Assessment of the coolant voiding dynamics following the failure of preirradiated LMFBR fuel pins

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 fission-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