

DOESTD300793
2.29) + 2.21 = 9.09 kg. Since all of this material is < 1.0 % enriched, the limiting value (from
Section 6.2) would be 13.1 kg of U235. Therefore, if the EBRII bundle were surrounded by 18
average TRR rods, the configuration could not go critical, since 9.09 kg < 13.1 kg. If six of the
TRR rods were assumed to contain the maximum plutonium possible (79 grams each), then the
total equivalent U235 loading, based on the values in Table 3, would be (2 x 2.29) + 3.26 + 2.21
= 10.0 kg. This is less than the 13.1 kg limit for the average enrichment of < 1.0 %, and equals
the 10.0 kg limit (from Section 6.2) for the maximum enrichment of 1.01 %.
The above 10.0 and 13.1 limits assume a spherical configuration of optimally moderated and
reflected fissile material, without neutron absorption in structural materials. KENO calculations
(Ref. 1) which account for the annular geometry of the dissolver, as well as the neutron
absorption in the steel walls of the dissolver, have obtained a maximum K = 0.94211 (Table 6,
case 5 of Ref. 1) for 19 metric tons (19,000 kg) of uranium in the annular dissolver with an
equivalent U235 enrichment of 0.9635 % (Table 1 of Ref. 1). This calculation was conservative,
since it did not restrict the fissile material to the inserts, ignored the neutron absorption in the steel
material of the inserts, and ignored neutron absorption in the nitrogen in the nitric acid. The total
uranium loading of all the EBR/TRR material in this dissolution is only 4223 kg (Table 1), with an
average equivalent*U235 enrichment of 0.78 %. Therefore, when the actual dissolver geometry
and wall materials are accounted for, the following conclusions can be made:
1. Even if all of the EBR/TRR material (4224 kg) were put into the dissolver at once, it could
not go critical. Therefore, double batching of any of the planned four batches is critically safe.
2. The neutronic isolation of the inserts does not have to be assumed to maintain the required
criticality safety margin.
3. The TRR material does not have to be placed into inserts to maintain the required criticality
safety margin.
6.3
Analysis With Uranium in Solution
Even if, as assumed in the above section, the dissolver solution initially contains no fissile material
at the start of the batch, the uranium and plutonium in the fuel rods will be going into solution as
the dissolution progresses. As they dissolve, the fuel rods will be decreasing in diameter, which
will cause the U238 absorption resonances to be less spatially self shielded, which will cause the
Keff to decrease. This decrease in Keff does not depend on the dissolved uranium and
plutonium being swept away from the fuel rods, for it will even occur for an infinite lattice as it
dissolves into a homogeneous mixture. This is shown by Figure 6, which is taken from Figure 22
of Ref. 11, where the curve for the homogeneous water reflected case lies entirely to the right of
the curve for the heterogeneous water reflected case. This is also implied by the results in Table 5
of Ref. 1. But in making this argument, two qualifications need to be made:
1. The monotonically decreasing Keff that results from a heterogeneous fuel geometry
dissolving into a homogeneous fuel geometry is only claimed for fairly low enrichments,
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