Volatile fission product removal

G8. Goode, J. H. (ed.) Volatile Fission Product Removal from LMFBR Fuels, Report ORNL-TM-3723, 1972. 

Voloxidation A process for removing volatile fission products and tritium from irradiated nuclear fuel in advance of other processing. Being developed from 2004 at the Argonne National Laboratory. 

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... 

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. 

In preparation for dissolution, step 1, cladding is opened to permit subsequent dissolution of the oxide fuel. For steel or zircaloy this is done by mechanical shearing or sawing. Off-gases from decladding contain up to 10 percent of the radiokrypton and xenon in the fuel and some of the 002," tritium, and other volatile fission products. If voloxidation (Sec. 4.3) is used after decladding to remove tritium, more of the other volatile radionuclides will then be evolved also. 

Although iodine and cesium are readily released from the fuel during core heatup and are expected to be removed by bubble dynamics in molten pools, frequently small fractions (3 to 10%) of these fission products remain in the molten and re-solidified fuel debris. The reason for this presence of volatile fission products in molten fuel debris is not yet fully understood, but the small surface-to-volume ratio of large pools and the possibility of chemical forms of cesium that are relatively stable at high temperatures may be contributing factors. This retention of smaller amounts of iodine and cesium in the molten material might be favored by rapid local heat-up, as would occur if the oxidation of Zircaloy by steam were a significant heat source. 

The fuel stream enters the bottom portion of the reactor vessel at a minimum Inilk temperature of 750°F, and flows upward through the core, hen fissions within the fuel cause the fluid to undergo a temperature rise of fttO°F. resulting in a maximum fuel temperature of 1050°F. Upon lea im the core, the fluid passes upward to a degassing area, where volatile fission products arc removed from the fuel stream. The reactor discharge (oivi ts of a lieader which splits the fuel flow into the primary heat-tiaiisport loops. 

The reactor vessel is constructed of 2 % Cr-1% Mo steel, 2 inches thick, designed for a temperature of 1150°F and maximum pressure of 120 psi. Three 28-in.-diameter pipes carry the fluid into the reactor at the bottom and leave at the top. The entire reactor vessel is doubly contained by a relatively thin-walled containment vessel. A drain line to the fuel dump tanks is also provided. The free space above the reactor core is used as the degasser to remove volatile fission products. The reference core design has the following specifications ... 

Uranium is converted by CIF, BiF, and BrP to UF. The recovery of uranium from irradiated fuels has been the subject of numerous and extensive investigations sponsored by atomic energy agencies in a number of countries (55—63). The fluorides of the nuclear fission products are nonvolatile hence the volatile UF can be removed by distiUation (see Nuclearreactors Uraniumand uranium compounds). 

Iodine is also given off to a small extent in dissolving the uranium metal in nitric acid, but larger amounts may be obtained on steam distillation after dissolution (5). Ruthenium is often removed from the fission products by distillation of the volatile tetroxide formed by oxidation with potassium permangate, sodium bismuthate, periodic acid (38) etc. The distillation goes readily and gives a product of good purity. 

When chain fission of the alkoxy radical occurs on the other side of the free radical group, the reaction will not yield volatile aldehydes but will instead form nonvolatile aldehydo-glycerides. Volatile oxidation products can be removed in the refining process... 

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. 

Horner et al. (9) have observed that in dissolution of irradiated fuels residues, ruthenium present as fission product Ru metal, destroys the Ce(lV) probably through formation of volatile RuO followed by its decomposition to solid Ru(>2 and return from the dissolver condenser to the solution. The Ru02 is then again oxidized to RUO4. Elimination of this problem will require removal of ruthenium from the solution. It should be noted that other fission products such as iodine are oxidized to high oxidation states by Ce(lV) and also will consume Ce(lV). 

In the Aquafluor process [G4] developed by the General Electric Company, most of the plutonium and fission products in irradiated light-water reactor (LWR) fuel are separated from uranium by aqueous solvent extraction and anion exchange. Final uranium separation and purification is by conversion of impure uranyl nitrate to UFg, followed by removal of small amounts of PuF , NpFg, and other volatile fluorides by adsorption on beds of NaF and Mgp2 and a final fractional distillation. A plant to process 1 MT/day of irradiated low-enriched uranium fuel was built at Morris, Illinois, but was never used for irradiated fuel because of inability to maintain on-stream, continuous operation even in runs on unirradiated fuel. The difficulties at the Morris plant are considered more the fault of design details than inherent in the process. They are attributed to the attempt to carry out aqueous primary decontamination, denitration, fluorination, and distillation of intensely radioactive materials in a close-coupled, continuous process, without adequate surge capacity between the different steps and without sufficient spare, readily maintainable equipment [G5, R8]. 

Calcines are products obtained by removing the volatile components of the waste, i.e., water and nitrate, at temperatures between 400 and 900° C. The result is a mixture of oxides of fission products, actinides, and corrosion products in particulate form with a specific surface of 0.1 to 5 ra /g. The plain calcine is not very stable chemically because of its large surface area and the chemical properties of some of the oxides, and it is highly friable. To improve the properties of calcines, advanced forms are developed. One such product is the so-called multibarrier waste form, a composite consisting of calcine particles with inert coatings, such as pyrocarbon, silicon carbide, or aluminum, embedded in a metal matrix. Another advanced calcine is the so-called supercalcine. This is essentially a ceramic obtained by adding appropriate chemicals to the HLW to form refractory compounds of fission products and actinides when fired at 1200°C. Supercalcine requires consolidation by embedding in a matrix but does not need to be coated, as the material is supposed to have inherent chemical stability. 

The most hazardous volatile constituents are the iodine and rudienium fission products. Though more than 95 % of the iodine is volatilized in the dissolver (as I2, HI and HIO mainly) most of it is caught in the off-gas scrubber and most of what remains is removed by the filters. With these techniques the retention of iodine in the plant is >99.5%. 

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