PH22647A - Immobilization of sodium sulfate radwaste - Google Patents

Description

IMMOBILIZATION OF SODIUM SULFATE RADWASTE

‘ BACBGROUND OF THE INVENTION

Sodium sulfate radwaste slurry is the prime liquid waste generated from boiling water re- actor facilities which use bead resin cleanup.

At the present time, the slurry is concentrated into an evaporator to about 25 wt.% and then is’ - 7 immobilized in cement. One drum of slurry gene- « rates about three drums of final solidified waste.

The soliflified waste is shipped to a burial site . 10 at a cost that approaches $1000 for the three drums. This situation is considered to be unsa- tisfactory in the industry due to the high cost involved. Also, to improve the public safety and the industry's image it is desirable to immobilize the waste in a material which has a lower leach 0 rate and a higher mechanical stress than does cement. . While glass is superior to cement as a con~ tainment material, until now it has not been possible to immobilize sodium sulfate radwaste in glass material as they are incompatible and tend to form more than one glass phase. : SUMMARY OF THE INVENTION ~ I have discovered that sodium sulfate rad-~ : ;. waste can be immobilized in glass if it is first on treated to remove sulfate. I have further found 2 Co that sulfate removal required the following two conditions to be present: (1) an unstable (to tem- perature) sulfate and (2) a reducing atmosphere or : matrix.

Sodium sulfate of itself meets neither of these conditions as it is a stable sulfate and it is a light oxidizer.

The stability of the sulfate is highly dependent on the cation present.

While sodium stabilizes the sulfate, I have found that iron compohinds cause instability.

Therefore, by adding — 10 an iron compound together with a strong reducing agent to sodium sulfate both conditions required for removing sulfate can be met.

Once the sulfate has been removed, the remaining radwaste can be combined with glass formers i ‘ to form a stable glass product. ) 15 The compatible glass product generated from a drum of slurry using the process of this invention fills only about a third of a drum rather than the _ three drums that using cement would generate.

As a result of this one-third reduction in waste volume, there is a teemendous savings in transportation and : storage cost of the drums.

Furthermore, glass immo- ’ bilized waste has a lower radio-nuclide leach rate and a higher mechanical strength than does cement im- mobilized waste.

For these reasons the containment of the radionuclides is safer as there is less chance of contamination with the environment.

RELEVANT ART

U.S. Patent 3,943,062 discloses the solid- dification of liquid nuclear waste which contains : 5 sodium or sodium compounds by calcifying in a fluid- ized bed.

U.S. Patent 4,028,265 discloses a method for ’ : converting sodimm nitrate containing liquid radio- active waste to a stable form by the addition of - ~ 10 clay.

DESCRIPTION OF THE INVENTION

. The process of this invention is applicable to any sodium sulfate containing aqueous slurry. The invention is particularly directed at sodium sulfate

LF 15 slurries containing radioactive waste that are the evapora tor bottoms of a bolling water reactor. These slurries are typically about 25% (all percentages herein are by weight) sodium sulfate (based on slurry . “ weight) , although in actual practice the sodium sulfate content can vary from about 15 to about 40%. The slurry may also contain various hydroxide, nitrate, , and boric compounds. These compounds are not incom- patible with the process of the invention and will aid in making a good quality glass, Certain refrac- tory type elements such as ‘aluminum, zirconium, thorium, .

and the rare earths, however, should be limited : to less than about 5% of the slurry solids because at higher percentage the melting temperature becomes excessive, Halide compounds, with the possible ex- ception of fluoride, should be avoided in excess of about 1 or 2% (based on slurry solids) as they tend

Ea to form a second glass phase. However, these compounds are generally excluded from the reactor fluid anyway because of their corrosive nature and stainless steel : = 10 piping, Phosphate and carbonate compounds may also be present, but they are generally compatible with the vitrification process used in this invention.

In the first step of the process of this inven- tion, the water in the sodium sulfate slurry is eva- porated to less than about 5% (based on the total slurry weight) in a stirrer drier to form solid gra=- nules or powder. The removal of water is necessary as the presence of too much moisture could cause foam formation or solids bumping, which means that escaping oo 20 steam blows the solids out of the reaction vessel.

The evaporation of the water can be accomplished by heating the slurry to 150°C for as dong as is necessary.

In the next step of the invention, a destabili- zation compound and a reducing agent are added to remove the sulfate. The addition of a destabilizing compound and the reducing agent may be made piior to evaporation if desired. The reason that sulfate must be removed when sodium is present is that so- ~ dium sulfate melts without decomposing at temperatures near 880°C and the resulting liquid is non-miscible with a typical glass melt, Glass immobilization of radioactive waste requires the radionuclides and wakte to be miscible with glass, and this can only occur after the sulfate fraction is removed. In the pe 10 process of this invention this is accomplished by causing the formation of sulfates which are less stable than sodium sulfate, followed by the decom- :

KX position of the unstable sulfate to various sulfurous gases. This is accomplished by the addition of cations that introduce instability (along with a reducing agent) .

The destabilizing compound is a salt of a metal which forms an unstable sulfate. An unstable sulfate is one which decomposes upon heating instead of ex- hibiting a melting point phase change. Unstable sul- fates generally decompose in the 400 to 800°¢ range,

Suitable distabilizing compounds include ferrous amo- nium sulfate, ferrous Sulfate, bismuth sulfate, cupric sulfate, aluminum sulfate, gallium sulfate, and manga- nese sulfate. Ferric compounds, such as ferric sulfate and ferric nitrate, can also be ‘used if a reducing agent in an amount of hbout 15 to about 20% ie added to reduce the ferric compound in place to the corres- ponding ferrous compound, Particularly preferred : 5 is ferrous ammonium sulfate which has been found to work quite well. The amount of destabilizing compounds should be about 50 to about 200% of the weight of the sodium sulfate in the slurry. If less than 50% is used, all of the sulfate ion may not be destroyed. More or 10 than 200% serves no useful purpose and will simply add to the amount of waste that must be disposed of.

Ferric ammonium sulfate is preferably added on a one- to-one weight ratio with sodium sulfate, and graphite 3 is added at about 10% of the total solids weight.

The reducing agent used in this invention should be at least as strong a reducing agent as hydrogen (Temp 400°C). Suitable reducing agents include high

B temperature hydrogen, dry ammonia, hydrazine, and some light hydrocarbon type amines such as methylamine, dime- thylamine and trimethylamine. The preferred reducing agent is carbon, especially in the form of graphite, as it has been found to work well, 4t is safe to use, and it reacts to produce carbon dioxide which is discharged and eliminated and, therefore, has no negative effects upon the glass product. The amount of reducing agent should be about 5 to about 20% based on the total weight of the sodium sulfate and the destabilizing compound. If less reducing agent is used, some of the sulfate may not be decomposed and if more is used, the glass vitrification temperature may be : : ) raised.

A suitable composition is about 20 to about 35% based on total composition weight, of a nuclear waste concentrate containing about 15 to about 40% oT 10 sodium sulfate and less than about 5% water, about 50 to about 208%, based on sodium sulfate weight, of the destabilizing compound, and about 5 to about 20%, - based on sodium sulfate plus destabilizing compound weight, of the reducing agent. 1s In the next step of the process of this inven- tion the slurry concentrate is heated at about 700 to about 900°C to decompose the sulfate to sulfurous gases mainly, sulflur oxide gases such as Sulfur dhoxide, and to force these gases out of the powder or granular solids. Heating sheould continue until the evolution of the sulfate gases substaitially ceases, which should not exceed eight hours.

In the next step of the process of this inven- tion the remaining concentrate is mixed with glass formers in an amount of about 65 to about 80%, based on total weight. Glass formers are compounds routine- ly used to form glass such as boron oxide, and silica mixed with a glass stabilizer such as alumina or lime. Several combinations of glass formers are suitable in this invention, wnd they may be selected according to the type of glass that is desired, as : ‘is well known in the art. A suitable range for a borosilicate glass composition is about 15 to about 40% silica, about 20 to about 40% boron trioxide, and

Le 10 about 1 to about 5% lime or alumina (to act as a i stabilizer by preventing the glass from fracturing after vitrification during cooling), and about 20 to 3 about 35% of the waste. If more waste is mixed in } with the glass formers, they will not dissolve at the melting temperature and a mniform product will not be obtained. If less waste is mixed in, the . amount of waste glass that must be stored will be . unnecessarily large. A borosilicate glass consisting of about 33% boron trioxide, about 31% silica, and about 2% alumina or lime, mixed with about 33% of the : waste concentrate is preferred. :

No clay or cryolite additives are necessary.

Halide salts should be avoided because they are difficult to remove and cuase the formation of a second phase in the glase melt.

Once the glass formers have been added, the mixttwe is heated to the melting temperature of : the glass, which is typically about 1050 to about } 1200°c. Below about 1050°C a homogeneous glass melt may not be achieved, and therefore a poor qua- lity glass or ceramic may result. Higher glass melting temperatures could be used if suitable con- tainers can be found. This temperature is main- tained until a homogeneous glass melt is obtained.

Generally, about two hours are required to produce o a homogeneous product; shorter melting times may result in an inhomogeneous glass melt and therefore a poor product. Longer vitrification times, up to eight hours, are acceptable and are limited only by economics and the corrosion of the container. The melt should be annealed by allowing it to cool gra~ dually to room temperature. This can be done either in the furnace itself or the melt can be poured into containers which are insulated so that the melt cools slowly. For a 6" diameter, 30" deep stainless steel can of glass a minimum annealing time of 4 hours is : typical and a maximum annealing time would be 24 hours.

The cold glass can then be packaged in drums, or etc. and be transported to storage facilities.

The following Examples further illustrates this 10 invention:

EXAMPLE 1

A sodium sulfate slurry made of 10 grams of sodium sulfate and 30 grams of water was mixed with 10 grams of ferrous ammonium sulfate and 4 grams of graphite. The mixtures was dried by heating at least 150°C under a partial vacuum for 2 hours to a moisture content of less than 5%.

It was then heated to about 800°C and allowed to react for 4 hours which decomposed the sulfates and drove off the sulfurous gases. The resulting calcine was cooled and 5 grams of it was mixed with 5 grams of silica, 5 grams of boron trioxide, plus a trace of lime stabilizer. The mix was vitrified

E 15 by melting at 1100°C until a homogeneous melt was achieved, which required over an hour. The resulting product was a good quality black glass. = EXAMPLE 2

A slimry containing 70 grams of sodium sulfate and 110 grams of water was mixed with 100 grams of ferrous ammonium sulfate and 40 grams of graphite } and was treated as in Example 1 except that the sul- fate removal time was 2 hours instead of 4 hours. : To ten grams of the calcine mix was added 10 grams of silica, 10 grams of boron trioxide, and a gram of n lime. This mix was vitrified at 1100°C to form a : good quality glass product.

Other experiments using an iron additive for promoting instability in the sodium sulfate were : 5 generally successful. The iron additive used was : ferric oxide. However, the use of other sulfates or reducing agents such as sinc,sulfate, urea, and carbon without any iron compound being present, were not successful,

CC ~ 10 CLAIMS: 1. A methed of immobilizing nuclear waste in an aqueous slurry containing sodium sulfate comprising: ~~ (a) evaporating water from said slurry to produce a concentrate; (B) mixing into said concentrate (1) about 50 to 200% by weight, based on sodium sulfate weight, of a destabi- — lizing compound of a metal which forms an unstable sulfate decomposing in the temperature range of 400 to 800°¢; and (2) about 5 to about 20% by weight, based on the total weight of said sodium

B sulfate and said destabilizing com- pound, of a reducing agent;

Claims (8)

1. . L (C) heating at about 700 to about 900°C until the evolution of sulfurous gases substan- tially ceases; (D) mixing with glass formers in an amount of about 65 to about 80%, based on total weight; (E) heating to about 1050 to about 1200°C; and (F) cooling to room temperature.
2. 2, A method according to Claim 1 wherein the step (B) occurs prior to step (Aa). a 10
3. 3. A method according ;to Claim 1 wherein the water in said slurry is evaporated to less than 5% : by weight of the dry solids weight.

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4. 4. A method according to Claim 1 wherein said concentrate is about 15 to about 40% by weight sodium sulfate.
5. 5. A method according to Claim 1 wherein said destabilizing compound is ferrous ammonium sulfate,
6. 6. A method according to Claim 1 wherein said reducing agent is carbon.
7. 7. A method according to Claim 6 wherein the amount of said carbon is about 5 to about 20% by weight based on total solids weight.
8. 8. Nuclear waste concentrate containing com- position adapted as immobilization as stable glass J 25 product comprising: oo

   (A) about 20 to about 35%, based on total composition weight of a nuclear waste concentrate containing about 15 to about 40% by weight sodium sulfate and less than about 5% by weight water; (B) about 50 to about 200%, based on said sodium sulfate weight, of a destabilizing compound of a metal which forms an unstable sulfate decomposing in the temperature — 10 range of 400 to 800°C; and (C) about 5 to about 20%, based ontthe total weight of maid sodium sulfate and said destabilizing compound, of a reducing agent. ’ 15 WILBUR ORME GREENHALGH Inventor