Method for judging dangerousness of nuclear industry post-treatment waste liquid in evaporation and concentration process

Abstract

The invention relates to a risk judgment method for nuclear industry post-treatment waste liquid in an evaporation and concentration process. Comprising the specific steps of long-distance sampling, high-temperature catalytic oxidation, NO2 absorption, CO2 measurement and risk evaluation and judgment. The long-distance sampling is used for collecting a high-level liquid waste sample in the evaporation and concentration tank; high-temperature catalytic oxidation refers to conversion of organic matters in the waste liquid sample into CO2; the NO2 absorption means that a hydrochloric acid solution is used for absorbing NO2 in the gas; cO2 determination refers to determination of the concentration of CO2 generated by high-temperature catalytic oxidation of the sample by using a non-infrared spectroscopy; the risk evaluation and judgment means that the concentration of CO2 generated by the waste liquid sample is compared with a threshold value, and if the measured value is higher than the threshold value, evaporation liquid is diluted in time; if the temperature is lower than the threshold value, the evaporation concentration tank can continue to operate normally. According to the method, an important reference index is provided for post-treatment safety in the nuclear industry, the possibility of'red oil 'explosion accidents of a spent fuel post-treatment plant can be reduced to a great extent, and technical support is provided for nuclear safety protection.

Description

Technical Field

[0001] The present invention belongs to the technical field of nuclear chemical engineering, and relates to a method for judging the hazards during the evaporation and concentration process of nuclear industry spent liquid. Background Art

[0002] The spent fuel reprocessing plant is mainly responsible for treating the waste after the use of nuclear fuel rods. In the Purex process of spent fuel reprocessing, tributyl phosphate (TBP), diluent, HNO3 and their degradation products undergo complex physical and chemical reactions under high-temperature irradiation conditions. When the temperature control or ventilation conditions are poor, violent thermal runaway reactions may occur and even lead to "red oil" explosion accidents, which will cause serious consequences:

[0003] Radiation leakage: Radioactive waste is stored in the spent fuel reprocessing plant after the accident. The explosion may lead to the leakage and diffusion of the waste, causing serious radiation pollution and threatening the health and safety of the surrounding environment and personnel.

[0004] Casualties: The explosion accident may expose the staff inside the plant near the radiation source, thus being directly affected by radioactive particles or radiation beams, resulting in injuries or deaths.

[0005] Environmental pollution: The explosion will release a large amount of toxic gases, soot and other hazardous substances, and the radioactive substances contained therein may spread to the surrounding environment through air, soil and water sources, having a long-term impact on the ecosystem and biodiversity.

[0006] Economic and social impacts: The explosion accident will lead to the shutdown of the spent fuel reprocessing plant and the damage of facilities, having a serious impact on the local economy. In addition, emergency measures such as evacuation, medical rescue and environmental restoration will also consume a large amount of resources and bring inconvenience and burden to the local community.

[0007] The composition of "red oil" has not been fully characterized yet. Generally speaking, "red oil" can be described as a mixture containing saturated extracted heavy metal nitrates and HNO3, and TBP and its degradation products exposed to high temperature and ionizing radiation. The possible components include dibutyl phosphate, butyric acid, butanol, butyl nitrite and nitration derivatives, etc. Its red color mainly comes from nitro and nitroso organic compounds generated during the process.

[0008] From the perspective of severe thermal runaway, its essence is a violent oxidation-reduction reaction among various substances. Among them, various complex reducing organic substances such as TBP, diluents, and their degradation, radiolysis products, and nitration derivatives are important factors for the system to undergo severe thermal runaway; from the perspective of the formation of "red oil", TBP, diluents, and their degradation products and nitration derivatives are the material basis for the formation of "red oil". Therefore, the total organic matter content in the waste liquid will be an important indicator in the evaporation and concentration process of reprocessing.

[0009] Due to the complex composition of "red oil", and the reprocessing waste liquid has high radioactivity and strong acidity, it is difficult to separate and analyze various organic components. Therefore, the total organic matter content is used as a reference index. The total organic matter content in the waste liquid can be reflected by the total organic carbon. Currently, the mainstream determination methods of total organic carbon include: high-temperature catalytic combustion oxidation - non-dispersive infrared detection (NDIR), wet oxidation (persulfate) - non-dispersive infrared detection (NDIR), ultraviolet oxidation - non-dispersive infrared detection (NDIR), ultraviolet (UV) - wet (persulfate) oxidation - non-dispersive infrared detection (NDIR). They all convert organic matter into carbon dioxide, and then use non-dispersive infrared spectroscopy to measure the concentration of carbon dioxide to obtain the total organic carbon content. Different methods have different oxidation methods for converting organic matter into carbon dioxide. Among them, high-temperature catalytic combustion is a method that can ensure the oxidation of all organic carbon, so it is also considered the most accurate method and can be used as a standard for calibrating other methods. The detection limit of the combustion oxidation - non-dispersive infrared method is 0.1mg / L, and the detection lower limit is 0.5mg / L, which is widely used. Summary of the Invention

[0010] The purpose of the present invention is to provide a method for judging the danger during the evaporation and concentration process of reprocessing waste liquid, which can effectively provide a reference for the safe operation of nuclear power plant spent fuel reprocessing plants and prevent the occurrence of "red oil" explosion accidents.

[0011] To achieve the accurate determination of the organic matter concentration in the nuclear power plant spent fuel reprocessing waste liquid, and further provide a reference for preventing the formation of "red oil", thermal runaway, and even "red oil" explosion accidents, the solution provided by the present invention is as follows:

[0012] A method for judging the danger during the evaporation and concentration process of nuclear industry reprocessing waste liquid, the specific steps are as follows:

[0013] S1: Sampling at a distance; collecting high-level radioactive waste liquid samples from the evaporation concentrator through remote automated operation to avoid operators being damaged by contacting high-level radioactive waste liquid.

[0014] S2: High-temperature catalytic oxidation; the sample will be fed into the catalytic combustion tube by a peristaltic pump. The catalyst is selected from platinum or cobalt, and the combustion chamber temperature is 800 °C. The organic matter in the sample reacts with oxygen under the action of the catalyst and high temperature to produce CO2, and nitric acid will produce NO2.

[0015] S3: NO2 absorption; the infrared absorption wavelength of NO2 generated by high-temperature catalytic oxidation treatment is close to that of CO2, which interferes with the accurate measurement of CO2. The gas is passed through the carrier gas into the hydrochloric acid solution for washing. The solubility of NO2 in the hydrochloric acid solution is relatively large, while the solubility of CO2 in the hydrochloric acid solution is relatively small. Therefore, a large amount of NO2 in the gas is absorbed while the CO2 content remains basically unchanged, so as to reduce the interference of NO2 on the measurement of CO2 concentration.

[0016] S4: CO2 measurement; the treated gaseous sample is introduced into the gas sample chamber, and the non-dispersive infrared spectroscopy method is used to accurately measure the CO2 concentration. The basic principle of carbon dioxide gas concentration measurement is Lambert-Beer's law. When infrared light passes through the gas to be measured, these gas molecules absorb infrared light of a specific wavelength, and their absorption relationship follows Lambert-Beer's law. Assume that the incident light is parallel light with an intensity of I0, and the intensity of the outgoing light is I, and the thickness of the gas medium is L.

[0017] When the light intensity attenuation caused by the absorption of the number of molecules dN in the gas medium is dI, according to Lambert-Beer absorption law, there is the following relationship:

[0018] dI / I = -KdN

[0019] In the formula, K is a proportionality constant. For the entire path, assume that N is the total number of molecules of the absorbing gas medium. After integration, we get:

[0020] lnI = -KN + a

[0021] In the formula, a is an integration constant. If the concentration of the gas is c, obviously N ∝ cL. Further, the formula is:

[0022] lnI = -μcL + a

[0023] In the formula, μ is a constant. After rewriting, we have:

[0024] I = e a e -μc L = I0e -μcL

[0025] I0 is the incident light intensity.

[0026] Let μ represent the attenuation coefficient of the gas to be measured, whose magnitude depends on the wavelength of the incident light. Let c represent the concentration value of the measured gas, and L represent the thickness of the gas medium. Under the test environment, L is a constant parameter, and μ is a constant for a constant wavelength. Therefore, as long as the intensities of the light before incidence and after passing through the gas are measured, the concentration value of the gas can be determined.

[0027] S5: Hazard assessment and judgment; compare the measured CO2 concentration with the CO2 concentration threshold when a violent thermal runaway reaction occurs in the spent fuel reprocessing waste liquid: if the CO2 concentration is higher than the predetermined threshold, the evaporation liquid should be promptly adsorbed or transferred and diluted; if it is lower than the predetermined threshold, the evaporation concentrator can continue to operate normally. The predetermined threshold is the CO2 concentration that may cause a violent thermal runaway reaction in the spent fuel reprocessing waste liquid obtained through experiments and calculations, which is 10,000 ppm.

[0028] The present invention has significant advantages compared with the prior art as follows:

[0029] The present invention provides an important reference index for the safety of nuclear industry reprocessing, can greatly reduce the possibility of "red oil" explosion accidents in spent fuel reprocessing plants, and provides technical support for nuclear safety protection. Brief Description of the Drawings

[0030] Figure 1 It is a schematic diagram of a catalytic combustion tube.

[0031] Figure 2 It is a schematic diagram of the NO2 absorption process.

[0032] Figure 3 It is a block diagram of the CO2 measurement system.

[0033] Figure 4 It is a graph of the experimental results of the 2500 ppm CO2 measurement error.

[0034] Figure 5 It is a graph of the experimental results of the 5000 ppm CO2 measurement error.

[0035] Figure 6 It is a graph of the experimental results of the 7500 ppm CO2 measurement error. Detailed Description of the Preferred Embodiments

[0036] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0037] A method for judging the danger during the evaporation and concentration process of nuclear industry reprocessing waste liquid, and the method is as follows:

[0038] S1: Remote sampling; using a remote control robotic arm, inserting an acid-resistant container into the evaporation concentrator and collecting a spent fuel reprocessing waste liquid sample in a sealable container. It should be noted that all containers used should be resistant to strong acids, and due to the high-level radioactive characteristics of the spent fuel reprocessing waste liquid, all staff should take protective measures and stay away from the site.

[0039] S2: High-temperature catalytic oxidation; using a peristaltic pump to pump a certain amount of the reprocessing waste liquid sample in the container into the catalytic combustion tube in the combustion chamber. The schematic diagram of the catalytic combustion tube is as shown in the appendix Figure 1 shown. The catalyst filler is platinum, the carrier gas is high-purity oxygen, and the temperature of the combustion chamber is 800 °C. The organic matter in the waste liquid sample will be oxidized to produce CO2 under the environment of high-purity oxygen, catalyst and high temperature, and is carried into the subsequent module by the carrier gas. Due to the presence of high-concentration nitric acid in the reprocessing waste liquid sample, NO2 will be generated under the condition of high-temperature catalysis, and the absorption peak of NO2 is close to that of CO2 in the infrared range, which will affect the accurate measurement of CO2.

[0040] S3: NO2 absorption; since NO2 has a high solubility in hydrochloric acid solution while CO2 has a low solubility in hydrochloric acid solution, in order to reduce the interference of NO2 gas on the measurement result of CO2, the gas after high-temperature catalytic oxidation is passed through 10% hydrochloric acid solution for washing. A large amount of NO2 is absorbed while the CO2 content remains basically unchanged, and then it is passed into the gas sample chamber for measurement. The process schematic diagram is as Figure 2 shown.

[0041] S4: CO2 measurement; passing the gas washed by 10% hydrochloric acid solution into the gas sample chamber, and accurately measuring the CO2 concentration by using non-dispersive infrared spectroscopy. The block diagram of its measurement system is as Figure 3 shown. The basic principle of carbon dioxide gas concentration measurement is Lambert Beer's law. When infrared light passes through the gas to be measured, these gas molecules absorb infrared light of a specific wavelength, and their absorption relationship follows Lambert Beer's law. Assume that the incident light is parallel light with an intensity of I0, and the intensity of the outgoing light is I, and the thickness of the gas medium is L.

[0042] When the light intensity reduction caused by the absorption of the number of molecules dN in the gas medium is dI, according to Lambert Beer's absorption law, there is the following relationship:

[0043] dI / I = -KdN

[0044] In the formula, K is a proportionality constant. For the entire path, assume that N is the total number of molecules of the absorbing gas medium. After integration, we get:

[0045] lnI = -KN + a

[0046] In the formula, a is the integration constant. If the concentration of the gas is c, it is obvious that N ∝ cL. Further, the formula is as follows:

[0047] lnI = -μcL + a

[0048] In the formula, μ is a constant. After rewriting, we have:

[0049] I = e a e -μcL = I0e -μcL

[0050] I0 is the intensity of the incident light.

[0051] μ represents the attenuation coefficient of the measured gas, the magnitude of which depends on the wavelength of the incident light. c represents the concentration value of the measured gas, and L is the thickness of the gas medium. In the test environment, L is a constant parameter, and μ is a constant for a constant wavelength. Therefore, as long as the intensities of the light before incidence and after passing through the gas are measured, the concentration value of the gas can be determined.

[0052] S5: Hazard assessment and judgment; Since the complex organic substances in the spent fuel reprocessing waste liquid are the material basis for the formation of "red oil", and various heat-sensitive organic substances generated in the high-level radioactive environment are also the main conditions for thermal runaway accidents, the total organic substance concentration of the spent fuel reprocessing waste liquid is used as an important indicator for the prevention of spent fuel reprocessing accidents. After the steps of the foregoing S1 - S4, the concentration of a certain amount of the reprocessing waste liquid converted into CO2 is obtained, and it is compared with 10,000 ppm. If it is higher than 10,000 ppm, the evaporation liquid should be adsorbed or transferred and diluted in a timely manner; if it is lower than 10,000 ppm, the evaporation and concentration tank can continue to operate normally.

[0053] Specifically, in the CO2 measurement step of S4, the system needs to be calibrated, a CO2 standard gas with a known concentration is configured, and the CO2 measurement device is verified and calibrated. The standard gas is obtained by the dynamic gas mixing method, and its raw material gases are N2 standard gas with a concentration of 99.99% and CO2 standard gas with a concentration of 99.99%. The working principle of the dynamic gas mixing system is to control the flow rates of the above two gases by using a high-precision flow controller to achieve the purpose of mixing gases with different concentrations into the required concentration gas. The CO2 sensor is used to measure the concentration of the configured CO2 gas sample to provide a comparison for the calibration experiment and the performance test experiment.

[0054] Dynamic gas mixing refers to the process of mixing different gases in a certain proportion into a system by controlling the flow rate and time of gas inlet and outlet. Assuming that the volume fractions of CO2 and N2 to be mixed are x and (1 - x) respectively, the total flow rate of the mixed gas is V, the flow rate of CO2 is Q1, and the flow rate of N2 is Q2, then there are the following formulas:

[0055] Volume fraction relationship between CO2 and N2:

[0056] x+(1 - x)=1

[0057] Calculation formula for gas volume flow rate:

[0058] Q1 = Vx

[0059] Q2 = V(1 - x)

[0060] Among them, V represents the total flow rate of the mixed gas, x represents the volume fraction of CO2, (1 - x) represents the volume fraction of N2, Q1 represents the flow rate of CO2, and Q2 represents the flow rate of N2.

[0061] It should be noted that during the calculation process, the temperature and pressure of the gas need to be kept constant, otherwise correction calculations are required. If the air pressure changes during the gas distribution process of the mixed gas, the calculation formula needs to be corrected. Generally speaking, the influence of the change in air pressure on the gas flow rate can be calculated by the following formula:

[0062] Q2 = Q1×(P2 / P1)×(T1 / T2)×(Z1 / Z2)

[0063] Among them, Q1 represents the gas flow rate under standard conditions (usually 101.325 kPa and 273.15 K), Q2 represents the gas flow rate under actual conditions, P1 and T1 represent the gas pressure and temperature under standard conditions, P2 and T2 represent the gas pressure and temperature under actual conditions, and Z1 and Z2 represent the compression factors of the gas under standard conditions and actual conditions respectively.

[0064] On this basis, substituting the total flow rate V of the mixed gas into the above formula and then substituting the volume fractions of CO2 and N2, the specific calculation formula can be obtained as follows:

[0065] Q1=(V×x×Z2×P1×T2) / (Z1×P2×T1)

[0066] Q2=(V×(1 - x)×Z2×P1×T2) / (Z1×P2×T1)

[0067] Among them, Q1 represents the CO2 flow rate under standard conditions (usually 101.325 kPa and 273.15 K), Q2 represents the N2 flow rate under standard conditions, V represents the total flow rate of the mixed gas, x represents the volume fraction of CO2, P1 and T1 represent the gas pressure and temperature under standard conditions, P2 and T2 represent the gas pressure and temperature under actual conditions, and Z1 and Z2 represent the compression factors of the gas under standard conditions and actual conditions respectively.

[0068] The performance is verified by repeatedly measuring CO2 gas at different concentrations. The error analysis experiment on the CO2 gas detection method adopted in the present invention is carried out, and the results are shown in the appendix Figures 4 - 6 . According to Figures 4 - 6 , it can be seen that the CO2 gas detection method adopted in the present invention can accurately measure the CO2 gas concentration including 2500 ppm, 5000 ppm and 7500 ppm, and the error is controlled within 1%.

Claims

1. A method for judging the danger during the evaporation and concentration process of nuclear industry reprocessing waste liquid, characterized in that, It includes the following steps: S1: Remote sampling; S2: High-temperature catalytic oxidation; Under the conditions of high temperature and noble metal catalyst, the organic matter in the water sample is oxidized to CO2 and carried into the subsequent process by the carrier gas; S3: NO2 absorption; The gas generated in step S2 is washed through a hydrochloric acid solution by the carrier gas to remove NO2 in the gas and reduce its interference with the CO2 measurement; S4: CO2 measurement; The gas washed in step S3 is introduced into the gas sample chamber for CO2 concentration measurement; S5: Hazard assessment and judgment; Compare the CO2 concentration obtained in step S4 with a predetermined threshold to evaluate and judge the hazard of the post-treatment waste liquid.

2. The method for judging the danger during the evaporation and concentration process of the spent liquid from nuclear industry reprocessing according to claim 1, characterized in that, The remote sampling step is to collect a high-level radioactive waste liquid sample from the evaporation concentrator through remote control automation operation.

3. The method for judging the hazards during the evaporation and concentration process of nuclear industry reprocessing waste liquid according to claim 1, wherein In the high-temperature catalytic oxidation step, the water sample will be sent into the catalytic combustion chamber by a peristaltic pump. The catalyst is selected from single or combined noble metal catalysts of platinum, palladium, rhodium, and copper, and the combustion chamber temperature is 800 °C; The organic matter in the water sample reacts with oxygen under the catalyst and high temperature to produce CO2, and nitric acid will decompose to produce NO2.

4. The method for judging the hazards during the evaporation and concentration process of nuclear industry reprocessing waste liquid according to claim 1, wherein In the NO2 absorption step, in the high-temperature catalytic oxidation treatment in S2, the infrared absorption wavelength of the generated NO2 is close to that of CO2, interfering with the accurate measurement of CO2; The gas is introduced into a 10% by volume hydrochloric acid solution for washing through the carrier gas. NO2 has a large solubility in the hydrochloric acid solution, while CO2 has a small solubility in the hydrochloric acid solution. Therefore, a large amount of NO2 in the gas is absorbed and the CO2 content remains basically unchanged, so as to reduce the interference of NO2 on the CO2 concentration measurement.

5. The method for judging the danger during the evaporation and concentration process of nuclear industry reprocessing waste liquid according to claim 1, characterized in that, In the CO2 measurement step, the treated gaseous sample is introduced into the gas sample chamber, and the non-dispersive infrared spectroscopy method is used to accurately measure the CO2 concentration.

6. The method for judging the danger during the evaporation and concentration process of nuclear industry reprocessing waste liquid according to claim 1, characterized in that, Based on the complex composition of the organic matter in the post-treatment waste liquid and the difficulty of separation and analysis, the total organic matter concentration in the waste liquid is overall controlled; The CO2 concentration measured in S4 is used to obtain the organic matter content in the high-level radioactive waste liquid sample; Compare the measured CO2 concentration with the CO2 concentration threshold when the spent fuel reprocessing waste liquid undergoes a violent thermal runaway reaction: If the CO2 concentration is higher than the predetermined threshold, the evaporation liquid should be adsorbed or transferred and diluted in time; If it is lower than the predetermined threshold, the evaporation concentrator can continue to operate normally.

7. The method for judging the danger during the evaporation and concentration process of nuclear industry reprocessing waste liquid according to claim 6, characterized in that, The predetermined threshold is the CO2 concentration that may cause a violent thermal runaway reaction in the spent fuel reprocessing waste liquid obtained through experiments and calculations, which is 10000 ppm.

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