JPS60159699A - Method of solidifying nuclear waste - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for immobilizing nuclear waste. Radioactive sodium sulfate waste slurry is the primary liquid waste generated in boiling water nuclear reactors using spherical resin purifiers. At present, this slurry is concentrated in an evaporator to a concentration of about 25% by weight and then fixed in cement. One drum of slurry yields approximately three drums of final solidified waste. The solidified waste is sent to a landfill, at a cost of as much as $100 per three drums. Due to such high costs, the above situation is considered unsatisfactory from a technical standpoint. From the standpoint of public safety and business image, it is desirable to fix the waste in a material that has a lower leakage rate and greater mechanical stress than cement i.

Although glass is a better material than cement as a sealing material, sodium sulfate L waste and glass cannot coexist and have a tendency to form one or more glass phases.
It has hitherto not been possible to immobilize sodium sulfate waste in glass materials.

US Pat. No. 3,1343,0E12 discloses a method for solidifying liquid nuclear waste containing sodium or sodium compounds by calcination in a fluidized bed. U.S. Pat. No. 4,028,285 discloses a method for converting sodium nitrate-containing liquid radioactive waste into a stable form by adding clay. A method for immobilizing nuclear waste in an aqueous slurry comprising:
The slurry water is evaporated to form a concentrate, and a destabilizing metal compound that forms an unstable sulfate is added to the concentrate in an amount of 50 to 200% based on the weight of sodium sulfate. mixed,
The amount of the destabilizing metal compound mixed is about 5 to 20% by weight based on the total amount of the sodium sulfate and the destabilizing metal compound as the reducing agent, so that the generation of sulfur dioxide gas is substantially stopped. heated to 700-900"0 and mixed with 65-80% of the total weight of dark glass forming agent,
It is an object of the present invention to provide a method characterized by heating to 1050 to 1200°C and cooling to room temperature.

According to the present invention, it has been found that radioactive sodium sulfate waste can be fixed in glass by treating the radioactive sodium sulfate waste to remove the sulfate. Two conditions need to be present for sulfate removal.

(1) the sulfate is temperature unstable; and (2) the presence of a reducing atmosphere or reducing tX) material. Sodium sulfate is a stable sulfate salt and a weak oxidizing agent, so
Sodium sulfate itself does not satisfy both conditions of −1;

Sulfate stability is greatly influenced by the cations present. Although sodium stabilizes 1 u of sulfuric acid, it was found that iron compounds cause destabilization. Therefore, by adding an iron compound to sodium sulfate together with a strong reducing agent, both conditions necessary for sulfate removal can be satisfied. Once the sulfate is removed, the remainder of the radioactive waste can be combined with glass formers into a stable glass product.

When the method of the present invention is used, the coexisting glass product produced from the slurry is only about 1/3 of one drum, which is different from the amount equivalent to three drums when cement is used. This reduction in waste volume by a third results in significant savings in drum transport and storage costs.

Additionally, glass-fixed waste has a lower radionuclide leakage rate and higher mechanical strength than cement-fixed waste.

For these reasons, there is little chance of environmental contamination, and it is highly safe as a sealed body for radioactive nuclides.

The method of the present invention can be applied to any aqueous slurry containing sodium sulfate. The present invention is particularly intended for the treatment of radioactive waste containing sodium sulfate slurry, which is the bottom residue of the evaporator of a boiling water nuclear reactor. In reality, the sodium sulfate content is 15-40%
Generally speaking, the above slurry is approximately 2
Contains 5% (all percentages herein are by weight) of sodium sulfate (based on the weight of the slurry). The slurry has 5f properties containing various hydroxides, nitrates, and boric acid compounds.

These compounds do not interfere with the implementation of the method of the invention and help produce good quality glasses. However, the melting temperature of refractory elements such as aluminum, zirconium, tritium, rare earth elements, etc. becomes excessively high when the content thereof becomes high, and therefore, the content of refractory elements such as aluminum, zirconium, tritium, and rare earth elements must be limited to about 5z or less in the slurry solid carving. Halogen compounds other than fluoride should not exceed 1% or 2% (based on slurry solids) because they form a second glass phase.

However, in any case, halogen compounds are corrosive and are usually excluded from reactor fluids to protect stainless steel piping. Although phosphates and carbonates may also be present, these compounds are generally compatible with the vitrification process utilized in the present invention.

In one embodiment of the present invention, as a first step, the water in the sodium sulfate is evaporated in a dryer with an agitator to reduce the water content to less than 5z (relative to the weight of the slurry);
Form into solid particles or powder. Solids pumping, where too much water is present, creating bubbles and escaping water vapor blowing solids out of the reaction vessel.
This causes water to be removed. 150°C
Heat the slurry for the required time to evaporate the water.

In the next step, a destabilizing compound and a reducing agent are added,
Remove sulfates. If desired, destabilizing compounds and reducing agents may be added prior to evaporation. The reason why sulfate must be removed when sodium is present is that sodium sulfate melts at around 880°C without decomposing, and the resulting liquid is incompatible with typical glass melts. It is a certain thing. In order to fix radioactive waste with glass, it is essential that the radionuclides and waste be compatible with the glass, and this essential requirement is 11! You will only be satisfied if you remove the part. In the method of the present invention, after forming sulfates which are less stable than sodium sulfate, M and acid salts are removed by decomposing the unstable sulfates into various sulfur dioxide gases.

For this reason, cations that cause destabilization are added together with a reducing agent.

Destabilizing compounds are metal salts that form unstable sulfates. Unstable sulfur nIIJ! ! decomposes when heated instead of causing a certain change in the melting point phase. Unstable sulfuric acids generally decompose in the range of 400°C to 800°C. Examples of compounds suitable as destabilizing compounds include ferrous ammonium sulfate, ferrous sulfate, bismuth sulfate, cuprous sulfate, aluminum sulfate, gallium sulfate and manganese sulfate. Approximately 15-20% reduction of ferric compounds to corresponding ferrous compounds
Ferric compounds such as ferric ammonium sulfate and ferric nitrate can also be used with the addition of a reducing agent. A compound with very good action properties is ferrous ammonium sulfate, and this compound is particularly preferred. The amount of the destabilizing compound added is 50 to 200% of the weight of sodium sulfate in the slurry.
%. If the amount used is less than 50%, all of the sulfate ions in the slurry cannot be destroyed. Even if the number exceeds 200, there is no point in using it, and the amount of waste that must be disposed of simply increases. Preferably, a 1:1 weight ratio of ferrous ammonium sulfate to sodium sulfate is added, and about 10% of the total solids weight of graphite is added.

The strength of the reducing agent used must be at least comparable to the reducing power of hydrogen as reducing agent at 400°C. Reducing agents suitable for use include hot hydrogen, dry ammonia, hydrazine and light hydrocarbon amines such as methylamine, dimethylamine, trimethylamine. Carbon in the form of graphite is
Carbon in the form of graphite is the preferred reducing agent because it exhibits good working properties, is safe to use, and produces a product in the form of carbon dioxide that is removed and thus has no adverse effect on the glassware. The amount of reducing agent is between 5 and 20% based on the total weight of sodium sulfate and destabilizing compound.

If the amount of reducing agent is less than the above range, some of the sulfates will not be decomposed, and if the amount used exceeds the above range, the vitrification temperature will increase. A suitable composition has about 20% of the total composition weight.
~35% nuclear waste concentrate with 15-402 N
50 to 20 o
z destabilizing compound and 5 to 20% reducing agent based on the combined amount of sodium sulfate and destabilizing compound.

In the next step, the slurry concentrate is heated to 700-900°C to decompose the sulfate into sulfur oxide gases, mainly carbon dioxide (sulfur dioxide gas), and convert these gases into powder or particulate form. Drive away from solids. Heating is continued until the volatilization of sulfur dioxide gases has substantially stopped, but for this purpose heating must be continued for 8 hours or more.

In the next step, the remaining concentrate is mixed with a glass-forming agent. Glass forming agents are compounds commonly used for glass formation, such as boron oxide and silica mixed with glass stabilizers such as alumina or lime. Several combinations of glass forming agents are suitable for the present invention and may be selected depending on the desired glass type, as is well known in the art. A suitable composition range for the borosilicate glass composition is 20-40% boron trioxide, 1-5% lime or alumina (which acts as a stabilizer to prevent the glass from cracking during cooling after vitrification), and waste material. 20
The composition consists of ~35%.

The amount of waste mixed into the glass forming agent is within the above range -1;
If it is, it will not melt at the melting temperature and a uniform product cannot be obtained. If the waste glass is reduced below the above range, the amount of waste glass that must be stored increases unnecessarily. A borosilicate glass comprising about 33% boron trioxide, about 311% silica, and about 2z alumina or lime is preferably mixed with about 331% waste concentrate.

There is no need to add clay or cryolite.

Halogen illuminants are difficult to remove and form a second phase within the glass melt and must be avoided.

After adding the glass former, the mixture is brought to the melting point of the glass,
Generally, it is heated to a temperature of 1050°C to 1200°C.

If the temperature is lower than 1050° C., homogeneous glass melting will not occur, resulting in poor quality glass or ceramic. If a suitable container is available, the glass melting temperature can be further increased. Continue heating until a homogeneous glass melt is obtained. Generally, it takes about 2 hours to obtain a homogeneous product, and shorter melting times result in a non-homogeneous glass melt, resulting in a lower quality product. Long vitrification times of up to 8 hours can be used, and long vitrification times are only limited by economic considerations and corrosion of the container. By slowly cooling the melt to room temperature,
Anneal the melt. Annealing can also be carried out inside the furnace itself, or the melt can be poured into a thermally insulated vessel that allows slow cooling of the melt. Diameter 15.2c+m for glass
(8 inches) depth? When using a 30 inch stainless steel can, the typical minimum annealing time is 4 hours and the maximum annealing time is 24 hours. Next, the cooled glass is loaded into a drum or the like and transported to a storage facility.

Next, the present invention will be explained in terms of examples.

Lidera LILi 10 g of ferrous ammonium sulfate and 4 g of Graphi I. were mixed with sodium sulfate waste consisting of 10 g of sodium sulfate and 30 g of water.

The mixture was dried by heating to 150° C. for at least 2 hours under reduced pressure to a moisture content of less than 5 g.

Next, the mixture was heated to about 800°C and reacted for 4 hours to decompose sulfates and expel sulfur dioxide gas. The resulting calcined product was cooled, and to 5 g of it were added 5 g of silica, 5 g of boron trioxide, and a trace amount of lime stabilizer. The mixture was melted and vitrified at 1100°C until a homogeneous melt was obtained, which required 1 hour. The resulting product was a black, high quality glass.

To a slurry containing 70 g of sodium sulfate and 210 g of water, 100 g of ferrous ammonium sulfate and 40 g of ferrous ammonium sulfate were added.
The same treatment as in Example 1 was performed, except that the sulfate removal time was 2 hours instead of 4 hours. 10 g of silica to 10 g of the firing mixture;
10 g of boron trioxide and 1 g of lime were added. This mixture was vitrified at 1100°C to obtain a glass product of good quality.

Other successful experiments included iron additives to help destabilize the sulfuric acid.

The iron additive used in other experimental examples was ferric oxide.

However, experiments were unsuccessful when using sulfates or reducing agents such as zinc sulfate, urea, and carbon in the absence of iron compounds.

Claims (1)

[Claims] 1. A method for immobilizing nuclear waste in an aqueous slurry containing sodium sulfate, the method comprising evaporating water from the slurry to create a concentrate and forming unstable sulfates. A destabilizing metal compound is added to the concentrate at an amount of 50 to 200% lIN based on the weight of sodium sulfate, and the destabilizing metal compound is mixed in! 11 is about 5 to 20% by weight based on the total Ql of the sodium sulfate and the destabilizing metal compound that is the reducing agent, and the temperature is set at 700 to 800 °C until the generation of sulfite 111 substantially stops. heated and mixed with a glass forming agent in an amount of 65-80% of the total weight, 1050
A method characterized by heating to ~1200°C and cooling to room temperature. 2. A method according to claim 1, characterized in that the destabilizing compound and the reducing agent compound are mixed into the slurry before evaporating the water from the slurry. 3. The method according to claim 1 or 2, characterized in that the water content in the slurry is evaporated until the water content becomes 5z or less of the dry solid electrical charge. 4. Process according to claim 1, 2 or 3, characterized in that the concentrate is 15-40% by weight of sodium sulfate. 5. The destabilizing compound is ferrous ammonium sulfate, ferrous sulfate, ferric sulfate, ferric nitrate, bismuth sulfate, cupric sulfate, aluminum sulfate, gallium sulfate, manganese sulfate, or mixtures thereof. 5. A method according to claim 1, 2, 3 or 4, characterized in that: 6. The method according to claim 1, 2, 3, or 4, wherein the reducing agent is carbon, hydrogen, dry ammonia, hydrazine, or light hydrocarbon amines. .