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

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heated to above 100 C) (EPA 1973). Although disposal strata containing more inert minerals,
particularly quartz-rich sandstones suitable for deep well liquid injection, might sustain
higher temperatures, thermal effects on containment formations, which may include temperature
sensitive minerals, would also have to be considered.
Deep Well Injection
In the deep well injection concept, the liquid wastes would be fed into porous or frac-
tured strata, such as depleted hydrocarbon reservoirs, natural porous strata, or zones of
natural or induced fractures. To protect freshwater aquifers from waste contamination, the
injection zone would have to be well below the aquifers and isolated by relatively imperme-
able strata, e.g., shales or salt deposits.
In general, injection requires pressure at the wellhead, although in some circumstances
gravity feed is sufficient. The controlling factors are the rate of injection and the perme-
ability of the disposal formation. The increase in the total fluid volume in an injection
zone is accommodated by compression of any fluid already present and expansion of the rock
formation. The relation between injection rates and pressures is based on extensive
oil-well and ground-water experience. Injection is possible at depths down to several
thousand meters.
For this concept, the activity of the injection waste has been assumed to be controlled
by the allowable gross thermal loading, the injection zone thickness, and the porosity in
that zone. It is also assumed that one injection zone with two wells would be used at each
site. In the long term, the waste might progressively disperse and diffuse throughout the
host rock and eventually encompass a large volume. The concentration might be variable and
unpredictable. Thus, criteria for permissible activity levels might be required. Determina-
tion of the dilution requirement is complicated by the sorption of nuclides onto the mineral
skeleton, to an extent determined by waste chemistry and rock mineral content. If sorption
were too high, concentration of heat-generating components might result in "hot spots".
Injected waste might be partially retrieved by drilling and pumping, but sorption of
nuclides onto the mineral skeleton and precipitation within the pores would limit the amounts
recovered.
Predisposal Treatment. In deep well injection, spent fuel would be shipped to a proces-
sing facility at the well injection site. The spent fuel would be dissolved in acid and the
hulls removed. (For recycle, the uranium and plutonium would be removed from the acid solu-
tion.) The acid solution would constitute the basic waste form for isolation.
The acid waste from reprocessing would contain both fission products and actinides. Be-
tween 60 and 75 percent of the heat generated in the initial emplacement years would be due
to 9 0 SR and 1 3 7 Cs. Partitioning strontium and cesium from the remainder of the waste