The Solubility of Helium in Uranium Dioxide

A mass spectrometry method for quantitative and kinetic analysis of gas release from nuclear materials and its application to helium desorption from UO2and fission gas release from irradiated fuel

“…The data on helium solubility obtained in our experiments are significantly lower than earlier studies performed using the infusion technique [29,30,34,35] which also show significant discrepancy. At the infusion temperature used, 1500 K, oxygen defects can form giving extra room for helium and resulting in an overestimation of the helium solubility.…”

“…Based on quantitative helium release measurements from UO 2 single crystal samples infused at 1500 K, 1 × 10 8 Pa and for a time ranging from 15 to 360 min, it was possible to understand the helium behaviour in this matrix and to determine the helium solubility in the UO 2 (5.8 × 10 −9 mol g −1 ). Comparing our result with literature data for the He solubility in UO 2 [29,30,34,35], we can conclude that the diffusion of helium is fast enough to achieve saturation of the octahedral sites within 120 min of infusion at 1500 K and 1 × 10 8 Pa, while too long infusion time might produce additional thermal defects that increase the apparent solubility. The second example involved the release of the fission products Kr and Xe from irradiated nuclear fuel, and shows that the fission gases can be accurately measured hence providing quantitative data on their fate during irradiation.…”

“…Around 1100-1200 K the first release can be observed, likely due to either helium adsorbed onto the surface [29,30] or to helium depletion from the subsurface (area A in Figure 5). The depletion of this volume, not deeper than a few micrometres, could contribute to the driving force for the helium bulk release.…”

Helium thermal diffusion in a uranium dioxide matrix

“…Although it exceeds that of the fission gases Xe and Kr in UO 2 , it is still below 0.5%. The solubility limit determined under gas pressure has been estimated as less than 6.7 · 10 À4 cm 3 (STP) (gUO 2 ) À1 atm À1 [1] and only 2.4 · 10 À5 cm 3 (STP) (gUO 2 ) À1 atm À1 in monocrystals [2].…”

“…Little published data is available concerning helium diffusion in waste containment matrices, but diffusion coefficients have been reported for UO 2 and PuO 2 by Stark and Rufeh [1,4], nuclear glass [6] and britholite [5]. Our study following the same procedure as [5] consisted of three steps:…”