To understand the phenomenology and consequences of a hypothetical mild (∼50¢ to $5/sec) overpower transient for a liquid-metal fast breeder reactor (LMFBR), a continuing series of in-pile fuel-failure experiments has been performed in the TREA T reactor for both fresh and preirradiated pins. The voiding history obtained from the failure of a fresh fuel element has been shown to result from a mild fuel-coolant-interaction process. However, preirradiated pins are accompanied by significant fission gas retention, which leads to a difference in failure criterion and voiding mechanism. Thus, the dynamics of coolant expulsion, following the failure of preirradiated pins under overpower transient conditions, must include the effects of gas discharge to the coolant channel. The flow transient for preirradiated fuel elements, brought to failure in a flowing sodium environment, was analyzed in terms of fission-gas-induced voiding. The analytic approach was based on single-phase isothermal expansion of the fis-sion-gas bubble, which was assumed to be ideal and to fill the coolant-channel cross section uniformly. Gas discharge through the cladding breach was calculated using either the equation for critical or for subcritical flow, depending on the pin-to-bubble pressure ratio. The coolant was considered incompressible. A parametric study indicates that the initial breach size is quite small, initiating a relatively slow rate of voiding without initial inlet-flow reversal. However, ∼100 msec after failure, rapid coolant expulsion and inlet-flow reversal occurs. To account for this condition, fission-gas communication between the storage plenum and core regions of the pin must be assumed, since the free gas inventory within the fueled section alone is quite small. At the time of flow reversal, however, significant melting and some vaporization of the fuel has occurred; thus, coolant expulsion by fuel-coolant interaction and/or fuel vapor pressure cannot be discounted. The release of such fission gas, however, will affect the energetics of fuel-coolant interaction and fuel sweepout; thus, gas effects must be taken into account in the analysis of LMFBR overpower transients.
