Risk insight of SNF degradation

Table 7.1 summarizes the dissolution rates for oxidizing and reducing dis­posal environments (Ahn et al. , 2011a) used in a performance assessment model (Markley et al., 2011). A range of environmental conditions are con­sidered, mostly near-neutral pH and ambient temperature. The variation of pH and temperature can be adjusted in terms of dissolution rate as user — defined parameters. For this base case, radionuclide release is estimated combining the reducing and oxidizing environments, to simulated residual radiolysis of water by actinides in the reducing environment. Figure 7.8 shows the estimated dose from the radionuclide release for this combined case.

Considering all radionuclide release fractions from the UO2 matrix, an exercise was conducted to estimate the doses to workers or members of the public from airborne fragments of the SNF matrix caused by SNF oxidation and SNF drop/collision (after Kamas et al., 2006). The most significant dose contributor in the release fraction is aerosol SNF fines (i. e., small solid particles). Tritium, noble gases, iodine, crud, ruthenium, caesium, strontium and SNF fines were part of the source term considered. In Fig. 7.11. the

image119

7.10 Comparison of the DOE handbook respirable fraction equation to experimental values of the specific energy input into the brittle material (NRC, 2007).

Table 7.1 Summary of SNF dissolution rates in oxidizing and reducing environments

The oxidizing environment is considered because of the potential alpha radiolysis in the reducing environment and the early waste package failure. The assessment is more based on immersion conditions that are considered in the alternative disposal sites in the future.9 The dissolution rate of commercial SNF is assumed to be bound to that of sMOX under immersion conditions.12 Both commercial SNF and sMOX have the particle size of ~1 mm after reactor irradiation. Other references include the references of [3] and [7].

An average factor of 0.03 (0.01-0.1) was factored in the oxidizing case. In the French and Belgian repositories, an average 2 x 10~6/ year was used13, similar to the current estimate. To be consistent, the dissolution rate of sMOX was assumed to be the same as the rate of commercial SNF12.

Because the alpha radiolysis may have limited effects on the dissolution rate of commercial SNF14 and sMOX, the combined case is separated to represent some effects of alpha radiolysis. If we consider the hydrogen effects to be produced by the container corrosion, this combined rate could be conservative. The hydrogen could inhibit the SNF dissolution rate.15 To be consistent, the dissolution rate of sMOX was assumed to be the same as the rate of commercial SNF.12

Подпись: Mobilization 3.00E-05, 6.00E-04 (degradation) of (log-uniform) commercial SNF and sMOX (spent MOX) under the oxidizing condition (fraction per year; minimum and maximum)
Подпись: Mobilization 9.00E-07, 2.00E-05 (degradation) of (log-uniform) commercial SNF and sMOX under the reducing condition (fraction per year; minimum and maximum)
Подпись: Mobilization 9.00E-07, 6.00E-04 (degradation) of (log-uniform) commercial SNF and sMOX under the combined condition (fraction per year; minimum and maximum)

Parameter name Value Description and basis

Spent MOX fuel is also included in the table and the reference numbers quoted are from the reference by Ahn et al. (2011a).

Подпись: Normal operations, SNF oxidation: building wake effects considered for worker dose CD S CD СЛ О О □ Worker dose DPublic dose image176

7.11 Example dose estimate for (a) oxidation and (b) collision (/drop) of SNF assemblies (after Kamas et al., 2006). Used with permission from American Nuclear Society (ANS).

radionuclide release fraction of the aerosol SNF fines, 2.0 x 10-6 for the drop/collision case and 1.2 x 10-3 for the SNF oxidation case, were used to estimate the dose to workers or members of the public (Ahn et al., 2011b; Kamas et al. , 2006). A site boundary was defined, for the dose to workers within the boundary and to members of the public outside the boundary.

The left figure is for SNF oxidation under normal operations. The wake effects are a modification of the radionuclide transport path right outside any storage building if any building shadow exists. Consequently, radionu­clide transport will stop. Within a short distance from the building, the radionuclide transport will not be reached. The right figure is for drop/col — lision cases. In both cases, arbitrary dose rate units are used for the log scale. The oxidation case gives a dose rate ten times higher than the collision case in the same log-scale unit.

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