PNNL-13501 - Pacific Northwest National Laboratory

Case 1: Full-density solid targets
Case 2: Full-density thick annular targets (IR=0.2 cm,
OR=0.41 cm)
Case 3: Full-density thin annular targets (IR=0.35 cm,
OR=0.41 cm)
Case 4: ½ density solid targets
Case 5: 1 /700 density solid targets
Case 6: Target material mixed directly with fuel.
Note that Case 6 is different than the other cases because
target rods are not used. In Case 6, the target material is
mixed directly with the fuel in a quantity that contains the
entire available Tc-99 and I-129 inventories in all
available reactors (both pressurized water and boiling
water reactors).
We estimated that 73 MT of Tc-99 and 20 MT of I-129
will be generated when all of the current operating light
water reactors in this country have been shut down. We
assumed for this report that the transmutation effort would
begin in the year 2010 and that the inventory of Tc-99 and
I-129 available for irradiation in 2010 would be 45 MT
and 14.5 MT, respectively.
Results and Accomplishments
Table 1 gives the yearly transmutation percentage per
reactor, the number of reactors required to contain the
inventory, and the yearly transmutation rate using all
necessary reactors for Cases 1 through 6, assuming
available inventories of 45 MT Tc-99 and 14.5 MT I-129.
Both Tc-99 and I-129 have the highest transmutation rates
Table 1. Transmutation rates for Tc-99 and I-129 (year 2010)
Transmutation
Rate Per Reactor
370 FY 2000 Laboratory Directed Research and Development Annual Report
when present in low concentrations in a reactor. This
effect results from the reduced self-shielding when these
isotopes are present in low quantities. However, as the
quantity per reactor is reduced, the number of reactors
required to burn the inventory increases. In particular,
Case 5, which has the best overall transmutation rate, has
the worst total throughput due to the large number of
reactors that would be required to contain the targets.
However, Case 5 does demonstrate the importance of
self-shielding to this problem, particularly for Tc-99.
These results may be extrapolated through the year 2030
to give an approximate life-cycle analysis, taking into
consideration generation of Tc-99 and I-129 in these
reactors during this time, and yearly decommissioning of
40-year-old reactors. We estimate that a typical reactor
will generate approximately 25 kg/yr of Tc-99 and 5 kg/yr
of I-129. For the life-cycle analysis, Case 6 was selected
for Tc-99 as it had the highest transmutation rate, while
Case 2 was selected for I-129 because the annular design
allowed for Xe gas buildup and because of the small
number of reactors required. Tc-99, Case 6, gives a total
20-year transmutation of 46.3 MT, or 63% of the total
projected inventory destroyed. I-129, Case 2, gives a total
20-year transmutation of 6.9 MT, or 35% of the total
projected inventory destroyed.
The effects of adding Tc-99 to the fuel matrix and the
implications regarding fabricability are important.
Adding Tc-99 to the fuel matrix would result in an
additional dose rate of 3 mrem/hr on the surface of a fuel
pellet. The contact dose rate of a pellet is a function of
both enrichment and age and can exceed an exposure rate
of 200 mrem/hr. Therefore, the dose effects of adding an
additional 3 mrem/hr from Tc-99 in the fuel matrix appear
to be inconsequential.
Tc-99 I-129
Approx. Number
of Reactors
Required
Total Annual
Transmutation
Rate (MT/yr)
Transmutation
Rate Per Reactor
Approx. Number
of Reactors
Required
Total Annual
Transmutation
Rate (MT/yr)
Case 1 1.43% 4 0.65 2.12% 5 0.31
Case 2 1.64% 6 0.74 2.21% 7 0.32
Case 3 2.46% 16 1.11 2.71% 19 0.39
Case 4 1.91% 9 0.86 2.50% 10 0.36
Case 5 8.35% 3028 0.09 (a) 3.05% 3589 0.01 a
Case 6 5.65% All (106) 2.54 2.69% All (106) 0.39
(a) Based on 69 PWRs.