A new four-factor formula is proposed for deriving a " finite multiplication factor "
k*-he ratio between neutron production and absorption rates in a finite medium-from experimentally determined lattice parameters in a slightly-enriched Pu-U lattice.
The "two-group multiplication factor"
k*
∞2-related to τ+
L2 and equal to (1 +τB
2C) (1+
L2B
2C) at criticality-is derived from
k*.
The experimental lattice parameters are corrected to account for neutron leakage, from which the " infinite multiplication factor "
k∞ is derived.
There are found differences between
k*,
k*
∞2 and
k∞ amounting to about 1-2% in the Advanced Thermal Reactor (ATR)-type heavy-water lattice, despite the fact that all these three quantities are often indiscriminately called " infinite multiplication factor."
The proposed four-factor formula for deriving
k* was applied to the Deuterium Criti-cal Assembly (DCA) of 1.2%-enriched UO
2 lattices of 28-pin clusters in square arrays spaced at 22.5 cm. The values of
k* calculated with a lattice design code METHUSELAH II were in fairly good agreement with those determined by experiment. The calculation tended to somewhat overestimate
k*, particularly in lattices of highly voided coolant. A similar tendency was observed in the case of 1.5%-enriched UO
2 lattices of 22.5 cm pitch in the ATR Sumitomo-Two-Region Critical Facility.
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