In the past (Ref. 8), a value of F = 2.0 was used based on calculations (Ref. 9) of the limiting
concentration of PuO2 mixed homogeneously with natural UO2 and H2O. And a value of F = 1.6
is used in the 221-F Building Technical Standards (Ref. 17). For a heterogeneous system of
EBR-II blanket rods, a "best" value of F = 1.71 was calculated (Ref. 10). To assure that a
conservative value of the equivalent U-235 enrichment would be used for this NCSE, the most
conservative value of F = 2.0 was assumed. Using 2.0 for the Plutonium Equivalency Factor
gives average equivalent U-235 enrichments of 0.77% for the EBR-II blanket rods in bundle
DU006, and 0.79 % for the TRR rods in the 54 and 27 TRR bundles, as shown in Table 1. These
values of the enrichment can then be used with published curves and tables to assure the criticality
safety of possible configurations.
Analysis for No Uranium in Solution
For the case where there is initially no fissile material in solution, then standard criticality figures
and tables for lattices in water can be used. The curve for uranium metal lattices in water, given
in Figure 5 (taken from Figure 1 of Ref. 6), indicates that optimally configured fuel rods with a U-
235 enrichment of 1.0 wt. % will be critically safe with up to approximately 12 or 13 kg of U-
235. The uncertainty about this value is a result of the difficulty in interpolating on the
logarithmic vertical scale in Figure 5. Since this figure is based on the data in Appendix B of Ref.
7, the exact value for 1.0 wt. % uranium metal lattices can be obtained from Table 6 of Ref. 7.
This value is 13.1 Kg U-235 to produce a safe K-eff of 0.98 with any rod diameter, any rod
spacing, and optimal water moderation in a spherical configuration with full water reflection. t
Since the equivalent U-235 enrichment of EBR-II bundle DU006 is less than 1.0 %, the rods in
DU006 will be critically safe (K < 0.98) in any configuration with full flooding as long as there is
less than 13.1 kg of U-235, or its equivalent. The equivalent U-235 weight in the EBR-II bundle
DU006 can be calculated to be 0.628 + (2.0 x 0.789) = 2.21 kg (Table 3), which is much less than
13.1 kg U-235. Therefore, if the rods in DU006 are isolated from other fissile material, they will
always be critically safe. This will be true whether they are intact in the bundle, or whether the
aluminum bundle and cladding have been dissolved and the EBR-II fuel slugs have fallen to the
bottom of the inner annulus in the dissolver.
By the same reasoning, six average TRR bundles that have been loaded into a Mark42/TRR insert
can not go critical. With an effective U-235 enrichment of 0.79 °/e, the limit is again 13.1 kg U-
235. The effective U-235 content in a Mark-42/TRR insert, according to Table 3, for average
TRR rods is 2.29 kg. This value is far below the 13.1 kg limit for criticality safety.
Variations in the fissile content of the TRR rods must also be considered. According to Table 3, a
Mark-42/TRR insert could contain an equivalent U-235 loading of up to 3.26 kg if all of the TRR
rods in it had a maximum plutonium content. The equivalent U-235 enrichment of this material
would be 1.01 %. From Figure 5, for a lattice of optimally configured and flooded uranium metal
rods at 1.01 % U-235, the critically safe mass limit for U-235 is at least 10 kg. This 10 Kg limit is
far larger than the 3.26 kg of U235 that could be present in a Mark-42/TRR insert. Therefore,
maximum plutonium TRR rods in a Mark-42/TRR insert will be critically safe. Maximum
plutonium TRR rods placed into the 3-well insert will also be critically safe for the same reason.