Helium release from radioisotope heat sources

Abstract

Diffusion of helium in 238 PuO 2 fuel was characterized s?. s a function of the heating rate and the fuel microstructure. The samples were thennally ramped in an induction furnace and the helium release rates measured with an automated mass spectro neter. The diffusion constants and activation energies were obtained from the data using a simple diffusion model. The release rates of helium were correlated with the fuel microstructure by metaJlographic examination of fuel samples. The release mechanism consists of four regimes, which are dependent upon the temperature. Initially, the release ss controlled by move nent of point defects combined with trapping along grain boundaries. This regime is followed by a process dominated by formation and growth of helium bubbles piong grain boundaries. The third regime involves volume diffusion controlled by movement of oxygen vacancies. Finally, the release at the highest temperatures follows the diffusion rate of intragranislar bubbles. The tendency for helium to be trapped within the grain boundaries diminishes with small grain sizes, slow thermal pulses, and older fuel. Activation energies and diffusion constants were correlated with previous measurements conducted on inert gases diffusing in ceramics. The helium release measured from a LightWeight Radioisotopic Heater Unit in an earlier study was verified, and the release behavior was predicted for a heat source containing the current fuel form.

Longevity of Plutonium 238 Heat Sources

“…4 For bulk diffusion by movement of point defects, activation energies of 49.9 to 52.9 kcal/mole have been reported, which compares well with 49.9 kcal/mole observed for movement of oxygen point defects through thoria. 8 Volume diffusion through interstitial sites within the oxide lattice occurs with activation energies ranging between 17 and 32 kcal/mole under a range of stimuli 4 , and an energy of 17.9 kcal/mole under static heating. 3 Activation energies have been reported for diffusion of helium through the ErH 2 fluorite lattice.…”

“…3,4 Examination of these data indicates that distinct helium transport processes can be distinguished by their various activation energies, 4 suggesting the possibility that two or more mechanisms are active simultaneously. 3,4 Examination of these data indicates that distinct helium transport processes can be distinguished by their various activation energies, 4 suggesting the possibility that two or more mechanisms are active simultaneously.…”

“…7 Mechanisms identified 4 are diffusion of bubbles within grains, migration of bubbles along grain boundaries, bulk diffusion by movement of point defects, and volume diffusion through interstitial sites within the oxide lattice. 4 of diffusion rates as a function of rate and temperature indicates the regions where the various mechanisms are active. Consolidation of the extensive compilation made by Peterson et.…”

Distended clads gas pressurization and material failure timescales

“…The quantity of helium sequestered varies with the microstructure of the fuel and the temperature, and helium release is known to be sporadic and apparently dependent on local conditions in the fuel. [10,11] Results are presented in Table 5 and illustrated in Figures 2 and 3.…”

“…Clads that have been stored at ambient temperature may have a substantial fraction of their helium inventory that is captured within the PuO2 lattice and exerts no pressure. [4,9,10,11] In clads that have been exposed to elevated temperature, it is likely that helium has been liberated from the matrix and is exerting pressure on the fuel.…”