EP2791945A2

EP2791945A2 - Processing of chlorine-containing carbon-based radioactive waste

Info

Publication number: EP2791945A2
Application number: EP12816710.3A
Authority: EP
Inventors: Gérard LAURENT
Original assignee: Electricite de France SA
Current assignee: Electricite de France SA
Priority date: 2011-12-16
Filing date: 2012-12-11
Publication date: 2014-10-22
Also published as: ZA201404590B; CN104170019A; FR2984583A1; WO2013088059A2; US20140350320A1; KR20140133500A; RU2014129056A; WO2013088059A3; JP2015505967A

Abstract

The invention relates to the processing of carbon-based radioactive waste, comprising at least: a) soaking in an acid solution (S2, S3), and b) a heat treatment, of thermal shock type (S4, S5), said acid solution recovering radioactive material resulting from said waste at least after the implementation of step b). (FIG.2)

Description

Carbonaceous radioactive waste treatment with

chlorine

The present invention relates to the treatment of radioactive waste, in particular but not exclusively based on graphite.

The decontamination of an irradiated graphite matrix may be carried out using a so-called "steam reforming" technique (or "steam reforming" in Anglo-Saxon terms) as defined in particular in document US-6. , 625, 248. However, the technique presented in this document does not ensure acceptable radioactive releases.

It has been proposed in the document WO-2010/103210 a solution to this problem by observing that only a first portion of the carbon in the graphite treated at a sufficiently high temperature is radioactive, the remaining carbon in the graphite being treated being, depending on the treatment time, much less or not radioactive so that the carbon dioxide resulting from its combustion can be released freely into the atmosphere.

The teaching of these two documents should allow sufficient decontamination of carbon 14, chlorine 36 and tritium. Since the other radionuclides are not volatile, they can be recovered in solid residues present at the end of the vapo-reforming phase. However, the decontamination of chlorine 36 may be more difficult than that of carbon because this radionuclide can be present in two forms:

a mineral form and

- another form, organic.

The latter form, which is strongly bound to graphite (in particular by "aromatic" C-Cl bonds), may not be totally released during a steam-reforming treatment.

X-ray photoelectron spectrometry (X-ray) spectrophotometric analyzes on irradiated graphites effectively showed the presence of two different chemical forms of chlorine in graphite:

a form of chlorine termed "organic" chlorine, defined by C-Cl bonds specific to "aromatic carbon" (in particular of the high energy double bond type), and directly linked to the carbon of the carbon matrix usually forming graphite, and

a form of chlorine called "inorganic chlorine", in the form of oxychlorides of undetermined composition, probably located in the porosity of the graphitic material (called "chlorite compounds (C102-) and chlorate (C103-)").

"Organic" chlorine is strongly bound to graphite and may not be completely released during a steam reforming treatment, even modified by the teaching of WO-2010/103210. The present invention improves the situation.

To this end, it proposes a process for the treatment of carbonaceous radioactive waste, comprising at least: a) a soaking in an acidic solution, and b) a thermal treatment, of thermal shock type, said acid solution recovering the radioactive material resulting from said waste at least after the implementation of step b).

Possible embodiments of this process are briefly presented hereinafter.

For example, the acidic solution may comprise sulfuric acid (H ₂ SO ₄ ). Tests with this type of acid have given good results.

It may be advantageous that the acidic solution further comprises an oxygen supply element in the acidic solution, for example hydrogen peroxide (H ₂ O ₂ ) in a range of proportions typically comprised between 0.1% and 20%. % (a proportion of 5% yielding good results presented below).

The soaking in the acidic solution can be carried out in a time range of between 15 and 20 hours, for example about 18 hours. The aforementioned thermal shock, made for example by roasting, can be carried out in a temperature range between 800 and 1200 ° C (for example about 1000 ° C) for a period of between 15 and 30 minutes (for example about twenty minutes).

It appears from the tests carried out that the radioactive material which escapes after stage b) of the carbonaceous waste (of graphitic type) contains at least chlorine 36. It very probably contains practically all the chlorine " organic "defined above, since the tests conducted showed that virtually all the chlorine 36 was found in the solution after the heat shock, and therefore it had been virtually completely extracted from carbonaceous waste.

The present invention thus makes it possible to extract chlorine 36, of organic type, as demonstrated in the exemplary embodiments presented in detail below.

Since the invention makes it possible to recover this type of radioactive material ("organic" chlorine), it can then advantageously be used in combination with a steam reforming treatment, as explained above. Thus, the treatment of the waste by the implementation of steps a) and b) of the process within the meaning of the invention can be preceded or succeeded by a steam-reforming type treatment.

Thus, leaching of the radionuclides out of the irradiated graphite can be obtained by soaking in a highly acidic and oxidizing solution, followed by a thermal shock.

The present invention also aims at a facility for the treatment of carbonaceous radioactive waste, for the purpose of process according to one of the preceding claims, characterized in that it comprises: a tank for storing said waste in an acidic solution, and

- Heating means shaped to apply a thermal shock to said waste after soaking in said acid solution.

Other advantages and features will become apparent upon reading the detailed description of exemplary embodiments, and examining the accompanying drawings in which:

FIG. 1 diagrammatically illustrates the main steps of the process within the meaning of the invention, and

- Figure 2 schematically illustrates an installation for the implementation of this method.

In the following exemplary embodiments, it is proposed to mix sulfuric acid (H ₂ SO ₄ ) and hydrogen peroxide (H ₂ O ₂ ) in appropriate proportions (detailed in the examples of embodiment below). after) to determine their ability to release chlorine 36 from the graphitic matrix.

Examples of realization

Four tests are presented in the table below, with hydrogen peroxide H ₂ O ₂ as oxygen supply material and sulfuric acid H ₂ SO ₄ as acid medium and a distribution of about 4 to 20 volumes of acid (at 95%) for 1 volume of hydrogen peroxide (at 30%).

Carbonaceous radioactive waste was crushed into powder form to constitute the different samples of the table above.

Each sample, having a mass of 5 grams and observed particle sizes distributed typically between 2380 and 4000 microns after crushing, was: soaked for 18 hours in a solution of the aforementioned type (H ₂ SO ₄ and H ₂ O ₂ ),

then rinsed before being neutralized in pH with 5% sodium hydroxide (NaOH),

then grilled:

o by rapid heating (in a range of 5 to 60 minutes, for example for 20 minutes), and o high temperature (between 900 and 1200 ° C, for example at 1000 ° C).

After this treatment, it has been observed that 90% of the chlorine 36 is released (in particular for the first sample - 19 ml of acid per 1 ml of H ₂ O ₂ ), with a tendency towards an improved yield by increasing in particular the acid concentration.

Other observations

In addition, during treatment, some characteristic behaviors were noted. For example, for each test, 5 grams of carbonaceous radioactive waste was added to a corresponding amount of solution (according to the table above) and soaked for 18 hours. During this phase, bubble formation was observed on the surface of the graphite particles.

It was also noted that the higher concentrations of H ₂ SO ₄ in the soaking solution (i.e. the first sample above) had the effect of swelling the graphite. The pores of the latter absorbed a large part of the mass of the solution.

In contrast, for the fourth sample of the above table, it was shown that swelling and penetration were very low, compared to the first sample. The collected solutions were highly acidic and required neutralization with 5% NaOH prior to analysis to determine the quantum of radioactivity (Cl-36) that leached the solution.

It turns out that the graphitic mass after soaking has increased significantly because the solution has penetrated into the pores of the graphite and has actually caused its swelling. The samples were then held at a temperature of 1000 ° C for twenty minutes to extract any solution present in the pores of the graphite and to extract any radioactivity. After twenty minutes, each sample was removed and collected for testing and to determine if significant amounts remained after soaking and baking.

It was observed after this heat treatment that the initial graphitic mass did not decrease significantly after soaking and subsequent baking in the electric oven at 1000 ° C. Similarly, examination of the percentage of total radioactivity captured shows that significant quantities of Cl-36 have been leached, in particular in the first sample of the table above (with the highest proportions of H ₂ SO ₄ ). Finally, one of the most important observations is that the solution captured 1080 Bq / g of Cl-36, a quantum very close to the initial Cl-36 value of pre-treatment graphite (1,200 Bq). /boy Wut) . Thus, after the treatment presented above by way of example, there remains 10% of Cl-36 in the radioactive carbonaceous waste.

FIG. 1 summarizes the main steps of the process within the meaning of the invention, comprising: for example, during a first step S1, the recovery of radioactive carbonaceous waste, for example in the form of graphite,

in the next step S2, it is triggered a soaking of these wastes in an acid solution, strongly oxygenated, for example sulfuric acid (H ₂ SO ₄ ) with about 5% hydrogen peroxide (H ₂ O ₂ );

after identifying a sufficient soaking time (for example 18 hours) in step S3, applying a heat treatment, by roasting, at a temperature of the order of 1000 ° C, in step S4;

after identifying a sufficient duration of heat treatment (for example 20 minutes) in step S5,

chlorine 36 can be recovered in the acid solution and treated separately, and the carbonaceous waste can then be subjected to a vapor reforming treatment as described, for example, in the document cited above. WO-2010/103210 (step S6).

The installation for the implementation of this process may then comprise, with reference to FIG. 2, a tank CU and a conveyor C1 of GR waste products pouring into a highly acidic and oxygenated solution (H ₂ SO ₄ H ₂ O ₂ ) contained in the tank CU, which is surrounded (in the example shown) heating means MC to apply a heat shock type treatment. The GR waste thus treated can then be recovered by the second conveyor C2 (after filtering, for example, the acid solution now containing chlorine-36) to be conveyed to a steam reforming installation. Chlorine 36 can be recovered for example from the solution remaining under the tank CU, as presented as a purely illustrative example in Figure 2.

Moreover, the present invention is not limited to the embodiment presented above by way of example; it applies to other variants.

Thus, it will be understood, for example, that another type of acid may be provided in combination or alternatively with sulfuric acid.

Similarly, hydrogen peroxide is a good oxygen delivery element in a solution. However, as an alternative to the embodiment presented above, it is possible for example to provide a bubbling of oxygen in the acid solution.

Thus, it will be understood that the proportions of the oxygen supply element in the acid solution are susceptible to variations depending on the acids and elements employed. Likewise, the soaking time in step a) is capable of variations. The same goes for the temperature and the duration of the thermal shock. Finally, a particular embodiment has been described above in which the above-mentioned step a) of soaking in the acid solution is preceded by a crushing of the carbonaceous waste to reduce it to powder. However, this implementation is not essential and it can be alternatively provided to directly dipping solid graphite for example up to heart in an acid solution.

Claims

1. Process for the treatment of carbonaceous radioactive waste, comprising at least:

a) a soaking in an acidic solution (S2, S3), and b) a thermal treatment, of heat shock type (S4, S5),

said acid solution recovering radioactive material from said waste at least after the implementation of step b).

2. Method according to claim 1, characterized in that the acidic solution comprises sulfuric acid (H ₂ SO ₄ ).

3. Method according to one of claims 1 and 2, characterized in that the acidic solution further comprises an oxygen supply element to said solution.

4. Method according to claim 3, characterized in that the acid solution comprises hydrogen peroxide (H ₂ O ₂ ).

5. Method according to one of claims 3 and 4, characterized in that the acid solution comprises an oxygen supply element in a range of proportions between 0.1% and 20%.

6. Method according to one of claims 3 to 5, characterized in that the acidic solution comprises an oxygen supply element in a proportion of 5%.

7. Method according to one of the preceding claims, characterized in that the soaking in the acid solution is carried out in a time range of between 15 and 20 hours (S3).

8. Method according to one of the preceding claims, characterized in that the thermal shock is conducted in a temperature range between 800 and 1200 ° C.

9. Method according to one of the preceding claims, characterized in that the heat treatment is carried out for a period of between 15 and 30 minutes (S5).

10. Method according to one of the preceding claims, characterized in that said radioactive material comprises at least chlorine 36.

11. Method according to one of the preceding claims, characterized in that the treatment of waste by the implementation of steps a) and b) is preceded or succeeded by a treatment type vapo-reforming (S6).

12. Method according to one of the preceding claims, characterized in that step a) is preceded by a crushing of the carbonaceous waste to reduce them to powder.

13. Method according to one of the preceding claims, characterized in that the thermal shock is carried out by roasting in step b).

14. Radioactive Waste Treatment Facility carbonates, for the implementation of the method according to one of the preceding claims, characterized in that it comprises:

a tank (CU) for storing said waste in an acidic solution, and

heating means (MC) shaped to apply a thermal shock to said waste after soaking in said acid solution.