The HGCR-1, a Design Study of a Nuclear Power Station Employing a High-Temperature Gas-Cooled Reactor with Graphite-UO₂ Fuel Elements Page: 65
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The model used for predicting the release of fission products assumes that release from
the equivalent sphere is the rate-controlling mechanism and that when the fission
products have been released from the spheres the subsequent diffusion out of the UO2
pores is very rapid.
A decision was made to limit this study to the diffusion of the fission products
xenon, krypton, bromine, iodine, rubidium, and cesium because the measured escape
rate data indicate that only these elements tended to diffuse out of UO2 in significant
quantities. However, the daughters of these elements would also contribute activity
to the primary system of an HGCR-1, even though they did not escape directly from
the UO2.
The results of the estimate of the diffusion of fission products from UO2 (App. A)
indicate that the release of the various nuclides of interest is dependent upon two
major factors, the temperature of the UO2 and the half life of the nuclide. As expected,
increasing the temperature increases the diffusion coefficient and thus increases fission-
product release. There is a sharp increase in the diffusion rate at a temperature of
10000C, with the amount of diffusion being about constant at temperatures of 10000C
or less. Therefore, it is important to minimize the amount of fuel that will be at temper-
atures in excess of 10000C. There is, however, a lower limit on the amount of activity
released from the UO2, since, even at the lowest temperatures, significant amounts of
activity will be released from the UO2 by the recoil mechanism, as indicated by Fig. C.1
of App. C. The effect of half life on the amount of activity release from UO2 is also
indicated in Fig. C.1 of App. C. For a given temperature, the fraction of a nuclide that
escapes from the UO2 increases quite rapidly with increasing half life. This means,
in effect, that the material through which the nuclide diffuses acts as a holdup medium.
If the half life of a nuclide is less than the time required to diffuse out of the medium,
the fraction of the nuclides escaping will be small. An increase in temperature would
reduce the time required to diffuse out of the medium, and thus the fractional release
of a nuclide increases with temperature.
Fission products are also released from the UO2 by the recoil of the fission frag-
ments. Since the range of the fission fragments is established by the energy of these
fragments, the fractional release of the fission fragments will depend only on the size
of the UO2 particles. As Fig. C.1 of App. C indicates, there is an incentive to go to
large particle sizes (greater than 100 ).
Release of Fission Products from Graphite
The release of fission products from graphite was estimated by the same method as
that used for estimating the diffusion of fission products from UO2. The analysis of
the diffusion of fission products from graphite is given in detail in App. B. It was
assumed that the source of the fission products which diffused through the graphite65
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Cottrell, William B.; Copenhaver, C. M.; Culver, H. N.; Fontana, M. H.; Kelleghan, V. J. & Samuels, G. The HGCR-1, a Design Study of a Nuclear Power Station Employing a High-Temperature Gas-Cooled Reactor with Graphite-UO₂ Fuel Elements, report, 1959; Oak Ridge, Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc100279/m1/75/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.