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3.2.2 Toxicity of 232Th
Given the potential for radiological hazard via the mining of western U.S. thorium
deposits as a result of implementation of 232Th-based fuel cycles, current difficulties in
estimation of 232Th DCFs must also be considered here.
As is evident in Fig. 3.0-1 (see Section 3.0), both 232U and 232Th decay to 228Th,
and then through the remainder of the decay chain to stable ‘OePb. 232U decays to 232Th
via a single 5.3-MeV alpha emission; 232Th decays via three steps, a 4.01-MeV alpha
emission to 228Ra, followed by serial beta decays to 228Th.
in the convergent decay chains is obviously nearly equal.
The total energy released
The ICRP’ lists effective energies (to bone, per disintegration) as 270 MeV for
232Th and 1200 MeV for 232U; these effective energies are critical in the determination
of dose conversion factors to be used in estimation of long-term dose commitments. The
large difference between the effective energies calculated for the two radionuclides is
based on the ICRP assumption (ref. 7) that radium atoms produced by decay in bone of a
thorium parent should be assumed to be released from bone to blood, and then redistributed
as though the radium were injected intravenously. As a result, the presence of 228Ra i n
the 232Th decay chain implies, under this ICRP assumption, that 90% of the 228Ra created
within bone is eliminated from the body. Therefore, most of the potential dose from the
remaining chain alpha decay events is not accrued within the body, and the total effective
energy for the 232Th chain is a factor of 4.4 lower than that for 232U, as noted.
Continuation and reevaluation of the early research” leading to the above dis-
similarity indicated that the presumption of a major translocation of 228Ra out of bone
was suspect (refs. 10-14) , and that sufficient evidence existed to substantiate retention
of 97% o f 228Ra i n bone. Recalculation of effective energies for the 232Th chain on this
basis results in a value of 1681 MeV as listed in ERDA 1451 (ref. 15), a substantial increase
implying the need for more restrictive limits with respect to 232Th exposures.
trast to this argument, the 1972 report of an ICRP Task Group of Committee 2 (ref. 16)
presents a newly developed whole-body retention function for elements including radium
which effectively relaxes 232Th exposure limits.
3.2.3 Hazards Related to Gamma-Ray Emissions
In con-
While fuel fabricated from freshly separated 233U emits no significant gamma radia-
tion, ingrowth of 232U daughters leads to buildup of 208T1 2.6-MeV gamma radiation, as
well as other gama and x-ray emissions. As discussed elsewhere in this report, it is
anticipated that occupational gamma exposures during fuel fabrication can be minimized by
such techniques as remote handling and increased shielding.
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