Risk insight of cladding performance

The values of crack opening from models need to be compared with those used in determining the radionuclide release fraction in storage from exper­imental work (Lorenz et al., 1980). Lorenz et al. (1980) conducted burst tests by heating a clad SNF rod, allowing an opening area of about 1.6 cm2 (-10-1 fraction of the total cladding surface). If the calculated value of the cladding area opened by cracking resulting from hydrogen effects is smaller than that from the experiments by Lorenz et al. (1980), the radionuclide release fraction will not increase with further cladding cracking from the embrit­tlement. Otherwise, the radionuclide release fraction from the UO2 matrix to the canister inside will be affected by the embrittlement. The current

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7.13

image125

Effects of radial hydrides on ductility (elongation) loss in a more severe case. The different orientations result in different magnitudes of ductility loss (Yagnik et al., 2004). Used with permission from American Nuclear Society (ANS).

7.14 A schematic of the DHC process showing hydrogen diffusion to an existing crack tip. The threshold stress intensity factor for DHC of Zircaloy is in the range of 5 MPa m1/2. Under storage conditions, such level of stress intensification has not been demonstrated, although a few possibilities are currently under study.

Подпись: Initial perforations cased from reactor damage, creep from storage at high temperature, damage during earthquakes, or local corrosion

Cone shape assumed for unaltered fuel matrix

7.15 Подпись: Pressurized water reactor fuel assemblyПодпись:Подпись:image127Cladding unzipping process by the oxidation/hydration of the SNF matrix or zirconium (DOE, 2002).

regulation for SNF storage requires that the cladding must be protected during storage against degradation that leads to gross rupture or the SNF needs to be otherwise confined.

The effects of crack opening on the radionuclide release in disposal were also studied. Ahn and Mohanty (2008) summarized the literature results. In the presence of partial protection from failed cladding by cracking, the
dissolution rate decreases significantly compared to bare SNF, with slit (-0.015 cm [5.9 x 10 inch] width and -2.54 cm [1 inch] length) or hole (-0.02 cm [7.9 x 10-3 inch] diameter) defective SNF cladding under immer­sion conditions in J-13 well water at 85°C (185°F). The tests were intended to simulate cladding partially failed by localized corrosion or SCC. The radionuclide release rates decreased by a factor of -140 for Tc-99, -7 x 105 for I-129, and -65 for Sr-90, compared to bare clad SNF. However, in the segment tests of clad SNF under immersion conditions, the radionuclide release rates did not decrease compared to bare SNF. In these tests, nearly half of the surface area was exposed. Similar conclusions can be drawn from tests with Canadian deuterium-natural uranium reactor clad SNF, immersed in Canadian granitic groundwater. These results suggest that cladding would not inhibit the dissolution rates if it fails catastrophically exposing a sub­stantial surface area to the solution (groundwater).

Elam et al. (2003) assessed the effects on nuclear subcriticality caused by the configuration changes due to cladding failure. The main assumption in this study is full water flooding in the cask. The report presented the reactivity for various SNF rod conditions. For uniform burnup of 45 and 75 GWd/MTU collapsed SNF rods, the variations of the neutron multiplica­tion factor, Akeff, were not significant for this changed cladding configuration.

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