RU2558899C1 - Method of removing radioactive 60co isotope from stillage residue of nuclear power plants and system therefor - Google Patents

Abstract

FIELD: atomic physics.

SUBSTANCE: invention relates to nuclear power engineering and specifically to processing liquid radioactive wastes, particularly still residue of evaporator systems for treating drain water at nuclear power plants. A method of removing the radioactive ⁶⁰Co isotope includes oxidising the stillage residue in circulation mode through a tubular reactor under the action of hard ultraviolet radiation of a xenon lamp, fed hydrogen peroxide and continuous injected air into the reactor, which is first fed into the inner electrode of the lamp, and the obtained ozone-air mixture is fed into the oxidised solution, and activated corrosion products are filtered out.

EFFECT: efficient removal of radioactive ⁶⁰Co isotope from stillage residue of nuclear power plants and saving reactants for coprecipitation post-treatment.

2 cl, 1 dwg, 1 ex

Description

The invention relates to the field of nuclear energy, namely to the processing of liquid radioactive waste, in particular bottoms (BO) of evaporation plants for the treatment of floor drain water from nuclear power plants. Vat residues can be processed as containing salts of boric acid (obtained from the operation of power plants at VVER reactors), as well as borate-free bottoms of power plants at RBMK reactors. If borates are present in the bottoms being processed, it is recommended that they be precipitated prior to the proposed removal of radioactive cobalt.

During operation of a nuclear power plant, a significant amount of liquid radioactive media is formed, which are collected, averaged and concentrated by evaporation. The bottoms obtained by evaporation are sent for temporary storage in special containers. The total amount of bottoms left over at Russian nuclear power plants is ~ 100,000 m ³ and is increasing by ~ 10% annually. The reserve of temporary storage is almost exhausted today. So, for example, at the Leningrad NPP, storage capacities are 96% full. Therefore, the task of processing bottoms is extremely urgent.

At present, bottoms of nuclear power plants are processed by the cementing method (less often by bitumen, calcination, and production of salt melt). Moreover, in all the mentioned methods, the final solid processing product is placed in reinforced concrete containers or metal barrels. This leads to large volumes of the radioactive product and, as a result, to large costs for obtaining packages of cured LRW and their subsequent long-term storage.

Therefore, at present, a method for processing KOs has been developed by separating radioactive elements into a small volume of the solid phase and obtaining practically no radionuclide salts belonging to the VLLW category (very low-level waste). Salts (VLLW) can be stored in simple hangar-type storage facilities and stored in industrial landfills in the future.

The main isotopes that determine the activity of KOs are ^134.137 Cs and ⁶⁰ Co. Cesium isotopes are contained in CO in ionic form; therefore, their isolation is not a problem (ferrocyanide ion-selective sorption method). At the same time, Co is present in CO in the form of strong complex compounds, such as, for example, oxalates and ethylenediaminetetraacetates. Therefore, to remove radioactive cobalt, it is necessary to destroy these complexes.

To date, two methods for the oxidation of cobalt complex compounds seem to be the most promising.

Ozonation (RF Patent No. 2268513, G21F 9/06, G21F 9/20, publ. 20.01.2006). The oxidation of organic ligands is accompanied by the precipitation of the solid phase of transition metal hydroxides (mainly iron (III) hydroxide), which captures most of the ⁶⁰ Co isotope. The resulting radioactive sludge is separated by filtration, and the filtrate is sent for purification from radioactive cesium. The disadvantages of ozonation are: insufficiently cleaned from ⁶⁰ Co, the danger of using ozone, the concentration of which in the air should not exceed 0.1 mg / dm ³ , the high cost due to the high consumption of reagent (10 kg of ozone per 1 m ^{3 of} cubic residue) and electricity . High power consumption, in addition, requires the installation of an ozonation station, which leads to an explosive process.

The oxidative-co-precipitation method of purification from ⁶⁰ Co, described in RF patent No. 2467419 G21F 9/30, publ. 11/20/2012. In this case, the bottom residue is first oxidized by the combined action of hard ultraviolet radiation and hydrogen peroxide, and then a two-stage (usually) co-precipitation treatment is carried out using cobalt or manganese diethyldithiocarbamate. This makes it possible to achieve a much deeper degree of purification of CO from radioactive cobalt and, after purification from cesium and evaporation of the bottoms, to obtain salts of the ONOO category. This approach improves the efficiency of the process and meets all safety requirements. The disadvantage of this method is the increase in the volume of radioactive concentrate with respect to ozonation due to the formation at the stage of coprecipitation of an additional amount of radioactive sludge. Despite this, this method allows to reduce the cost of processing KO in 1,5 ÷ 2 times compared with ozonation. The basis of the invention is the creation of a method for removing the radioactive isotope ⁶⁰ Co from the still bottoms of nuclear power plants and a system for its implementation, which provides improved cleaning of radioactive cobalt at the oxidation stage and, as a result, the opportunity to save on the amount of reagents for co-precipitation treatment, while it is possible to reduce the number of stages of coprecipitation by treating the bottom residue with an ozone-air mixture obtained by blowing compressed air into the space, etc. leg to the internal electrode ultraviolet lamp.

The solution to this problem is provided by the fact that in the method of removing the radioactive isotope ⁶⁰ Co from the bottom residues of nuclear power plants by oxidizing the bottom residue in a circulation mode through a tubular reactor, exposing the bottom residue to hard ultraviolet radiation from a xenon lamp while continuously injecting air into the reactor and releasing activated corrosion products by filtration, the injected air is first sent to the inner electrode of the lamp, and the ozone-air mixture obtained after this l sent to the oxidizable solution.

To implement the method, a system is proposed for removing the radioactive isotope ⁶⁰ Co from bottoms of nuclear power plants, including an oxidation circuit containing a pump, a UV reactor, an air supply system to the oxidation circuit, and also including a filter with a reagent supply system for co-precipitation treatment, in which air supply to the oxidation circuit provides a preliminary air supply to the space adjacent to the inner electrode of the ultraviolet lamp, and further supply of the obtained ozone-air with Mesi after passing the reactor into the bottom residue; the pore size of the filter is selected no more than 0.2 μm.

The essence of the invention is that, using an oxidation step similar to that described in RF patent No. 2467419 (UV reactor based on a xenon UV lamp with radiation λ = 172 nm + hydrogen peroxide), purge the air to cool the internal electrode and, in order so that the ozone-air mixture formed in this space is fed into the bottom residue.

In the patent of the Russian Federation No. 2467419 provides purging with air in order to better mix KO in the UV reactor. In contrast, in the present invention, the air ejected into the bottom residue must first pass through the inner space of the lamps, where it captures the ozone generated in the region adjacent to the inner electrode during the barrier discharge. Thus, the resulting ozone-air mixture not only mixes the bottom residue, but also takes a direct part in its oxidation.

An important point is the fact that the additional oxidizing agent is a by-product of the operation of the ultraviolet lamp and does not require any additional devices or energy costs for its production.

In addition, the ozone concentration in the resulting mixture is much lower than in the ozonizer, which guarantees its complete destruction in the still residue and eliminates the entry into the atmosphere of the room where liquid radioactive waste is processed.

An increase in the oxidation rate and the degree of evolution of ⁶⁰ Co at the oxidation stage leads to the fact that a solution with a significantly lower concentration of this radionuclide enters the co-precipitation stages of purification. Therefore, the number of co-precipitation steps in the waste processing line can be reduced (for example, leaving one instead of two). This allows to reduce the amount of co-precipitating reagents, and therefore, to reduce the amount of solid radioactive waste generated. As already noted, it is the amount of secondary waste generated that is the bottleneck of the oxidation-precipitation method, therefore, reducing their amount by almost half is a very significant improvement of the method.

Filters with a pore diameter of not more than 0.2 μm are used to separate the phases after the oxidation step, since the resulting precipitate consists of very small particles, and the use of larger pores can lead to the passage of radioactive cobalt into the solution to be cleaned. In particular, membrane curtain filters, which do not clog for a long time and are easily regenerated by the reverse filtrate stream, have proven themselves well.

The invention is illustrated in FIG. 1, which shows a UV reactor.

In the proposed technology, the bottom residue is oxidized by circulating in the circuit: (heated tank → pump → UV reactor → heated tank) at a temperature of 45 ÷ 100 ° C under the influence of: hard ultraviolet radiation, hydrogen peroxide dosed into the bottom residue, ozone-air mixture obtained by purging with compressed air the space adjacent to the internal electrode of the ultraviolet lamp (Fig. 1), and ejected in the KO.

In FIG. 1 shows the following notation:

1 - input stream of bottoms,

2 - output stream of bottoms,

3 - reactor vessel,

4 - air flow for cooling the inner electrode of the UV lamp, which is proposed to be fed further into the volume of the KO, for example, using an ejector

5 - inner and outer walls of the lamp housing,

6 - xenon inside the lamp,

7 - slaughter residue inside the reactor,

8 - sealing of the reactor,

9 - the inner electrode of the lamp,

10 - high voltage cable for supplying a voltage pulse.

The resulting radioactive solid phase is then separated by microfiltration on a filter with a pore diameter of not more than 0.2 μm and sent for further processing (cementing) and disposal. The filtrate is subjected to a one-stage co-precipitation treatment with the addition of sodium diethyldithiocarbamate (DDTC-Na) and cobalt (II) salts. The formed amount of the solid phase at this stage is 0.75–1% of the volume of KO, which is two times less than in the oxidation-precipitation method described in RF patent No. 2467419. It should be borne in mind that this method can be applied not only for RBMK reactors, but also for VVER reactors due to an increase in productivity with preliminary elimination of borates from the CO.

Concrete example

The bottom residue of LRW was subjected to preliminary pH adjustment to 7-7.5 with nitric acid. The solution was heated to 95 ° C, 30% peroxide solution was introduced in an amount of 15% of the volume of CO. The bottoms were circulated for 10 hours through a UV xenon ultraviolet reactor. During the first three hours, a uniform additional supply of hydrogen peroxide was carried out in an amount of 15% of the volume of KO. In addition, compressed air was introduced into the bottom residue, which previously passed through the interelectrode space of the xenon ultraviolet lamp. At the end of the oxidation, the suspension was fed to the filtration on a curtain filter. Next, sodium diethyl dithiocarbamate and cobalt (II) nitrate and two cobalt (II) nitrates were introduced into the filtered suspension at 25 ° C in the amount of 10 and 6 g of salts per 1 liter of the treated solution, respectively, and kept under stirring for 2 hours. The solid phase of cobalt diethyldithiocarbamate was separated by filtration on a curtain filter.

The result of the separation was a decrease in the activity of the bottoms residue of ⁶⁰ Co from 4 · 10 ⁴ Bq / dm ³ to 1.2 · 10 ² Bq / dm ³ , which, after the isolation of cesium by the ferrocyanide method, is sufficient to obtain the required degree of purification of the bottoms.

Thus, the proposed method has the following advantages compared to analogues:

1. Compared to ozonation:

a) avoids the use of an ozonation station and makes the process safer;

b) allows for more efficient purification from ⁶⁰ Co;

c) consumes less power;

d) is more productive.

2. Compared with the oxidation-deposition method described in the patent of the Russian Federation No. 2467419:

(a) Less secondary radioactive waste;

b) an increase in the rate of the oxidation process;

c) lower cost of reagents for coprecipitation.

Claims (2)

1. The method of removing the radioactive isotope ⁶⁰ Co from the bottom residue of nuclear power plants by oxidizing the bottom residue by circulation through a tubular reactor, exposing the bottom residue to hard x-ray radiation from a xenon lamp by introducing hydrogen peroxide and continuously injecting air into the reactor and releasing activated corrosion products by filtration, characterized in that the injected air is first sent to the inner electrode of the lamp, and the resulting ozone-air mixture is not in the layers control the oxidizable solution. 2. The system for removing the radioactive isotope ⁶⁰ Co from the bottoms of nuclear power plants, including an oxidation circuit containing a pump, a UV reactor, an air supply system to the oxidation circuit, and also including a filter with a reagent supply system for co-precipitation treatment, characterized in that in the system air supply to the oxidation circuit provides a preliminary air supply to the space adjacent to the inner electrode of the ultraviolet lamp, and further supply of the resulting ozone-air mixture after passing the reaction a torus in the bottom residue; the pore size of the filter is selected no more than 0.2 μm.