An explicit analysis of bubble-size equilibration by volume diffusion has been derived and incorporated into the FRAS2 code. This code was developed for mechanistic analysis of transient fission-gas behavior in LMFBR fuel, but had been limited by the approximation that the time required for bubble-size adjustment, following coalescence or changes in temperature or pressure, was negligible. The equilibration phenomenon is illustrated by several idealized examples. If the fuel temperature rises linearly with time, an isolated bubble will expand rapidly toward its equilibrium size only after the temperature reaches a critical value that depends on both bubble size and heating rate. Conversely, if the temperature is reduced uniformly, an isolated bubble will shrink only until diffusion becomes too slow for further size change. The final size that is ''frozen in'' may be significantly larger than the final equilibrium size. Finally, isothermal equilibration following coalescence of two equal bubbles can be described conveniently in terms of relaxation times, which depend on bubble size and temperature. However, a complete picture of the role of equilibration in transient fission-gas behavior can only be gained through an investigation of the bubble-size distribution and its evolution. Such an investigation is carried out with the FRAS2 code, in which the explicit equilibration model is implemented. Since nonequilibrium bubbles in a thermal transient are generally overpressured, the gas represents a source of energy that can conceivably disrupt the fuel. This ''dispersive potential'' is also calculated in the FRAS2 code.

Calculations of fission-gas behavior have been carried out with an updated version of the FRAS3 code for the FGR-40 series of transient tests of irradiated PNL-10 fuel. This same series of tests was used in an earlier evaluation study with a preliminay version of the code. While that study provided positive support for the modeling approach, it also indicated deficiencies in some areas. Although a number of improvements have been implemented in the current version of the code, this study examines the effect of an explicit treatment of bubble growth within the grains, including the effect of vacancy depletion caused by the competition of overpressured bubbles for available vacancies. The result is a reduction by as much as 90% in the predicted swelling, accompanied by an increase in transfer of gas from the grains to boundaries. Both swelling and gas release predictions are brought into much better agreement with the observed values.