ORNL-TM-7207 - the Molten Salt Energy Technologies Web Site

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Variation of fuel composition with time. Fission sf 235U in the
operating reactor will result in a decrease of that isotope and an in-
growth of fission products and in the generation of (1) 234U from 232Th,
(2) 239~u from 238~, and (3) numerous transuranium isotopes. ~urther, a
once-through DMSR will require additions of uranium at intervals during
its lifetime. For example, a DMSR with a fuel containing 9.5 mole X heavy
metal ~ ~ u contain l d about 110,000 kg of 232Thf 3,456 leg of 235U9 and
14,000 kg of 2 3 8 ~ at startup.
factor, it would require the addition of 4,470 leg of 235Y and bEi9400 kg
Of 238u
~uring 30 years of operation at 75% plant
If Such a reactor received only additions Of UF4 and UF3 and if
no fuel were removed,* the final quantities and concentrations of heavy
metals in the fuel would be those indicated in Table 24. The end-of-
life fuel would also contain about 1.4 mole % of soluble fission-product
species and would have a total of nearly 2.4 mole Z of uranium isotopes,
about 6.053 mole % of plutonium isotopes (about 32% of which is 239Pu)9
and less ThFk than the original fuel. Thus, the concentration of heavy
metal in the fuel changes very little although the species do change;
total heavy metals in the end-of-life fuel equal about 9.3 mole X compared
with an initial 9-5 mole X. Therefore, the physical properties of the
fuel would not be likely to change appreciably during reactor life al-
though a gradual change in some chemical properties would be expected.
Additions of uranium can be made conveniently as a liquid EiF-UFb
mixture19 (liquidus 490"~) while the reactor is operating, as was done
many times during operation of the MSRE.22 To keep a proper concentration
of UF3 in the fuel and possibly to remove tramp oxide-ion contamination
from the fuel, some fuel maintenance operations will be necessary. The
combination of these relatively simple operations likely will result in
sufficient addition of LiF and BeF2 to require on-site removal and stsr-
age of a small fraction of the fuel before reactor end-of-life. These
operations and the resulting fuel management options are described in a
later section after the chemical basis for them has been presented.
* This type of speration may be p~ssible
although subsequent discus-
sion will show other more likely modes of operation.