Fission Product Behavior in High-Burnup Water Reactor Fuel Subjected to Slow Power Increases

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Abstract
Data are presented on fission gas release for UO2-Zr fuel pins that were subjected to slow power increases late in life. These tests were performed with fuel pins that had been previously irradiated to average burnups of 27000 to 35000 MWd/ton U (peak pellet 43 700 MWd/ton U). The subsequent power increases were to 301 to 444 W/cm (peak pellet), and the hold time was 24 h, with one test at 72 h. Emphasis was given to extensive axially and radially local measurements, rather than to integral pin data. Cross-sectional releases increased with transient powers above 350 W/cm; at 415 W/cm they seemed to saturate at 40 to 45 % within 24 h. Radially local releases started at calculated local temperatures at ∼700°C, reaching a constant level of 90 to 95% above 1100°C. Local swelling appeared to begin at ∼650 to 700°C, with maximum swelling levels at 10%. From the observed transient release data, an effective diffusion coefficient could be calculated that was more than three orders of magnitude higher than commonly accepted values.

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