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

Waste-Type Compatibility
6.30
It is assumed for the reference case that only liquid HLW and liquid RH-TRU would be
injected into the rock melting cavity. Because of uncertainties associated with emplacement,
such as additional criticality concerns, and a sufficient heat generation rate for the
volume, spent fuel is not considered suitable for this reference case. Therefore, spent fuel
and other wastes that may have low heat generation per unit of volume, such as solid RH-TRU
and CH-TRU, are assumed to be sent to a geologic repository. Note that the suitability of
spent fuel and other wastes for rock melt disposal may be improved by safely and economically
putting them into a slurry form.
Waste-System Description
Basically, rock melting would work in the following manner. In the charging phase, HLW
in aqueous solution would be injected into a mined cavity. The heat generated by the radio-
active decay of the waste would drive off steam, which would be piped to the surface. When
the boil-off rate reached a certain level, liquid transuranic wastes would be added to the
charge. Periodically, high-pressure cleaning water would be flushed through the injection
piping to minimize contamination and solid particle buildup. This cleaning water would also
flow into the waste, providing a coolant to prevent the rock from melting during the waste
charging phase. Cooling would be by evaporation or the heat of vaporization. At the surface,
the steam driven off from the waste would be condensed and recirculated to cool the charge in
the cavity. The closed system would be designed to prevent the release of radioactivity to
the environment (Bechtel 1979a).
After about 25 years, when a substantial fraction of the cavity volume was filled, charg-
ing would be stopped. After the water was allowed to boil off and the waste to dry, the in-
let hole would be sealed. The cavity temperature would rise rapidly and rock melting would
begin, with radioactive materials dissolving in the molten rock. As the mass of molten rock
grew, its surface area would expand and the rate of conductive heat loss to the surrounding
rock would increase. Preliminary calculations indicate that at about 65 years, the rate of
conductive heat loss from the melt pool would exceed the rate of heat input from radioactive
decay. At this point, the melt would begin to slowly solidify. During the rock melting
phase, the heat from the melt would inhibit ground water from entering the area and should
prevent the leaching of the radionuclides. This is referred to as the "heat barrier" effect
(DOE 1979). Following resolidification, when the heat barrier had dissipated, fission
products would have decayed to very low levels. The relative toxicity of the residual radio-
nuclides in the solidified waste-rock matrix is expected to be significantly less on a volu-
metric basis than that of a typical uranium ore from which nuclear fuel was originally
extracted. The final product of the melt is expected to be a relatively insoluble sphere or
resolidified silicate rock conglomerate, with a highly leach-resistant matrix, which would be
deeply isolated from the biosphere (Bechtel 1979a).