Page 221 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

NUCLEAR WASTE STORAGE 199

3.32.2 Cask Design for Permanent Storage
In addition to the unavoidable material aging because of exposure to radiation from
the radioactive material and hence corrosion is expected to be a concern in the
long-term storage of nuclear waste. It has been further suggested that heat generation
from radiation can drive the corrosion rate higher. Many cask designs have been
proposed, and each with different materials of construction. The most common
proposed materials are carbon steel, stainless steel, and concrete construction.
At the present time, all the nuclear waste generated is solid waste. Hence, the waste
is relatively noncorrosive, which minimizes the risk for internal corrosion damage to
storage and transportation banks. There is, however, a significant amount of old liquid
nuclear waste in storage, which can corrode the containers internally. In addition,
the presence of water in the solid waste could potentially cause corrosion problems.
External corrosion is a potential problem, because the older liquid waste is stored in
buried tanks, and these tanks are therefore exposed to groundwater. The consequences
of leaks are numerous and hence the structural integrity of the storage containers must
be assured for centuries.
The potential for corrosion of permanent storage canisters has been and continues
to be under investigation. A literature review and summary of plutonium oxide and
metal storage packaging failures was published (54). Metal oxidation in nonairtight
packages with gas pressurization was identified as the most common mechanism of
packaging failure. An example of a possible corrosion problem was further described
in a study on hydrogen/oxygen recombination and generation of plutonium storage
environments (55). There are also literature citations available with respect to the
predicting of service life of steel in concrete used for the storage of low-level nuclear
waste (56).
In a September 2000 meeting on key technical issues regarding container life, the
US Nuclear Regulatory Commission (NRC) and representative of the DOE discussed
the ongoing research into the effects of corrosion processes on the lifetime of the
containers (57).
In the above meeting, a wide range of material issues that designers are facing were
discussed. In nuclear waste containers, both corrosion from the inside and from the
outside should be considered. The issues included, but were not limited to, general
and localized corrosion of the waste package outer barrier; methods for corrosion
rate measurements; documentation on materials such as Alloy 22 and titanium; the
influence of silica deposition on the corrosion of metal surfaces; passive film stability,
including that on welded and aged material; electrochemical potentials; MIC; stress
distribution because of laser peening and influence on rock fall impact strength; and
dead load stressing and the effects of fabrication sequence and welding.

3.32.3 Effect of Location on Corrosion of Nuclear Storage Containers

The current plans for a permanent nuclear storage repository are to build it at a
relatively dry site at a depth of several hundred meters below earth’s surface. It is
thought that the presence of water will eventually corrode the storage containers.