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Submitted 10/17/02
Q:
When discrete TENORM is present underneath a slab,
what are some common methods employed to 1) determine the source term so
that modeling could be performed to demonstrate compliance with the 100 mrem/y
all controls fail scenario and 2) for remediation? The contamination is in
the form of dime-sized chips that are below a 8" steel-reinforced concrete
slab and could be 4 feet deep. The chips read over several 100,000 cpm, with
some over 1,000,000 cpm.
A: It
appears that you are considering restricted release for this site. For
example, placing a deed restriction on the property can prohibit excavation
in the area of the slab. This would certainly minimize the radiation dose
provided that the restrictions remain in place. However, your question is
right on target - how can you demonstrate compliance with the 100 mrem/y
dose criterion if controls/restrictions fail?
If you are willing to have the slab removed, then the answer is fairly
straightforward. That is, you could characterize the source term by removing
the pad and collecting soil samples and borehole readings. Remediation would
be low-tech. Excavating the soil and spreading the spoils in
10 to 30 cm lifts, and scanning with a NaI detector for gamma radiation
levels from the TENORM. Once the pad is removed, the chips are easily
detected provided that they are not buried more than 1 foot or so. A
high-tech, more costly option would be to use a soil sorter to segregate
clean soil from the TENORM chips.
The problem is not so easy if the pad must remain in place. Given the
discrete nature of the TENORM contamination (dime-sized chips), any sort of
statistical sampling regime might not be cost effective. [That is, if the
contamination were somewhat disperse, rather than in dime-sized chips, then
you could design a statistical sampling regime and collect soil samples by
coring through the concrete slab.] A very large sample size would likely be
necessary to provide sufficient confidence that the discrete contamination
does not result in a dose greater than 100 mrem/y if the controls were to
fail.
Bottom-line: It is unlikely that collecting a large number of core samples
will be as cost effective as simply removing the pad and excavating the
soil.
Submitted 10/17/02
Q:
How long is the liability period for a normal
Radioactive Waste Repository? If the wastes are long lived alpha bearing
materials such as containing Thorium will the liability period of the
Construction of the Repository similar.
A:
In general the owners/operators of a nuclear waste disposal site are
considered liable for that site forever. Any exceptions to this would come
through legally binding agreements which would document the transfer of
liability to another organization. Examples of this are as follows.
(1) Attempts have been made to hold the waste generator responsible for
the waste, rather than the repository operator. In general these
agreements have not been very successful because of the issues associated
with multiple generators using a single repository.
(2) Transfer of ownership/operation of the repository. In cases where
ownership of the repository changes, the liability usually also changes to
the new owner. Again, the new owner would retain liability for the waste
forever, or for as long as they owned the site or transferred it to someone
else.
(3) I believe there are some cases where by prior agreement ownership and
responsibility for the waste was transferred after a certain amount of time
to a state, or other government body. In these cases certain cleanup or
stabilization actions were required by the owner/operator, and specific
conditions met before the facility (repository) was turned back over to the
government. In this case the original owner might still be required to
retain responsibility for the stability and safety of the waste for at
least some extended period of time.
Specifically in the U.S., operators of these sites must accept that
operational liability. At one US waste disposal site (the Barnwell Site
located in South Carolina) it has been assumed that nearly 90% of all the
low level radioactive waste disposed of at its site will be decayed to
background levels in a period of 100 years. Once the site stops operation,
the control of the site will revert back to the local state government. The
state government has control of a fund established by the operator and the
state that is funded through a tax on waste shipped to that site for
disposal. The State of South Carolina then uses those funds to cap and close
the waste disposal sites as well as to maintain and repair the site cap as
needed into the future.
At some US Department of Energy sites that are undergoing decommissioning
using entombment, an institutional control period of 300 years has been
assumed for the use of this decommissioning option which is basically
creating a waste disposal cell. Many of these same US DOE sites also now
have on-site disposal cells at their sites for wastes from environmental
restoration projects and decommissioning projects. The disposal cells are
designed to have varying design lives but a typical site like the Fernald
Site disposal cell - is designed to limit migration of contaminants and
remain stable for 1,000 years to the extent reasonable, and in any case no
less than 200 years. Now that is for that one site that is taking lots of
uranium contaminated materials. By the way you might find this Web site
useful -
http://www.em.doe.gov/profiles/index.html - it describes the DOE
waste disposal cells that are being designed around the country at various
sites. Its a very useful website for you I believe. Now the same US DOE
also remediated many older uranium mining and milling sites in the US and
their design criteria for these disposal cells was - These facilities are
designed to meet the radiological protection criteria set forth in 40 CFR
192(Uranium Mill Tailings Remedial Action regulations for closure
standards).
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