Environmental Engineering Reference
In-Depth Information
Impact limiter
Neutron shielding shell
Neutron shielding
Closure Lid
Outer steel shell
Lead Gamma shielding
Inner steel shell
Basket
Figure 8.6 Generic truck cask (Courtesy of the Nuclear Regulatory Commission)
both the steel and the lead provide shielding that absorbs gamma rays emitted
by the used fuel. The closure lid is actually a very complex component, usually
composed of an inner and outer lid and many fasteners. It must be removed to
load and unload the used fuel, but it must also provide an air-tight seal during
transportation. The outer neutron shielding shell is typically made of a polymer
that has high hydrogen content. Water is sometimes used. On the ends of the
cask are impact limiters. Made of balsa wood or some synthetic material that
will collapse slowly when struck, the impact limiter is designed to cushion the
cask in the event of an accident.
Figure 8.7 is an illustration of a generic rail cask for shipping used nuclear
fuel. It is similar in structure to the truck cask but can weigh about 250,000
pounds, have a diameter of 8 feet, and be about 25 feet long.
In 1998, the Oak Ridge National Laboratory issued, for the Department of
Energy, a document titled “The Radioactive Materials Packaging Handbook”
(Shappert 1998). It provided, in one document, the information that a company
designing and building a Type B cask would need to manufacture a cask that
could meet all applicable NRC and DOT requirements. Authors of the document
took into account not only the regulations but also fabrication and operational
experience with earlier Type B casks, since any difficulties with fabrication and
operation can often be overcome by design modifications.
Factors to be considered in cask design include the characteristics of the mate-
rial to be shipped, not only the type and amount of radiation emitted but also
the chemical and physical form of the material as well as its mass, density, and
volume. In addition, heat generated by the used fuel must be considered. The
total number of shipments to be made in any one cask is also important. If each
 
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