Management of Commercially Generated Radioactive Waste - U.S. ...

N.2
Fission product activity in thorium wastes is about the same as that in LWR wastes, with
only slight aggregate differences because of the mass distribution of 2 3 3 U fission fragments
and the greater thermal efficiency of HTGRs. Some of the specific isotope yields are differ-
ent by a factor of about two, but these differences are not among controlling long-lived
isotopes and thus neither simplify nor complicate long-term waste storage as visualized and
being developed for the slightly enriched uranium (SEU) cycle in LWRs.
Radiogenic heating is of importance when considering storage and isolation of certain
radioactive wastes. Heat generation rates in the thorium wastes are essentially the same
as in LWR wastes for the first several thousand years. They reach a maximum at less than
twice the LWR rate in about 100 years, then decrease and finally peak again at 50 to
100 thousand years. Although the latter peak can exceed the LWR rate by a factor of 15, the
actual value of the heat generation rate is quite small by that time.
For the first few thousand years, actinide and heavy element radioactivity in LWBR
wastes is somewhat less than that in the LWR wastes. The radioactivity in HTGR wastes at
these times exceeds that in LWR wastes by up to a factor of 7.because of the plutonium
(primarily 23 8 Pu) which is present. After hundreds of thousands of years, the radioactivity
in both HTGR and LWBR wastes exceeds that of LWR wastes by factors of 10 to 20. As in the
case of heat generation, however, the absolute activity at these long times is relatively
small.
In the instance of thermal neutron reactors, the more 233U recycled, the lower will be
the releases of transuranium isotopes formed by successive neutron captures in the fuel.
233
This is due mainly to the fact that the capture-to-fission ratio is less for U than for
235U 2 3 9 Pu, or 24 1 Pu. On the other hand, more (5 to 10%) 233U in thorium fuel cycles must
be fissioned than 235U or plutonium in the SEU because the energy yield per fission for
233U is less, and because thorium has about one-fifth the fast-fission effect of 238U.
The actinide radioactivity and the heat generation rate differences are also influenced
by the way the transuranic isotopes are managed, in particular regarding the yields on pro-
cessing and the goal exposure of the fuels. However, when the gross characteristics of the
LWBR-generated waste (total activity, heat output, chemical and physical form) are compared
to LWR-generated waste, these characteristics are very similar (DOE 1979). As a result, no
special waste management requirements are posed by the LWBR concepts which do not already
exist for the LWR and no changes are anticipated to be necessary in the waste isolation pro-
gram for LWR systems to accommodate a thorium-based system. ERDA (1976) performs an envi-
ronmental assessment of a thorium-uranium fuel cycle and should be referred to for detailed
information.
Gaseous releases from a facility reprocessing thorium- 2 3 3 U fuel would be somewhat
85
greater than those from a reprocessing facility. This is particularly true for Kr,
although the xenon yields are more nearly equal. Because of the greater 85 Kr release, an
analysis is required to determine the significance of the release.
The 14C release from an HTGR reprocessing facility could be up to 15 times larger than
that of SEU in LWRs because of the large amount of graphite in the fuel and the burning