Isotope exchange reaction between 235UF5 nanoparticles and 238UF6 gas

Abstract

The equilibrium in the isotope exchange reaction between 235U-enriched UF5 nanoparticles and natural UF6 gas was experimentally investigated. From the relationship between the isotopic fraction of UF6 gas at equilibrium and the specific surface area of the UF5 nanoparticles, it was experimentally demonstrated for the first time that all UF5 molecules on the outermost layer of the particles participated in the reaction until the isotopic fraction of UF5 on the outermost layer became equal to that of the UF6 gas. A fast rate process during the early period of the reaction and a subsequent slow rate process were quantitatively observed. These facts contradict the previously reported assumption not supported by experimental evidence that the UF5 molecules in the underlying layer of the particle participate in the reaction. A new model based on our experimental findings was proposed. Our model suggests that there are two kinds of molecules on the outermost layer with different reactivities and all UF5 molecules on the outermost layer contribute to the reaction. The rate equations were derived from the proposed model and analytically solved. The time dependency of the isotopic fraction of the UF6 gas and UF5 particles was expressed by three kinetic parameters such as the rate constant of the high reactivity molecule, ks, the rate constant of the low reactivity molecule, ks′, and the fraction of high reactivity molecules on the outermost layer, ξ. Our model reproduced well all the experimental data determined in the present study with ks=5.5×10−20 cm3/(s molecules), ks′=4.5×10−22 cm3/(s molecules), and ξ=0.078.

Determination of Nonlinear Parameters Included in Rate Equations by Taylor's Differential Correction Method Intermolecular Transfer of a Fluorine Atom from UF6 to UF5

“…Since the first observation by Grigor'ev et al, 3) particular attention [4][5][6][7][8][9][10] has been paid to the uranium isotope exchange between gaseous UF 6 and solid UF 5 expressed by…”

“…The discussions include also a short review of the previous researches and the possibility of a new rate equation for this reaction. 6 and UF 5 Since the first observation by Grigor'ev et al, 3) particular attention [4][5][6][7][8][9][10] has been paid to the uranium isotope exchange between gaseous UF 6 and solid UF 5 because this reaction may be an important factor, which results in a drop in the final selectivity of molecular laser isotope separation. Grigor'ev et al 3) have pointed out the reaction may easily occur through intermolecular transfer of a fluorine atom from UF 6 to UF 5 .…”

“…However, this assumption leads to another difficulty: if there is an increase in the UF 5 surface area, the surface energy must increase with time, while there is no way to supply the required energy to the particles under the isothermal and isobaric conditions, as pointed out by Kuga and Takeuchi. 9) Recently, Kuga and Takeuchi 10) have carried out the experiment under the condition of high concentration of UF 6 over a long time, and has concluded that only the UF 5 molecules on the outermost layer of the particles participate in the reaction until the isotopic fraction of UF 5 on the surface becomes equal to that of UF 6 gas. Their model suggests that there are two kinds of molecules on the outermost layer with different reactivities.…”

A Note on the Inhibition of the Isotope Exchange Reaction between 235U-enriched UF5 Nanoparticles and Natural UF6 Gas

“…12 of Ref. (6). On the other hand, as shown in Table 1, the thermal annealing treatment at 323 and 348 K exhibits a remarkable inhibition effect on the IER.…”

“…The details of the IER experiments have already been described in Refs. ( 4) and (6). Table 1 summarizes the effect of those inhibition treatments on the isotopic ratio for the IER.…”