Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Radioactive metal
Reexamination Certificate
2000-12-15
2003-09-23
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Radioactive metal
C075S396000, C420S002000, C423S251000, C976SDIG009
Reexamination Certificate
active
06623712
ABSTRACT:
The present invention relates to a plutonium or plutonium alloy dissolution process which uses a dissolution medium composed of a mixture of nitric acid, a carboxylic acid and an amino compound.
The field of the invention may generally be defined as that of the conversion of alloyed or non-alloyed plutonium into plutonium oxide, particularly within the scope of the dismantling of plutonium contained in nuclear weapons with a view to its use in civilian nuclear reactors, particularly in the form of MOX fuel.
However, the process according to the invention can clearly be used within the scope of any application requiring the conversion and transformation of plutonium or one of its alloys.
The use of plutonium obtained by dismantling nuclear weapons in the form of MOX fuel for civilian nuclear electricity requirements is indeed one of the options studied on a global level for the “excess” stocks declared by the USA and the Russian Federation, each amounting to approximately 50 tonnes. This is the purpose of the AIDA/MOXI programme defined in the French-Russian Intergovernmental agreement of 12
th
November 1992.
The plutonium obtained by dismantling nuclear weapons is generally plutonium alloyed with gallium (Ga) with a gallium content of up to 5% by weight.
Plutonium contained in nuclear weapons to be burned in civilian reactors is generally recovered according to a processing pattern organised around the following main steps: denaturation of plutonium, conversion of plutonium into PuO
2
oxide and, finally, manufacture of MOX fuel.
As regards the conversion step, known processes to convert alloyed plutonium into PuO
2
or into (U,Pu)O
2
can be classified into three main categories:
dry processes;
pyrometallurgic processes;
aqueous processes.
In all cases, irrespective of the type of process, the process must particularly comply with the following constraints:
risk control;
high gallium purification;
supply of sinterable oxide compatible with MOX fuel manufacture;
effluent and waste limitation.
The main difficulty converting alloyed plutonium into PuO
2
or into (U,Pu)O
2
lies in the first conversion steps such as dissolution which have not been validated industrially.
In recent years, numerous studies have been conducted in this area by the French and Russians and by the Americans.
Dry processes include the HYDOX process used in the United States for conversion into PuO
2
.
This process comprises the two operating sequences: hydridation (HYD) of plutonium into PuH
2+x
and Ga, followed by oxidation (OX) of the hydride into oxide to produce plutonium oxide.
Both operations can be carried out either in the same apparatus or in two separate confinements which limits the risk related to the presence of the explosive mixture H
2
/O
2
.
The PuO
2
obtained from this treatment has the disadvantage of having a low sintering capacity and a high gallium contamination level.
Pyrometallurgic processes are based on the solubilisation of plutonium in a bath of molten chlorides followed by its conversion into PuO
2
or into (U,Pu)O
2
, by chemical displacement or electrolysis. Any gallium present is eliminated by distillation in the form of GaCl
3
at the start of the process.
The sintering capacity of the PuO
2
obtained is subject to caution and the management of the waste induced is not defined. For this reason, pyrometallurgic processes have known no industrial applications.
Aqueous process conversion processes which firstly comprise a dissolution step offer the advantages of operating at low temperatures and requiring minimum research and development.
Indeed, apart from the actual dissolution, all the steps subsequently used to produce MOX fuel, such as purification of the plutonium generally in the form of plutonium nitrate in solution, conversion into sinterable oxide, fuel manufacture, are known and perfectly controlled, particularly in France.
Numerous dissolution media have been proposed for alloyed or non-alloyed plutonium.
In this way, metal plutonium is dissolved rapidly at ambient temperature, at a rate of 30 to 50 mg/min/cm
2
and with a good yield (98%) in a dissolution medium composed of sulphamic acid (NH
2
SO
3
H).
Dissolution takes place according to the following reaction:
Pu+3H
+
→Pu3++3/2H
2
Said sulphamic acid process has been used for 25 years in the United States at Savannah River and in France at Valduc to recycle certain metal or alloyed plutonium waste.
However, dissolution with sulphamic acid involves a number of risks meaning that it cannot be used in current or future reprocessing plants. Said risks are essentially chemical risks related to a significant release of hydrogen and the potential and uncertain formation of pyrophoric compounds and industrial environmental problems related to the difficulty managing the effluents generated containing sulphate ions.
Dissolution of alloyed plutonium in a nitric medium was firstly dismissed since it results in the formation (passivation phenomenon) of a layer of adherent plutonium oxide on the metal surface which prevents the chemical reaction from continuing, as described in the document by J. M. CLEVELAND “Plutonium Handbook”, Chapter 13, WICK O. J Ed, ANS Publication, 1980.
Dissolution of alloyed plutonium with hydrochloric acid was studied in the document by A. S. POLYAKOV, J. BOURGES, A. BOESCH, E. CAPELLE, H. BERCEGOL, P. BROSSARD, P. BROS, B. SICARD and H. BERNARD “AIDA/MOX: Progress Report on the Research concerning the Aqueous Processes for alloyed Plutonium Conversion to PuO
2
or (U,Pu)O
2
”, Global 95, Versailles, France, 11-14 September 1995.
The use of a hydracide instead of an oxacide potentially prevents the oxidation of plutonium (III) into plutonium (IV) and eliminates the passivation phenomenon encountered in nitric medium. It is also possible to recycle HCl, by distillation, upstream from the process.
Experiments, conducted with 6 mol/l HCl, at ambient temperature, in a glass apparatus with identification of the type and measurement of the volume of gas released, demonstrated that 1.5 moles of hydrogen were produced per mole of plutonium dissolved, according to the reaction:
Pu+3HCl→Pu
3+
+3Cl
−
+3/2H
2
with little residue produced (≈2% by weight).
However, the dry insolubles are pyrophoric and the use of chloride ions appears to be difficult to manage on an industrial scale due to corrosion and effluent problems which have resulted in this process being abandoned.
Subsequently, dissolution in nitric medium of metal or alloyed plutonium was studied in-depth to remedy the disadvantages due to the formation of the passivating plutonium oxide layer.
In this way, the document by D. G. KARRAKER “Mechanism of Plutonium Metal dissolution in HNO
3
—HF—N
2
H
4
solution” DP-MS-85-45, Sep. 6, 1985 describes a process wherein the formation of the passivating layer is avoided by the use of a powerful reduction agent: hydrazine nitrate (NH
2
—NH
2
, HNO
3
).
This process offers the advantage of solubilising &agr; or &bgr; phase plutonium in nitric medium, with an acceptable rate (20 to 40 mg/min/cm
2
) while reducing the release of hydrogen to a yield less than 3% (H
2
/mole Pu)
However, the disadvantages of this process are that it uses undesirable fluoride ions ([HF]=0.1 M) for corrosion problems and generates compounds involving a chemical risk such as nitrohydric acid, HN
3
at a rate of 10
−3
moles of HN
3
per mole of Pu.
An improvement to this process is described in the document U.S. Pat. No. 5,135,728, which relates to a plutonium, particularly delta phase plutonium, dissolution process, by heating a mixture of nitric acid, hydroxyl ammonium nitrate (HAN) instead of hydrazine, and potassium fluoride, and immersing the metal in said mixture. Due to the replacement of hydrazine nitrate by hydroxyl ammonium nitrate, said process does not induce any risk of nitrohydric acid formation, but it remains penalising in terms of waste, since it introduces fluoride ions.
Another nitric dissolution process is referred to as ano
Adnet Jean-Marc
Bourges Jacques
Bros Pascal
Brossard Philippe
Bos Steven
Commissariat a l'Energie Atomique
Pearne & Gordon LLP
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