Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Radioactive metal
Reexamination Certificate
2001-02-26
2003-02-11
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Radioactive metal
C423S158000, C423S166000, C423S181000, C423S184000
Reexamination Certificate
active
06517788
ABSTRACT:
DESCRIPTION
1. Technical field
The present invention relates to a method for separating caesium, strontium and transuranium elements present in sodium waste, and to a device for implementing this method.
The present invention may, for example, be applied to the treatment of radioactive sodium waste derived from nuclear waste treatment.
In particular it provides the possibility of limiting the inventory of materials involved to separate or isolate the above-cited elements from the waste, of controlling and managing chemical reaction time, of conducting separation as a continuous process and of guaranteeing the safety of personnel and equipment throughout the separating process.
It also provides for optimum adjustment of the quantities of reagents added, and limits undesired chemical reactions due in particular to the instability of these reagents.
2. Prior art
A method for decontaminating alkaline waste of caesium, based on the insolubility of caesium tetraphenylborate, was implemented in a single precipitation reactor with a long stay time in conjunction with a liquid-solid separation by tangential filtration.
However, the reaction is very difficult to control since tetraphenylborate is unstable and implementation of the method is accompanied by a strong release of benzene and other unstable aromatic compounds in the reactor, jeopardising the safety of the method. Moreover, the quantity of reagents used is high and is reflected in the cost of the decontamination process.
DESCRIPTION OF THE INVENTION
The purpose of the present invention is precisely to overcome the above-cited disadvantages by providing a method to separate caesium, strontium and tranuranium elements contained in sodium waste, which entails the use of TPB
−
.
The method of the invention comprises, in line, the following successive steps:
(a) filling at least one of two feed tanks with the waste,
(b) analysis of the content of caesium, strontium and other transuranium elements present in the waste,
(c) pre-treatment of the waste consisting of, in one of the feed tanks filled with the waste, mixing said waste with an insolubilising agent for strontium and transuranium elements in a quantity adapted in particular in relation to the analysis of step (b), mixing being conducted for a first stay time and at a first temperature that are adequate for insolubilising the strontium and transuranium elements, said pre-treated waste forming a first suspension.
(d) an optional separation step to separate, from the first suspension, the insolubilising agent bound to the strontium and the transuranium agents, and to obtain a first liquid phase, free of said insolubilising agent, containing the caesium and sodium in solution.
(e1) a first, continuous, caesium separation treatment conducted on the first suspension derived from step (c) or on the first liquid phase derived from step (d), consisting of:
firstly mixing, in a first precipitation reactor, said first suspension or said first liquid phase with TPB
−
in appropriate quantity relative in particular to the analysis of step (b), and to a first measurement of the caesium content made at step (f1) downstream from this first separation treatment, mixing being conducted for a second stay time and at a second temperature adequate to precipitate the caesium, contained in the first suspension or in the first liquid phase, with the TPB
−
and to limit degradation of the TPB
−
, so as to obtain a second suspension containing, in suspension in a second liquid phase, the insoluble TPB
−
containing the TPB
−
bound to the caesium, and if present the insolubilising agent bound to the strontium and transuranium elements, and
secondly, separating from the second suspension the insoluble TPB
−
containing the TPB
−
bound to the caesium, and if present the insolubilising agent bound to the strontium and transuranium elements, in order to obtain the second liquid phase, said first precipitation reactor being continuously supplied with TPB
−
and the first suspension or the first liquid phase through the alternate use of the different feed tanks for said supply,
(f1) the first measurement of the caesium content remaining in said second liquid phase,
(g) recovery of the insoluble TPB
−
containing the TPB
−
bound to the caesium, and if present of the insolubilising agent bound to the strontium and transuranium elements derived from the preceding steps, and optionally,
(h) extraction treatment of the caesium bound to the recovered TPB
−
.
The method of the invention may also, between steps (f1) and (g), comprise the following steps:
(e1a) a second continuous separation treatment of the caesium conducted on the second liquid phase derived from step e1) consisting of:
firstly mixing, in a second precipitation reactor, said second liquid phase with TPB
−
in appropriate quantity relative in particular to the first measurement of step f1) and to a second measurement of the caesium content made at step f1a) downstream from this second separation treatment, mixing being conducted for a third stay time and at a third temperature suitable for precipitating the caesium present in the second liquid phase with the TPB
−
and for limiting degradation of the TPB
−
, such as to obtain a third suspension containing, in suspension in a third liquid phase, the TPB
−
bound to the caesium, and
secondly, separating from the third suspension the insoluble TPB
−
containing the TPB
−
bound to the caesium in order to obtain said third liquid phase, said second precipitation reactor being continuously supplied with TPB
−
and the with the second liquid phase derived from the first separation treatment,
(f1a) said second measurement of the caesium content in said third liquid phase.
TPB
−
, for example in the form of sodium tetra-phenylborate, may be used to extract caesium from a solution. However, this reagent is easily degraded into benzene and other aromatic products, in particular by radiolysis in contact with the radioactive elements present in the waste and by chemical reaction with various constituents of said waste. With the method of the present invention it is possible, among others, to better manage the caesium separation process using TPB
−
than with the methods of the prior art, to control the quantity of TPB
−
used and hence to limit benzene emissions.
If TPB
−
is used in NaTPB form, persons skilled in the art will easily understand that the TPB
−
-precipitate recovered at step (e1) and optionally at step (e1a) also contains NaTPB.
With the present invention, it is possible to conduct the continuous separation of caesium, in particular in solid CsTPB form, from sodium waste while ensuring the necessary safety guarantees for the treatment of radioactive solutions.
At step b) of the method of the invention, analysis may be carried out using any technique known to persons skilled in the art to determine the quantity of each of the above-cited elements, for example by means of elementary analysis such as emission spectrometry, or for example by &ggr; or &bgr; spectroscopy for caesium 137 (
137
Cs).
Preferably, the first analysis is made after homogenisation of the waste in the feed tank. Homogenisation may be conducted mechanically.
Pre-treatment of the sodium radioactive waste at step c) makes it possible to insolubilise the strontium and transuranium elements using a first insolubilising agent. This agent is preferably chemically stable in the presence of the sodium waste and radioelements it contains. It is added in sufficient quantity to fix the above-cited elements. This quantity is determined and adjusted in relation to the analysis made at step b).
According to the invention, the insolubilising agent may be chosen from among any agent which adsorbs and precipitates strontium and transuranium elements, such as for example a titanate, a zeolite, barium sulphate or a mixture thereof. This agent may for example be monosodium titanate.
This agent may als
Debreuille Marie-Françoise
Hubert Nathalie
Moulin Jean-Paul
Bos Steven
Compagnie Generale des Matieres Nucleaires
Pearne & Gordon LLP
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