Fully Moderated and Reflected Solids cont'd

DOE-HDBK-3010-94

6.0 Inadvertent Nuclear Criticality; Material Release in Criticality Excursions

ultimately terminated by density changes within the metal itself and the moderator due to heat

generated.

The products contained in the matrix of the solid fissionable materials are not exposed to the

ambient environment and would not constitute a source term hazard unless released from the

matrix. Of the 11 historical excursion involving metal system listed on Table 6-1 (b), only 1

(with a total fission yield estimated at 4E+17 fissions) exhibited any melting of the metal.

Warping/oxidation are listed as consequences of 3 additional events (total fission yields

ranging from 6E+16 to 1E+17 fissions). Therefore, a limited amount of softening/melting

of the solid would be anticipated at the reference yield level of 1E+18 fissions, although this

level is likely to be conservative. This softening/melting would not, however, cause

significant disruption of the solid mass itself. It would most likely produce physical

distortion of the material due to almost instantaneous melting and congealing from heat

transfer to and interaction with water.

It is assumed that 10% (0.01 fraction) of the metal melts/softens due to the heat generated,

thus allowing noncondensible gases and volatile radionuclides (radioiodine) in that fraction to

be released. For powders or fines in solutions, the surface area to volume ratio of individual

particles is considered sufficiently large that all noncondensible gases and radioiodines will

escape. Restrepo (April 1992) reviewed the post NUREG-0772 research and experiments on

fission product release rates from heated/melted spent fuel. The specific elements involved

were categorized into 12 discrete chemical groups (as shown below in Table 6-10) based

upon the similarity of their responses to thermal stress. "... release fractions within each

group varied by as much as two to three orders of magnitude ... the geometric mean and

standard deviation of the release fractions for each of those groups were obtained." The

geometric mean of the airborne release for each chemical groups, rounded off to a single

digit, was selected as the release fraction for that group (except the noble gases) and is listed

in Table 6-10. The upper limit estimate for release was selected for the noble gases. Also,

no release fractions are reported for chemical group 12 (boron) and, since the behavior of

that group's components is similar to that of the trivalent group (chemical group 9), the

release rate for the trivalent elements is used for the boron elements in Table 6-10.

Thus, for metals, 5E-1 of the noble gases is assumed to be released from the solid matrix to

the moderator and all of the noble gases released to the moderator are released to the

ambient environment around the moderator. Likewise, 5E-2 of the halogen (iodine) is

released from the matrix to the moderator. The 0.25 capture factor for iodine in solution is

not assumed since the iodine release is already less than that for solutions and the water

depth associated with this configuration is assumed to be limited. The 2E-3 factor assigned

for radioruthenium may also be used without considering solution capture as the ruthenium

may be generated as discrete gas bubbles escaping from the solid surface. As with solutions,

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