Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Radioactive metal
Reexamination Certificate
2000-09-07
2002-10-08
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Radioactive metal
C423S021100
Reexamination Certificate
active
06461576
ABSTRACT:
FIELD OF THE INVENTION
This invention is a method for treating spent nuclear fuel through the use of pyroprocessing.
BACKGROUND OF THE INVENTION
There is an ongoing problem concerning the treatment of nuclear waste based on uranium oxide nuclear fuel. Currently, the oxide fuel is reduced chemically by lithium to a metal in a molten salt. This head-end reduction step, with the elimination of oxygen, precedes an electrometallurgical process. There are some difficulties in the application of the lithium head-end process when applied to chopped oxide fuel where massive pellets of oxide inside Zircaloy cladding must be attacked by the reductant. Among these are the inability of the lithium to penetrate the massive pellets in a reasonable time and consequently, the need to remove the cladding and reduce the size of the massive pellets. Another problem fuel is Thorium oxide, ThO
2
, clad with zirconium. Lithium will not reduce this oxide.
Applicants have found that massive solids of ThO
2
and UO
2
will dissolve in a select reagent of LiF-50 mol % ZrF
4
(eutectic mp-500° C.). As a result, the cited reagent is effective in treating spent nuclear fuel of the form UO
2
—ThO
2
—ZrO
2
in a ceramic matrix and having a Zr cladding and other oxide residues containing Transuranic (TRU) Oxides.
Thus, the objective of this invention is to provide a reagent to chemically attack spent nuclear fuel in particular, but not limited to, fuel from light water reactors (LWR) or light water breeder reactors (LWBR).
Another objective is to combine the initial treatment of the spent nuclear fuel with plutonium recovery.
Additional advantages, objects and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.
SUMMARY OF THE INVENTION
To achieve the foregoing and other advantages, this invention is a method for treating spent nuclear fuel through the use of a pyroprocessing technique which is carried out in one vessel in a single molten salt bath. The chosen reagent is LiF-50 mol % ZrF
4
(eutectic mp 500° C.), but other ZrF
4
rich salt could be used. The reagent is heated to a temperature between 550-650° C. where is in a molten liquid state. The spent nuclear fuel is chopped into pieces and placed in a basket which is lowered into the molten salt. The products of the reaction between the salt and the spent nuclear fuel, in the form of UO
2
and ThO
2
, are UF
4
, ThF
4
and ZrO
2
. All of the products of the reaction dissolve in the liquid salt and thus, are effectively separated from the solids which remain in the basket. These solids include PuO
2
and the noble fission product elements (Mo, Ru, Tc, Rh, Pd, etc). The liquid salt together with the dissolved constituents are subjected to hydrofluorination to convert the ZrO
2
to H
2
O gas which is separated from the salt, and ZrF
4
which remains with the salt bath. To recover the uranium product, the salt bath is then subject to fluorination yielding a gaseous UF
6
product. The residual salt bath is then either recycled or converted to a fluorapatite waste. The Pu remaining in the basket can be recovered using a Pu recovery process.
REFERENCES:
patent: 3981960 (1976-09-01), Brambilla et al.
patent: 5202100 (1993-04-01), Nagel et al.
Miller William E.
Tomczuk Zygmunt
Bos Steven
Caress Virginia B.
Dvorscak Mark P.
Smith Bradley W.
The United States of America as represented by the United States
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