Recriticality calculation with GENFLO code for the BWR core after steam explosion in the lower head
Recriticality of the partially degraded BWR core has been studied by assuming a
severe accident phase during which the fuel rods are still intact but the control
rods have experienced extensive damage. Previous NKS and EU projects have
studied the same case assuming reflooding by the ECCS system In the present
study it was assumed that coolant enters the core due to melt-coolant interaction
in the lower plenum. In the first case specified the relocation and fragmentation
of the molten control rod metal causes the level swell in the core but no steam
explosion. In the second case a steam explosion in the lower head was assumed.
In the first case a prompt recriticality peak can occur, but after the peak no
semistable power generation remains. In the second case the consequence of
the slug entrance into the core is so violent that the fuel disintegration and melting
during the first power peak may occur. After the large power peak water is
rapidly pushed back from the core and no semistable power generation maintains.
The fuel disintegration studies have been based on a coarse assumption
that the acceptable local energy addition into the fresh fuel may be 170 cal/g, but
with increasing burn-up it can be as low as 60-70 cal/g. In the level swell variations
the maximum energy addition was between these limits, but in most of the
steam explosion variations much above these limits. Additional variation of the
assumptions related to the neutronics demonstrated that for the converged
analysis result some interactions would be useful with respect to the boundary
conditions and neutronic options.