GB2028297A - Process for treating hydrazine- containing water effluents from nuclear power stations - Google Patents

Abstract

In a method of treating hydrazine- containing water effluent from nuclear power generating plant by distilling the effluent to concentrate non-volatile components and passing the distillate over ion exchangers, an ozone containing gas is passed through the effluent before it is distilled to oxidize the hydrazine. Preferably a catalyst, such as potassium iodide or a manganese salt, is added to the water prior to passing ozone therethrough.

Description

SPECIFICATION Process for treating hydrazine-containing water effluents from nuclear power stations t This invention relates to a process for treating hydrazine-containing water effluents from nuclear power stations.

In nuclear power stations, large cooling circuits are operated with fully demineralised water and, in order to prevent corrosion, hydrazine is added to the water, the hydrazine content of the water generally being up to 200 mg/litre. As these cooling water systems have various branches, certain leakages occur which, depending on the construction of the individual units, can amount to up to 2 cubic metres of water per day. This leakage water is combined with other radioactive water effluents and they are processed together. Because of the radioactivity, the water must be "made safe" by special treatment. For this purpose, the water is firstly concentrated in an evaporator, and the distillate from the evaporator is then fed through a mixed bed of anionic and cationic ion exchangers.The residue in the evaporator and the mixed bed must be handled as special wastes, due to possible radioactive contamination and transported away.

When hydrazine-containing water is treated in this way, the hydrazine, or ammonia derived from decomposition of the hydrazine, is present in the distillate, and, as a consequence, the mixed ion exchanger bed becomes spent relatively quickly. As, for safety reasons, the mixed bed must not be regenerated, this leads to an increased quantity of special waste.

In order to overcome this problem it has already been proposed or attempted to remove the hydrazine from the water before evaporation. This can be achieved without difficulty by treatment with chlorine or sodium hypochlorite. However, in this case considerable quantities of sodium chloride arise, and remain in the evaporator in the form of an additional residue, and thus the evaporator has to be de-sludged and emptied after a quarter of the otherwise usual time, and the waste has to be transported away as special waste. This considerably increases costs.

Similarly, the use of hydrogen peroxide to oxidize the hydrazine does not give satisfactory overall results. Hydrogen peroxide reacts with hydrazine only in the presence of a catalyst, e.g. a halogen complex, which likewise remains in the evaporator as a salt residue and therefore necessitates earlier emptying of the evaporator.

It has now been found that these drawbacks do not occur if an ozone-containing gas mixture is drawn through the water before evaporation, distillation then being carried out in the normal manner and the distillate passed through a mixed ion exchanger bed.

According to the invention therefore there is provided a process of treating water effluent from nuclear power generating plant by distilling the effluent to concentrate non-volatile components and passing the distillate over ion exchangers, in which the water effluent is one containing hydrazine and an ozone containing gas is passed through the effluent before it is distilled.

Suitable ozone-gas mixtures for use in accordance with the invention are ozone-air and ozone-oxygen mixtures in all contentrations. Ozone concentrations of at least 10 g ozone per cubic metre of accompanying gas have proved particularly satisfactory.

It has been found that the rate of reaction between hydrazine and ozone reduces towards the end of the reaction with decreasing hydrazine concentration and increasing ozone concentration. By adding small quantities of an oxygen subcarrier as catalyst, this can be prevented and an accelerated and quantitative conversion can be attained up to the end of the reaction. Suitable catalyst are potassium iodide or manganese salts such as MnCl2 or MnSO4, and these are suitably added to the water in amounts of from 0.5 - 50 cubic metre, preferably 1 - 2 cubic metre.

The end point of the reaction can be determined by measuring the ozone content of the overlying gas phase, in that the amount of excess ozone dissolved in the water is proportional to the corresponding amount of ozone in the overlying gas space. The measurement can be made with an ozone analyser, e.g. by UV absorption at 254 nM. The end point of the reaction can, in this manner, be reliably determined without danger, without necessitating contact with the sometimes radioactive effluent water which may occur using conventional analytical methods.

Decomposition of the hydrazine using ozone is a particularly suitable process in the case of nuclear power stations, as it leads to no yield of radioactive waste, in that the reaction products, namely nitrogen and water, give rise to no problems.

Afurther advantage when used in nuclear power stations is that ozone can be prepared from air using electrical energy, of which there is an adequate supply in nuclear power stations, and thus no additional chemicals, chemical transportation, safety devices or the like are necessary.

In addition to hydrazine, nuclear power station water effluents also contain other impurities, for example traces of detergents etc. It has been found that the hydrazine is quickly and nearly quantitatively oxidised even in such variously contaminated water.

The objective of all nuclear power stations, namely to keep the amount of radioactive waste as small as possible, is thus achieved to the invention, at least on the water effluent side.

In order that the invention may be well understood, the following Examples are given by way of illustration only.

Example 1 An ozone-air mixture containing 12 g of ozone per cubic metre of air was passed through a radioactive water effluent containing 50 mg/litre of chloride ion and 50 mg/litre of hydrazine, until the ozone content of the overlying air had reached 0.4 g ozone/m3.

The water treated in this manner was fed to an evaporator and concentrated to a chloride content of 10,000 mg/litre, i.e. to 200 times its initial concentration (10,000 mg/litre of chloride being the highest allowable concentration for corrosion reasons). No ammonia was found in the distillate.

The distillate was passed through a mixed ion exchanger bed containing 1,900 litres of cation exchange resin corresponding to 1,900 grams equivalent of exchange capacity. With a throughput of 90 cubic metres of water per day, the mixed bed was still not spent after 200 days.

Example 2 The procedure of Example 1 was repeated, with the difference that potassium iodide were added to the effluent water before the ozone treatment in an amount of 2 g of potassium iodide per cubic metre of effluent water.

An ozone content of 0.4 g ozone/cubic metre in the overlying gas phase was almost instantaneously reached, in contrast to Example 1. No ammonia was found in the distillate, and after 200 days the mixed bed was still not spent.

Comparative Example 1 A radioactive water effluent containing 50 mg/litre of chloride ion and 50 mg/litre of hydrazine was fed, without oxidation treatment, to an evaporator until a chloride content of 10,000 mg/litre was achieved, i.e.

a concentration of 200 times the original, and led off as radioactive sludge.

The 50 mg/litre of hydrazine gave by thermal decomposition, 38 mg/litre of ammonia, correspond- ing to 2.1 mg equivalent/litre and this distilled over.

The distillate was passed through a mixed ion exchanger bed containing 1,900 litres of cation exchange resin. After a throughput of 900 cubic metres, the ion exchanger was spent, thus signifying that for a daily production of 90 cubic metres of evaporator product, the mixed bed filter would have to be stored as a radioactive special waste after only ten days.

Comparative Example 2 A radioactive water effluent containing 50 mg/litre of chloride ion and 50 mg/litre of hydrazine was treated with about 1.5 mlllitre of sodium hypochlorite (150 litre active chloride). The 50 mgllitre of hydrazine produced 180 mg/litre of NaCI, i.e. 114 mg/litre of chloride ion, which was additionally present in the water effluent. The water treated in this manner was fed to an evaporator and concentrated. The allowable chloride content of 10,000 mg/litre was attained by concentration to only about 80 times the original, i.e. the radioactive quantity in the evaporator was doubled with respect to untreated water (Comparative Example 1). No ammonia was noticeable in the distillate.

Claims (7)

1. A method of treating water effluent from nuclear power generating plant by distilling the effluent to concentrate non-volatile components and passing the distillate over ion exchangers, in which the water effluent is one containing hydrazine and an ozone containing gas is passed through the effluent before it is distilled.

2. A process as claimed in claim 1, in which the ozone-containing gas is an ozone-air or ozoneoxygen mixture containing at least 10 g of ozone/ cubic metre of accompanying gas.

3. A process as claimed in claim 1 or claim 2, in which a catalyst is added to the water effluent, prior.

to passing ozone therethrough, an amount of from 0.5 - 50 cubic metres.

4. A process as claimed in claim 3 in which the catalyst is added in an amount of from 1 - 2 cubic metre.

5. A process as claimed in claim 3 or claim 4, in which the catalyst is potassium iodide or a manganese salt.

6. A process as claimed in any one of the preceding claims in which the end product, of the reaction between hydrazine and ozone is determined by measuring the ozone content of the overlying gas phase.

7. A process as claimed in claim 1 substantially as hereinbefore described with reference to the Examples.