Method to confine plutonium in apatitic ceramics and...

Details Method to confine plutonium in apatitic ceramics and... Method to confine plutonium in apatitic ceramics and...

2001-03-09

2003-09-23

Bos, Steven (Department: 1754)

Hazardous or toxic waste destruction or containment

Destruction or containment of radioactive waste

By fixation in stable solid media

C588S002000, C588S015000

Reexamination Certificate

active

06624339

ABSTRACT:

DISCLOSURE
1. Field of the Invention
The present invention relates to a method to confine plutonium in apatitic ceramics and the products obtained using said process.
More specifically, it relates to a method to fix plutonium in atomic form in a ceramic mineral phase providing a durable confinement material.
With the end of the Cold War, the signing of the Nuclear Weapon Non-proliferation Treaty and the discontinuation of nuclear testing, some countries are in possession of a large quantity of plutonium which requires storage.
Plutonium is a radioactive element with a long life (Pu-239: T=24390 years), hence the need to develop extremely stable conditioning matrices. To avoid criticality problems, the matrix must prevent concentration in the form of clusters.
The purpose of this invention is to provide a method for preparing a confinement matrix intended to store plutonium, the durability and long-term stability of which are confirmed by the existence of natural britholites (OKLO “natural” reactor) containing long-life actinides.
2. State of the Related Art
It has already been envisaged to condition radioactive waste in silicate apatites, as described in the document WO-A-95/02886 [1]. In this document; the radioactive waste conditioned may be lanthanides and actinides.
As indicated in this document, the use of phosphosilicate apatite is very advantageous since apatites have the following remarkable properties:
these structures are very chemically and thermally stable in the natural environment. In addition, natural silicate apatites containing actinides have existed for hundreds of millions of years.
Apatites have a very low solubility in water; in addition, their solubility decreases as the temperature increases, which is a positive point for actinide conditioning since their high activity involves an increase in temperature in the matrix containing them.
Apatitic structures are capable of withstanding radioactivity since the irradiation damage incurred is unstable at temperatures above 60° C. Apatites have the property of restructuring at temperatures above 60° C.
Apatitic structures have the very advantageous property of being able to incorporate numerous metallic elements, particularly actinides and lanthanides, in their structure.
Natural apatites may be represented in the following formula:
Ca
10
(PO
4
)
6
F
2
which corresponds to the structure of fluoroapatite. However, in this structure, numerous substitutions may be made, particularly by replacing Ca
2+
cations by various metallic elements such as lanthanides, actinides and alkaline metals, substituting PO
4
3−
ions by other anions such as SiO
4
4−
, and also substituting F
−
anions by S
2−
, Cl
−
, Br
−
, I
−
or OH
−
anions.
The formulations used in this document take the following form:
M−Ca
x
Ln
y
A
z
(PO
4
)
u
(SiO
4
)
6−u
X
where
M=an alkaline metal,
Ln=Y or a lanthanide,
A=an actinide
X=S
2−
, 2
F
−
, 2Cl
−
, 2OH
−
, 2Br
−
or 2I
−
.
0≦t≦3,
0<u<6,
0<x≦10,
0≦y≦10,
0<z≦7, and
y+z>0
These apatite formulations may comprise many SiO
4
4
ions, therefore few PO
4
3−
ions, and for this reason show poorer “self-curing” against irradiation from &agr; radiation emitted by the Pu. Indeed, phosphate tetrahedrons are more rigid than tetrahedrons formed by silicates.
In addition, the method to manufacture said apatites requires the use of excess CaF
2
, for example, a 10% excess with reference to the stoichiometric quantity required to obtain the fluorinated apatite, and several calcination steps with intermediate grinding to obtain the fluorinated apatite.
When a dense ceramic is required, the powder obtained is subjected after the calcination steps to sintering, for example hot compression sintering. Therefore, this method requires numerous calcination steps followed by sintering to obtain a dense ceramic.
The present invention specifically relates to new phosphosilicate apatite formulations, suitable for plutonium confinement, and a method to prepare said apatites in dense ceramic form which is easier to use.
DESCRIPTION OF THE INVENTION
In this way, the invention relates to a plutonium confinement block comprising a phosphosilicate apatite matrix containing the plutonium to be confined and any radioactive lanthanides, the plutonium and lanthanides being included in the chemical structure of the phosphosilicate apatite and said apatite complying with the following formula:
M
t
Ca
x
Ln
y
Hf
w
Pu
z−w
(PO
4
)
6−u
(SiO
4
)
u
F
2
  I
wherein:
M represents at least one alkaline metal;
Ln represents at least one cation selected from the lanthanides, and
t, x, y, z, w and u are such that:
0≦t≦1,
8≦x≦10,
0≦y≦1,
0<z≦0.5,
0≦w<z, and
0<u≦y+2z,
and the total number of positive charges provided by the M, Ca, Ln, Hf and Pu cations are equal to (20+u)
According to the invention, in the phosphosilicate apatite in compliance with formula I given above, the quantities of plutonium, and Ln, Hf and M cations may be varied within relatively small limits with reference to the calcium content x of the apatite, which is 8 to 10.
Similarly, on the tetrahedral sites (PO
4
and SiO
4
), it is possible to vary the SiO
4
/PO
4
ratio. Each substitution has an effect on the physico-chemical properties of the conditioning matrix and it is the specific choice of formulation that gives the matrix specific physico-chemical properties. The SiO
4
/PO
4
ratio particularly controls the leaching resistance and the irradiation strength. Therefore, it is necessary to adjust said ratio with reference to the quantity of plutonium introduced. For this reason, according to the invention, the quantity of SiO
4
cannot exceed the value y+2z, which corresponds to the quantities of Ln and Pu cations present in the phosphosilicate apatite.
In the formula I given above, the total number of negative charges is supplied by the PO
4
3−
, SiO
4
4−
and F
−
anions. Said charges are balanced by the M monovalent cation, the Ca divalent cation and the trivalent and tetravalent cations which may be lanthanides, Hf and Pu.
To ensure that the charges are balanced, the quantities t, x, y, w and z are selected according to the quantity (6−u) of the PO
4
−
anions and the valence of the Ln, Hf and Pu cations present to obtain neutrality. For this reason, said quantities may or may not be integers.
The alkaline metal used in said apatite may be any alkaline metal. Na and K are selected advantageously for their easy use. The lanthanides may be any lanthanide from La to Gd. They may be radioactive or not. The choice of Ln cation is important since it makes it possible to substitute the apatite with a neutrophage product such as Gd, to reduce criticality risks.
According to the invention, the confinement block may be composed exclusively of phosphosilicate apatite containing Pu, which preferentially, contains no alkaline metal. However, it is also possible to have, around this apatite matrix containing the plutonium to be conditioned, at least one non-active silicate apatite layer and, if applicable, other layers of silicate apatite or not, of different compositions, so as to form appropriate successive barriers between the plutonium and the environment.
Preferentially, the phosphosilicate apatites of the block and/or the most inner layers directly in contact with the plutonium, do not contain any alkaline metal for improved resistance to irradiation damage, and the most outer layers may be selected to withstand attacks from the outer environment. It is possible to use, for one or more layers, the fluoroapatite composition Ca
10
(PO
4
)
6
F
2
which is particularly resistant.
In addition, the layers are chosen so as to improve the overall mechanical properties.
According to a first embodiment of the invention, the phosphosilicate apatite formulation is such that the number of silicates is equal

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