JP4129237B2 - Glass for solidifying radioactive waste - Google Patents

Description

  The present invention relates to a glass for solidification treatment of radioactive waste made of magnesium phosphate glass. In addition, the present invention relates to a radioactive waste-containing vitrified body using the above glass and a method for solidifying radioactive waste.

Nuclear power generation has the great advantage of processing spent fuel and reusing energy (reprocessing), but also has the problem that various radioactive wastes involved in the process are inevitably generated.
Among these radioactive wastes, high-level radioactive waste that is taken out during reprocessing contains a lot of radioactive materials with high radioactivity levels and long half-lives, and generates heat, so it is safe to use cement, glass, etc. Research and development of a method for processing into solids is underway, and now it is possible to reduce the volume and make it the most chemically stable vitrified body, and then isolate it in a stable formation at a depth of 300 m or below. "Geological disposal" will be performed.

Glass is used because waste contains various radioactive materials with different ionic radii, and glass having a network structure with irregular atomic arrangement is advantageous for containing them. This is because it can be made into a small solidified body and is easy to handle, and glass is inherently a stable substance that hardly elutes into water.
Examples of inorganic substances that can be vitrified include oxides such as boron, silicon, phosphorus, and germanium, chlorides, and sulfurates. Among them, SiO ₂ is one of the oxides that most easily generate glass, but melting of SiO ₂ requires a high temperature of 1800 ° C. or more, and has a problem that it is difficult to form because of its high viscosity. is borosilicate glass having a reduced melting temperature by adding boron to SiO ₂ (component ratio B ₂ O ₃ / SiO ₂ of about 0.3) is widely used as a solidification glass radioactive waste (See, for example, Patent Document 1, Patent Document 2, and Patent Document 3).
Such borosilicate glass has a low melting temperature, excellent corrosion resistance to chemicals, high resistance to heat and radiation, a relatively high waste content, and is particularly difficult to adapt to a glass structure. There are advantages such as relatively high tolerance for melting.

However, it has been pointed out that borosilicate glass is difficult to solidify a large amount of waste, has a poor leaching rate with respect to water, and has problems in cases where there are many phosphoric acid components in the waste.
For example, in the vitrification treatment with borosilicate glass, 75% of glass raw material and 25% of waste are generally used, and a large amount of glass is used for waste. This is because when a large amount of waste components are contained, a phase separation phenomenon in which precipitates containing Mo as a main component occurs, the containment function of the solidified glass is remarkably lowered, and the decay heat of fission products in the wastes. This is because, since there is always heat generation, increasing the waste content in the vitrified body raises the temperature of the central portion of the solidified body and changes the properties of the solidified glass.
In addition, if the phosphoric acid (P ₂ O ₅ ) concentration in the waste is increased, the glass is phase-divided and the stability is lowered. Therefore, the waste content is adjusted so that the P ₂ O ₅ concentration is 1 to 3% by mass. It is necessary to limit the amount.
Furthermore, there remains a problem of so-called “sodium effect” in which the amount of sodium elution increases.

On the other hand, phosphate glass, like borosilicate glass, has the advantages of having a low melting point and being easy to mold and incorporating more types of elements. Application to solidification treatment of radioactive waste is being studied. However, the iron-based and lead-based phosphate glasses that are easily corroded and have a high sodium elution amount have not yet been solved.
Therefore, at present, there is a demand for providing a glass that can stably solidify a large amount of high-level radioactive waste containing many elements and having high risk.
JP 57-197500 A JP-A-8-233993 Japanese Patent Laid-Open No. 9-72998

  The problem of the present invention is to solve the above-mentioned problems in the prior art, (a) the waste content can be increased and a large amount of waste can be solidified, (b) the leaching rate with respect to water is small, (c) Provided is a solidified glass for radioactive waste solidification treatment and a solidification treatment method for radioactive waste, which have characteristics such as good stability even if there are many phosphoric acid components in the waste, and (d) a small amount of sodium elution. There is.

  The present inventors also have a low melting temperature, a very wide range of vitrification, a chain / short chain, even in a phosphate glass having a completely different composition from the borosilicate glass widely used in the current solidification treatment. Intensive research focusing on the fact that a large amount of network-modifying ions can be introduced into the structure and ortho-salt, and that chemical durability generally improves as the number of network-modifying ions increases. As a result, it was found that a phosphate glass having a specific magnesium content, which is a salt with magnesium among many phosphates, can solve the above-mentioned problems in the prior art, thereby completing the present invention. It came to.

That is, the present invention is a radioactive waste characterized in that MgO and P ₂ O ₅ are made of magnesium phosphate glass containing a molar ratio (MgO / P ₂ O ₅ ) of 0.25 to 2.35. This is a glass for solidification treatment of objects.

  Further, the present invention is characterized in that in the radioactive waste vitrified body in which the radioactive waste is encapsulated in glass, the glass for solidification treatment is used, and the content of the radioactive waste is 5 to 60% by mass. It is a vitrified material containing radioactive waste.

  Furthermore, in the vitrification treatment in which the radioactive waste is mixed with molten glass and contained in the glass, the present invention is for the radioactive waste and the solidification treatment so that the radioactive waste content in the mixture is 5 to 60% by mass. After mixing with glass, the mixture is heated and melted, and then cooled and solidified, thereby solidifying the radioactive waste.

  According to the glass for solidification processing of radioactive waste and the solidification processing method of the present invention, a large amount of waste can be taken in, and it is excellent in chemical stability and durability, and the leaching rate with respect to water is small. There is no limitation on the content of the phosphoric acid component, and the sodium elution amount is extremely low.

Hereinafter, the present invention will be described in detail.
(1) Magnesium phosphate glass The magnesium phosphate glass (MgO-P ₂ O ₅ glass) of the present invention has a molar ratio of phosphoric acid and magnesium oxide (MgO / P ₂ O ₅ , that is, the number of moles of MgO). ) Divided by the number of moles of P ₂ O ₅ ) of 0.25 to 2.35, preferably 0.65 to 1.5, and particularly preferably 0.8 to 1.25. When the molar ratio is less than 0.25, the hygroscopicity is increased and the stability of the glass is lowered. On the other hand, when it exceeds 2.35, the glass is not vitrified and devitrified, which is not preferable.

Regarding the raw material of the magnesium phosphate glass of the present invention, orthophosphoric acid (H ₃ PO ₄ ), diphosphorus pentoxide (P ₂ O ₅ ), ammonium dihydrogen phosphate (NH ₄ H ₂ ) as a phosphorus (P) source. Examples thereof include phosphorus compounds such as PO ₄ ) and diammonium hydrogen phosphate ((NH ₄ ) ₂ HPO ₄ ), while magnesium (Mg) sources include magnesium oxide (MgO), magnesium carbonate (MgCO ₃ ), magnesium chloride ( Examples include magnesium compounds such as MgCl ₂ ). Among these raw materials, a combination of orthophosphoric acid and magnesium oxide or magnesium carbonate is preferable from the viewpoint of safety in which handling is easy and no toxic gas is generated during production, and a combination of orthophosphoric acid and magnesium oxide is particularly preferable. preferable. Incidentally, magnesium phosphate glasses none glass obtained by reacting a phosphorus compound and the magnesium compound is such raw material consists of MgO · P ₂ O _5.

The glass for solidification processing of this invention can be manufactured as follows.
That is, the raw material phosphorus compound and magnesium compound are weighed by a weighing machine such as a hopper scale so that the phosphoric acid (P ₂ O ₅ ) and magnesium oxide (MgO) contained in the glass after production are in the above molar ratio. And then mixed in a melting furnace such as a tank furnace, a crucible furnace, an electric furnace, etc., at 900 to 1600 ° C., preferably about 1300 ° C. from the viewpoint of stirring effect by heat convection and reduction of volatilization amount of components. It can be produced by heating and melting for 30 minutes to 48 hours under temperature conditions, preferably about 3 hours from the viewpoint of stirring effect and reduction of volatilization amount of components, and then rapidly cooled and vitrified.

In the solidification processing of the radioactive waste described later, the produced glass is used after being agglomerated or powdered so as to be uniformly mixed with the waste.
Furthermore, for the purpose of improving mechanical properties such as chemical durability and strength, nitrogen can be contained as a component other than the above. Further, iron oxide or lead oxide can be added for the purpose of improving resistance to radiation.

(2) Radioactive waste The object of vitrification in the present invention is radioactive waste, especially high-level radioactive waste, which is used during solvent extraction and separation in the reprocessing process of spent fuel used in nuclear power plants. The discharged waste liquid or a solidified body obtained by drying the waste liquid.
That is, after spent fuel is dissolved in nitric acid in the reprocessing process, it is discharged from the process of extracting uranium and plutonium by organic solvent tributyl phosphate (TBP), and most of fission products, It contains actinide elements such as neptunium (Np), americium (Am), and curium (Cm), has a high level of radioactivity, and generates large decay heat.

In Japan, this liquid high-level radioactive waste is melted together with glass material, put into a stainless steel container (canister), cooled and hardened, stored for cooling for 30-50 years, and finally for geological disposal. Attached.
In the vitrified material, the radioactive material constituting the high-level radioactive waste is homogeneously and stably incorporated into the irregular network structure of the glass, and the vitrified material is broken because the glass component and the radioactive material are integrated. However, the loss of radioactive material is prevented.
As will be described later, in the examples of the present specification, in order to safely and easily evaluate glass, instead of actual high-level radioactive waste, various kinds of elements (Na, Sr, Non-radioactive simulated waste containing La, Mo, Mn, Fe, Te) was subjected to vitrification.

(3) Solidification method The above-mentioned magnesium phosphate glass is pulverized with a mixer (pulverizer) or the like to be agglomerated or powdered. Next, the crushed glass and the solid or liquid waste in the form of a lump or powder are mixed.
Radioactive waste, magnesium phosphate glass powder, alumina, and platinum so that the content of radioactive waste in the mixture is 5 to 60% by mass, preferably 15 to 50% by mass, and particularly preferably 25 to 45% by mass. , Put into a stirring vessel made of stainless steel or glass lining and mix with a mixer until the waste is evenly dispersed in the glass powder. If the content of waste is less than 5% by mass, there is a problem that the stability (chemical durability) of the solidified glass is lowered. On the other hand, if it exceeds 60% by mass, phase separation occurs, without vitrification. Problems arise such as devitrification and reduced chemical durability.

  Next, the glass / waste mixture is melted at 900 to 1600 ° C., preferably about 1300 ° C., for 30 minutes to 48 hours, and then rapidly cooled to −50 to 100 ° C., preferably close to room temperature to solidify the glass and radioactive. A waste-containing vitrified body is obtained.

EXAMPLES Next, although an Example is shown and this invention is demonstrated further more concretely, this invention is not limited to these Examples.
<A> Preparation of Simulated High-Level Radioactive Waste Simulated high-level radioactive waste for evaluating the glass for solidification treatment and the solidification treatment method of the present invention was obtained from M. Ishida, T. Yanagi and R. Terai, Journal of Nuclear. Prepared by referring to Science and Technology, 24 [5] (May 1987) 404-408. Specifically, a predetermined amount of each raw material containing the waste elements shown in Table 1 was mixed and prepared.

<B> Production of Magnesium Phosphate Glass (1) Production of Glass with MgO: P ₂ O ₅ = 45: 55 (Molar Ratio 0.82) Magnesium Oxide and Phosphoric Acid at a Molar Ratio, MgO: P ₂ O ₅ = 45: 55. Weighed magnesium oxide in water in a beaker and mixed with phosphoric acid. The product was water removed to some extent using a mantle heater and then placed in a platinum crucible and heated in an electric furnace at 600 ° C. for 3 hours to further remove the water. The obtained batch was further melted at 1250 ° C. for 1 hour, then poured onto a stainless steel plate and rapidly cooled (natural cooling in air at room temperature) to prepare a magnesium phosphate glass. This glass is abbreviated as “45M55P”.

(2) Preparation of glass with MgO: P ₂ O ₅ = 55: 45 (molar ratio 1.22) Weigh phosphoric acid and magnesium oxide in a molar ratio of MgO: P ₂ O ₅ = 55: 45. Except for mixing, a magnesium phosphate glass was obtained in the same manner as 45M55P. This glass is abbreviated as “55M45P”.

<C> Production of pseudo radioactive waste-containing glass-solidified body The produced 45M55P glass was made into a powder form having a size of about 10 mesh using a stainless mortar. Next, the simulated waste listed in Table 1 was mixed so that the content was 25% by mass, then placed in an alumina crucible and melted at 1250 ° C. for 2 hours, and then rapidly cooled (natural cooling in air at room temperature). A waste-containing vitrified body (abbreviated as “45M55P25W”) was produced.
Moreover, after mixing so that a simulated waste content rate might be 45 mass% using 45M55P glass, the waste containing glass solidified body (it abbreviates as "45M55P45W") was obtained on the same conditions.
Further, 55M45P glass was used under the same conditions to obtain waste-containing glass solids having a simulated waste content of 25% by mass and 45% by weight, respectively (abbreviated as “55M45P25W” and “55M45P45W”, respectively).

<D> Physical Property Evaluation of Magnesium Phosphate Glass and Pseudo-Radioactive Waste Glass Solidified Body A total of 6 samples of the simulated waste-containing glass solidified body obtained in <C> and the base glass not containing waste, Density measurement, qualitative analysis by X-ray diffraction (XRD), and differential thermal analysis (DTA) were performed.
(1) Measurement of density The density of the sample was measured at room temperature using the Archimedes method.
Using kerosene as the immersion liquid, first, the density (d ₀ ) of kerosene was obtained from the following formula (I) by the pycnometer method (the corrected pycnometer volume was 24.999 cm ³ ).
Next, the density (d) of the sample was determined from the following formula (II) by the Archimedes method.

d ₀ = (W ₁ −W ₀ ) /24.999 Formula (I)
[Wherein W ₀ represents the weight of kerosene and W ₁ represents the weight of the pycnometer]
d = [W / (W−W ′)] × d ₀ formula (II)
[Wherein, W represents the weight of the sample in the air, and W ′ represents the weight of the sample in the liquid]
Table 2 shows the measurement results.

It is known that MgO—P ₂ O ₅ glass has a minimum density (about 2.20 (g / cm ³ )) of meta composition (MgO: P ₂ O ₅ = 50: 50 (molar ratio 1)). (Phosphate abnormal phenomenon). In the glass with a molar ratio of 0.82, the density of the vitrified body is almost constant even when the waste content increases, whereas in the glass with a molar ratio of 1.22, when the waste content increases, the glass The density of the solidified body was increased to 2.88 (g / cm ³ ) in the 55M45P45W sample. This indicates that the glass having a molar ratio of 1.22 contains the elements in the waste more stably in the glass network than the glass having a molar ratio of 0.82.

(2) Analysis by X-ray diffractometry (XRD) Powder X-ray diffraction of the vitrified body is performed on each of the prepared sample and the sample heat-treated at 500 ° C. for 2 hours to determine whether it is crystalline or amorphous. It was judged. Table 3 shows the diffraction results. A mark with an halo pattern peculiar to an amorphous was marked with a circle and a mark with a crystallization peak was marked with a cross.
In general, when glass is crystallized, the solubility in water increases and the leaching rate with respect to water increases. As a result, the waste containment function is lowered, which is not preferable. However, the glass having a molar ratio of 0.82 has a waste content of 25 mass. %, The glass is amorphous, and the glass having a molar ratio of 1.22 is amorphous even when the waste content is 45%.

  The XRD measurement was performed under the following measurement conditions using “RINT2000” manufactured by Rigaku Corporation as a measuring instrument.

(3) Differential thermal analysis (DTA)
DTA measurement was performed to investigate the thermal properties of the vitrified body. As a measuring instrument, “Rigaku Thermoflex TAS-200 TG8101D” manufactured by Rigaku Corporation was used, and the measurement was performed under the following conditions.

  The DTA measurement result is as shown in FIG. 1, (a) shows a sample 45M55P0W, (b) shows 45M55P25W, (c) shows 45M55P45W, (d) shows 55M45P0W, (e) shows 55M45P25W, and (f) shows 55M45P45W. In the figure, “Exo.” Represents heat generation and “Endo.” Represents heat absorption.

Table 6 shows a crystallization start temperature (Tx), a glass transition point (Tg), and a temperature difference (Tx−Tg) serving as an index of glass stability of each sample. In general, the higher the glass transition point, the more thermally stable the glass, and the larger the temperature difference (Tx−Tg), the higher the stability of the glass.
As is apparent from Table 6 in the case of the magnesium phosphate glass of the present invention, in the glass having a molar ratio of 0.82, the value of (Tx−Tg) gradually decreases as the waste content increases. Stability declines. The glass with a molar ratio of 1.22 has a stability of 55M45P25W (Tx-Tg = 57 ° C.) containing 25% by mass of waste compared to 55M45P0W (Tx-Tg = 128 ° C.) of the base glass not containing waste. Although the properties decreased, the stability of 55M45P45W (Tx-Tg = 80 ° C.), which was further increased by increasing the waste content to 45% by mass, increased. This indicates that the glass having a molar ratio of 1.22 contains the elements in the waste more stably in the glass network than the glass having a molar ratio of 0.82.

As a comparative example, iron phosphate glass measured under the same conditions (compositions were P ₂ O ₅ : 60.06 mass%, Fe ₂ O ₃ : 25.74 mass%, CeO ₂ : 9.70 mass%) Table 7 shows the DTA measurement results of the waste-containing vitrified product using Li ₂ O: 4.50% by mass).
Such iron phosphate _{(P 2 O 5 -Fe 2 O} 3 -CeO 2 -Li 2 O) based glass, a raw material of _{(NH 4) 2 HPO 3,} Fe 2 O 3, CeO 2, Li 2 CO 3 The mixture was mixed in a magnetic mortar for 1 hour, the mixed powder was transferred to an alumina crucible, melted at 1250 ° C. for 2 hours in an electric furnace, melted, poured into a stainless steel plate, and cooled.
In addition, the waste-containing vitrified body is made by powdering the produced glass with a stainless mortar, mixing a predetermined amount of the simulated waste shown in Table 1 with an alumina mortar, and melting in an electric furnace at 1250 ° C. for 2 hours. After melting, it was poured into a stainless steel plate and cooled.

  In the iron phosphate glass of the comparative example, the Tx-Tg temperature difference rapidly decreases from 167 ° C. to 59 ° C. when the waste content increases from 0% to 45%, whereas the phosphorus of the present invention In the case of magnesium acid-based glass, the temperature difference decreases from 104 ° C. to 62 ° C. or from 128 ° C. to 80 ° C., but is not so severe, and the temperature difference itself is large, so it can be said that it is more stable.

<E> Chemical durability The elution of the waste-containing vitrified product into water was measured by ICP emission spectroscopic analysis.
About 1 g of a glass solid sample ground to a size of 10 to 20 mesh using a stainless mortar to adjust the specific surface area is weighed, put into a stainless steel screw can, immersed in 50 ml of pure water, and the screw can Was kept in a dryer at 90 ° C. for 20 days, and then the filtered sample was dried and weighed again.
Further, the filtrate containing the eluate from the vitrified body was subjected to qualitative and quantitative analysis using an ICP emission spectroscopic analyzer (“Inductively Coupled Plasma Emission Spectrometer SPS 7000” manufactured by Seiko Denshi Kogyo Co., Ltd.). The results are shown in Table 8.
The surface area S of the portion where the glass is in contact with water was determined by the following formula.
S = W · S ₀ / ρ
[W is the amount of vitrified glass immersed in water, S ₀ is the specific surface area, and ρ is the density of the glass]

As is clear from Table 8, the leaching rate tended to decrease as the waste content increased, and the glass solid (55M45P45W) containing 45% by mass of simulated waste in a 1.22 molar ratio glass. The leaching rate was the lowest. It can be seen that the leaching rates of the glass samples (45M55P0W, 55M45P0W) containing no simulated waste are high, and the chemical durability of the glass sample itself to water is not high.
Next, Table 9 shows the results of qualitative and quantitative analysis of the filtrate using an ICP emission spectroscopic analyzer.

As apparent from Table 9, every glass solidified body had a high leaching rate of Mg and P, which are the main components of glass. When attention was paid only to simulated waste, the leaching rate of Na and Mo was high.
The most excellent vitrified body among the above samples was 55M45P45W containing 45% by mass of simulated waste with a glass having a molar ratio of 1.22 having a low leaching rate with respect to water and a relatively high stability of the glass. . This is considered to be closely related to the structure of the magnesium phosphate glass.

The glass structure having a molar ratio of 0.82, that is, MgO: P ₂ O ₅ = 45: 55 is a structure containing tetrametaphosphate ions. On the other hand, it is considered that the glass structure containing 45% by mass of the simulated waste at a molar ratio of 1.22, that is, MgO: P ₂ O ₅ = 55: 45 contains pyrophosphate ions. Therefore, although 45% by mass of simulated waste is contained at a molar ratio of 0.82 including a two-dimensional cyclic structure, 45% of the simulated waste is present at a molar ratio of 1.22, which has more one-dimensional chain structure than glass. It is possible to introduce a larger amount of various ions in the glass containing mass%.
In addition, since the chemical durability of phosphate glass is improved as the amount of network-modifying ions increases, among the glass with a molar ratio of 1.22, the content of simulated waste is 45% by mass of 55M45P45W. It is thought that chemical durability became high.

Next, an observation result by a scanning electron microscope (SEM; “S-2380N” manufactured by Hitachi, Ltd.) is shown. FIG. 2 is an SEM photograph, in which (a) shows a sample 45M55P25W, (b) shows 45M55P45W, (c) shows 55M45P25W, and (d) shows 55M45P45W. Similar to the XRD measurement results, the glass state was observed in (a) and (d), and the formation of crystals was observed in (b) and (c). Further, when (a) and (d) were compared, the leaching rate was the lowest (d) was a single phase, whereas in (a) a phase separation was observed.
It is clear from the SEM photograph that 55M45P45W containing 45% by mass of simulated waste at a molar ratio of 1.22 is the best among the vitrified bodies.

The borosilicate glass actually used in the nuclear fuel cycle mechanism has a glass transition point of around 500 ° C. and a leaching rate with respect to water of 2.5 × 10 ⁻⁵ (g / cm ² · day). It can be seen that the vitrified body (55M45P45W) containing 45% by mass of simulated waste at such a molar ratio of 1.22 has excellent characteristics.

As a comparative example, lead phosphate glass (composition is P ₂ O ₅ : 58.5 mass%, PbO: 27.3 mass%, CeO ₂ : 9.7 mass%, Li ₂ O: 4.5 mass%) ) Waste solidification capacity.
The lead phosphate glass to be evaluated was mixed with 11.70 g of (NH ₃ ) ₂ HPO ₄ , 2.73 g of PbO, 0.97 g of CeO ₂ , 0.45 g of Li ₂ CO ₃ at 950 ° C. After melting for 1.5 hours, it was produced by naturally cooling in air at room temperature.
Next, the prepared lead phosphate glass powder and the simulated radioactive waste listed in Table 1 were mixed so that the waste content was 25% and melted at 1150 ° C. for 2 hours to solidify the waste-containing glass. Got the body. Similarly, waste-containing vitrified bodies having waste contents of 30%, 35%, and 45% were prepared.

  The presence or absence of phase separation of the prepared vitrified body was observed with a stereomicroscope ("Stereoscope SPH-40L" manufactured by Carton Optical Co., Ltd.). Further, the presence or absence of a crystal phase was measured by X-ray diffraction. For the measurement of X-ray diffraction, the prepared vitrified material was pulverized with a stainless mortar and then finely pulverized with an agate mortar. The powder was subjected to powder X-ray diffraction measurement. The results are shown in Table 8.

As shown in Table 10, in lead phosphate (P ₂ O ₅ —Pb—CeO ₂ —Lii ₂ O) glass, phase separation occurs when the waste content exceeds 25%, and a crystal phase is generated. The rate limit is 25%.

It is the graph which described the differential thermal analysis (DTA) measurement value of the vitrified body sample, a horizontal axis represents temperature and a vertical axis | shaft represents calorie | heat amount. It is a scanning electron micrograph of a vitrified body sample.

Claims (1)

Glass for radioactive waste solidification treatment, characterized in that MgO and P ₂ O ₅ are made of magnesium phosphate glass containing a molar ratio (MgO / P ₂ O ₅ ) of 0.25 to 2.35. .

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