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Development of Processing Technology for Radioactively Contaminated Water at Chernobyl NPP (sorbents NIKET-M & MODIKS, published at WM2019 Conference, March 3 – 7, 2019, Phoenix, Arizona, USA)

Development of Processing Technology for Radioactively Contaminated Water at Chernobyl Nuclear Power Plant - 19656

Igor Voinov*, Victor Remez*, Alexey Myasnikov** and Analoly Terzi**

* METOIL s.r.o. ** Chernobyl Nuclear Power Plant

ABSTRACT

During the operation of the Chernobyl Nuclear Power Plant, distillation by evaporation was the main method for processing radioactively contaminated water. When the plant was decommissioned, the evaporation facility was mothballed and floor drain treatment was suspended in 2016. At present, the existing tanks with the capacity of 26,000 m3 are more than 60% full with radioactively contaminated floor drains, which requires alternative processing methods.

The current process flow for radioactive waste management at State Specialized Enterprise "Chernobyl NPP" includes processing of accumulated liquid radioactive wastes at a liquid radioactive wastes processing plant. Under the processing technology, liquid radioactive waste is immobilized at the plant by cementation and the resulting solid radioactive waste is then buried in a near-surface storage.

Key factors for the selection of alternative radioactively contaminated water processing methods are the possibility to clear the processed water (no control monitoring), which requires the same degree of processing as for drinking water, and minimization of radioactive waste generation due to the lack of free space for solid waste disposal in the near-surface storage.

Floor drains that require processing contain radionuclides such as 137Cs, 90Sr, 60Co and 40К. The current level of the total beta and gamma activity is about 4*105 Bq/kg.

The following criteria are set for the Chernobyl NPP site for water to be disposed to the sewerage system:

-Max. 37 Bq/kg for the total beta and gamma containing radionuclide activity

-Max. 0.7 Bq/kg for the total alpha containing radionuclide activity.

METOIL produces inorganic sorbents for removing radionuclides from liquids and develops sorbent- based technology for a variety of applications related to liquid radioactive waste processing.

In 2017-2018, experts from the Chernobyl NPP and METOIL ran several joint laboratory tests to evaluate the efficiency of inorganic sorbents made by METOIL for processing floor drains at the Chernobyl NPP.

The test procedure included treatment of radioactively contaminated water in a two-stage sorption process: static sorption (sorbents are mixed in a liquid) and dynamic sorption (the liquid is filtered through a sorbent column). NIKET and MODIKS sorbents were provided for the tests.

The results of the test measurements showed that the total alpha containing radionuclide activity is less than 0.026 Bq/kg and the total beta and gamma containing radionuclide activity is 9.2 Bq/kg. The physical and chemical parameters after processing also met the water disposal criteria.

High sorption capacity of the sorbents used for the tests allows processing hundreds of water volumes with just one sorbent volume. At present, preparations are underway for full-scale tests at a pilot plant with a capacity of minimum 10 m3/day.

At a later stage, spent sorbents may be immobilized by inclusion into the cement matrix, which will substantially reduce the amount of radioactive waste to be buried.


INTRODUCTION

On 26 April 1986, a disaster occurred at the Chernobyl Nuclear Power Plant. By that time, the station operated four RBMK-1000 reactors, with two more reactors under construction. The explosion destroyed Reactor No. 4 but the other reactors were not affected. A confinement structure called the Shelter Object was erected to cover the remains of the reactor building. The construction of the new reactors was never completed; the remaining reactors were put back into operation after decontamination and emergency repairs. Following a number of incidents such as a fire at Reactor No. 2 in 1991, a final decision was made to decommission the Chernobyl plant. On 15 December 2000, the Chernobyl plant was shut down forever.

In 2016, due to a high level of risk related to the potential destruction of the Shelter Object, another confinement structure known as the New Shelter, or New Safe Confinement, was built to enclose the original Shelter Object.

The generation of liquid radioactive waste (LRW) at the Shelter Object is subject to a large number of internal and external factors. Water enters the Shelter Object from a variety of sources, including precipitation, condensate, dust suppression solutions, and low-activity leaks on the side of the cascade wall. Thus, the micro-component composition of LRW is affected by organic components of dust suppression solutions, such as surfactants (OP-7), glycerin, silicone-acrylic emulsion, oxalic acid, oleic acid, ethyl alcohol, and products of interaction of atmospheric water with structural materials, mainly with concrete, namely: potassium, sodium, alkaline earth metals, pH, carbonates, bicarbonates, silicates, etc. The radionuclide composition of liquid radioactive waste depends on the interaction of moisture with various accident-derived modifications of nuclear fuel and consists mostly of 137Cs, 90Sr, 60Co, 40К, uranium, and trans-uranium elements.

During the operation of the Chernobyl plant, distillation by evaporation was the main method for processing radioactively contaminated water (RCW). When the plant was decommissioned, the evaporation facility was mothballed and LRW treatment was suspended in 2016 due to power shortages. Generated waste is accumulated in temporary storage tanks. By now, the tanks with a capacity of 26,000 m3 are more than 60% full. Every year, another 12,000 m3 or so is added, which means that LRW treatment should be recommenced and alternative, less energy-consuming methods should be deployed.

Key factors for the selection of alternative RCW treatment methods are the possibility to dump processed water to the central sewage system and the minimization of radioactive waste generation due to the lack of free space for solid waste disposal in the near-surface storage.

These goals can be achieved by using sorbents for LRW treatment; after the treatment, the processed solution is dumped to the central sanitary sewage system and spent sorbents are immobilized into the cement matrix.

Processed water to be dumped to the central sewage system must meet the criteria listed in Table I.

TABLE I. Sewage Acceptance Criteria for Processed Waste Water

 

Waste Water Quality Parameter

Allowable Values

 

Temperature, °С

Max. 40

 

pH value, units

6.5-9.0

 

Suspended matter, mg/dm3

Max. 500

 

Insoluble oils, resins and fuel oil

Not allowed

 

Oil and petroleum products, mg/dm3

Max. 20

 

Vegetable and animal fats, mg/dm3

Max. 50

 

Chlorides, mg/dm3

Max. 350

 

Sulfates, mg/dm3

Max. 400

 

Sulfides, mg/dm3

Max. 1.5

 

BODtotal, mg/dm3

Max. 350

 

Ammonia, mg/dm3

Max. 30

 

Synthetic surfactants, mg/dm3

Max. 20

 

(Total) Volumetric radionuclide activity, Bq/dm3

Max. 37

DESCRIPTION

Methods

METOIL develops selective inorganic sorbent-based technology for a variety of applications related to processing of low- and medium-activity liquid radioactive waste. One of the company's methods uses inorganic sorbents in the form of fine powder in a static sorption process: they are added and mixed in a liquid and are then separated from the processed solution. This approach offers numerous benefits compared to similar methods, of which the use of granulated sorbents in filtration columns is the most common.

Experience has shown that liquid radioactive waste with a high level of gamma-emitting isotopes such as cesium-137 and cobalt-60 can be deactivated in a column filled with granulated sorbents. However, in this case local oversaturation of sorbed nuclides occurs at the flow inlet to the column, which results in high background gamma radiation. This requires special protection measures to prevent personnel exposure when the plant is in operation and the columns are reloaded. Moreover, the column and the sorbent are just 25-30% utilized because the sorbent is oversaturated at the column inlet and little of it is used in the middle of the column and the section farthest from the inlet. This results in increased amounts of expensive sorbents and secondary solid radioactive waste in the form of spent sorbents.

In a static sorption process, powder sorbent is 100% used. Fine powders have a higher sorption surface compared to granulated sorbents, which delivers increased sorption efficiency. The enormous capacity of fine powder makes it possible to concentrate radionuclides in a very small amount, thus reducing the volume of final radioactive waste for disposal.

The proposed method that uses fine powdered sorbents combines the advantages of static sorption and column sorption.

The treatment of medium-activity liquid radioactive waste with powder sorbent at the initial stage is a quick and safe way to reduce the specific activity of the solution by five to six orders of magnitude to several hundreds of Bq/kg. At a later stage, it is further treated in a column filled with granulated sorbent to single-digit values in Bq/kg; in this case, no hazardous gamma fields are generated, the column has a very long operational life, and the entire amount of the sorbent is utilized.

High selectivity of sorbents offers the possibility to match them to a specific radionuclide and chemical composition of liquid radioactive waste at a nuclear power plant to maximize the recovery of radionuclides in line with the targets. The sorbents have been tested on a variety of actual liquid radioactive waste from nuclear power plants in Kazakhstan, France, Germany, Japan, and Slovakia. The tests demonstrated that these highly efficient sorbents are capable of recovering radionuclides even from challenging solutions that contain organic compounds not destructible by ozone oxidation and from solutions with high salt content.

In 2017-2018, experts from the Chernobyl NPP and METOIL ran several joint laboratory tests to evaluate the efficiency of inorganic sorbents made by METOIL for processing floor drains at the Chernobyl plant. The test procedure included treatment of radioactively contaminated water in a two-stage sorption process: static sorption (sorbents are mixed in a liquid) and dynamic test sorption (the liquid is filtered through a sorbent column).

For these tests, sorbents based on transition metal ferrocyanides (NIKET) and manganese dioxide (MODIKS) were used. Sorbent properties are listed in Table II.

TABLE II. Properties of Sorbents

Modification of Sorbent

NIKET-М

MODIKS

Sorption base

Nickel copper ferrocyanide

Manganese dioxide

Extracts

Cs, Co, U, Pu, Am, etc.

Sr, Am, Pu, U, etc

Operating range of salt content in a solution, g/dm3

up to 600

up to 600

Operating pH range

2-13

3-13

Operating temperature, °С

up to 80

up to 80

 

Properties of liquid radioactive waste treated during the tests are shown in Table III.

TABLE III. Physical, Chemical and Radiometric Parameters of Liquid Radioactive Waste

 

Parameter

Measured Values

1

pH value, units

8.06

2

Suspended matter, mg/dm3

22.5

3

Salt content (evaporated residue), g/dm3

0.8

4

Concentration of petroleum products, mg/dm3

0.5

5

Total hardness, mg-eq/dm3

1.1

6

Mass concentration of sulfate ions, mg/dm3

75.0

7

Mass concentration of nitrate ions, mg/dm3

295.0

8

Mass concentration of chloride ions, mg/dm3

28.5

9

Mass concentration of ammonium ions, mg/dm3

4.0

10

Mass concentration of sodium ions, mg/dm3

320.0

11

Mass concentration of oxalate ions, mg/dm3

11.8

12

Concentration of synthetic surfactants, mg/dm3

0.05

13

Concentration of dust suppressants, mg/dm3

25.0

14

Σβ activity, Bq/m3

9.8 * 107

15

Σα activity, Bq/m3

1.8 * 104

 

 

A 5 g sample of sorbent/sorbent mixture is placed in 500 ml of radioactive solution. The solution is mixed for 60-120 minutes with a magnetic mixer. After mixing, the solution is left for 20 minutes for the residue to settle down. Then the solution is decanted and the sorbent is separated with a paper filter. The filtrate is sent to a radiometry lab for activity measurements. The results are shown in Table IV.

RESULTS

TABLE IV. Activity Measurement after Sorption Processing

 

Sorbent Composition

a-Activity Sum, Bq/kg

P-Y Activity Sum,, Bq/kg

Initial

Processed

Initial

Processed

1

NIKET-M + MODIKS

16

0.028

1.1 * 105

9.2

2

NIKET-M + MODIKS

16

0.026

1.1 * 105

10

3

NIKET-M + MODIKS

18

Non- Detectable

9.8 * 104

13.2

4

NIKET-M + MODIKS

18

Non- Detectable

9.8 * 104

4.5

 

CONCLUSIONS

The results have demonstrated that the required level of processing may be achieved without the dynamic sorption stage. The physical and chemical parameters after processing also met the water disposal criteria. High sorption capacity of the sorbents used for the tests allows processing hundreds of water volumes with just one sorbent volume. At present, preparations are underway for full-scale tests at a pilot plant with a capacity of minimum 10 m3/day.

At a later stage, spent sorbents may be immobilized by inclusion into the cement matrix, which will substantially reduce the amount of radioactive waste to be buried.

Яндекс.Метрика