LOFT-I shutdown margin during initial fuel load: an independent Monte Carlo analysis

KENO Monte Carlo calculations have been performed to provide an independent assessment of the criticality safety of the LOFT reactor during initial loading operations. The Monte Carlo model representation, with some minor exceptions, describes the geometry of the reactor internals exactly, i.e., each fuel pin, poison pin, guide tube, etc., is explicitly described. All nuclear data used in the calculations were from the ENDF/B libraries (all data was Version IV with the exception of the stainless steel which was Version III). Groupwise averaged cross sections, required by KENO were derived using integral transport theory for resonance shielding and S/sub n/ transport theory to collapse to the sixteen-group structure used in the Monte Carlo model. Results of the calculations show that the criticality-safety margin is very high for the planned operations, i.e., the normalized k/sub eff/ for the fully-loaded reactor with rods in at cold 3000 ppM coolant conditions is 0.807 +- 0.009/sub 95/. The results also show that shutdown margins are not decreased significantly if for some unforeseen reason, loss of borated water occurs in any region inside the vessel. Additional calculations show that the reactor is criticality safe if it becomes necessary to remove any (or even all) controlmore » rods from the core. The normalized k/sub eff/ for the unrodded core at cold 3000 ppM coolant conditions was computed to be 0.898 +- 0.016/sub 95/.« less