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.
I Warranty, i ]m0lBlenwj "nisb0o* was prepared as an acoouni of wo-fc sponsored by an agency Of the United States Government, (either the Uniled Slates Government nor any agency thereof. W any ol Iheir employees, mafcesany Arrant. 0c implied, or assumes any legal liability or rewonsibilily 'of lh^ accuracy, usefulness ol any information, apparatus, product. or process disclosed, i. ,» us? vkOu' rJ not infringe privately owned rights. Reference herein to any specifi-. commercial product, process, or lerviu* by fade name, trademark, manufacturer, or otherwise, does J not necessarily constitute or imply its endorsement, recommend*lion, or favoring By the United Sidles Government or any agency thereof. The views and opinion) of authors expressed herein do r* necessarily stati> or reflect those o! the United Steles Government jr any agency thereof.
Thin report wai »rrpar*d mm an acanmt of work ina-arwl by Ibe Hailed Slate* tractor**, fubrontrartera. or their emaloyeei. makes any warranly-npmifr imtliod. or •HUHK* any leRal liability or rnpoMihilily Tor th* ooeuracv, campletenean. or uaeftilaetHof ••v intmrmutitm. aa««rattw. pradurl. or arocnt dlaclaaad. or that iU UM> wMilc not infrinRr arlvately owned riRhlN THERMODYNAMICS AND TRANSPORT OF GASEOUS IRIDIUM
Two LightWeight Radioisotope Heater Units (LWRHU) were exposed to thermal ramps simulating atmospheric reentry. The helium release rates were measured during each test and modeled after simple diffusion theory. The reentry pulses did not result in swelling of the claddings or degradation of the fuel pellets.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.