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External Criticality Risk of Immobilized Plutonium Waste Form in a Geologic Repository

Abdelhalim A. Alsaed
John A. McClure
Publication Date


This technical report provides an updated summary of the waste package (WP) external criticalityrelated
risk of the plutonium disposition ceramic waste form, which is being developed and
evaluated by the Office of Fissile Materials Disposition of the U.S. Department of Energy (DOE).
The ceramic waste form consists of Pu immobilized in ceramic disks, which would be embedded
in High-Level Waste (HLW) glass in the HLW glass disposal canisters, known as the "can-incanister"
concept, that are externally identical to standard HLW canisters (DOE 1996, p. 5). The
cans with the ceramic disks would occupy approximately 12% of the HLW canister volume, while
most of the remaining 88% of the volume would be occupied by HLW glass. An extremely conservative approach was employed in estimating the overall probability of criticality. Many parameters are optimized to result in the highest possible probability of criticality
without taking probability credit for them. In the degradation analysis (CRWMS M&O 2001a),
the source term was optimized by varying the degradation rate of the three main components of a
plutonium-bearing waste package, i.e., SS, HLW glass, and ceramic. The source term was
optimized such that its chemistry is favorable for high solubility of fissile material while
maintaining favorable chemistry for accumulation in the external environment with reasonable
dilution. Probability credit for the conservative degradation rates is not taken and neither was any
credit taken for source term optimization. It is important to understand that the estimation of the probability of criticality external to waste
packages bearing plutonium disposition ceramic waste form is highly dependent on the theory
behind accumulation mechanisms. The probability presented in this report is linked to the
neutralization accumulation mechanism in an assumed geometry. Currently, this is the mechanism
that is believed to result in the highest accumulations of fissile materials. The assumed geometry
is needed because of the limitations of PHREEQC, which is the only tool available for such
purposes. Even though the extreme conservatism presented in this report is defendable and
reliable, it is not comprehensive of all accumulation mechanisms.

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