Fission-product elements

The use of larger particles in the cyclotron, for example carbon, nitrogen or oxygen ions, enabled elements of several units of atomic number beyond uranium to be synthesised. Einsteinium and fermium were obtained by this method and separated by ion-exchange. and indeed first identified by the appearance of their concentration peaks on the elution graph at the places expected for atomic numbers 99 and 100. The concentrations available when this was done were measured not in gcm but in atoms cm. The same elements became available in greater quantity when the first hydrogen bomb was exploded, when they were found in the fission products. Element 101, mendelevium, was made by a-particle bombardment of einsteinium, and nobelium (102) by fusion of curium and the carbon-13 isotope. 

Gross dissolution of the waste can be measured from the loss of weight of the sample or by determination of the appearance in the leaching medium of the major matrix constituent e.g. glass). However, the selective leaching of important fission product elements has been observed in this work to be significantly different from that of the bulk waste matrix. The units normally used to describe leach rates, g/cm day, appear to imply... 

The population of fission product elements as a function of time is changing rapidly. These may be estimated from a knowledge of the half-lives of the fission product chain members, the mass chain yield, and the independent yield distribution along the mass chains. Although there are some uncertainties in these procedures largely because of lack of data on short-lived species, and a less than perfect understanding of the charge distribution function, reasonable estimates of radioactive atom... 

Element uptake from soil and transfer into the edible parts of plants have been addressed in several other studies. Soil-to-plant transfer factors in fruit and vegetables grown in various agricultural conditions have been determined for, for example, Pt [100], T1 [101], and various other metal contaminants [102], In a study on stable isotopes of fission product elements (Ce, Cs, Sr), an in vitro enzy-molysis method has been applied to investigate the solubilization of the analytes from fodder in a simulated ruminant digestion [103], The effect of inhibitors of fission product solubility was also considered and essential elements were determined simultaneously to evaluate potential nutrition problems for the animals from the use of such inhibitors. Selective leaching of individual classes of metal complexes with different ligands and sequential enzymolysis have been recently applied to estimate the potential bioavailability to humans of Cd and Pb in cocoa powder and related products [104]. 

Salt Transport Processing (8, 9, 10, 11) The selective transfer of spent fuel constitutents between liquid metals and/or molten salts is being studied for both thorium-uranium and uranium-plutonium oxide and metal fuels. The chemical basis for the separation is the selective partitioning of actinide and fission-product elements between molten salt and liquid alloy phases as determined by the values of the standard free energy of formation of the chlorides of actinide elements and the fission products. Elements to be partitioned are dissolved in one alloy (the donor... 

For convenience, the fission product elements are divided into four groups, FP-1, FP-2, FP-3, and FP-4, that correspond to the order in which separations occur, Table I. The FP-1 fission products are volatile and do not react with either the salts or metal elements. They are removed during the head-end processing. Elements that are sufficiently active to be oxidized by CaCl2 are designated as FP-2 fission products. Also designated as FP-2 fission products are iodine, bromine, selenium and tellurium, that are removed with the salt during the oxide reduction step. 

Each process step is being investigated for fission product behavior and distribution with respect to the desired actinide recovery. The effect of added constituents on the behavior of actinide and fission-product compounds in molten nitrates will be studied. Soluble species in molten nitrates are to be identified and a determination made as to whether fission product elements are present, either as soluble species or solids, as anionic or cationic species. 

Molten-Tin Process for Reactor Fuels (16). Liquid tin is being evaluated as a reaction medium for the processing of thorium- and uranium-based oxide, carbide, and metal fuels. The process is based on the carbothermic reduction of UO2 > nitriding of uranium and fission product elements, and a mechanical separation of the actinide nitrides from the molten tin. Volatile fission products can be removed during the head-end steps and by distilling off a small portion of the tin. The heavier actinide nitrides are expected to sink to the bottom of the tin bath. Lighter fission product nitrides should float to the top. Other fission products may remain in solution or form compounds with... 

Initial experiments have demonstrated the feasibility of car-bothermic reduction of UO2 and nitriding of uranium in molten tin. Nitriding of the product of carbothermic reduction of mixed U02 Pu02 and added fission product elements is one of the steps to be confirmed for this process to be deemed potentially useful. 

For convenience purposes, the fission product elements are divided into four groups FP-1, FP-2, FP-3, and FP-4. FP-1 fis-... 

FISSION PRODUCT ELEMENT RELATIVE REMOVAL IMPORTANCE... 

Percent of neutrons absorbed by a specific fission product element compared to the neutrons absorbed by all the fission products in a PWR neutron spectrum. 

Analyses of the final solids indicated that all fission product elements of concern were present antimony, cerium, cesium,... 

We may be able to change the chemistry of uranium, plutonium, and fission-product elements reported in this paper with specific additives. 

In addition, precipitation of fission-product elements has been demonstrated. The addition of sodium carbonate or sodium sulfate to molten equimolar sodium-potassium nitrate containing soluble strontium nitrate results in precipitation of strontium carbonate or strontium sulfate, respectively. Molybdenum trioxide, M0O3, added to an equimolar sodium-potassium nitrate melt resulted in evolution of nitrogen dioxide and dissolution of the molybdenum, presumably as the molybdate anion, MogOy (14). Addition of soluble strontium nitrate to this nitrate melt produced an insoluble precipitate that was also insoluble in water. The existence of an aqueous insoluble strontium molybdate is known, and it is believed that a similar species is formed in the melt. 

Volatilization. Many fission-product elements, including krypton, xenon, iodine, cesium (normal boiling point 705 C), strontium (1380°C), barium (1500°C), the rare earths (3200 C), and plutonium (3235°C), are more volatile than uranium (3813°C). Cubicciotti [C17], McKenzie [M5], and Motta [M8], in laboratory experiments, showed that around 99 percent of these more volatile elements could be separated from uranium by vacuum distillation at 1700 C. Because of the high temperature and severe materials problems, volatilization has not been used as a primary separation process, but does contribute to removal of the most volatile fission products in conventional reprocessing. In fractional crystalUzation or extraction with liquid metals, distillation is used to separate uranium and plutonium from more volatile solvent metals. 

An important objective of dissolution and the preconditioning of feed solution prior to extraction is to convert these fission-product elements into states that will not contaminate uranium, plutonium, or solvent in subsequent solvent extraction. 

Table 11.2 Amounts of fission-product elements and actinide elements in the waste from 1 MT LWR uranium fuel (30,000 MWd/MT bumup) at discharge from reprocessing (150 days cooled fuel elements) and 6 years after discharge (contributions of more than 0.1 percent) accmding to ORIGEN...

Early Work. The irradiated fuel, upon discharge from the reactor, comprises the residual unbumt fuel, its protective cladding of magnesium alloy, zirconium or stainless steels, and fission products. The fission process yields over 70 fission product elements, while some of the excess neutrons produced from the fission reaction are captured by the uranium isotopes to yield a range of hew elements—neptunium, plutonium, americium, and curium. Neutrons are captured also by the cladding materials and yield a further variety of radioactive isotopes. To utilize the residual uranium and plutonium in further reactor cycles, it is necessary to remove the fission products and transuranic elements and it is usual to separate the uranium and plutonium this is the reprocessing operation. 

Table 3.3. Fission product element concentrations (g/kg HM) in irradiated LWR uranium fuel (initial enrichment 4.0%...

Chemical compounds which are thermodynamically stable as isolated, pure compounds under the relevant conditions are not necessarily formed in the fuel matrix. There are problems due to kinetic interferences, to the very high UO2 excess, and to the unfavorable mass ratios of some of the fission product elements under consideration for the formation of compounds that are possible in principle. 

There are transitions between the different groups which are partly caused by the oxygen potential of the fuel, while the formation and composition of precipitates, in particular, mainly depends on the concentrations of the individual fission product elements in the fuel. 

Figure 3.13. Typical properties of fission product elements in oxide fuels...

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