RU2753380C1 - Method for determining volumetric activity of radionuclides fission products and activated corrosion products in aqueous coolant of first circuit of nuclear power plant - Google Patents

Abstract

FIELD: nuclear energy.

SUBSTANCE: method relates to the field of nuclear energy, namely, to the determination of the volumetric activity of reference fission products, for example, iodine nuclides, and activated corrosion products in water coolants of the first circuits of nuclear power plants under neutral and alkaline water-chemical conditions. A fixed volume of the coolant sample is introduced into the measuring tank. The initial radiation level of activated corrosion products is measured. The sample is subjected to repeated filtration through the sorbent remotely in the circulation mode with the return of the filtrate back to the container. During the filtration process, the concentration of activated corrosion products from the sample is controlled by measuring the residual activity of these products in the filtrate after their isolation on the sorbent after a certain period of time. Filtration is completed under the condition that at least 98% of radionuclides of activated corrosion products are isolated, and a sorbent modified with antimony crystalline acid is used as a sorbent.

EFFECT: extension of the range of methods for determining the volume activity of reference fission products.

1 cl, 1 dwg, 1 tbl

Description

The technical solution relates to the field of nuclear energy, namely to methods for determining the volumetric activity of fission products, which are reference for monitoring the state of the core, for example, iodine nuclides, and activated corrosion products (APC) in water coolants of the first circuits of nuclear power plants (NPP) at neutral and alkaline water-chemical regimes.

The existing methods for monitoring the tightness of the claddings (CGO) of fuel elements (fuel elements) of the active zones (AZ) of the nuclear power plant include periodically carried out measurements of the activity of reference radionuclides in the coolant samples. Levels and changes in the activity of reference radionuclides in the coolant during operation of the reactor facility characterize the current state of the fuel element cladding and allow predicting the residual core life. Reliable determination of the activity levels of activated corrosion products and their changes during the reactor run allows one to judge the development of corrosion processes in the circuits.

To assess the state of the core, for example, use the total specific activity of nuclides 131 ÷ 135-iodine, or the specific activity of individual iodine nuclides. The identification and determination of the activity of gamma-emitting radionuclides is carried out by the method of gamma spectrometry.

The activity of the taken samples of the primary coolant during the operation of the nuclear power plant at the energy levels of power is determined by the fission products of uranium, radionuclides formed during neutron activation of corrosion products, and coolant impurities. The lower threshold of the measured activity of reference radionuclides is determined by the detector sensitivity, exposure time and background Compton radiation of other radionuclides contained in the sample. The superposition of the emission spectra of radionuclides during gamma-spectrometric measurements of samples significantly complicates the reliability of the determination of the activity of reference nuclides [L.N. Moskvin et al. Chemical problems of atomic energy. Volume 2. Radiochemical analysis and radiochemical technologies - St. Petersburg: Publishing house of VVM, 2013. Pp. 21-25].

A separate problem is the measurement of the activity of activated corrosion products - radionuclides ⁵¹ Cr, ⁵⁴ Mn, ⁵⁶ Mn, ⁵⁹ Fe, ⁵⁸ Co, ⁶⁰ Co, ⁹⁵ Zr, which are formed during neutron activation of the components of structural materials (CM) of the primary circuit passing into the coolant due to corrosion processes. APC are distributed between the coolant and particles of corrosion products (PC) of structural materials and can be in the coolant in “dissolved” (ion-dispersed and colloidal) and undissolved states. Mostly APCs exist in the coolant in the form of suspensions - about 70% of the total activity. At the same time, for a number of radionuclides, in particular for ⁵⁴ Mn and ⁵⁶ Mn, the proportion of ionic and colloidal forms of existence reaches 50% [V. Dua, The speciation analysis of colloids in the primary coolant in nuclear power plant. Radiation Physics and Chemistry. Vol. 159, 2019. PP. 81-88].

In the case of sampling into transport containers for subsequent measurement of samples in laboratory conditions, the loss of APC radionuclides occurs due to sorption and sedimentation on the surfaces of the container. Also, in the process of gamma-spectrometric measurements of APC in the samples of the coolant, they are deposited and sorbed on the surfaces of the measuring vessel, which distorts the calibrated geometry of the measurement and increases the error of the result. In this regard, before measuring the APC activity in coolant samples, it becomes necessary to isolate these radionuclides on a solid carrier (filter, sorbent, etc.).

Various methods are used to ensure the efficiency of performing gamma spectrometric measurements of the activity of individual groups of radionuclides, including reference radionuclides, APC and activated impurities in the coolant, and to reduce the error of measurement results.

From the prior art, a method is known according to US patent No. 4978506 (publ. 12/18/1990), which is designed to control corrosion products circulating in the first or second circuit of a nuclear reactor. In accordance with the method, a coolant sample is taken, a sample is introduced into the recirculation loop and insoluble particles are separated from soluble impurities. The solid particles are then suspended and collected in a particle collection vessel. The method is implemented using a system that, in particular, includes a container for isolating a representative sample of the coolant on the sampling line, a pump for circulating an isolated sample through membrane filters for concentrating "insoluble" corrosion products with the return of the filtered volume to the container, valves for switching the coolant flows from its isolated volume through the elements of the system, including ion-exchange columns to remove "soluble" corrosion products. According to the authors of the solution, the method and system allow the use of higher sampling rates, minimizing the deposition of corrosion products on the sampling line, which leads to an increase in the accuracy of determining the concentration of insoluble particles. In addition, the solution significantly reduces the time required to obtain sufficient solids for accurate analysis.

The disadvantages of this solution are the isolation of activated corrosion products on several counting samples, the complexity of the isolation with switching valves and circulation through different circuits through various filter elements, the inability to remotely replace the filter elements.

Known solution [Comprehensive solution to the problems of radioecological safety of nuclear power facilities, industry and fleet. Ed. L.N. Moskvin - SPb .: Publishing house of VVM, 2009. Pp. 22-23], in which a storage filter is used to isolate PC and APC directly on the sampling lines, which includes a membrane microfilter and a mechanical filter made of porous polytetrafluoroethylene.

After the filtration of the coolant is completed, the filter is disconnected and transported to the laboratory. The disadvantages of the solution are that the AIC is isolated on several counting samples, which increases the time of their measurement; there is no possibility of remote change of the storage filter or its filter elements. Also, the installation of a filter directly on the sampling line requires significant mechanical strength of the filter elements and does not allow controlled release of APC.

The closest to the claimed method of the known technical solutions is the method of complex express radiochemical analysis (ERA), based on the division of the nuclides present in the coolant into separate groups with comparable levels of radionuclide activity in each group [L.N. Moskvin et al. Chemical problems of atomic energy. Volume 2. Radiochemical analysis and radiochemical technologies - St. Petersburg: Publishing house of VVM, 2013. Pp. 43-48].

The separation of groups of radionuclides by the ERA method is achieved by a single filtration of a selected sample of the coolant of a certain volume through a column with a set of block selective sorbents. Sorbents have a porous matrix and are made of sintered polytetrafluoroethylene. The selectivity of sorbents in relation to various groups of radionuclides is achieved by introducing organic extractants and inorganic ion exchangers into a porous matrix based on polytetrafluoroethylene. The sample is filtered first through a mechanical filter, on which colloidal and insoluble forms of APC are concentrated, namely, ⁵¹ Cr, ⁵⁴ Mn, ⁵⁹ Fe, ⁵⁸ Co, ⁶⁰ Co, ⁹⁵ Zr, etc., then through a sorbent impregnated with trioctylamine (TOA) , selective with respect to iodine radionuclides. Preliminarily, before carrying out the operation of radionuclide isolation on sorbents, the composition of the coolant sample of the primary circuit of the nuclear power plant is adjusted, for which a sulfite-acetate buffer solution is introduced into the sample. After the separation of the sample into groups of radionuclides is completed, the sorbents are subjected to separate gamma spectrometric measurements. The advantage of this method is the ability to carry out a comprehensive determination of radionuclides (APC and PD) in the coolant of the 1st circuit of the nuclear power plant.

This method is the closest to the claimed method and is adopted by the authors as a prototype.

The disadvantages of the prototype include the following.

1. Numerous preparatory work preceding the gamma-spectrometric measurement of samples: operations of sampling the coolant; preparation of a sample for separation; selective separation of radionuclides into groups. As a result, the minimum time for determining the activity of radionuclides from the moment of sampling to obtaining the result of gamma-spectrometric measurement is 3-4 hours. All described operations are also accompanied by personnel contact with a highly active sample, the duration of which is at least 1-2 hours and leads to additional exposure staff;

2. Distribution of activated corrosion products between several counting samples: a mechanical filter, block sorbents impregnated with cation exchange materials, filtrate - depending on the forms of existence of AIC after adjusting the composition of the sample with a sulfite-acetate buffer solution. To determine the activity of APC, it is necessary to measure several counting samples, which, firstly, leads to a decrease in the efficiency of the analysis, and secondly, it complicates the determination of small amounts of APC isolated on individual samples (in particular in the filtrate) against the background of the activity of other radionuclides;

3. Time delay from the moment of sampling to separations in the radiochemical laboratory (RCL), which affects the reliability of the results of determining the activity of groups of radionuclides. Activated corrosion products, which are in the coolant mainly in an "insoluble" form, partially pass from the sample onto the surface of the sampling vessel and the separation unit. Losses of agro-industrial complex in the course of determination by the considered method reach 20%.

The task to be solved by the claimed technical solution is to create a method for determining the volumetric activity of radionuclides of fission products and activated corrosion products in a water coolant, which makes it possible to increase the coefficient of release of APC from coolant samples, to increase the efficiency of determining the activity of groups of radionuclides and, first of all, radionuclides. benchmarks that determine the state of the core, to reduce their minimum measurable activity (MIA) with a simultaneous decrease in the time of personnel contact with a highly active sample.

To solve the problem and achieve the specified technical results in the method, a fixed volume of the coolant sample is introduced into the measuring vessel, controlled phase concentration of a group of isotopes on a block sorbent by filtering the sample through a sorbent based on polytetrafluoroethylene, followed by gamma-spectrometric measurement of the activity of nuclides on the sorbent and in the filtrate , according to the claimed solution, before filtering the sample, the initial level of radiation of the activated corrosion products is measured, the sample is remotely in circulation mode subjected to repeated filtration through the block sorbent with the return of the filtrate back to the container, thereby reducing the background activity of the sample and, as a consequence, the minimum measurable activity radionuclides reference fission products. In the process of filtration, the concentration of activated corrosion products from the sample is controlled by measuring after a certain period of time the residual activity of these products in the filtrate after their separation on the sorbent, and filtration is completed provided that at least 98% of the radionuclides of the activated corrosion products are released, while the block sorbent is used sorbent modified with crystalline antimony acid.

It has been experimentally established that, as a selective block sorbent, the most effective is the use of a sorbent based on polytetrafluoroethylene modified with crystalline antimony acid (SCA). Moreover, in contrast to the prototype, one counting sample is used - a sorbent based on polytetrafluoroethylene, modified by SCC, on which all forms of AIC existence are distinguished: suspended, ionic and colloidal.

The achieved technical result is expressed in a significant decrease in the background load on the measuring path, due to the presence of AIC in the coolant, which makes it possible to reduce the MIA of radionuclides-reference points. In this case, in parallel, there is a remote separation and concentration of activated corrosion products from the coolant on a solid carrier - a selective sorbent, which excludes the stages of sampling, their transportation and separation in the RCL and provides:

- an increase in the coefficient of release of AIC from a coolant sample,

- reducing the time of personnel contact with a highly active sample.

The method can be implemented using a device connected on a bypass to a measuring tank with a coolant and including a circulation pump, shut-off valves, pipelines and a filter with a block sorbent that selectively extracts all forms of activated corrosion products from the coolant sample. The device is controlled and the filter element is replaced completely remotely from a workplace protected from radiation. The figure shows a diagram of the device, where the positions indicate:

1 - sample supply valve;

2 - sampling line;

3 - measuring capacity;

4 - measuring circuit;

5 - gas removal line;

6 - gas removal valve;

7 - sample circulation valve;

8 - gamma-spectrometric complex (GSK);

9 - wall of the radiation protection chamber;

10 - penetration in the wall of the radiation-shielding chamber;

11 - circulation pump;

12 - filter with sorbent impregnated with SCC;

13 - flow meter;

14 - sample drain valve;

15 - drainage pipeline.

The inventive method for determining the volumetric activity of radionuclides of activated corrosion products and fission products in the water coolant of the primary circuit of a nuclear power plant is carried out as follows.

After the remote opening of valve 1, the sample of the coolant from the sampling line 2 enters the measuring metal container 3 and the measuring circuit 4.

In this case, the removal of the gas-air mixture that was in the measuring container 3 and the measuring circuit 4 before the sample supply, as well as the degassing of the sample, is carried out through the gas removal line 5 with the valve 6 open. 4 provides a closed valve 7. After filling the tank 3 and circuit 4, valves 1 and 6 are closed. Measurement of gamma radiation of the sample in the measuring container 3 using the gamma spectrometric complex GSK 8.

GSK 8 is located behind the wall of the radiation-shielding chamber 9. Gamma radiation from the sample, which is in the measuring tank 3, enters the GSK 8 through the penetration 10 in the wall. The operation of GSK 8 is controlled by the operator from a remote workplace protected from radiation. The measurement time is determined by the specific activity of reference radionuclides. After measuring the gamma spectrum, valve 7 is remotely opened and pump 11 is turned on, which circulates the sample through filter 12 with a sorbent impregnated with SCC. The filtration time is 10-30 minutes at a sample flow rate through the filter 100-200 ml / min. The flow rate during filtration is measured using a flow meter 13. After the filtration of the sample with the help of GSK 8 is completed, the sample is re-measured in the measuring vessel 3. In the filtered sample, the activity of iodine radionuclides after a decrease in their MIA and the residual activity of APC are determined. If the extraction efficiency of APC equal to 98% is achieved, the valve 14 is opened and the sample is drained into the drainage pipeline 15. If the declared efficiency of APC extraction is not achieved, the filtration operation is repeated. After the final completion of filtration and drainage of the sample, the sorbent is remotely removed from the filter 12 and transferred for measurement. A new sorbent is remotely installed in filter 12.

An example of the implementation of the method

A sample of the primary coolant with the pH value of the medium pH = 10 and the total activity of the APC 1.3⋅10 ⁷ Bq / kg with a volume of 0.4 l is fed into the measuring tank and the measuring circuit. The activity level of the sample is measured using a HSC. Then the pump is connected and with a sample flow rate through the sorbent of 180 ml / min, filtration is carried out for 25 min. Personnel contact with a highly active sample occurs only during the transportation of the SCC sorbent from the sampling site to the laboratory. The table shows a comparison of the characteristics obtained in accordance with the inventive method, and the characteristics of the prototype method:

- the ratio of MIA of radionuclides ¹³¹ I and ¹³⁵ I before and after filtration of the sample,

- the efficiency of APC isolation on one counting sample modified with SCC and on a set of block sorbents,

- the minimum time for determining the activity of radionuclides and iodine radionuclides from sampling to completion of measurements of personnel contact with a highly active sample,

- time of personnel contact with a highly active sample.

Thus, the claimed method, when using only one counting sample - a sorbent based on polytetrafluoroethylene modified with crystalline antimony acid, by performing filtration of the sample in a circulating mode, makes it possible, in comparison with the prototype, to significantly reduce the MIA of reference radionuclides (by 6-7 times) and increase the accuracy determination of agro-industrial complex, the efficiency of allocation of agro-industrial complex reaches 98%. In comparison with the prototype, the time for determining the activity of radionuclides from the moment of sampling to obtaining the result of gamma-spectrometric measurement is reduced by 2 times, and the time of personnel contact with a highly active sample is 3-6 times.

Claims (1)

A method for determining the volumetric activity of fission product radionuclides and activated corrosion products in the water coolant of the primary circuit of a nuclear power plant, including the introduction of a fixed volume of the coolant sample into the measuring vessel, controlled phase concentration of a group of isotopes on the sorbent by filtering the sample through a sorbent based on polytetrafluoroethylene with subsequent gamma spectrometric measurement of activity nuclides on the sorbent and in the filtrate, characterized in that before the filtration of the sample, the initial level of radiation of the activated corrosion products is measured, the sample is remotely in circulation mode subjected to repeated filtration through the sorbent with the return of the filtrate back to the container, thereby reducing the background activity of the sample and, as a consequence , the minimum measurable activity of radionuclides of reference fission products, while in the filtration process the concentration of activated corrosion products from the sample is controlled by measuring through a certain the period of time of the residual activity of these products in the filtrate after their isolation on the sorbent, the filtration is completed provided that at least 98% of the radionuclides of the activated corrosion products are released, and a sorbent modified with crystalline antimony acid is used as the sorbent.

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