Spent nuclear fuel (SNF) degradation

This section presents the degradation behaviour of SNF in mild and near­neutral environments under (i) oxidizing or reducing aqueous disposal conditions, and (ii) in dry storage environments. During the aqueous dis­solution of SNF, highly soluble fission products such as Tc-99 or I-129 are released congruently with (i. e., in proportion to) the SNF matrix (UO2 ) dissolution. On the other hand, actinides such Pu-239 or Np-237 are released at a concentration below or equal to their solubility limits (or colloid con­centration), which are in turn determined by the SNF matrix dissolution rate, groundwater flow rate and solubility limit. Colloids are suspended solid particles of less than 1 micrometer in size that can contain actinides. An oxidizing aqueous environment promotes electrochemical dissolution of the SNF matrix in soluble species with the aid of oxidants such as dis­solved oxygen and hydrogen peroxide (Shoesmith, 2000) . In a reducing environment, the UO) matrix will dissolve chemically in soluble species (Sunder and Shoesmith, 1991). Generally, the electrochemical dissolution rate is faster than the chemical dissolution rate. In the presence of radiolysis effects, the SNF matrix may dissolve in either an electrochemical or a chemical process, depending on the magnitude of the radiolysis (Ahn et al, 2011a). In conjunction with container failure and sorption and/or flow behaviour of backfill, the SNF matrix dissolution serves as the source term of radionuclide release in the PA. In a dry storage environment, mechanical degradation of the SNF matrix could occur by air oxidation/humid air hydration or impact fragmentation upon the canister failure under normal conditions (e. g., SCC failure) or external hazard conditions (e. g., aircraft or seismic impact). In the canister, if incomplete drying of SNF assemblies occurs, the residual water may increase RH sufficiently to oxidize (by oxygen from the radiolysis of water molecules) or hydrate the SNF matrix. With severe external hazards, high temperatures or impact stress may frag­ment the SNF matrix by oxidation or mechanical disintegration. The respir­able SNF particles (i. e., suspended aerosol, less than 10 pm [3.9 microinch] in size) produced by the fragmentation serve as the primary source term for radionuclide release in air.

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