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

Issue
128
RISK PERSPECTIVES
Several commenters noted that the period of time cited as critical for containment of
radioactive wastes within a repository was not presented in a consistent fashion.
Draft p. 1.5--Five hundred years rather than 1000 years in the period that is most
critical On p. 1.9 it is stated that high level wastes must be retained safely for up to
one million years. This is patently ridiculous. (147)
Draft p. 1.5--The 1000 years noted as the time period necessary for decay of 90Sr and
137Cs is too long a period. (154)
Draft p. 1.9--The time required for isolation is inconsistent with the discussion pre-
sented in Section 3.1.3. (124)
Draft p. 1.9--Uranium, plutonium, and the balance of the TRU's do not need to be iso-
lated for up to one million years. (154)
Draft p. 3.1.16--The statement that "High level wastes must be kept isolated from the
biosphere for a long time period, perhaps hundreds of thousands of years" is at variance
with statements made elsewhere in the draft document (eg., p. 1.9-- up to one million years
and p. 1.28--500 years). (198, 218-DOI)
Draft p. 3.1.64--It is stated that "..after several hundred years of decay, the wastes
do not exceed the natural radioactivity of the ones from which they came." Elsewhere in the
report, however, references is made to isolation times of one million years or isolation
until the'waste has decayed to harmless levels. (38, 218-DOI)
Draft p. 3.1.65--The statement was made that "containment times of 500 years are the
most important." However, on page 3.1.59 it is stated that a significant release "could
occur at 1000 years" and on page 3.1.64, it stated that after "700 years, the radioactivity
in the repository poses a greatly reduced threat." Some consistency should exist in the
document for the period of concern and basis for arriving at this time should be clearly
delineated. (208-NRC)
Statements which point out the essentially "back to nature" radioactive decay time of
500 years may not be clear to the general public. (166)
Response
The material has been thoroughly revised and attention paid to such consistency. The
"500 year" and "700 year" numbers are from the referenced work of other authors. The value
used in this Statement is 1,000 years.
The 1,000 years allocated for the decay of mixed fission products typified by 90 Sr
and 137 Cs is based in part on conservative calculations of the percentage impact of these
nuclides on the total base and on proposed canister design criteria. The language used on
draft p. 1.5 has been modified to omit reference to general acceptance. For the inventories
involved, 500 years would be adequate (or more than) for just 9Sr and 1Cs.