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Conference Article: Effect of water vapor on the release of fission gases from UCO in HTGR coated fuel particles.
Myers, B.F. (Oak Ridge National Lab., TN (United States))Abstract
The release of fission gas from uranium oxycarbide fuel was
measured under hydrolyzing conditions during irradiation of fuel
compacts in the High Flux Isotope Reactor. Four water vapor
injection experiments at partial pressures between 21 and 199
Pa and temperatures between 755 and 779 deg. C were
conducted at selected times. Three isotopes of krypton (85mKr,
87Kr, 88Kr) and of xenon (133Xe, 135Xe, 138Xe) were
measured. The prehydrolysis fission gas release was found to be
dependent on the fission rate density. During and after a water
vapor injection experiment, there were typically three distinct
responses: (1) an initial, rapid release. (2) a steady release. And
(3) a decline in release after termination of injection. The major
contribution to the initial release was stored fission gas. The
quantity released depended on the square of the partial pressure
of water vapor but was independent of the fission gas element or
isotope. The iodine precursors of 133Xe and 135Xe contributed
significantly to the initial release of the xenon isotopes. The
interaction of H2O with the UC2 and UO2 was found to be
sequential and was attributed to a slow movement of water
vapor into the fuel compact as a result of reaction with the
carbonaceous pore walls of the compact matrix material. The
release of fission gas from each fuel kernel was rapid (80 s)
following carbide hydrolysis but was slow (2 h) during the
H2O-UO2 interaction. The steady release, diffusional in nature,
was dominant after the initial rapid release and persisted until the
water vapor injection was terminated. The ratio of the steady to
the prehydrolysis release was dependent on the element,
independent of the isotope of the element, and larger after the
carbide portion was hydrolyzed. At the termination of water
vapor injection, the fission gas release declined as the fuel
sintered.
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key words: Gas Cooled Reactor, Nuclear Technology