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

331
ALTERNATIVE DISPOSAL CONCEPTS
the release of radioactivity to the environment and minimize the amount of water required
compared to a once-through system.
Draft p. 3.4.1-22
Issue
The Rock Melt Concept discussed in Section 3.4 assumes that the cavity is loaded over
a period of years. This prolonged loading time has at least two disadvantages. First, the
physical integrity of access and venting shafts must be maintained for the duration of the
loading. Second, the cooling water itself will be contaminated and must be carefully con-
tained and eventually the contamination must be disposed of as yet another waste.
Another loading scheme should be considered. The waste could be stored at the surface
until the full load for the cavity has been accumulated. The waste could then be rapidly
loaded into the cavity and the cavity quickly sealed.
It appears that the quick loading of the cavity is a practical alternative to the pro-
longed loading suggested in the GEIS. Further variations should also be considered, such
as the use of an array of cavities (a few to maybe 10's of cavities). This would reduce
the loading rate (in the case of the quick load) and distribute the heat load over a large
volume. (208-NRC)
Response
While rapid loading of the cavity in the rock melt alternative has the advantage of
minimizing the time that the access shaft must be kept open, it has the disadvantage of
storing waste above ground for a long period of time. The risk of possible population
exposure to radiation is considered to be higher from waste stored above the ground than in
the case for waste downhole. The storage tanks required for approximately 40,000 MTHM of
liquid high-level waste (33 million liters) which is necessary to fill each cavity (see
Section 6.1.2.2 of the final Statement) is a primary consideration in the decision. Since
the rock melt process was developed to handle a liquid waste steam the problem of disposal
of contamination liquid (either residual from the waste stream of from evolving water) is
one of the important factors which would eventually be addressed in the detailed engi-
neering design studies of the process. See Section 3.4.2.1 and 3.4.2.2 of the draft State-
ment. In addition, further discussion is contained in Section 6.1.2.2 of the final
Statement.
In addition, the problem of maintaining physical integrity of the cavity, shafts, and
vents was identified as a disadvantage in Section 3.4.1.4 of the draft Statement and is
similarly addressed in Section 6.1.2. of the final Statement. The approach of rapidly
loading the cavity is not considered to be technically conservative or to be consistent
with a step-wise approach, two requirements which are inherent to the DOE philosophy.