensive grain boundary flssuring caused by interllnlcing of discrete pores. Solid fission product precipitates remain on fissured surface... 

The first successflil production method for the separation of Pu from U and its fission products was the bismuth phosphate process, based on the carrying of Pu by a precipitate of BiPO (126). That process has been superseded by Hquid-Hquid extraction (qv) and ion exchange (qv). In the Hquid-Hquid... 

The Tj-carbides are not specifically synthesized, but are of technical importance, occurring in alloy steels, stelUtes, or as embrittling phases in cemented carbides. Other complex carbides in the form of precipitates may form in multicomponent alloys or in high temperature reactor fuels by reaction between the fission products and the moderator graphite, ie, pyrographite-coated fuel kernels. 

Cesium isotopes can be recovered from fission products by digestion in nitric acid, and after filtration of waste the radioactive cesium phosphotungstate is precipitated using phosphotungstic acid. This technique can be used to prepare radioactive cesium metal or compounds. Various processes for removal of Cs isotopes from radioactive waste have been developed including solvent extraction using macrocycHc polyethers (62) or crown ethers (63) and coprecipitation with sodium tetraphenylboron (64). 

Plutonium is recovered from uranium and fission products by solvent extraction, precipitation, and other chemical methods. In most chemical processes, plutonium first is converted to one of its salts, usually plutonium fluoride, before it is recovered in purified metallic form. The fluoride is reduced with calcium metal to yield plutonium. Electrorefining may produce material of higher purity. 

Uraninite crystals retain most of the actinides produced by the fission reactions and most of the fission products that have ionic radii close to that of uranium. When uraninite becomes hydrothermally altered or transformed during supergene weathering, that is, in the weathered zone of the Bangombe reactor, the reduced conditions in the close vicinity of the U ore allows its precipitation in newly formed Si-P-REE-uranium minerals (coffinite). 

The radioactivity ratio of potentially unfractionated fission product radionuclides in precipitation should be independent of the amounts of aerosol and water vapor removed from the air masses. For an air mass containing uniformly mixed radioaerosols from the same nuclear explosion, the ratios should be the same by time and collection-site latitude along the coast. The ratios at storm date may be calculated for depositions following a specifically known atmospheric nuclear explosion with known initial production quantities. The presence of longer lived radio-... 

The analysis was made on the assumptions that the differences in gaseous-precursor fission products resulted in a potential fractionation of the three alkaline-earth radionuclides and the radioaerosols participating in the precipitation processes in the Pacific cyclones over the... 

The most important chemical parameter affecting the deposition and subsequent mobility of radioactive aerosols, such as the nuclides 90Sr and 137Cs examined in this study, is their solubility in rainwater. If these aerosols are dissolved in precipitation, the main factor in their transport is the movement of the rainwater, not the transport of insoluble aerosol particles. Huff and Kruger (2) examined the solubility products of strontium and chemically similar compounds which may carry trace amounts of 90Sr, and they estimated that strontium should be soluble in precipitation. Solubility tables also indicate that cesium compounds likely to exist in precipitation should be soluble. It was noted that the possibility did exist that some of the fission product "Sr and 137Cs might be bound within the structure of insoluble natural aerosols or nuclear weapon debris. 

Even though in the THOREX process 233U can be preferentially recovered from irradiated thorium fuel by using an extraction flowsheet based on 5% TBP n-dodecane as the extractant, further lowering of the concentration of TBP in the solvent has certain advantages in terms of reduced co-extraction of thorium and fission products (195, 196). Ramanujam et al. reported a sequential precipitation technique... 

The presence of fission products in suspensions or precipitates also increased the y-activity undergone by the solvent (103). 

An example of removing multiple interfering elements is strontium purification in the presence of fission products. Ferric ion is added as a holdback carrier for the rare earths (and other radionuclides) and then precipitated as Fe(OH)3, the scavenger that carries these radioactive impurities. This or any other step can be repeated for enhanced removal of impurities. 

In laboratory tests using simulated HLW solution spiked with fission product tracers, Am and Cm, the denitration step proved to be a sensitive process, but Am/Cm recoveries of ca. 90% in the aqueous supernate could be realized under optimized conditions. Decontamination factors (DF) > 1000 for Zr, Nb, Mo, and 100 for Ru and Fe were obtained in the precipitation step. The solvent extraction cycle gave > 98% recovery of Am/Cm and DF > 10 for rare earths, Sr and Cs. Appreciable decontamination was also obtained for Zr/Nb (DF = 20), Ru (50), U (650), Pu (250), Np (800) and Fe (420). The ion exchange cycle served mainly for Am-Cm concentration and for removal of DTPA and lactic acid based on tests with europium as a stand-in for trivalent actinides, concentration factors of about 50 could be expected under optimized conditions. 

The long half-life of the two end products makes them especially dangerous. In an atmospheric nuclear explosion, the tertiary fission products are formed in the stratosphere and gradually come down to earth. Every spring about one-half to two-thirds of the fission products in the stratosphere come down and are eventually deposited by precipitation. Figure 11-6 gives a schematic outline of the pathways through which the fallout may reach us. 

Aliquots of fission product solution with added known amounts of cesium and strontium were passed through a column of Amberlite IRA-400 (OH) and eluted with several column volumes of water. The strontium and cesium in the eluate were separated from each other by the oxalate and perchlorate precipitation method. By this means, the determinations of these two elements were accomplished rapidly and with good precision. By using an anion exchange column in the carbonate form, only cesium in fission products can be eluted (186,271). This type of separation should have. application to rapid procedures. ... 

Jacobi and Andre (113) found a longer time of 20 to 50 days, which is consistent with the lifetime calculated by Bates and Hays (11) for odd nitrogen. However, Francis, Chester and Haskin (66) found that dust containing fission products had been aged and that analysis of filtered rainwater gave a much shorter lifetime of approximately 10 days. Recently Poet, Moore, and Martell (195) estimated a mean atmospheric residence time of about four days for particles in the lower troposphere and about one week for particles in precipitation. 

The HLW solutions obtained by reprocessing are collected in tanks near the reprocessing plants. They contain relatively high concentrations of HNO3 which can be decomposed by addition of formaldehyde or formic acid. At low acid concentrations, however, hydrolysis and precipitation of elements of groups III and IV occurs. Concentration by heating may lead to volatilization of fission products, such as Ru isotopes in the form of RUO4. 

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