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

5.8
releases so that substantial nuclide decay occurs before entering the geologic system where
the natural barriers would prevent or delay releases to the biosphere.
5.1.2.2 Waste Packages Components
Components of a generalized waste package were shown in Figure 5.1.2. The following
discussion addresses each component separately; however, it is the performance of the entire
system of components taken as a whole that is of most importance in the final analysis.
Waste Form
The waste forms include all radioactive materials that may potentially be sent to deep
geologic repositories, and are divided into three major categories: spent fuel, high-level
waste and TRU waste forms, which are described in more detail in Sections 4.3.2, 4.3.3,
and 4.3.4, respectively. The current primary emphasis on waste package design is for spent
fuel and for HLW, the reference waste forms considered throughout the following discussion.
Due to their high radiation levels and heat generation, spent fuel and HLW place the most
stringent requirements on the waste package. However, when most of the fission products
have decayed (after a few hundred years), the properties of the TRU waste become dominant.
The waste form is an inert solid designed to be chemically, thermally and radiolyti-
cally stable. The waste form itself is the first containment barrier for the waste.
Canister
The canister provides physical containment for the waste forms and thus isolates the
waste from near-field surroundings. The extent to which the canister can delay or minimize
waste-water interactions is important. Moreover, the canister is expected to provide physi-
cal protection during interim storage, transportation, handling, emplacement, and any waste
retrieval operations that may be required. The canister material chosen must be compatible
with the waste form. The ductility, weldability and impact resistance of metals make them
primary candidates as canister materials.
High-level waste forms will generally fill the canister 80 to 90% full. The remaining
space will be occupied by air. Stabilizer materials are being considered for use in spent
fuel canisters. Gaseous stabilizers, such as helium, have been considered from the stand-
point of providing a heat transfer medium without causing chemical or mechanical attack on
the spent fuel/cladding assembly or the canister. Particulate or solid stabilizers, such
as lead, glass, clay, or sand, can provide additional functions, including maintaining the
position of the spent fuel within the canister; preventing canister collapse under litho-
static pressures; acting as a corrosion resistant protective barrier; improving heat trans-
fer; increasing radiation attenuation; and enhancing nuclide sorption.
Overpack
The overpack is similar in principle to the canister. An overpack offers several
options to the package designer: it may function as a redundant canister, applied (if
necessary) for all stages of package handling, transportation, and emplacement; it can
exhibit corrosion or mechanical properties superior to those of the primary canister,