Confinement of caesium and/or rubidium in apatitic ceramics

Details Confinement of caesium and/or rubidium in apatitic ceramics Confinement of caesium and/or rubidium in apatitic ceramics

2001-05-30

2002-12-03

Silverman, Stanley S. (Department: 1754)

Hazardous or toxic waste destruction or containment

Destruction or containment of radioactive waste

By fixation in stable solid media

C588S014000, C588S015000, C588S016000, C588S020000

Reexamination Certificate

active

06489531

ABSTRACT:

TECHNICAL FIELD
The present invention relates to the aim of a method for fixing caesium and/or rubidium in a mineral phase of durable containment. More precisely it relates to the fixation of radioactive caesium and rubidium resulting from the reprocessing of irradiated fuels.
The caesium resulting from the reprocessing of irradiated fuels is a long half-life fission product with extremely high volatility and diffusibility. It is thus necessary to fix it in extremely stable matrices.
In the extraction solutions issuing from reprocessing installations of spent fuels, caesium is present under the form of the following isotopes:
135
Cs,
137
Cs and
133
Cs.
Thus, in a solution obtained from a fuel UO
x
irradiated at 33000 MWj/t, with decay time of 3 years, these three isotopes are found in the quantities shown in the following table 1.
TABLE 1
Quantities
Average
formed
energy
Isotopic
Isotope
Half-life
(g.t-1)
(KeV)
content
135
Cs
2.3 · 10
6
yr
 357
 56.3
10%
137
Cs
30 yr
1130
174.3
31%
133
Cs
stable
/
remainder
Because of the long-term storage, it is thus necessary to condition the caesium in a matrix which is physically and chemically stable. In fact, this element, not inserted in the network of a defined structure not incorporated in the network of an unstable structure, tends to diffuse outside the matrix under the influence of outside agents, water in particular. Furthermore, during the production of a conditioning material, carried out at high temperature, caesium, which is very volatile, is difficult to incorporate into the matrix.
Effective storage of caesium requires its incorporation into a solid matrix which resists transport, irradiation, which is thermally stable and is inert in geological storage conditions or in long-term storage.
PRIOR ART
The present policy for conditioning non-separated nuclear waste is vitrification in borosilicate glass. In this case, the caesium is processed together with the other waste whereas it would be of great interest to have specific matrices, specially adapted to the containment of caesium separated from the other waste and/or rubidium.
More recently, it was envisaged conditioning radioactive wastes by coating them in an apatitic matrix possibly containing the actinides and lanthanides to be conditioned, as described in WO-A-95/02886 [1]. However, these matrices were not designed for conditioning caesium and/or rubidium specifically and separately whereas the policy developed nowadays is to manage long-lived radionuclides separately such as Cs. These radionuclides can be separated during reprocessing, under the form of aqueous solutions of caesium and/or rubidium nitrate or carbonate.
DESCRIPTION OF THE INVENTION
The present invention has the specific aim of providing a containment matrix intended for the conditioning of caesium and/or rubidium, with confirmed long-term durability and stability, and which guarantees not only the containment of the waste but also the protection of the environment.
According to the invention, the material for containing the radioactive caesium and/or rubidium comprises a phosphosilicated apatitic matrix including in its chemical structure the radioactive caesium and/or rubidium to be contained, this apatite corresponding to the following formula:
M
t
Ca
x
Ln
y
(PO
4
)
6−u
(SiO
4
)
u
X
in which:
M represents Cs and/or Rb,
Ln represents at least one trivalent cation,
X represents at least one anion chosen from among 2F
−
, S
2−
, 2Cl
−
, 2Br
−
, 2I
−
, 2OH
−
and O
2−
, and
t, x, y, and u are such that:
0<t·2.5
2·x·8
1·y·7
0·u·6
x+y+t=10,
and the total number of positive charges provided by the cations M, Ca and Ln are equal to (20+u).
The utilisation in this containment material of a phosphosilicated apatite matrix is very interesting. In fact,
135
Cs is a &bgr;
−
emitter not producing any damage to this matrix, which thus remains stable to these emissions. Furthermore, since the caesium is incorporated in the same network as the apatite, it is thus fixed and cannot diffuse through this matrix. Finally, since apatite is an extremely stable material thermally (up to 1200° C.), the
137
Cs thermal effect will be of no consequence. As a result,isotopic separation between
135
Cs and
137
Cs will not be necessary within the framework of caesium conditioning.
Moreover, apatites have very low solubility in water, which diminishes when the temperature rises. This is a positive point for caesium conditioning, since the
137
Cs present in the waste, has high thermal power, which implies a rise in temperature of the matrix containing it, but in the case of apatite, will reduce the solubility of the latter in water.
According to the invention, in order to evacuate better the heat involved by the presence of
137
Cs in the apatite, its thermal conductivity can be raised by a slight substitution of iron, that is by using, for example, for Ln in the formula given above, a lanthanide and iron.
Phosphosilicated apatites corresponding to the formula given above are especially intended for caesium conditioning, but they are also appropriate for conditioning rubidium. This is particularly interesting in the case where it is not possible to separate the rubidium and caesium present together in an aqueous solution.
According to the invention, the quantity t of caesium and/or rubidium included in the apatite matrix can be varied from 0 atoms per mesh to 2.5 atoms per mesh. A quantity lower than 0.1 atoms per mesh is not of much interest since this formulation only corresponds to 1% by mass of caesium. Generally it is preferable for the quantity of Cs and/or Rb to be less than 1.5 atoms per mesh (t≦1.5) since an apatite containing a greater quantity, higher than 1.5 atom per mesh is difficult to synthesise because of the high size of the caesium ion.
In this case, there is the risk that the caesium does not enter the apatite network in substitution but in insertion. As a result, because of its high mobility, the caesium would be less linked to the network and could diffuse through the matrix.
Preferably, according to the invention, one uses an apatite in which u is equal to 1, that is a composition containing 5 phosphate groupings and 1 silicate grouping, since studies on the phosphate-silicate solid solution, that is for u ranging between 0 and 6 have shown that the caesium is incorporated best within the apatitic structure for the value u=1. For compositions apart from this value, there is a risk of seeing the caesium crystallise in the secondary phases and being less well included in the chemical structure.
In the phosphosilicated apatite corresponding to the formula given above, the total number of negative charges is brought by the anions PO
4
3−
, SiO
4
4−
and X
2−
. These charges are balanced by the positive charges of Ca
2+
, M
+
and the trivalent cation Ln.
As an example X
2−
, can represent (FO
0,5
)
2−
. For the trivalent cation Ln, various trivalent cations can be used, in particular those belonging to the lanthanide group as well as iron and aluminium. As an example, Ln can be constituted of La alone, La in combination with Fe or furthermore Nd alone.
The present invention also has the aim of a process for containing the caesium and/or rubidium in a phosphosilicated apatite matrix corresponding to the following formula:
M
t
Ca
x
Ln
y
(PO
4
)
6−u
(SiO
4
)
u
X
in which:
M represents the Cs and/or Rb to be contained,
Ln represents at least one trivalent cation, chosen for example among the lanthanides, iron and aluminium,
X represents at least one anion chosen from amongst 2F
−
, S
2−
, 2Cl
−
, 2Br
−
, 2I
−
, 2OH
−
and O
2−
, and
t, x, y, and u are such that:
0<t≦2.5 and preferably 0<t≦1.5
2≦x≦8
1≦y≦7
0≦u≦6 and preferably u=1
x+y+t=10
and the total number of positive charges provided by the cations M, Ca and Ln are equal to (20+u), which comprises the foll

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