Modeling Fragmentation and Spallation of Oxide Reactor Fuel During Transient Heating Including the Effects of Burnup and Fission Product Distribution

Abstract

Characteristics of solid fuel fragmentation and energetic spallation during hypothetical accident transients are calculated. Fission gas, having migrated to the grain boundaries as intergranular bubbles, is assumed to provide the potential energy sufficient to fracture the fuel and induce fuel particle motion radially. Fuel cladding is assumed to maintain a radial constraint on the fuel until the cladding fails by melting. It is shown that the vapor pressure of elemental cesium contained in the fuel cladding gap can impose a compressive stress on the fuel sufficient to maintain a large quantity of intergranular fission gas trapped within equilibrium bubbles. On cladding melting and relaxation of constraint, the extent of fuel fracture initiated at the overpressured bubbles is calculated. It is shown that expanding fission gas in the fractured grain boundaries is sufficient to eject fuel particles at substantial velocities.

The results of calculations are consistent with experimental observations. The calculated compressive constraint of 3 to 5 MPa is in agreement with cladding failure stresses near the melting point and with those tensile stress levels necessary to cause observed cladding swelling. In further concurrence with the experiment, it was found that higher burnup fuel (~9.9 at.% burnup) containing more fission gas was more likely to undergo energetic spallation, with larger quantities of fuel moving at greater speeds, than fuel irradiated to intermediate levels (4.7 at.% burnup). In the high burnup case, fuel particles of 0.5 to 1.0 mm in size were ejected radially at speeds >0.5 m/s. In the intermediate burnup case, smaller particles ≍ 0.2 mm in size move somewhat slower at speeds ≤0.4 m/s. A low burnup fuel (2.3 at.%) was calculated not to fragment and spall at all. Since the calculated vapor pressure of elemental cesium was used to account for the compressive constraint, the analysis requires the presence of cesium to predict spallation.