Noble metal fission products

The report on the disassembly and postoperative examination of the ARE pointed to the ease of removal of the noble gases and the deposition of certain noble metal fission products on metallic surfaces. It was also learned that because of the evolution of chemistry that occurs during radioactive decay, it is important to account for the kinetics of noble gas removal from the salt. 

Hulls, noble metal fission products Most fission products ... 

The liquid fuel of MSRs allows fission product gases such as xenon and krypton to be released from the liquid. Along with xenon and krypton, noble metal fission products may entrain and exit as a fine particulate smoke (ORNL 4865,1975). Iodine tends to form a stable iodide within the liquid salt but a fraction is expected to entrain as well. Finally, while alkali, alkaline, and lanthanide fission products will bind to fluorine to form salt-stable fluorides when bom within the salt, there are many unstable isotopes of xenon and krypton that after leaving the salt subsequently decay to daughter isotopes including i37Cs (30 year) and (2.3 million year) along with numerous other isotopes of Ba, La, Ce, Rb, and Sr. 

Noble metal fission products will not form stable fluorides in the fuel salt but will tend to plate out on surfaces in the primary loop. The main complication that arises from this is in terms of the primary heat exchangers. Work with the MSRE indicated that upward of 40% of noble metals plated-out on the walls of heat exchanger. If shell and tube designs are proposed, then the heat generation by noble metals attached to tube surfaces can be a concern if both primary and secondary salts are drained from the heat exchangers, a rare but plausible event. ORNL determined this a manageable but concerning situation (ORNL TM-3145,1971). 

Numerous studies by other workers (I, 10) have shown that the releases of iodine and the noble-gas fission products from pyrolytic carbon-coated fuel particles are controlled by diffusion of these nuclides through grain boundaries, cracks, and defects in the isotropic pyrolytic carbon coating. When coatings are intact, however, the release of these fission product nuclides is low. However, the pyrolytic carbon coating constitutes only a delaying barrier to the metallic nuclides barium and strontium through which they diffuse with diffusion coefficients of the order of 10 9 cm.2/sec. (at — 1400°C.). The steady-state release of these metallic nuclides is controlled instead by diffusion out of the fuel kernel,... 

A 140-page summary report describing the behavior of fission products in the MSRE is the most complete source of information on fission product behavior in molten fluorides. In all instances, the evidence confirms what basic thermodynamics tells us only the noble gases (Xe, Kr) and tritium are released from the salt. All of the alkali (eg., Cs), alkaline earth (e.g., SrX rare earth (e.g., Y, Ln), and most metallic fission products (e g., Zr) are dissolved in the salt as fluorides and are relatively nonvolatile. A few of the metallic fission products (the noble group Ag, Pd, Ru, Mo, Tc, Rh, Sb) are not dissolved (or are partially dissolved), but remain as metallic species and tend to deposit on the colder metallic surfaces. 

The release of metallic fission products, e.g., Ba , Cs , and Sr , tends to be by surface diffusion rather than by pore diffusion, as in the case of noble gases. Work done by the Dragon project (/, 42) has... 

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. 

Trummer M, Nilsson S, Jonsson M. (2008) On the effects of fission product noble metal inclusions on the kinetics of radiation induced dissolution of spent nuclear fuel. J Nucl Mater 378 55-59. 

Jensen, G.A. dL al.. J., Recovery of noble metals from fission products, Nucl. TechnoL,... 

In addition, Brambilla has claimed that both iodine and ruthenium are volatilized when the molten nitrate reacts with the oxide fuel (9). In contrast to this volatilization, other literature claims that both iodine and ruthenium will be found in the molten phase (6, 13). Avogadro and Wurm state that most of the fission products, other than the noble metals, are either volatile or soluble in the nitrate melt, even without addition of nitric acid vapor (12). In a later paper, however, Avogadro reports that iodine is stable as iodide in molten nitrates, and that ruthenium is partially soluble in the molten phase, and partially volatilizes, while the majority remains with the... 

Activities were estimated on the basis of long time operation where saturation exists. The escape of fission products by recoil was estimated to be about 10 . This could be stopped by an exceedingly thin coating (. 1mm.). I am informed by Mr. Mulliken that recent work on the diffusion of flssion products in U metal at 600 °C (hotter than in the He plant) fails to disclose any mechanism for escape other than recoil. Of order 1/10 of the recoil activity will be transmitted through a permanent gas (1/20 activity in permanent gases and 1/20 resulting from their decay). Another 1/20 of the recoil activity will be volatile at the helium temperature of 400 C (largely Iodine). We may then expect 10" of the total activity to deposit on walls as soon as it can. 10 of the activity will deposit as soon as it can after the helium is cooled, and 10 will remain as a noble gas. 

As can be concluded from this figure, the apparent ionic radii of La ", the tervalent lanthanides and Ce" " are almost identical to that of The radii of most of the other ions and also of the atoms of the platinum metals are within a range of about 30 M> around the calculated value of the lattice vacancy position thus, one can expect that their incorporation into the lattice will be possible without major difficulties. The same apphes for both the neutral atoms and the tetrava-lent ions of molybdenum and technetium, which means that the question of lattice compatibiUty will give no preference to one of the two valency states. On the other hand, the atomic radii of the fission product noble gases krypton and xenon are... 

Tech. 1970) during the operation of the Molten Salt Reactor Experiment showed that only the noble gases (Xe, Kr) and tritium are released to the cover gas. Most fission products are dissolved in the molten salt (CsF, SrF2, BeE) although some exist as metals and tend to deposit on metallic surfaces (Ag and others). This barrier to the release of radionuclides reduces the fuel quality requirements. 

Figure 7.6 Schematic representation of the fuel salt treatment with two loops. On the left is the online treatment with gas bubbhng in the core to extract noble gases and metallic particles (fission products [FPs]). On the right is the mini-batch on-site reprocessing with two objectives removing FPs (Zr, Ln) and adjusting the fuel content in fissile and fertile isotopes.

Oxidizing nature of the fission process. The fission of a mole of UF.1 would yield more equivalents of cation than of anion if the noble gas isotopes of half-life greater than 10 min were lost and if all other elements formed fluorides of their lowe.st reported valence state. If this were the case the system would, presumably, retain cation-anion equivalence by reduction of fluorides of the most noble fission products to metal and perhaps by reduction of some U + to U +. If, however, all the elements of uncertain valence state listed in Article 12-6.2 deposit as metals, the balance would be in the opposite direction. Only about 3.2 equivalents of coml)iiicd cations result, and since the number of active anion equivalents is a minimum of 4 (from the four fluorines of UF4), the deficiency must 1)0 alleviated by oxidation of the container. The evidence from the Aircraft Reactor Experiment, the in-pile loops, and the in-pile capsules has not shown the fission process to cause serious oxidation of the container it is possible that these experiments burned too little uranium to yield significant results. If fission of UF4 is shown to be oxidizing, the detrimental effect could be overcome by deliberate and occasional addition of a reducing agent to create a small and stable concentration of soluble UF3 in the fuel mixture. 

---