Recriticality Considerations in Disrupted Gas-Cooled Fast Reactor Core Geometry

The margin to criticality of selected distorted core configurations derived from the hypothetical loss of shutdown cooling accident in a 300-MW(electric) gas-cooled fast reactor has been investigated using two-dimensional transport theory. Configurations representing crumbled cores, declad fuel columns in nearly intact geometry, and partial slumping of the fuel on the core floor with subsequent radial spreading have been studied. Three minimum postulated conditions for re-criticality have been identified for the configurations characterized by declad fuel in nearly intact geometry:

1. If the fuel is fully compacted (no spaces among the crumbled fuel pieces), then 55% of the fuel rods in the core must crumble for recriticality.
2. If 65% of the volume in the crumbled fuel region is space occupied by fuel, then 46% of the fuel rods must be crumbled (with the remaining 54%) standing erect but declad).
3. If 28% of the volume in the crumbled fuel bed is space occupied by fuel, then 82%) of the rods must crumble for recriticality.

For cores that are postulated to melt and slump in a radially outward progression, the reactivity changes from a“safe shutdown ” with K_eff = 0.93 to supercritical with K_eff = 1.103. If the compact, but molten, core spreads radially outward, the reactivity decreases to K_eff = 1.041.