Method for dissolving nuclear fuel

Abstract

A process for dissolving nuclear fuel, in particular irradiated nuclear fuel, comprising immersion of the nuclear fuel in a nitric acid solution. This dissolution process further comprises mechanical milling of the nuclear fuel, this mechanical milling being performed in the nitric acid solution during the immersion. The disclosure also relates to the use of a mill equipped with mechanical milling structure to implement the dissolution process.

Claims

1. Process for dissolving nuclear fuel, comprising immersion of the nuclear fuel in a nitric acid solution, wherein it comprises mechanical milling of the nuclear fuel, this mechanical milling being performed in the nitric acid solution during said immersion.

2. The dissolution process according to claim 1, wherein the nitric acid solution is heated to between 90 C. and 105 C.

3. The dissolution process according to claim 1, wherein the molar concentration of the nitric acid solution is between 1 mol/L and 10 mol/L.

4. The dissolution process according to claim 1, wherein the nitric acid solution also comprises a neutron poison such as gadolinium.

5. The dissolution process according to claim 1, wherein mechanical milling is performed throughout the entire duration of immersion.

6. The dissolution process according to claim 1, wherein the nuclear fuel comprises at least one plutonium oxide and/or at least one mixed oxide of plutonium and of at least one second metal selected from among uranium, thorium, neptunium, americium and curium.

7. The dissolution process according to claim 6, wherein, the second metal being uranium, the nuclear fuel containing at least one mixed oxide of uranium and plutonium is a MOX fuel.

8. The dissolution process according to claim 1, wherein the nuclear fuel is irradiated nuclear fuel.

9. The dissolution process according to claim 1, wherein the nuclear fuel comprises fabrication rejects of non-irradiated nuclear fuel.

10. The dissolution process according to claim 1, further comprising, when the nuclear fuel is confined within a cladding, a step to de-clad the nuclear fuel, this decladding step being prior to immersion.

11. The dissolution process according to claim 1, further comprising the implementation of a mill equipped with mechanical milling means.

12. The dissolution process according to claim 11, wherein the mill is a bead or pebble mill.

13. The dissolution process according to claim 3, wherein the molar concentration of the nitric acid solution is between 3 mol/L and 8 mol/L.

14. The dissolution process according to claim 12, wherein the beads or pebbles are in zirconium dioxide.

15. Process for dissolving irradiated nuclear fuel comprising the following successive steps, taken in this order: (a) dissolving irradiated nuclear fuel by immersion in nitric acid solution, after which a nitric dissolution solution is obtained containing dissolution fines; (b) separating the dissolution fines from the nitric dissolution solution; and (c) dissolving the dissolution fines separated at step (b), wherein the dissolving step (c) comprises immersion of the dissolution fines in a nitric acid solution and mechanical milling of the dissolution fines, the mechanical milling being performed in the nitric acid solution during said immersion.

16. The process according to claim 15 further comprising, when the irradiated nuclear fuel is confined within a cladding, a step to de-clad the irradiated nuclear fuel, this decladding step preceding step (a).

17. The dissolution process according to claim 15, wherein the irradiated nuclear fuel comprises at least one plutonium oxide and/or at least one mixed oxide of plutonium and of at least one second metal selected from among uranium, thorium, neptunium, americium and curium.

18. The dissolution process according to claim 17, wherein, the second metal being uranium, the nuclear fuel comprising at least one mixed oxide of uranium and plutonium is MOX fuel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 gives a graph translating the change, as a function of time (denoted t and expressed in min), in the weight concentration of cerium (denoted [Ce] and expressed in g/L), in nitric dissolution solutions obtained when implementing two dissolution processes, one conforming to the invention (P.sub.i), the other being a reference process conforming to the state of the art (P.sub.r).

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

Example 1

Comparison Between Two Ceria Dissolution Processes in (5M) Nitric Acid Solution

(2) In this example, a bead mill available from Wma-Getzmann under the trade name Dispermat SL5 having a 50 mL milling chamber volume and zirconium dioxide beads were used.

(3) A three-way valve was connected to the outlet pipe of this mill for sampling purposes to determine the state of progress of ceria dissolution in the nitric acid solution by monitoring the concentration of cerium [Ce] in the resulting nitric dissolution solution, this concentration being determined by Inductively Coupled Plasma, Atomic Emission Spectrometry (ICP-AES).

(4) For the first test, 20 g of ceria were immersed in 100 mL of nitric acid solution at a molar concentration of 5 mol/L (5 M) in the milling chamber of the bead mill, in the presence of the beads, so as to monitor the progress of ceria dissolution when implementing a reference dissolution process denoted P.sub.i.

(5) For a second test, 20 g of ceria were immersed in 100 mL of nitric acid solution at a molar concentration of 5 mol/L (or 5 M) in the milling chamber of the bead mill, but in the absence of said beads, so as to monitor the progress of ceria dissolution when implementing a reference dissolution process denoted P.sub.r.

(6) With reference to FIG. 1 giving the change, as a function of time, in the weight concentration of cerium in each of the nitric dissolution solutions obtained when implementing the dissolution processes P.sub.i and P.sub.r, it is observed that: after 400 min (i.e. a little more than 6 h), the weight concentration of cerium in the nitric dissolution solution is 0.09 g/L for process P.sub.r against 4.22 g/L for process P.sub.i, which corresponds to 0.1% dissolution of ceria with process P.sub.r against 5% with process P.sub.i; and after 1350 min (about 22 h), the weight concentration of cerium in the nitric dissolution solution is 0.31 g/L for process P.sub.r against 17.75 g/L for process P.sub.i, which corresponds to only 0.2% dissolution of ceria with process P.sub.r against 11% with process P.sub.i.

(7) In other words, it is observed an increase by a factor of 50 in the dissolution kinetics of ceria in 5 M nitric acid solution, justifying the advantage of simultaneously performing ceria immersion and milling.

Example 2

Comparison Between Two Ceria Dissolution Processes in (5 M) Nitric Acid Solution

(8) In this example, an oscillating mill was used comprising two compartments denoted C.sub.I and C.sub.R.

(9) In compartment C.sub.I, comprising a grinding bead in zirconium dioxide, a dissolution process conforming to the invention was implemented, denoted P.sub.I. 2 g of ceria were immersed in 10 mL of nitric acid solution at a molar concentration of 5 M. After an immersion time and simultaneous milling of 7.5 h of the ceria in the nitric acid solution, the nitric dissolution solution obtained denoted S.sub.I was analysed by ICP-AES.

(10) In compartment C.sub.R, comprising a grinding bead in zirconium dioxide, a reference dissolution process was implemented, denoted P.sub.R. 2 g of ceria were immersed in 10 mL of deionized water. After an immersion time and simultaneous milling of 7.5 h of the ceria in the deionized water, the solution comprising the milled ceria was filtered and dried. The milled, dried ceria was then placed in a beaker and immersed in 10 mL of nitric acid solution at a molar concentration of 5 M, under agitation with a magnetic stir bar. After an immersion time of 7.5 h with agitation of the milled ceria in the nitric acid solution, the nitric dissolution solution obtained denoted S.sub.R was also analysed by ICP-AES.

(11) The weight concentrations of cerium measured in solutions S.sub.I and S.sub.R respectively were 4 g/L and 0.75 g/L.

(12) An increase is therefore observed in this example by a factor of 5 in the dissolution kinetics of ceria in the 5 M nitric acid solution.

(13) Such results clearly evidence the synergy of the dissolution process conforming to the invention which applies simultaneous immersion and milling, compared with a dissolution process applying milling followed by immersion.

BIBLIOGRAPHY

(14) EP 2 345 041 A1