GB2130783A

GB2130783A - A process for the solidification of aqueous radioactive waste

Info

Publication number: GB2130783A
Application number: GB08323443A
Authority: GB
Inventors: Martin Kadner; Dr Werner Halwachs; Henricus Muller; Dr Daniel Neupert
Original assignee: Nukem GmbH
Current assignee: Nukem GmbH
Priority date: 1982-10-21
Filing date: 1983-09-01
Publication date: 1984-06-06
Also published as: GB2130783B; DE3238962C2; JPS5975196A; GB8323443D0; DE3238962A1; FR2535102A1

Abstract

For the solidification of aqueous, nitrate-containing radioactive waste solutions by evaporating with phosphoric acid and converting the resulting phosphates into a difficultly soluble solid, a process is required which allows the production of anhydrous final storage products, with a good compatibility with the known embedding agents which can be highly charged with the solid waste. If the alkali metal phosphate residue remaining after evaporation is mixed with alkaline earth metal oxides and/or alkaline earth metal hydroxides, and heated at from 500 to 950 DEG C, alkali metal-alkaline earth metal phosphates of the rhenanite type are obtained which are practically insoluble and may be embedded in known binders in large quantities and with a good compatibility.

Description

SPECIFICATION A process for the solidification of aqueous radioactive waste This invention relates to a process for the solidification of aqueous, nitrate-containing radioactive waste by evaporating the waste solution in the presence of phosphoric acid and converting the resulting phosphates into a difficultly soluble solid.

In nuclear technology, radioactive liquid waste having a high (HAW), medium (MAW) or low (LAW), specific activity is produced. A large amount of this waste is made up of aqueous solutions of nitric acid which are produced in the waste disposal centres during the re-processing of spent nuclear fuels.

For a safe ultimate waste disposal, this liquid waste has to be converted into a final storage product which is as resistant to leaching as possible. To this end, it is necessary to neutralise the nitric acid, which is effected for the most part using soda or sodium hydroxide. However, a high sodium nitrate content is produced in the radioactive waste solutions as a result of this operation, which content has a negative effect on the properties of the final storage products or which complicates the processes for treating this waste.

A number of processes are known for the solidification of HAW liquid waste, by which the radionuclides are converted into a leaching-resistant matrix, such as phosphate- or borosilicate glass.

In these processes, the nitrate is decomposed for the most part under reducing conditions, and the remaining alkali is incorporated into the matrix.

Thus, for example in the vitrification process which is known under the name of "PAMELA" for highly radioactive waste (W. Heimerl, Behandiung hochradioaktiver Abfalie, Chemie in unserer Zeit 12, No. 3 (1978), P. 82-88), the HAW solution of nitric acid is converted into phosphate glass by evaporation, denitration in the presence of phosphoric acid, drying, calcination and melting; In this process, the evaporation is carried out according to German Offenlegungsschrift 2,240,929 under a reducing atmosphere using formaldehyde as the reducing agent. A disadvantage of this process is that the calcined material is relatively easily soluble in water.For this reason, it is converted into a waterinsoluble form by melting at in particular, from about 1 ,0000C (for example German Auslegeschrift (2,245,149) to about 1 ,2000C. This melting operation necessitates a high technical expenditure which is in fact only justified during the treatment of an HAW solution.

A modification of the PAMELA process according to German Offenlegungsschrift 2,807,324, in which, inter alia sodium dihydrogen phosphate (NaH2PO4) is used instead of phosphoric acid also requires the calcined material to be melted in order to improve the properties, such as reducing the solubility The production of a calcined material from highly active liquid waste by evaporation and denitration with the addition of pulverulent red phosphorus according to German Offenlegungsschrift 2,125,915 also necessitates the later embedding of the fission products in a glass meit.

Another method of calcining radioactive waste containing sodium nitrate is disclosed in German Offenlegungsschrift 2,603,1 57, in which the particles are prevented from bonding together in the fluidized bed by the formation of NaFeO2 by the addition of iron turnings or iron powder. However, this product is not suitable for ultimate waste disposal either due to its water-solubility.

Furthermore, European Patent Application Publication No, 43 397 discloses a process for embedding HAW waste in a glass block of polymeric aluminium phosphate. The production process of the primary phosphate Al- (H2PO4)3 which is used as the starting material for the glass matrix must be carried out carefully, so that no other undesired aluminium phosphates are produced which do not polymerise into phosphate glass. A disadvantage is also the high consumption of H3PO4 (molar ratio of H3PO4:AI3+ at least 6:1) and the high melting temperature of above 1 ,0000C. Moreover, a process which includes the fixing of HAW calcined material without a proportion of NaNO3 cannot be readily transferred to the treatment of an MAW/LAW solution having a high content of NaNO3.

The so-called MINERVA process developed by Eurochemic (C. Sombret, Present Status of Techniques Used for High-Level Liquid-Waste Solidification, Radioactive Waste Management 2 (1) (1981) p. 58-60 also uses an aluminium phosphate as a matrix for embedding the radioactive waste, which phosphate is present as granulated material in a fixed bed moved by stirring and which is continuously newly formed by the metered addition of fission product solution, phosphoric acid and aluminium hydroxide. A "mineralised" calcined material is produced at from 300 to 5500 C, the main component of which is tertiary aluminium phosphate (AIPO4).This calcined material is then either embedded in metal alloys or processed into a phosphate glass by melting at a temperature above 1 ,0000C. A particular disadvantage of this process is the producfion of a large volume of waste of radioactive substances by the high proportion of aluminium phosphate.

Unlike in the known processes for HAW conditioning, in the processes for fixing MAW- or LAW liquid waste, the dissolved sodium nitrate is for the most part also included in the matrix. As a result of the high salt content, significant incompatibilities with the matrix may occur under certain disturbance conditions, for example when cement is used as the binder, if water forces its way into the ultimate store, or when organic binders are used, in the case of fire. These incompatibilities substantially restrict the content of radioactive waste in the matrix and do not allow a further reduction in volume of the waste by increasing the concentration.

Thus, an object of the present invention is to provide a process for the solidification of aqueous, nitrate-containing radioactive waste by evaporating the waste solution in the presence of phosphoric acid, and converting the resulting phosphates into a difficultly soluble solid, which process allows the volume of the final storage products to be reduced by increasing the quantities of waste which may be embedded, provides a product having a good compatibility with known binders and allows the production of an hydros final storage products.

The present invention provides a process for the solidification of aqueous, nitrate-containing radioactive waste by evaporating the waste solution in the presence of phosphoric acid, mixing the alkali metal phosphate residue remaining after evaporation with alkaline-earth metal oxides and/or alkaline-earth metal hydroxides and then converting the product into difficultly soluble alkali metalalkaline-earth metal phosphates by a heat treatment at a temperature of from 500 to 9500 C. During the process according to the invention, the radionuclides and other impurities are fixed in phosphate mixed crystals, preferably of the rhenanite type, an NaCa(Mg) P04 compound having an apatite-like structure, which is very difficultly water-soluble.

By adding phosphoric acid to a waste solution which contains, dissoived, 300 g of Nans$1, and evaporating the solution to dryness, the nitric acid is expelled and a residue is produced which essentially consists of water-soluble sodium phosphate which contains radionuclides and other impurities.

Surprisingly, it has now been found that the water-soluble proportion of this residue is greatly reduced by mixing the residue with alkaline earth metal oxide powder and/or alkaline earth metal hydroxide powder, in particular compounds of the elements magnesium and calcium, and by heating to a temperature of from 500 to 9500C. In contrast to this calcined mixture, the water-solubility of the alkali phosphate residue only slightly decreases by a heat treatment without an alkaline earth metal addition.Thus, it has been found that the insoluble proportion of the waste converted into phosphate only increases by 2 to 10% with an increasing annealing temperature from about 10% to 3000 to 1 2 to 20% at 7000 0. However, by mixing the waste converted into alkali metal phosphate with CaO powder and heating to 7000 C, the insoluble proportion increased to 80%. A quantity of CaO sufficed for this purpose which was adequate for the formation of NaCaPO4, corresponding to a molar ratio of Na:Ca=1 :1. A maximum of only 4 g of nitrate per litre were still to be found in the aqueous extract of the annealed powder mixture, compared to at least 219 g/l in the untreated aqueous waste which contained 300 g of Nans$1.

It has proved to be particularly advantageous to adjust the molar ratio of nitrate to phosphoric acid which is added to 2 to 3 before evaporation of the solution and expulsion of the nitric acid. This measure promotes the preferred formation of Na2HPO4 as an intermediate product.

Furthermore, it has been found that the heat treatment of the alkali phosphate residue is particularly favourable at a temperature of from 300 to 5000C before mixing with alkaline earth metal oxides and/or alkaline earth metal hydroxides. This also promotes the formation of Na2HPO4 as an intermediate product.

MgO and/or CaO or hydroxides thereof are advantageously used because of their low molecular weight and low price, in particular because commercial magnesia, dolomite or caustic lime are particularly favourable in price. The addition of MgO has proved particularly advantageous. The insoluble proportion of the waste converted into alkali-metal-alkaline earth metal phosphate increased to 95% after mixing with stoichiometric quantities of MgO, and after annealing at 7000C.

The compound Na4Mg(PO4)2 which is difficultly soluble in water was detected by radiography.

The sodium content of this compound of about 30% by weight is even higher than that of the readily soluble sodium nitrate in the waste (27%). Nitrate could no longer be detected in the aqueous extract of the annealed powder mixture. The contents of soluble Cs and Sr were at the most 0.25% and at the most 0.05% respectively, and thus were lower by factors 400 and 2,000 respectively than in the aqueous waste.

A further reduction in the water-solubility was advantageously achieved by the addition of calcium hydrogen phosphate (CaHPO4) to the mixture of the waste converted into alkali metal phosphate, and CaO powder with subsequent annealing at 7000 C. The insoluble proportion increased to 97% by using stoichiometric quantities of CaO and CaHPO4 with respect to Na2HPO4 which suffice for the formation of NaCaPO4. In this case, it has proved favourable to subject the waste coverted into alkali metal phosphate to a previous heat treatment at 5000C in order to render it almost completely free of nitrate. The result of a radiographic analysis was that the insoluble compound NaCaPO4 had formed with an orthorhombic lattice structure (Buchwaldite). Upon the addition of about 10% by weight of silicon dioxide, mixed crystals of the rhenanite type (3 NaCaPO4. Ca2SiO4) having an apatitelike structure are also obtained (H. Remy, Lehrbuch der Anorganischem Chemie, 10th edition (1960), volume 1, P. 752. The smail quantities of radionuclides and other impurities of the waste are also practically completely incorporated in these insoluble mineral compounds. Surface-active SiO2 having a specific surface of, for example from 50 to 200 m2/g has proved to be particularly advantageous.

A particularly great reduction in volume is obtained by compressing the powder mixture of alkali metal phosphate residue and the additives into mouldings, for example cylinders or cuboids, before the heat treatment. It may be possible to dispense completely with the operation of binding into a leaching-resistant matrix as a result of the difficult solubility of the resulting alkali metal-alkaline earth metal phosphates.

Due to the good compatibility of the difficultly soluble, chemically practically inert alkali metalalkaline earth metal phosphate produced in this way with the known leaching-resistant matrix substances, for example cement or bitumen, a high reduction in volume may be achieved in a particularly advantageous manner by a high charging of the matrix with the alkali metal-alkaline earth metal phosphate.

The process according to the present invention will now be described in more detail using the following Examples.

Example 1 The starting material is a simulated material of the MAW/LAW liquid waste of a waste disposal centre (EZ waste simulated material) having the following composition: Na NO3 300 g/l Al 0.23 g/l Ca 1.50 gel Cr 0.08 g/l Cu 0.15g/l Fe 0.38 g/l K 0.08 gel Mg 0.75 gel Mn 0.08 g/l Mo 0.38 g/l Ni 0.08 g/l Zn 0.15g/l Zr 0.08 g/l Cs 10 9/1 Sr 10 g/l Na2HPO4 5 g/l TBP 0.2 g/l DBP 0.2 g/l Kerosene 0.2 g/l All the cations are present as nitrates, Mo as Na-molybdate.

1 litre of this EZ waste simulated material was mixed with 80 ml of H3PO4 (85% by weight) into a light green clear solution. This corresponded to a molar ratio of NaNO3:H3PO4 of 3:1. The solution was evaporated to dryness by heating with a heating hood in a 2 litre flask,71.1 by weight of the quantity of HNO3 which may be formed by the quantitative reaction of NaNO3 with HYPO4 being distilled off together with the water. This corresponds to slightly more than 2/3 of the conversion and thus of the conversion of Na NO3 into Na2HPO4. The maximum temperature in the flask was about 300 C. The residue consisted of a pink, solidified melt which was easy to crush and pulverise.

The water-insoluble proportion of this residue was 12%, and the pH of the solution was 7.5.

Further heating the residue to 7500C produced an insoluble proportion of 19%, and the pH of the aqueous extract was 9.5.

The same experiments were carried out using pure NaNO3 and with EZ waste simulated material without the addition of phosphoric acid, and with pure NaN 03 with the addition of H3PO4. The residues were heated in each case to 3000C and 7500C. The water-insoluble proportion was below 1% in each case.

Example 2 10 g of finely-powdered pink residue of Example 1 were mixed intimately with 4.67 g of CaO powder, and annealed for 1 hour at 7500C. The quantity of CaO which is used corresponds to an excess of 69%, based on the compound Na2HPO4. The water-insoluble proportion of the annealed and pulverised solid was 68.9%. The aqueous extract reacted in a strongly alkaline manner (pH 12.5). The same experiment with 4.51 g of Ca(OH)2 instead of CaO produced a water-insoluble proportion in the annealed alkali metal-alkaline earth metal phosphate of 69.5%.

Example 3 10 g of finely-powdered pink residue of Example 1 were mixed with 3.36 g of MgO powder, and annealed for 1 hour at 7500C. The water-insoluble proportion of this product was 92.5%, and the pH of the aqueous extract was 11.5.

The same experiment using 3.55 g of Mg(OH)2 instead of MgO produced a water-insoluble proportion in the annealed alkali metal-alkaline earth metal phosphate of 83.9.

Example 4 1 litre of EZ waste simulated material of the composition stated in Example 1 was mixed with 203.5 g of H3PO4 (85% by weight), corresponding to a molar ratio NaNO3:H3PO4 of 2:1. The solution was evaporated to dryness as in Example 1 , the residue reaching a temperature of about 3000 C.

Further heating of test samples of this residue produced only a slight increase in the water-insoluble proportion to 11.7%.

Insoluble Temperature proportion pH of the aqueous { C} {o/oJ extract 400 9.5 8 600 11.2 11 700 11.7 11.5 The nitrate content of the residue heated to 4000C was 30.6 mol %, based on the NaNO3 which was used. The residue was free of nitrate at 5000 C, which corresponds to the formation of Na2HPO4.

Example 5 10 g of finely-powdered residue of Example 4 which had been heated to 3000C were mixed intimately with 4.67 g of CaO. This quantity of CaO corresponded to approximately 1.7 times the stoichiometric quantity which is necessary for the formation of NaCaPO4.

The heat treatment of this powder mixture produced a considerable increase in the insoluble proportion up to 79.9% at 7000 C. The results of the parallel experiments without the addition of CaO are also stated in the following Table.

Insoluble Temperature proportion pH of the aqueous CaO-content (0C) {%) extract with 400 68.0 12.5 without 400 9.5 8 with 600 73.1 12.5 without 600 10.8 11 with 700 79.9 13 without 700 11.5 11 A clear proportion of nitrate and nitrite of a few gjl was established in the aqueous extract in every case. The solution reacted in a strongly alkaline manner (pH 12.5-1 3) due to the CaO excess.

Example 6 10 g of finely powdered residue of Example 4 which had been heated to 3000C were mixed with 3.36 g of MgO, and heated to 7000C. As a result of this measure, the water-insoluble proportion increased to 94.8%, as indicated by the following Table.

Insoluble Temperature proportion pH of the aqueous { C) {%) extract 400 54.5 11 600 90.5 12 700 94.8 11.5 The comparative values without the addition of MgO are stated in the Table of Example 4.

No nitrate and only traces of nitrite could be detected in the aqueous extracts in the case of the samples heated to 600 and 7000 C.

Furthermore, the result of a chemical analysis of the aqueous extract was that of the total quantity of Cs and Sr, only 0.14% of Cs and 0.03% of Sr in the alkali metal-alkaline earth metal phosphate of the 7000C annealing were soluble. Radiographic analysis showed that, in addition to excess MgO and y-Na3PO4, the water-insoluble compound y-Na4Mg(PO4)2 had formed as the main constituent. This compound crystallises in a cubic lattice and contains 30.0% by weight of Na.

This Example shows that the conversion of the very readily water-soluble NaNO3 (solubility 920 g/l of water) into a water-insoluble sodium compound having an even higher Na content (30%) than in the starting substance NaNO3 (27%) was achieved in an extremely economic manner.

Example 7 10 g of finely powdered residue of Example 4 which had been heated to 3000C were mixed with the stoichiometric quantity of MgO of 1.99 g for the formation of NaMgPO4, and heated to 7000 C. The product was insoluble in water to 93.4%, and the aqueous extract had a pH of 11.5 and was free of nitrate.

Example 8 lOg of finely powdered residue of Example 4 which had been pre-annealed to 5000C were mixed intimately with 2.76 g of CaO and 8.48 g of CaHPO4.2H20, and annealed for 1 hour at 7000C.

The quantities were calculated for the quantitative reaction of the Na2HPO4 into NaCaPO4. The annealed product was insoluble in water to 97%. The result of a radiographic analysis was that only the insoluble compound NaCaPO4 had formed with an orthohombic lattice structure (Buchwaldite).

The alkali metal-alkaline earth metal phosphates produced according to these Examples may be bound into known binders, such as cement, bitumen or organic plastics with a good compatibility and in large quantities.

Claims

1. A process for the solidification of aqueous, nitrate-containing radioactive waste by evaporating the waste solution in the presence of phosphoric acid, mixing the alkali metal phosphate residue remaining after evaporation with alkaline earth metal oxide and/or alkaline earth metal hydroxide, and converting the product into difficultly soluble alkali metal-alkaline earth metal phosphates by a heat treatment at a temperature of from 500 to 9500C.

2. A process as claimed in claim 1, wherein the molar ratio of nitrate to phosphoric acid is adjusted to 2 to 3.

3. A process as claimed in claims 1 or 2, wherein the alkali metal phosphate residue is heated to from 300 to 5000C before mixing with alkaline earth metal oxide and/or alkaline earth metal hydroxide.

4. A process as claimed in any of claims 1 to 3, wherein MgO and/or CaO or Mg(OH)2 and/or Ca(OH)2 are used as alkaline earth metal oxide or alkaline earth metal hydroxide.

5. A process as claimed in any of claims 1 to 4, wherein burnt dolomite, caustic lime and/or magnesia is used as the alkaline earth metal oxide mixture.

6. A process as claimed in any of claims 1 to 5, wherein magnesium is added as alkaline earth metal.

7. A process as claimed in any of claims 1 to 6, wherein the alkali metal phosphate residue is also mixed with calcium hydrogen phosphate before heating with alkaline earth metal oxides and/or alkaline earth metal hydroxides.

8. A process as claimed in any of claims 1 to 7, wherein the alkali metal phosphate residue is mixed with calcium hydrogen phosphate and surface-active silicon dioxide powder before heating with alkaline earth metal oxide and/or alkaline earth metal hydroxide.

9. A process as claimed in any of claims 1 to 8, wherein the heat treatment of the powder mixture of alkali metal phosphate residue and the additives takes place at from 650 to 8500C.

10. A process as claimed in any of claims 1 to 9, wherein the powder mixture of alkali metal phosphate residue and the additives is compressed into mouldings before the heat treatment.

11. A process as claimed in any of claims 1 to 10, wherein the difficultly-soluble alkali-alkaline earth metal phosphates are embedded in a leaching-resistant matrix.

1 2. A process for the solidification of aqueous nitrate containing radioactive waste substantially as described with particular reference to the Examples.