Reactor strain localization

It was determined for 7 x 7 BWR fuel that strain localization due to pellet-to-cladding interaction at pellet interfaces (ridging) and pellet cracks can cause a small but statistically significant number of fuel rod perforations during normal reactor operation. The fuel design improvements listed below have been made for 8 x 8 BWR/6 fuel to reduce pellet-to-cladding localized strain. 

The most sensitive ecosystems affected at Chernobyl were toe soil fauna and pine forest communities toe majority of toe terrestrial vertebrate communities were not adversely affected by released ionizing radiation. Pine forests seemed to be toe most sensitive ecosystem. One 400 ha stand of Pinus silvestris died and probably received a dose of 80-100 Gy. Other stands experienced heavy mortality of 10-12-year-old trees and as much as 95% necrotization of young shoots these pines received an estimated dose of 8-10 Gy. Abnormal top shoots developed in some Pinus, and these probably received 3-4 Gy. In contrast, leafed trees in toe Chernobyl Atomic Power Station zone, such as birch, oak, and aspen, survived undamaged, probably because they were about 10 times more radioresistant than pines. Extremely high radioresistance was documented in genetically adapted strains of toe filamentous fungus Alternaria alternata isolated from toe reactor of toe Chernobyl power plant other strains of this species are supersensitive to radiation. There was no increase in mutation rate of spiderwort (Arabidopsis thaliana), a radiosensitive plant, suggesting that toe dose rate was <0.05 Gy/h in toe Chernobyl locale. 

As shown in Table 4.2, large break LOCA events involve the most physical phenomena and, therefore, require the most extensive analysis methods and tools. Typically, 3D reactor space-time kinetics physics calculation of the power transient is coupled with a system thermal hydraulics code to predict the response of the heat transport circuit, individual channel thermal-hydraulic behavior, and the transient power distribution in the fuel. Detailed analysis of fuel channel behavior is required to characterize fuel heat-up, thermochemical heat generation and hydrogen production, and possible pressure tube deformation by thermal creep strain mechanisms. Pressure tubes can deform into contact with the calandria tubes, in which case the heat transfer from the outside of the calandria tube is of interest. This analysis requires a calculation of moderator circulation and local temperatures, which are obtained from computational fluid dynamics (CFD) codes. A further level of analysis detail provides estimates of fuel sheath temperatures, fuel failures, and fission product releases. These are inputs to containment, thermal-hydraulic, and related fission product transport calculations to determine how much activity leaks outside containment. Finally, the dispersion and dilution of this material before it reaches the public is evaluated by an atmospheric dispersion/public dose calculation. The public dose is the end point of the calculation. 

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