METHOD FOR DEHALOGENATION AND VITRIFICATION OF RADIOACTIVE METAL HALIDE WASTES

Abstract

The present disclosure relates to a method for dehalogenation and vitrification of radioactive metal halide wastes. The dehalogenation method of radioactive metal halide wastes includes the following steps: mixing the radioactive metal halide wastes with oxalic acid, and performing a thermal treatment to remove halogens from the radioactive metal halide wastes. The vitrification method includes a following step: immobilizing the dehalogenated wastes treated by the dehalogenation method of radioactive metal halide wastes into a vitrified form by adding glass additives. The benefits of the method for dehalogenation and vitrification of radioactive metal halide wastes provided by the present disclosure include not only low dehalogenation temperature, high dehalogenation efficiency and high waste loading in the vitrified form, but also no new substances introduced after dehalogenation, which is easy to be integrated with the existing vitrification process. Therefore, the present disclosure shows a promising application.

Claims

1. A dehalogenation method of radioactive metal halide wastes, comprising the following steps: mixing the radioactive metal halide wastes with an oxalic acid; and performing a thermal treatment to remove halogens from the radioactive metal halide wastes.

2. The dehalogenation method of radioactive metal halide wastes according to claim 1, wherein a temperature of the thermal treatment spans from 100° C. to 600° C.

3. The dehalogenation method of radioactive metal halide wastes according to claim 1, wherein a temperature of the thermal treatment spans from 280° C. to 400° C. and a duration of the thermal treatment spans from 20 min to 1000 min.

4. The dehalogenation method of radioactive metal halide wastes according to claim 1, wherein both the radioactive metal halide wastes and the oxalic acid are solid powders.

5. The dehalogenation method of radioactive metal halide wastes according to claim 1, wherein a molar ratio of the oxalic acid to the halogens is more than 0.5 to mix the oxalic acid with the radioactive metal halide wastes.

6. The dehalogenation method of radioactive metal halide wastes according to claim 1, wherein a molar ratio of the oxalic acid to the halogens spans from 1.2 to 3 to mix the oxalic acid with the radioactive metal halide wastes.

7. The dehalogenation method of radioactive metal halide wastes according to claim 1, wherein the radioactive metal halide wastes are chloride molten salt wastes or/and fluoride molten salt wastes generated from a dry reprocessing of spent nuclear fuel.

8. A vitrification method, comprising a following step: immobilizing the dehalogenated wastes treated by the dehalogenation method of the radioactive metal halide wastes according to claim 1 into a vitrified form by adding glass additives.

9. The vitrification method according to claim 8, wherein the glass additives for forming a vitrified form are borosilicate glass forming chemicals; in terms of the weight percentage of oxides, a waste loading of the vitrified form for the dehalogenated wastes spans from 15% to 35%.

10. A vitrified form, wherein the vitrified form is prepared by the vitrification method according to claim 8.

11. A vitrification method, comprising a following step: immobilizing the dehalogenated wastes treated by the dehalogenation method of the radioactive metal halide wastes according to claim 2 into a vitrified form by adding glass additives.

12. A vitrification method, comprising a following step: immobilizing the dehalogenated wastes treated by the dehalogenation method of the radioactive metal halide wastes according to claim 3 into a vitrified form by adding glass additives.

13. A vitrification method, comprising a following step: immobilizing the dehalogenated wastes treated by the dehalogenation method of the radioactive metal halide wastes according to claim 4 into a vitrified form by adding glass additives.

14. A vitrification method, comprising a following step: immobilizing the dehalogenated wastes treated by the dehalogenation method of the radioactive metal halide wastes according to claim 5 into a vitrified form by adding glass additives.

15. A vitrification method, comprising a following step: immobilizing the dehalogenated wastes treated by the dehalogenation method of the radioactive metal halide wastes according to claim 6 into a vitrified form by adding glass additives.

16. A vitrification method, comprising a following step: immobilizing the dehalogenated wastes treated by the dehalogenation method of the radioactive metal halide wastes according to claim 7 into a vitrified form by adding glass additives.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 is a flow diagram of the dehalogenation and vitrification route for radioactive metal halide wastes of the present disclosure.

[0019] FIG. 2 is the effect of molar ratio of oxalic acid to chlorine on chlorine removal efficiency in embodiment 1 of the present disclosure.

[0020] FIG. 3 is the effect of temperature of the thermal treatment on chlorine removal efficiency in embodiment 1 of the present disclosure.

[0021] FIG. 4 is the effect of dwelling time on chlorine removal efficiency in embodiment 1 of the present disclosure.

[0022] FIG. 5 is an XRD pattern of the vitrified form prepared in embodiment 1 of the present disclosure.

[0023] FIG. 6 is an XRD pattern of the vitrified form prepared in embodiment 2 of the present disclosure.

DETAILED DESCRIPTION

[0024] In order to make the purpose, technical scheme and advantages of the present disclosure clearer, this disclosure is further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present disclosure, but not to limit itself.

[0025] Referring to FIG. 1, the first aspect of the present disclosure provides a dehalogenation method of radioactive metal halide wastes, which includes the following steps: mixing the radioactive metal halide wastes with oxalic acid, and performing a thermal treatment to remove halogens from the radioactive metal halide wastes.

[0026] In the present disclosure, the temperature of the thermal treatment spans from 100° C. to 600° C. Further, the temperature of the thermal treatment spans from 250° C. to 500° C., wherein the dehalogenation efficiency could reach more than 90%. Further, the temperature of the thermal treatment spans from 280° C. to 400° C. wherein the extremely high dehalogenation efficiency could be achieved at such a low temperature, and the migration risk of volatile nuclides could be reduced as well.

[0027] Furthermore, a duration of the thermal treatment spans from 20 min to 1000 min. Further, the duration of the thermal treatment spans from 60 min to 600 min. Further, the duration of the thermal treatment spans from 90 min to 300 min.

[0028] In some embodiments of the present disclosure, a resulting mixture of radioactive metal halide wastes with oxalic acid is maintained in an environment preheated to the target temperature to perform a thermal treatment. In this process, the duration of the thermal treatment spans preferably from 30 min to 500 min, more preferably from 60 min to 300 min.

[0029] In some embodiments of the present disclosure, a resulting mixture of radioactive metal halide wastes with oxalic acid is heated to the target temperature in the furnace at a heating rate of 1° C./min to 20° C./min. In this process, the duration of the thermal treatment includes heating time and dwelling time. Further, a resulting mixture of radioactive metal halide wastes with oxalic acid is heated to the target temperature in the furnace at a heating rate of 1° C./min to 10° C./min, and maintaining at the target temperature for 0 min to 180 min.

[0030] Preferably, the heating rate spans from 4° C./min to 8° C./min.

[0031] In some more embodiments of the present disclosure, the thermal treatment process is as follows: a resulting mixture of radioactive metal halide wastes with oxalic acid is heated to 300° C. in the furnace at a heating rate of 5° C./min, and maintaining at 300° C. for 0 min to 120 min.

[0032] Preferably, when the resulting mixture of radioactive metal halide wastes and oxalic acid is heated to the target temperature at a heating rate of 1° C./min to 10° C./min for thermal treatment, the dwelling time spans preferably from 30 min to 90 min.

[0033] In the present disclosure, both the radioactive metal halide wastes and oxalic acid are solid powders, which are helpful to mix the mixture evenly and increasing the dehalogenation efficiency. Further, an average grain size of radioactive metal halide wastes and oxalic acid is less than 100 mesh.

[0034] In some embodiments of the present disclosure, the radioactive metal halide wastes and oxalic acid solid are mixed and crushed before performing a thermal treatment, and an average grain size of the mixture is less than 100 mesh.

[0035] In the dehalogenation process of the present disclosure, the molar ratio of oxalic acid to halogens is more than 0.5. Further, the molar ratio of oxalic acid to halogens is more than 0.8. Further, the molar ratio of oxalic acid to halogens is more than 1. Further, the molar ratio of oxalic acid to halogens spans from 1.2 to 3, wherein the dehalogenation efficiency could reach over 90%. Further, the molar ratio of oxalic acid to halogens spans from 1.5 to 2.5, wherein the dehalogenation efficiency could also reach over 90% and the amount of oxalic acid is reduced.

[0036] In some preferred embodiments of the present disclosure, the molar ratio of oxalic acid to halogens is 2.

[0037] In the present disclosure, the radioactive metal halide wastes include at least one of chloride molten salt wastes and fluoride molten salt wastes generated from dry reprocessing of spent nuclear fuel. Further, the chloride molten salt wastes include at least one of alkali metal chlorides, alkaline earth metal chlorides and rare earth metal chlorides; and fluoride molten salt wastes include at least one of alkali metal fluorides, alkaline earth metal fluorides and rare earth metal fluorides. Further, chloride molten salt wastes include LiCl, KCl, NaCl, CsCl, SrCl.sub.2 and rare earth metal chlorides; fluoride molten salt wastes include LiF, NaF, KF, CsF, MgF.sub.2, SrF.sub.2, and rare earth metal fluorides.

[0038] The second aspect of the present disclosure provides a vitrification method, which includes a following step: immobilizing the remaining dehalogenated wastes treated by the first aspect of the present disclosure into a vitrified form by adding glass additives.

[0039] In the present disclosure, the glass additives for forming a vitrified form are borosilicate glass forming chemicals. Further, the glass additives for forming a vitrified form include the following components: 63 wt % to 70 wt % of SiO.sub.2, 17 wt % to 22 wt % of B.sub.2O.sub.3, 6 wt % to 8 wt % of Al.sub.2O.sub.3 and 5 wt % to 10 wt % of CaO.

[0040] In the present disclosure, in terms of the weight percentage of oxides, the waste loading of vitrified form for radioactive wastes spans from 15% to 35%. Further, the waste loading of vitrified form for radioactive wastes spans from 20% to 35%. Further, the waste loading of vitrified form for radioactive wastes spans from 25% to 35%.

[0041] In the present disclosure, immobilizing the remaining radioactive wastes treated by the first aspect of the present disclosure into a vitrified form by adding glass additives includes the following steps: mixing the dehalogenated wastes with glass additives, and preparing a vitrified form by heating, melting and cooling.

[0042] In the present disclosure, a temperature of the heating and melting spans from 1000° C. to 1400° C. Further, the temperature of the heating and melting spans from 1100° C. to 1200° C. A duration of the heating and melting spans from 1 hour to 6 hours. Further, the duration of the heating and melting spans from 1 hour to 3 hours.

[0043] In some embodiments of the present disclosure, the temperature of the heating and melting is 1200° C. and the duration of the heating and melting spans from 1 hour to 2 hours.

[0044] The third aspect of the present disclosure provides a vitrified form, which was prepared by the vitrification method provided in the second aspect of this disclosure.

Embodiment 1

[0045] In this embodiment, non-radioactive chlorides were used to simulate electrorefining salt wastes generated from electrochemical processing of spent nuclear fuel, as shown in Table 1. A total weight of 20 g of oxalic acid and chloride molten salt wastes were weighed and fully mixed in different proportion; the resulting mixtures were placed in 100 mL corundum crucibles; the samples were heated to 100° C. to 600° C. in the furnace at a heating rate of 5° C./min and maintained at target temperatures for 0 min to 120 min; afterwards, the crucibles were taken out and cooled in air to room temperature to obtain dechlorinated wastes. The chlorine removal efficiency (CRE) was calculated using the following formula,

[00001]$\mathrm{CRE}=\frac{M_1-M_2}{M_1}\times 100\%$

where M.sub.1 and M.sub.2 were the mass of chlorine in the original waste and dechlorinated waste, respectively.

TABLE-US-00001 TABLE 1 Composition of the chloride molten salt waste (wt %) Component wt % LiCl 32.32 KCl 38.68 NaCl 9.00 CsI 7.00 SrCl.sub.2 3.00 CeCl.sub.3 5.00 NdCl.sub.3 5.00 SUM 100.00

[0046] The effects of molar ratio of oxalic acid to chlorine, a temperature of thermal treatment and dwelling time on chlorine removal efficiency were shown in FIGS. 2 to 4, respectively. The temperature of the thermal treatment in FIG. 2 was 300° C., and the dwelling time was 60 min; the molar ratio of oxalic acid to chlorine in FIG. 3 was 2, and the dwelling time was 0 min; the molar ratio of oxalic acid to chlorine in FIG. 4 was 2, and the temperature of the thermal treatment was 300° C. According to FIGS. 2 to 4, the optimal parameters for dechlorination was as follows: the molar ratio of oxalic acid to chlorine was 2, the temperature of the thermal treatment was 300° C., and the dwelling time at 300° C. was 60 min, resulting in the high dechlorination efficiency up to 99%. Thus obtained dechlorinated waste was then immobilized into a vitrified form: a total weight of 20 g of dechlorinated waste and glass additives was weighed and fully mixed according to the glass formula designed in Table 2 (the waste loading was 35 wt %); the resulting mixture was placed in a 50 mL corundum crucible; the sample was maintained in a muffle furnace at 1200° C. for 1 hour; glass melt was poured on a preheated copper plate mold and cooled to obtain a vitrified form.

TABLE-US-00002 TABLE 2 The percentage of the dechlorinated waste and glass additives in the designed glass formula of embodiment 1 (wt %) Component Dechlorinated waste Glass additive SiO.sub.2 45.01 B.sub.2O.sub.3 12.34 Al.sub.2O.sub.3 4.32 CaO 3.33 K.sub.2O 15.83 Li.sub.2O 7.54 Na.sub.2O 2.90 Cs.sub.2O 2.36 CeO.sub.2 2.10 Nd.sub.2O.sub.3 2.03 SrO 1.10 I 0.88 Cl 0.26 SUM 35.00 65.00

[0047] The XRD diffraction pattern (FIG. 5) of the vitrified form prepared in embodiment 1 presents a typical amorphous hump, which proves that the prepared waste form is a glass. An Archimedes principle was used to measure a density, which was 2.58 g/cm.sup.3. The chemical durability of vitrified form was evaluated according to the 7-day Product Consistency Test (PCT-7) and the normalized releases of major elements from PCT-7 of the vitrified form in embodiment 1 are shown in Table 3. The values of each element normalized release were lower than 2 g/m.sup.2, which meets the requirements of chemical durability of HLW vitrified form.

TABLE-US-00003 TABLE 3 Normalized releases of major elements from PCT-7 of the vitrified form in embodiment 1 Normalized releases of Elements elements (r.sub.i) (g/m.sup.2) B 0.7328 Na 1.2499 K 0.6588 Li 1.4559 Ca 0.0030 Al 0.3977 Si 0.3422 Sr 0.0167 Cs 0.3980 Ce 0.0021 Nd 0.0040

Embodiment 2

[0048] In this embodiment, non-radioactive fluorides were used to simulate fluoride molten salt wastes generated from dry reprocessing of spent nuclear fuel of MSRs, as shown in Table 4. A total weight of 20 g of oxalic acid and fluoride molten salt wastes were weighed according to the molar ratio (2) of oxalic acid to fluorine and fully mixed; the resulting mixture was placed in a 100 mL corundum crucible; the sample was heated to 300° C. at a heating rate of 5° C./min and maintained at 300° C. for 60 min; afterwards, the sample was taken out and cooled in air to room temperature to obtain the defluorinated waste. The fluorine removal efficiency (FRE) was calculated using the following formula.

[00002]$\mathrm{FRE}=\frac{M_3-M_4}{M_3}\times 100\%$

where M.sub.3 and M.sub.4 were the mass of fluorine in the original waste and defluorinated waste, respectively

TABLE-US-00004 TABLE 4 Composition of the fluoride molten salt waste (wt %) Component wt % KF 68.31 NaF 20.61 LiF 10.06 MgF.sub.2 0.13 CsF 0.39 SrF.sub.2 0.32 CeO.sub.2 0.18 SUM 100.00

[0049] The fluorine removal efficiency could reach 91% through above defluorination process. Thus obtained defluorinated waste was then immobilized into a vitrified form: a total weight of 20 g defluorinated waste and glass additives was weighed and fully mixed according to the glass formula designed in Table 5 (the waste loading was 25 wt %); the resulting mixture was placed in a 50 mL corundum crucible; the sample was maintained in a muffle furnace at 1200° C. for 1 hour; glass melt was poured on a preheated copper plate mold and cooled to obtain a vitrified form.

TABLE-US-00005 TABLE 5 The percentage of defluorinated waste and glass additives in the designed glass formula of embodiment 2 (wt %) Component Defluorinated waste Glass additive SiO.sub.2 47.83 B.sub.2O.sub.3 15.86 Al.sub.2O.sub.3 5.38 CaO 5.93 K.sub.2O 16.29 Li.sub.2O 2.13 Na.sub.2O 5.26 Cs.sub.2O 0.11 CeO.sub.2 0.09 SrO 0.10 F 0.98 SUM 25.00 75.00

[0050] The XRD diffraction pattern (FIG. 6) of the vitrified form prepared in embodiment 2 presents a typical amorphous hump, which proves that the prepared waste form is a glass. An Archimedes principle was used to measure a density, which was 2.48 g/cm.sup.3. The chemical durability of vitrified form was evaluated according to the 7-day Product Consistency Test (PCT-7) and the normalized releases of major elements from PCT-7 of the vitrified form in embodiment 2 are shown in Table 6. The values of each element normalized release were lower than 2 g/m.sup.2, which meets the requirements of chemical durability of HLW vitrified form.

TABLE-US-00006 TABLE 6 Normalized releases of major elements from PCT-7 of the vitrified form in embodiment 2 Normalized releases of Elements elements (r.sub.i) (g/m.sup.2) B 0.6031 Na 0.5806 K 0.6256 Li 0.4243 Ca 0.2392 Al 0.5649 Si 0.1370 Mg 0.2782 Sr 0.3189 Cs 1.4215 Ce 0.0527

[0051] Preferred embodiments of the present disclosure are described above, which don't limit the protection scope of the present disclosure. Any variation or substitution that may be easily made by those skilled in the art within the technical scope disclosed of the present disclosure should be covered by the protection scope of this disclosure.