Method for treating an absorber pin containing contaminated boron carbide and sodium

Abstract

Disclosed is a method for treating an absorber pin, wherein the pin comprises a cladding in which a sintered boron carbide-based material having cracks is located, the material having porosity less than 1% of the volume of the material, the cracks containing sodium and at least one radioactive material. The method includes contacting the material with a treatment reaction mixture including carbon dioxide and water, in such a manner that the production of sodium carbonate and the expansion thereof cause the opening of cracks and of the sheath from at least one slit provided in the sheath as well as the propagation of the treatment process within the material. The process overcomes the physical-chemical properties of a sintered boron carbide-based material as much as possible. These properties prevent an easy treatment of the sodium and radioactive material contained in the cracks of the material.

Claims

1. A method for treating an absorber pin, said pin comprising a cladding in which there is a material based on sintered boron carbide whose porosity represents less than 1% of the volume of the material, the material having cracks that contain sodium and at least one radioactive substance, the method comprising: a pretreatment step of contacting the material with a pretreatment reaction mixture consisting essentially of molar percentage 0.5% to 25% of carbon dioxide and a chemically inert gas for the remainder; and a treatment step in which the sodium is converted to sodium carbonate by a carbonation reaction by contacting the material with a treatment reaction mixture comprising in molar percentage 0.5% to 5% of steam, 5% to 25% of carbon dioxide and 74.5% to 94.5% of a chemically inert gas, in such a way that expansion of the carbonate causes opening up of the cracks and of the cladding starting from at least one slit made in the cladding as well as the propagation of said method of treatment within the material.

2. The method according to claim 1, wherein the material is in the form of pellets.

3. The method according to claim 1, wherein the boron carbide has a composition of carbon atoms between 8.8% and 20%.

4. The method according to claim 1, wherein the inert gas is nitrogen, argon or a mixture thereof.

5. The method according to claim 1, wherein said contacting with the pretreatment or treatment reaction mixture is carried out at a temperature between 40 C. and 55 C.

6. The method according to claim 1, wherein the method is carried out in a confinement enclosure.

7. The method according to claim 6, wherein the confinement enclosure is a glove box, a hot cell or a chemical reactor.

8. The method according to claim 6, wherein the treatment or pretreatment reaction mixture is introduced into the confinement enclosure at a flow rate allowing continuous renewal at least once per hour.

9. The method according to claim 2, wherein the boron carbide has a composition of carbon atoms between 8.8% and 20%.

10. The method according to claim 2, wherein the inert gas is nitrogen, argon or a mixture thereof.

11. The method according to claim 3, wherein the inert gas is nitrogen, argon or a mixture thereof.

12. The method according to claim 2, wherein contacting with the pretreatment or treatment reaction mixture is carried out at a temperature between 40 C. and 55 C.

13. The method according to claim 3, wherein contacting with the pretreatment or treatment reaction mixture is carried out at a temperature between 40 C. and 55 C.

14. The method according to claim 4, wherein contacting with the pretreatment or treatment reaction mixture is carried out at a temperature between 40 C. and 55 C.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) In the present description of the invention, verbs such as comprise, contain, incorporate, include and their conjugated forms are open terms and therefore do not exclude the presence of additional element(s) and/or step(s) that may be added to the initial element(s) and/or step(s) stated after these terms. However, these open terms further relate to a particular embodiment in which only the initial element(s) and/or step(s), excluding all others, are aimed at; in which case the open term further entails the closed term consist of or compose of and its conjugated forms.

(2) The expression and/or is understood to link elements with a view to denoting their individual presence, but also a mixture or combination thereof.

(3) Moreover, unless stated otherwise, the values at the limits are included in the ranges of parameters indicated.

(4) Despite the presence of a radioactive substance, for example cesium or tritium (fission products), cobalt 60 or manganese 54 (activation products), the method of the invention treats the sodium present in the cracks in the material based on sintered boron carbide.

(5) This material is generally in the form of pellets. It is constituted wholly or partly of sintered boron carbide, the composition of which in carbon atoms is generally between 8.8% and 20%, and may therefore optionally vary in this range relative to the stoichiometric formula B.sub.4C corresponding to 20% of carbon atoms, or even have an excess of carbon up to 1 wt %.

(6) The method of treatment according to the invention comprises a treatment step during which the material is brought into contact with a treatment reaction mixture comprising in molar percentage 0.5% to 5% of steam, 5% to 25% of carbon dioxide and 75% to 94.5% of a chemically inert gas. The minor proportion of water in the treatment reaction mixture prevents any condensation of water on the walls of the cladding and thus allows treatment of the sodium in complete safety.

(7) The time of contact of the treatment or pretreatment reaction mixture with the material depends respectively on the amount of crust or sodium to be treated, or also on the composition of the reaction mixture. A person skilled in the art can easily adapt this time in order to obtain treatment of the sodium contained in the absorber pin that is as complete as possible, which is indicated for example by the end of release of hydrogen.

(8) Contacting with the treatment or pretreatment reaction mixture is for example carried out for a duration between 5 hours and 15 days.

(9) It is preferably carried out at a temperature between 40 C. and 55 C. Regarding the treatment step, this prevents the water condensing, even at the maximum concentrations of 5 mol % of steam, and reacting violently with the sodium.

(10) Since the material contains at least one radioactive substance, the treatment according to the invention is most often carried out in a confinement enclosure such as a glove box or a hot cell.

(11) The treatment or pretreatment reaction mixture is then generally introduced into the confinement enclosure at a flow rate allowing its continuous renewal at least once per hour, typically once to twice per hour.

(12) Other aims, features and advantages of the invention will now be stated in the following description of a particular embodiment of the method of the invention, given for purposes of illustration, and not limiting.

DESCRIPTION OF PARTICULAR EMBODIMENTS

(13) The ends of a metal cladding comprising sintered boron carbide pellets that have cracks and that contain sodium and a radioactive substance are cut off using a laser.

(14) Then a longitudinal slit is made in the metal cladding.

(15) In a glove box thermostatically controlled at 45 C., the pellets are brought into contact with a pretreatment reaction mixture with the aim of removing the crust of soda that forms on the surface of the cracks. This mixture contains, in molar percentage, 10% of carbon dioxide and 90% of nitrogen.

(16) The pellets are then brought into contact with a treatment reaction mixture containing, in molar percentage, 3% of steam, 10% of carbon dioxide and 87% of nitrogen.

(17) After some hours, absence of release of hydrogen indicates the end of treatment.

(18) The solid waste obtained, namely sodium carbonate, boron carbide, the cladding and the radioactive substance, may be packaged in order to be removed to the appropriate channels.