Volatile fission products

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). 

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. 

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,... 

Table 2.4 shows the activities of fission products, relative to 137Cs, in the reactor fuel, the stack filter and sampling filters in the environment. All the environmental samples showed activity of the relatively volatile fission products, isotopes of Te, I and Cs, and only traces of the refractory elements, such as Zr, the rare earths and the alkaline earths. 

One milliroentgen is 84 jj.Gy, and 1 /uCi is 3.7 x 104 Bq, so the above relation is equivalent to 227 pGy h-1 per Bq m-2, close to the result of Fig. 3.8, curve A. As Knapp emphasised, the relation assumed that there was no chemical fractionation of fission products. If there was fractionation, this might give a higher proportion of volatile fission products, such as iodine, in the long-range fallout (Section 2.3). 

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. 

Admissible condition means the presence of microcracks in claddings through which gaseous and volatile fission products (noble gases, iodine isotopes) are released to coolant. 

There is virtually no contact between the primary circuit medium and coolant in case of the first-type cracks. During operation only gaseous and volatile fission products could be released via such cracks to the primary circuit medium. The second-type cracks could cause fuel corrosion and washing out of soluble fission products (cesiiun, strontium) and fuel particles to the coolant circuit. 

Gaseous or volatile fission products such as Kr, I (mainly as I2, HI and HOI),... 

Ceramic fuels can be fabricated into precise shapes (usually cylindrical pellets) that are clad in tubular thin-walled metal sheathing (cladding), which is back-filled with helium and end-capped. The cladding in water-cooled reactors in Zircaloy-2 [an alloy of Zr containing 1.4%Sn, 0.13% Fe, 0.1% Cr, Cr. 0.05% Ni, 0.01% N (max)] or stainless steel. It protects the fuel from the reactor coolant, retains the volatile fission products, and provides geometrical integrity. The clad fuel pins are assembled into fuel elements. 

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... 

The AIROX (Atomics international Reduction Oxidation) process is being developed to reprocess spent uranium oxide-based fuel. It is a cyclic oxidation-reduction process that employs only gaseous and solid materials no liquids are used. Hence, this process is often referred to as a dry process which simultaneously declads and pulverizes the fuel. Pulverization permits release of volatile fission products and comminutes the fuel for reenrichment and recycle. It also provides gaseous access to unreacted fuel in the center of the pellet. 

The behavior of volatile fission products is largely unknown. Brief literature references to iodine and ruthenium are contradictory. It is likely that elemental iodine is the stable species in the melt (16 ), and that some will be volatilized. Possible process modifications to guarantee a unique path for ruthenium have not been considered. The rare gases should escape because of the elevated temperature crystal modification. However, experience with the voloxidation process suggests that this release may not be complete. The behavior of both Kr-85 and tritium must thus be investigated. 

Sufficient knowledge of fission-product behavior in molten nitrate systems is not available in the literature. Laboratory investigations must define the behavior of the three types of fission products of concern volatile fission products, high... 

The preferred method of disposal of radioactive krypton isotopes, after being separated from other volatile fission products, is by dumping at sea as the compressed gas, confined in steel cylinders. According to a report by Bryant and Jones the cumulative quantities of Kr and in the environment by the year 2000 are such that these nuclides will pose no significant health problem. 

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. 

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

Off-gas purification. As a high-temperature process, any type of vitrification process will have to have a very effective off-gas cleaning system. In fact, besides the remote operation and maintenance technique, off-gas treatment will be among the most important waste-processing problems to be solved. The off-gas may contain volatile fission products, such as ruthenium and cesium, as well as aerosols and dust. Multistage systems will be required with wet and dry cleaning procedures to obtain an off-gas sufficiently clean for release to the atmosphere. 

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