Nuclear fuel reprocessing using hydrophilic substituted...

Details Nuclear fuel reprocessing using hydrophilic substituted... Nuclear fuel reprocessing using hydrophilic substituted...

2000-07-11

2002-09-03

Bos, Steven (Department: 1754)

Chemistry of inorganic compounds

Treating mixture to obtain metal containing compound

Radioactive metal

Reexamination Certificate

active

06444182

ABSTRACT:

This invention relates to nuclear fuel reprocessing and is particularly concerned with the separation of uranium from plutonium and neptunium.
Most commercial reprocessing plants use the Purex process, in which the spent fuel is dissolved in nitric acid and the dissolved uranium and plutonium are subsequently extracted from the nitric acid solution into an organic phase of tributyl phosphate (TBP) dissolved in an inert hydrocarbon such as odourless kerosene. The organic phase is then subjected to solvent extraction techniques to partition the uranium from the plutonium.
More particularly, the organic phase is subjected to separation of fission products by solvent extraction and in some cases then to removal of technetium, before the so-called U/Pu split. In the U/Pu split, Pu(IV) is reduced to Pu(III) which is inextractable into the organic phase and therefore follows the aqueous stream while the U remains in the organic stream. Usually, the reducing agent used in the U/Pu split is U(IV). Np(VI) in the solvent stream is also reduced by the U(IV) to Np(IV). Np(IV) is extractable into the solvent and so exits the contactor in the solvent steam with the U product. Hydrazine nitrate is normally used to stabilise the U(IV) and Pu(III) against oxidation by, in particular, HNO
2
. The unit for carrying out the partitioning of the U and Pu in practice comprises a contactor having a multiplicity of stages, for example six stages might be used in a modern centrifugal contactor.
There are disadvantages with such a process:
hydrazine is catalytically decomposed by Tc(VII) ions
under certain conditions hydrazine can form undesirable oxidation products (e.g. ammonium salts)
U(IV) must be produced in a separate process on plant, thus increasing costs
two reagents are needed
Np is not separated from U so additional downstream processes are needed to remove Np from U.
It is also a disadvantage of current commercial Purex processes that they use a three cycle flowsheet [(1) the so-called HA cycle in which fission products are separated and the U/Pu split is performed; (2) the UP cycle in which the uranium stream is purified; (3) the PP cycle in which the plutonium stream is purified]. It is therefore desired to develop an Advanced Purex process in which there is a single solvent extraction cycle.
Moreover, neptunium valency control can be a significant problem in Purex reprocessing. Neptunium is present in the Purex process as a mixture of three different valence states: Np(IV), (V) and (VI). Np(IV) and (VI) are both extractable into the solvent phase whereas Np(V) is in-extractable into this phase. In order to direct Np to raffinate streams, Np is normally stabilised in the (V) oxidation state. This is a complex matter, since not only is it the middle oxidation state of three but Np(V) also undergoes competing reactions, such as disproportionation to Np(IV) and (VI) and is oxidised to Np(VI) by nitric acid. Neptunium control is therefore difficult and efficient neptunium control is a major aim of an advanced reprocessing programme. In commercial Purex reprocessing plants, Np is typically separated from uranium during the uranium purification (UP) cycle. Np(IV) is converted to Np(V) and Np(VI) by heating in the aqueous phase in a conditioner at a high temperature. The conditioned liquor is fed to an extract and scrub mixer-settler where the Np(V) is rejected to the aqueous raffinate. Any Np(VI) present in the aqueous feed is reduced to Np(V) by hydroxylamine which is fed to the scrub section of the contactor. In a typical process, two or three mixer-settlers are required to decontaminate the uranium product from Np.
Numerous studies have been conducted to find replacements for the U(IV)+hydrazine system with an efficient reductant for Pu(IV) and Np(VI). Amongst the reductants studied have been butyraldehydes, hydroquinones, substituted hydroquinones and substituted hydroxylamines, such as N-methylhydroxylamine and N,N-diethylhydroxylamine (Yu-Keung Sze, Gosselin Y. A,
Nucl. Technology,
1983, vol 63, No. 3, pp 431-441; Koltonov V. S, Baranov S. M.,
Radiokhimiya,
1993, vol 35, No. 6, pp 11-21; Koltunov V., Baranov S., International Conference on Evaluation of Emerging Nuclear Fuel Cycle Systems (Global-95), September 1995, Versailles, France, Proceedings, vol. 1, pp 577-584).
A disadvantage of such known Np and Pu reduction processes is that their kinetics are potentially too slow for the short residence times of centrifugal or other intensified contactors which would be used in a modern Purex reprocessing plant. In particular, it is difficult to find reducing agents that rapidly reduce Pu(IV) and this is even more difficult if centrifugal contactors are to be used.
It has now been found that the incorporation of an OH (hydroxyl) group into organic derivatives of hydroxylamine will surprisingly increase the rate of Np(VI) and Pu(IV) reduction.
The present invention provides a spent fuel reprocessing method in which an organic phase containing Np(VI) is contacted with a hydrophilic substituted hydroxylamine in which at least one N—H hydrogen of the hydroxylamine is replaced by an organic substituent and there is at least one such organic substituent having one or more hydroxyl groups. The organic substituents are preferably alkyl groups or, in the case of hydroxyl-containing substituents, hydroxyalkyl and especially mono-hydroxyalkyl. Suitable alkyl groups include those having up to 4 carbon atoms but methyl and more particularly ethyl are preferred. The most preferred substituted hydroxylamine is ethyl(hydroxyethyl)hydroxylamine (EHEH) which has the formula HOC
2
H
4
(C
2
H
5
)NOH. The substituted hydroxylamine reduces Np(VI) to Np(V), which may then be backwashed into an aqueous phase.
Preferred reductants are hydroxyalkyl hydroxylamines of the formula
wherein
R
1
is H or C
1
-C
4
alkyl, and
R
2
is C
1
-C
4
hydroxyalkyl.
The organic phase preferably contains U(VI) and Pu(IV) as well as Np(VI), in which case the EHEH reduces the Pu(IV) to inextractable Pu(III), which may be backwashed into the aqueous phase together with the Np(V). At least in preferred processes using intensified contactors such as centrifugal contactors which have very short residence times, especially those using EHEH, no reduction of Np(V) to Np(IV) occurs so no downstream purification of the U product stream is necessary. It is a significant advantage of, in particular EHEH that the rate of Np(V) to Np(IV) reduction is low compared with the rate obtained using other reducing agents such as U(IV), C
6
H
5
NHNH
2
and ascorbic acid. We have found that when [HNO
3
]=2.0M, [reductant]=0.2M and T=45° C. that pseudo first order rate constants for Np(V) reduction are:
U (IV)
k
1
= 0.38 min
−1
C
6
H
5
NHNH
2
k
1
≅ 0.027 min
−1
Ascorbic acid
k
1
= 0.071 min
−1
EHEH
k
1
= 0.0024 min
−1
If downstream purification of the U product stream should be necessary or desirable, the treated solvent (organic) phase may then be contacted with a hydrophilic complexant-reductant (preferably formohydroxamic acid) to complex with any Np(IV) and reduce any Np(VI), which is backwashed into a second aqueous phase (WO 97/30456 and The applications of formo- and aceto-hydroxamic acids in nuclear fuel reprocessing, R. J. Taylor, I. May, A. L. Wallwork, I. S. Denniss, N. J. Hill, B. Ya Galkin, B. Ya Zilberman and Yu. S. Fedorov, J. Alloys & Cpds., forthcoming).
In a preferred class of methods, the organic phase is contacted with the EHEH or other substituted hydroxylamine and the Pu(III) plus Np(V) are backwashed in a first contactor unit into the aqueous phase. U as U(VI) remains in the organic phase and may then be backwashed into dilute nitric acid in a subsequent contactor in the usual manner. The contactors are suitably multi-stage contactors.
The invention includes a method of reducing a species selected from Np(VI) and Pu(IV) to, respectively, Np(V) and Pu(III), which method comprises contacting the species with a substituted hydrox

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