Encased Nuclear Material

DOE-HDBK-3010-94

4.0

Solids; Nonmetallic or Composite Solids

sufficiently cooled so that it will no longer melt simply from the absence of water without a

fire.

Releases from commercial power generation SNF has been extensively studied (Restrepo,

1991; Soffer, July 1993; Mendel et al., December 1988; Nourbakihsh, Khnatib-Rahbar and

Davis, March 1988). Although the thermal stress in severe reactor accidents is well beyond

the range normally encountered in industrial-type fires, the release fractions are certainly a

"bound". Current NRC proposed releases are for PWRs [Pressurized Water Reactors, the

more limiting case (Soffer July 1992)]:

"Gap" Activity

Fuel Melting/Slumping

Fraction

Noble Gases

0.05

0.95

Iodine

0.05

0.22

Cesium

0.05

0.15

Tellurium

0.11

Strontium

0.03

Barium

0.03

Ruthenium

0.007

Cerium

0.009

Lanthanum

0.002

Other radionuclides not specifically covered would be assumed to behave in the manner as

their chemical analogues (e.g., Pu = lanthanum).

The conditions represented here are extreme and represent fuel melting and slumping,

behavior not anticipated for the long-cooled SNF found in DOE facilities. Melting of long-

cooled SNF from the temperatures typically encountered in even large industrial fires is not a

major concern. The principal concern is the loss of the "gap activity" on loss of integrity of

the cladding (e.g. corrosion, puncture, impact) and even that is significantly diminished by

the long cooling times.

If the base material (sintered, compacted, uranium dioxide) is exposed to the ambient

atmosphere, further oxidation may occur. Iwasaki et al. (December 1968) presented the

results of a study to recycle off-spec fuel pellets. Production pellets were exposed to air at

temperature from 400 oC for 3-hr to 500 oC, 600 oC and 700 oC for 1-hr and size distribution

of the resultant powder measured. All the UO2 was converted to higher oxides under the test

conditions. The highest fraction of particles measured in the respirable size range was 0.18

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