ORNL-5388 - the Molten Salt Energy Technologies Web Site

3-1 0
3.2. RADIOLOGICAL HAZARDS OF DENATURED FUEL ISOTOPES
H. R. Meyer and J. E. Till
Oak Ridge National Laboratory
Consideration of the denatured 233U cycle has created the need to determine the
radiological hazards associated with extensive use of 233U as a nuclear fuel. These
hazards will be determined by the toxicity of the various isotopes present in the fuel
and in thorium ore, which in turn is influenced by the path through which the isotopes
enter the body--that is, by inhalation or ingestion.
from the denatured fuel present a potential hazard.
3.2.1. Toxicity of 233U and 232U
In addition, the gamma rays emitted
Only limited experimental data are available on the toxicity of high specific activ-
ity uranium isotopes such as 233U and 232U.
hazard, is the limiting criterion for the long-lived isotopes of uranium (23JU and 238U)
which are of primary concern in the light-water reactor uranium fuel cycle.' In order
to establish the relative radiotoxicity of denatured 233U fuel, it is helpful to consider
specific metabolic and dosimetric parameters of uranium and plutonium isotopes.
3.2-1 lists several important parameters used in radiological dose calculations.
effective half life for 239Pu i n bone is approximately 240 times that of uranium.
ever, the effective energy per disintegration for 232U is about three times greater than
that for any of the plutonium isotopes.
plutonium isotopes would be significantly greater than the dose from uranium isotopes
for the inhalation pathway, assuming inhalation of equal activities of each radionuclide.
Doses via the ingestion pathway, again on a per VCi basis, are much lower than those esti-
mated for the inhalation pathway.
Chemical toxicity, as opposed to radiological
In general, the time-integrated dose from
It is currently assumed that all bone-seeking radionuclides are five times more
effective in inducing bone tumors than 226Ra.
Table
have been conducted with 233U (ref. 2) and 232U (refs. 3-5) suggest a reduced effectiveness
in inducing bone tumors for these isotopes and may result in use of exposure limits that
are less restrictive than current limits.
The
How-
However, the limited number of studies that
The last two columns in Table 3.2-1 represent dose conversion factors (DCFs) for
uranium 'and plutonium isotopes calculated on the basis of mass rather than activity.
may be seen that the 232U "Mass DCFs" are more than four orders of magnitude greater than
those for fissionable 233U, due largely to,the high specific activity of 232U. This factor
contributes to the overriding importance of 232U content when considering the radiotoxicity
of denatured uranium fuels.
Figure 3.2-1 illustrates the importance of 232U content with respect to potential
toxicity of 233U fuel. This figure presents the estimated dose commitment to bone calcu-
It