Fission product volatilization

In particular accident sequences with high-pressure, low-flow conditions the buildup of a significant vapor pressure of volatilized fission products in the reactor pressure vessel over the surface of the overheated core may limit the release rates (e. g. Taig, 1984). Since the release rates used in the codes have mostly been determined in experiments in which vapor pressures did not limit the release, this effect has to be taken into account in order not to overestimate the fission product volatilization rates to be expected in such accidents. 

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). 

The rest of the less volatile fission products along with constituents of zircalloy, stainless steel, and the control rods are assumed to be in condensed form as inert aerosols that are treated together in TRAPMELT as "other aerosols." The aerosols are modeled as agglomerating and depositing on surfaces by several mechanisms (e.g., gravitational settling). 

The behaviour of irradiated uranium has been studied mainly with respect to the release of fission products during oxidation at high temperatures The fission products most readily released to the gas phase are krypton, xenon, iodine, tellurium and ruthenium. The release can approach 80-100%. For ruthenium it is dependent upon the environment and only significant in the presence of oxygen to form volatile oxides of ruthenium. 

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. 

Tc-99, which has a half life of 2.12 x 10 years, can be recovered from nuclear fission waste in kilogram quantities. Solvent extraction, ion exchange, and volatilization processes are employed to separate it from the numerous other fission products. Because of its long half life and its emission of a soft (low energy) beta particle, it can be safely handled in milligram quantities. Almost all chemical studies of the element have been carried out with this isotope. 

Concern about fission-product release from coated reactor fuel particles and fission-product sorption by fallout particles has provided stimulus to understand diffusion. In a fallout program mathematics of diffusion with simple boundary conditions have been used as a basis for (1) an experimental method of determining diffusion coefficients of volatile solutes and (2) a calculational method for estimating diffusion profiles with time dependent sources and. time dependent diffusion coefficients. The latter method has been used to estimate the distribution of fission products in fallout. In a fission-product release program, a numerical model which calculates diffusion profiles in multi-coated spherical particles has been programmed, and a parametric study based on coating and kernel properties has provided an understanding of fission product release. 

Fractionation of fission products during fallout formation was recognized by Freiling (4) in early studies of fallout particles. He also recognized that this phenomenon involved the volatility of the fission products. In an attempt to describe fractionation quantitatively, Miller (9) devised... 

The refractory nuclide chain, 95, and the volatile chain, 137, were investigated. If the fission product absorption process began at 2700°K. and proceeded using 200°K. steps to 900°K., the radial distributions of the fission products in these chains at the final temperatures can be obtained. Calculations show little variation in concentration of any of... 

In considering the case of maximum release, it is apparent that complete mixing in the liquid phase will lead to a greater release rate than that expected in cases where diffusion operates in two phases. Therefore, consider the case where both the solvent (Na) and the solute (volatile fission product) diffuse through a gas layer of constant thickness. It follows from the solution to Fick s law with appropriate boundary conditions that... 

The greatest Cs/Mn ratio observed in the fallout tray samples was 0.1 at 16,500 feet or 10 times that seen in the crater lip ejecta. Air filter samples, on the other hand, exhibited the expected enrichment in the volatile fission product, 137Cs, relative to the highly refractory 54Mn. The relationships of crater ejecta and fallout materials to volatile and refractory nuclear detonation products, shown in Figures 14 and 15, conform to the generally accepted theories on the contrasting behavior of these two radionuclides. 

A similarity in the leaching behavior of 137Cs, a volatile fission product, and residual tritium, a gaseous detonation product which probably behaves in its own unique manner, has been demonstrated. 

Ashing. Sections of filter papers containing the samples from Johnie Boy were ashed in a Trapelo low temperature asher to prevent volatilization of some of the fission products of interest for different portions... 

Elements from selenium through the middle rare earths will be present in the mixed fission product population they exhibit a wide variety of volatilities (1). The elements Y, Zr, and Nb and the rare earth oxides are high boiling and condensable at low partial pressures, whereas the noble gases, and the alkali metals Mo, Tc, Pd, Ag, Cd, Sn, Sb, Te, Ru, and perhaps Rh, are very volatile in a relative sense Sr and Ba are predicted to be of refractory or intermediate behavior. 

If only a few percent of the energy is expended in volatilizing the soil, several tens of thousand tons of soil will be entrained in the vapor cloud. The partial pressure of the vaporized soil constituents will then be of the order of a million times that of the fission products. There will... 

We note that fission product activity balance requires that the specific activity of a volatile chain increase more rapidly than a refractory chain with decreasing diameter because of the large depletion factors noted in 89Sr/144Ce in large sizes. 

If uniform mixing of the fission product vapors and volatilized materials results, the recondensed particles might be expected to have a constant specific activity of elements having similar boiling points. Note parenthetically that studies of fission-product incorporation into the metal and oxide products of vaporized iron wires (in which iron-metal spheres and iron-oxide irregulars are formed) indicate no simple relationship between specific activity and size. For example, a refractory element like zirconium is found most enriched in particles of intermediate size. This is probably in part caused by a concentration effect—i.e.y in these experiments the zirconium represented a mole fraction of about 10"9. As indicated earlier, the fission products are a minor constituent in the fireball, and a very complex pattern of incorporation can be anticipated, especially if coagulation with melted but unvaporized particles ensues. 

The 45Ca and fission product data, taken as a whole suggest two disparate sources of large debris one, originating at more distant soil horizons, consists of unmelted or partially melted soil particles the other, originating in the immediate vicinity of the fireball, is completely melted, with perhaps some volatilization. 

Fission products can be released from defects in the fuel rods or from tramp uranium on the fuel cladding. Of special importance are the volatile fission products 131—i35j M7>i38Xe in BWR steam). Cations include the Sr and Cs isotopes,... 

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