GB2130428A - A process for reducing the volume of aqueous radioactive waste - Google Patents

Abstract

In order to reduce the volume of, and to solidify aqueous, nitrate-containing containing radioactive waste solutions in the LAW- and MAW- ranges, a process is required in which larger quantities of waste than before may be bound into the matrix, with a good compatibility and with as low a nitrate content as possible. To this end, the waste solution is evaporated with phosphoric acid, one mol of phosphoric acid being added per 2 to 2.5 mols of nitrate, and the residue is heated to a temperature of from 250 to 450 DEG C, with the formation of Na2HPO4. The resulting solid is bound in cement, bitumen or synthetic resin binders.

Description

SPECIFICATION A process for reducing the volume of aqueous radioactive waste This invention relates to a process for reducing the volume of, and solidifying aqueous, nitrate-containing radioactive waste by evaporating the waste solution in the presence of phosphoric acid, and binding the resulting solid in cement, bitumen or organic binders based on synthetic resins.

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.

In the process for fixing MAW- or LAW liquid waste using known binders, 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 final store, or when organic binders are used, in the case of fire.

Thus, for example if the disturbance is the forced entry of water into a salt dome as the final store, there is the danger of cement corrosion by the action of concentrated brine, as a result of which, the diffusion-determined leaching rates may change in an unstable manner. The higher the salt content in the matrix, the more apparent this effect becomes. Therefore, an upper limit at about 10% is normally imposed on the salt content, precluding a further reduction in volume. On account of the high nitrate content in LAW/MAW liquid waste, bitumen is not included as a binder, because in the event of fire, combustion of the bitumen is substantially promoted by nitrate. A reduction in volume would be precluded in this case as well, because the risks in the event of fire would be increased even further with the waste concentration.The same applies to other organic binders based on synthetic resins.

In the case of HAW waste, it is known to evaporate the solutions with phosphoric acid optionally under a reducing atmosphere and to vitrify the residue. However, these processes are not used for LAW/MAW waste on account of the considerable technical expense.

Therefore, an object of the present invention is to provide a process for reducing the volume of and solidifying aqueous, nitrate-containing radioactive waste by evaporating the waste solution in the presence of phosphoric acid, optionally under reducing conditions, and binding the resulting solid in cement, bitumen or organic binders based on synthetic resins, in which process it is possible to bind larger quantities of waste in the matrix, with a good compatibility with the binder and with as low a nitrate content as possible.

The present invention provides a process for reducing the volume of and solidifying aqueous, nitrate-containing radioactive waste, which comprises evaporating the waste solution in the presence of 1 mol of phosphoric acid per 2 to 2.5 mols of nitrate, heating the residue to a temperature of from 250 to 4500C and binding the resulting solid in cement, bitumen or organic binders based on synthetic resins.

Evaporation in the process of phosphoric acid optionally takes place under reducing conditions. During evaporation anhydrous disodium hydrogen phosphate (Na2HPO4) is formed.

By adding phosphoric acid to an LAW/MAW waste solution which contains, dissolved, 300 g of NaNO3/l, and by evaporating the solution to dryness, nitric acid is expelled and a residue is produced which essentially consists of water-soluble sodium phosphate which contains radionuclides and other impurities.

The greatest reduction in weight is achieved by converting 3 mols of NaNO3 into 1 mol of Na3PO4, namely by 35.7%. This corresponds to a reduction in volume of 40.3% when the increase in density of 2.25 g/cm3 for Na NO3 to 2.54 g/cm3 for Na3PO4 is considered. However, it has been found that trisodium phosphate (Na3PO4) is incompatible with cement. In the case of Portland cement of quality PZ 45 F-HS, a greatly delayed hardening was observed within the first two months, even with 10% by weight of Na3PO4 in the end product, whereas in the case of blast furnace slag cement of quality HOZ 35 L-HS, the desired strength was not obtained at all.

No decisive improvement in the quality of the cement product with respect to NaNO3 could be achieved either by binding disodium hydrogen phosphate, containing water of crystallisation and corresponding to the formula Na2HPO4 x 2H2O in the cement. Similarly to the cementing of NaNO3 salt with blast furnace slag cement HOZ 35 L-HS (water/cement ratio W/C = 0.35), the setting procedure with Na2HPO4 x 2 H2O (W/C = 0.40) takes place very slowly. There is no appreciable increase in temperature in either case. Furthermore, it has been found that although the cement product is solidified above the 10% by weight of NaNO3 salt in the end product to about 16% by weight of NaN03, the remaining water is only absorbed very slowly.Above about 20% by weight of NaNO3 in the end product, the cement no longer hardens correctly, and the quality deteriorates to an increasing extent.

Surprisingly, it has now been found that a particularly firm crack-free cement product is obtained by converting the sodium phosphate residue into an hydros disodium hydrogen phosphate (Na2HPO4). In this case, although the reduction in weight is not as great as in the case of Na3PO4, it is still significant at 16.5%.

The good compatibility of the anhydrous Na2HPO4with the binder cement is of particular importance. It allows an increase in the salt content in the end product up to 35.7% by weight. This means that, converted, 42.8% by weight of NaN03 may be incorporated in the cement product in the form of Na2HPO4. This value is produced by bearing in mind the factor 0.835 which is to be used in the conversion of 2 mols of NaN03 into Na2HPO4.

Thus according to the process of the present invention, the sodium content in the cement product may be increased to more than four times by converting the NaNO3 contained in the MAW/LAW liquid waste into anhydrous Na2HPO4. This measure thus leads to a reduction in volume to a quarter of the quantity of waste to be ultimately stored, and the costs for the ultimate waste disposal of the radioactive waste are also reduced accordingly.

The residual nitrate may be removed from the MAW/LAW phosphate residue, which remains after expelling the nitric acid with phosphoric acid, in a known manner by the addition of reducing agents.

However, in the case of a quantity of phosphoric acid in a molar ratio to the NaNO3 dissolved in the aqueous radioactive waste of 1 :2.5, the resulting solid is completely dinitrated while heating to a temperature of from 250 to 4500C, even without the addition of reducing agents.

The conversion according to the present invention of NaN03 into Na2HPO4 also has an advantageous effect during the embedding of LAW/MAW waste in an organic matrix, such as bitumen or synthetic resin.

Since all these materials are combustible, NaNO3 which promotes the combustion as a strong oxidising agent cannot be bound in for safety reasons. On the other hand, disodium hydrogen phosphate is capable of substantially reducing the combustibility of organic binders due to its fire-inhibiting property.

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

Example 1 80 g of crystalline Na2HPO4 x 2H20 were mixed with 120 g of blast furnace slag cement of the HOZ 35 L-HS type and stirred with 54 g of water into a slurry which was easy to process.

The Na2HPO4 salt content was 25.1% by weight, based on the end product, with a water/cement value (W/C) of 0.45. Bearing in mind the conversion factor of 0.835 which is to be used in the conversion of 2 mols of NaN03 into Na2HPO4, this salt content corresponds to 30.1% by weight of NaNO3. The mass had solidified after 7 days, but a water residue was clearly visible. The surface was still wet in places even after 2 months.

A mixture of 60 g of crystalline NaNO3 and 140 g of HOZ 35 L-HS was mixed with 49 g of water into a slurry which was still soft after 7 days and exhibited a water residue. The NaNO3 content was 24.1% by weight, based on the end product, with a W/C value of 0.35. The product had hardened after 2 months, but deposits of salt crystals were visible on the surface.

80 g of anhydrous Na2HPO4 in powder form were mixed with 120 g of HOZ 35 L-HS and stirred with 78 g of water into a slurry which was easy to process. During this operation, a considerable evolution of heat was established, as a result of which the temperature rose to 600C. The Na2HPO4 content in the end product was 28.8 % by weight, corresponding to an NaNO3 value of 34.5%, with a W/C value of 0.65. Solidification commenced after 24 hours with the formation of a dry product. The product had hardened even after 7 days and was dry and scratch resistant. No change was observed within the next two months.Moreover, it was possible to bind into cement even greater quantities of anhydrous Na2HPO4: 100 g of anhydrous Na2HPO4 were mixed with 100 g of HOZ 35 L-HS and processed with 80 g of water into an easily stirrable slurry.

With a W/C value of 0.80, the Na2HPO4 salt content in the end product was 35.7% by weight, corresponding to an NaNO3 value of 42.8% by weight. Solidification cemmenced after 24 hours. After 48 hours, the product was dry and scratch-resistant and it had completely hardened after 7 days. No change could be established in the following period of 2 months. The moulding was then subjected to a leaching test in carnallite liquor at 55C over a period of 2 months. No changes in form were established after this test, and only individual, fine hair cracks were observed on the surface. The strength of the product was not changed by the action of the liquor. The carnallite liquor itself remained clear.

Example 2 The starting substance was a simulated material of the MAW/LAW liquid waste of a waste disposal centre having the following composition: NaNO3 300 g/l Al 0.23 gel Ca 1.50 g/l Cr 0.08 g/l Cu 0.15 g/l Fe 0.38 g/l K 0.08 g/l Mg 0.75 g/l Mn 0.08 g/l Mo 0.38 g/l Ni 0.08 g/I Zn 0.15g/l Zr 0.08 g/l Cs 10 g/l 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 were present as nitrates, Mo as Na-molybdate.

1 litre of this waste simulated material was mixed with 203.5 g of H3P04 (85% by weight) into a light-green clear solution. This corresponded to a molar ratio of NaNO3: H3P04 of 2:1. The solution was evaporated to dryness by heating with a heating hood in a 2 litre flask and the residue was then heated to 3000C for 3 hours, dilute nitric acid being distilled off and NOX being expelled. The remaining nitrate was removed by heating to 450"C for one hour. By analysing a sample quantity, the formation of sodiumortho phosphate (precipitation as Ag3PO4 in neutral solution with Ag ions) was detected and the absence of nitrate (colour reaction with FeSO4/H2S04) was established.

100 g of this powdered residue were mixed with 100 g of blast furnace slag cement of the HOZ 35 L-HS type and stirred with 68 g of water into a slurry which was easy to process.

The salt content in the end product was 37.3% by weight with a W/C value of 0.68. Bearing in mind the conversion factor of 0.835 which is to be used in the conversion of 2 mols of Na NO3 into Na2HPO4, this salt content corresponds to 44.7% of NaN03.

The product started to solidify after 24 hours, and it was already dry and scratch-resistant 48 hours after the stirring step.

It had completely hardened after 7 days and no changes could be established in the following period of 2 months.

The product could also be embedded analogously in bitumen and organic binders based on synthetic resins.

Claims (3)

1. A process for reducing the volume of and solidifying aqueous, nitrate-containing radioactive waste, which comprises evaporating the waste solution in the presence of 1 mol of phosphoric acid per 2 to 2.5 mols of nitrate, heating the residue to a temperature of from 250 to 450"C and binding the resulting solid in cement, bitumen or organic binders based on synthetic resins.

2. A process as claimed in claim 1, wherein the waste solution is evaporated in the presence of phosphoric acid under reducing conditions.

3. A process for reducing the volume of and solidifying aqueous nitrate-containing radioactive waste, substantially as described with particular reference to the Examples.