Criticality safety assessment of tank 241-C-106 remediation Page: 40 of 83
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WHC-SD-W320-CSA-001 REV 0
amorphous to somewhat crystalline and the crystalline material became less
crystalline with time. We could conclude that in an aqueous solution, the
fresh precipitate and the high temperature crystalline solid will both
transform to the somewhat crystalline material that we will call "aged"
hydrous Pu(IV) oxide. This would suggest that it is this "aged" hydrous
Pu(IV) oxide that could be controlling the fate of Pu in the Hanford tanks,
noting that the tanks have had about 40 years and elevated temperature
conditions (=25 - 90 "C) to promote the aging.
Using the Ks for the "aged" hydrous Pu(IV) oxide as calculated by Rai or
Kim, the lines on Figure 3-4 can be recalculated to get a new figure
(Figure 3-5) that provides the solubility limit of the "aged" precipitate.
There is some difference in Rai's and Kim's experiments, which leads to two
lines being shown for the high pH solution condition. The Kim line was left
out for low pH just for clarity because few tanks are expected to have low pH.
Data from Tables 3-2 and 3-3 are also shown for the two specific tanks in
question. Essentially, the aging process lowers the lines from Figure 3-4
down about 2 orders of magnitude for the Rai data and 2.4 orders of magnitude
for the Kim data. Tank 241-C-106 is one of the few tanks known to have a
relatively low pH and a very high carbonate content and, thus, should plot on
the upper Rai or upper Kim line (not shown in the figure but just below the
Rai line). The plotted point is very close to the extension of the Rai upper
line. The tank 241-AY-102 supernatant solution should plot on the lower Rai
and Kim lines but is significantly above the line and curiously near the line
expected for freshly formed hydrous Pu(IV) oxide. Figure 3-5 shows that all
the available Hanford supernatant solution Pu values fall near or above the
appropriate predicted line for aged solid. Tank 241-AY-102 Pu solution data
and perhaps some other tanks data fall near the freshly precipitated
solubility line. We suggest that the available data do not allow a definitive
statement to be made as to whether the observed Pu solution concentrations in
tank supernatant liquors are controlled by solubility or adsorption.
However, observed and calculated Pu solution concentrations in the two
tanks of interest are in the range of 0.003 to 0.35 ppm and are at least
8 times lower than values needed to allow nuclear criticality to occur in
solution. Therefore, a key conclusion of this report is that criticality is
implausible in tank supernate solutions from these two tanks and that
criticality will require the concentration of Pu solids and separation of
Pu-bearing or pure Pu solids from the large excess of neutron absorber solids
found in the tank sludge.
3.4 SOLID-SOLID SOLUTION
Another coprecipitation mechanism is solid solution formation, in which a
plutonium ion can replace an ion of another metal present at higher
concentrations in a crystalline lattice. Because the ionic radius of Pu(IV)
differs so much from those of Fe(III) and Al(III), such solid solution
formation may be discounted for those metals. However, the ionic radii of
Pu(IV), Zr(IV), La(III) and Bi(III) are quite similar. Thus, it is possible
that some solid solution formation occurred in neutralized zircaloy cladding
wastes, bismuth phosphate wastes, and perhaps other wastes. Though
statistically possible, this formation mechanism is very unlikely for the3-17
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Waltar, A. E. Criticality safety assessment of tank 241-C-106 remediation, report, July 19, 1996; Richland, Washington. (https://digital.library.unt.edu/ark:/67531/metadc702698/m1/40/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.