Nuclear fuel reprocessing

Details Nuclear fuel reprocessing Nuclear fuel reprocessing

1998-08-07

2000-07-25

Dunn, Tom

Chemistry of inorganic compounds

Treating mixture to obtain metal containing compound

Radioactive metal

423 9, 423 10, B01D 1100

Patent

active

060933755

DESCRIPTION:

BRIEF SUMMARY
This invention relates to nuclear fuel reprocessing and is particularly concerned with the control of neptunium in the reprocessing of spent fuel.
Reference will be made hereinafter to the use of the present invention in the Purex Process in an Advanced Reprocessing Plant (ARP). However, the present invention may have application in other processes for reprocessing spent fuel.
In Purex reprocessing, neptunium valency control can be a significant problem. 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 (tributyl phosphate (TBP) diluted in an inert hydrocarbon such as odourless kerosene (OK)) whereas Np(V) is inextractable into this phase. In order to direct Np to raffinate streams, Np has to be 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.
After fuel dissolution, Np is likely to be present as a mixture of all three oxidation states. Np(V) will be separated with the aqueous phase at an earlier stage. Np(IV) and (VI) will follow the solvent (containing uranium and plutonium) into the so-called U/Pu split. In the U/Pu split, Np is reduced by U(IV) to Np(IV) which follows the uranium stream into the solvent product. Np is then separated from uranium during the uranium purification clyde. 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(IV) is rejected to the aqueous raffinate. Any Np(VI) present in the aqueous feed is reduced to Np(V) by hydroxylarmine 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.
It has now been surprisingly discovered that formohydroxarnic acid (FHA) may be used to control neptunium in spent fuel reprocessing. According to the present invention there is provided a spent fuel reprocessing method which includes at least the formation of an aqueous solution and at least one solvent extraction step, characterised in that formohydroxamic acid is used to reduce any Np(VI) to Np(V) and/or to form a complex with Np(IV) whereby substantially all the neptunium present will be retained in the aqueous phase during solvent extraction.
A particularly useful property of FHA in the context of the present invention is that it is easily destroyed either by acid hydrolysis to formic acid and hydroxylamine or by nitric acid to component gases. Therefore the Np can be recovered from the FHA solution and the destruction of FHA will reduce radioactive liquid waste volumes.


BRIEF DESCRIPTION OF FIGURES

FIG. 1 is a flowsheet for second embodiment of the invention.
FIG. 2 illustrates the Np Polish contactor.
FIG. 3 is a graph of [HNO2] vs. time.
FIG. 4 is the absorption spectra between 920 and 1280 nm before and after addition of FHA solution in 1.7M nitric acid.
FIG. 5 is a graph of Np(VI) concentration vs. time after the addition of excess FHA.
FIG. 6 illustrates graphically the reaction between Np(V) and FHA at room temperature in a solution containing 1.7M HNO3 and 0.333M FHA.
FIG. 7 is the near infra-red spectrum of the Np(IV)-FHA complex and Np(V) in 0.1 M nitric acid.
FIG. 8 shows the effect of increasing FHA concentration on the Np(IV) distribution value in 1M HNO3.
The kinetics of the hydrolysis of FHA have been determined. In HNO.sub.3 the rate of reaction is: ##EQU1## where k=2.54.times.10.sup.-4 dm.sup.3 mol.sup.-1 s.sup.-1 and the energy of activation E.sub.ACT is 77.3.+-.1.6 kJ/mol
The hydrolysis of FHA is a potential problem in solvent extraction contactors which have long residence times such a

REFERENCES:
patent: 3900551 (1975-08-01), Bardoncelli et al.
patent: 4229421 (1980-10-01), Chapman et al.
patent: 4659551 (1987-04-01), Kolarik et al.
Nuclear Energy, vol. 26, No. 4, Aug. 1987, London, GB, pp. 253-258. V.A. Dracke: "Predicting the behavior of the neptunium during nuclear fuel reprocessing".
JOM, vol. 45, No. 2, Feb. 1993, Warrendale, US, pp. 35-39. Mac Toth et al.: "Aqueous and pyrochemical reprocessing of actinide fuels".

Affiliated with

Also associated with

Rating

Nuclear fuel reprocessing does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Nuclear fuel reprocessing, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Nuclear fuel reprocessing will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-1334533