RU2547447C1 - Method of tightness monitoring at heat exchange surface of steam generator of reactor plant with heavy liquid metal heat carrier - Google Patents

Abstract

FIELD: heating.

SUBSTANCE: method is based on gas content change registration in a first loop in case of a leak at heat exchange surface of steam generator and heat carrier bubbling with steam and water mix. In stationary operation of reactor plant, shielding gas (argon) used to compensate for heat carrier expansion is treated to remove vapours and sprays and pumped through a metering vessel, gas temperature and pressure are measured in sequence, and spectrometric measurement of ⁴¹Ar component activity is performed in the metering vessel, ⁴¹Ar activity per volume rated for standard conditions is calculated, its stationary value is determined. Further, heat exchange surface leaks in steam generator are diagnosed by ⁴¹Ar activity overshooting its stationary value.

EFFECT: enhanced sensitivity of leak detectors at heat exchange surface of steam generator of reactor plant with heavy liquid metal heat carrier.

1 dwg

Description

The invention relates to nuclear technology, and in particular to methods for monitoring the tightness of the heat exchange surface of steam generators of nuclear power plants, in which a heavy liquid metal coolant, for example, lead or molten lead with bismuth, is used to cool the core.

Bypass circuits are proposed for this type of installation. The pressure in the second steam-water circuit is significantly higher than in the first circuit, therefore, the hypothetical situation with the loss of tightness of the heat exchange surface of the steam generator will lead to the penetration of the steam-water medium into the primary circuit and bubbling of the liquid metal coolant.

In the installations under consideration, a protective gas cushion made of argon is provided in the upper part of the reactor space above the mirror of the liquid metal coolant, which serves as a compensator for the thermal expansion of the liquid metal coolant and a collector of the gas or water vapor leaving it in the event of a steam generator leak.

Operational and reliable control of the inter-circuit tightness of steam generators is of great importance for ensuring the safe operation of installations of this type. Meanwhile, the use of a liquid-metal coolant complicates such a diagnosis and narrows the range of technical solutions that could be applied for this.

A known neutron-noise method for diagnosing a leak in the heat exchange surface of a steam generator of a reactor installation with lead-bismuth coolant (S. A. Morozov “Application of reactive methods for diagnosing the ingress of steam and undissolved gas into the coolant in the reactor core of stand 27VT / 5.” Third inter-industry scientific and practical conference "Heavy liquid metal coolants in nuclear technology. Obninsk, 2010, proceedings, vol. 1, p.29-42). The method uses the effect of changing the reactivity of a nuclear reactor when replacing part of the volume of the coolant in the core with steam bubbles. In this case, a spectral analysis of the neutron reactivity noise of a nuclear reactor or a cross-correlation analysis of reactivity and pressure noise in the primary circuit is performed. The disadvantage of this method is its relatively low sensitivity. It turns out to be insufficient to diagnose a violation of the tightness of the heat exchange surface of the steam generator at an early stage of development of a through defect.

Also known is the induction-bubble method, selected as the closest to the claimed. It is used in a gas monitoring device in a liquid metal coolant to diagnose leakages of water-liquid lead or water-lead melt with bismuth steam generators (RU 2426111 C1, publ. 10.08.11). In this solution, the liquid metal coolant is pumped through a pipeline on which a measuring and signaling device in the form of an excitation winding and inductively connected measuring windings are pre-installed, and a vortex-forming cowling is installed inside the pipeline. Using constant measurements in real time, the gas content in the liquid metal circuit is monitored. When bubbling occurs, a change in this indicator is recorded and a decision is made on the presence of a leak in the steam generator.

The disadvantage of this method is its binding to the pipelines of the primary circuit, inside of which it is necessary to install additional structural elements - vortex-shaped cowls, the need for duplication, that is, its use on each steam generator to provide the required sensitivity, as well as the use of measuring sensors that must operate at high temperatures and increased radiation exposure. These circumstances limit the reliability and safety of this method.

The objective of the present invention is improved in comparison with the known solution, a combination of properties - increased reliability, security, sensitivity and ease of implementation.

The technical result achieved in this case is that the proposed method allows to diagnose leakage without using complex structural, measuring devices inside the liquid metal coolant circuit, for example, profiling elements, as in the prototype, or sampling, does not require duplication, that is, the installation of the same measuring devices each steam generator.

The proposed method is based on physical phenomena that manifest themselves irrespective of which design of the reactor installation and its technological scheme are selected and how pipelines of the primary circuit are located. That is, the implementation of the method is not tied to the design features of the reactor installation.

As will be shown below in the example of its specific application, the method provides an order of magnitude greater sensitivity for diagnosing inter-circuit leakages of a steam generator than known methods, in particular neutron noise.

The reliability of the method is also associated with its high selectivity, eliminating the adoption of false conclusions about leaks under the influence of external factors. Since it uses a different diagnostic feature than in the known methods, then its simultaneous use with these methods will increase the reliability of recording leaks in the heat exchange surface of the steam generator.

In the proposed method, in contrast to the known solutions, as a diagnostic feature, such a manifestation of the effect of bubbling the coolant with a steam-water medium is used, such as leaching of inert gases dissolved in it — gaseous fission and activation products. When the reactor installation is in stationary mode, most of the radioactive gaseous products of fission and activation reach equilibrium activity in primary media within a few days. The steam generator leak is accompanied by bubbling of the coolant, the gases dissolved in it are transferred to the volume of steam bubbles by diffusion, transferred with bubbles along the coolant circuit and then carried into the protective gas cushion above the coolant mirror.

Among the other components, among the gaseous activation products, there is the ⁴¹ Ar isotope, the activation product of the argon protective gas dissolved in the liquid metal coolant, therefore, one of the manifestations of the leakage of the heat exchange surface of the steam generator will be an increase in the content of this isotope and, therefore, its activity in the primary gas system, which may be recorded by spectrometric measurements with high sensitivity and reliability.

The basis of the proposed method is to register the growth of ⁴¹ Ar isotope activity in the gas system as a diagnostic sign of leakage of the heat exchange surface of the steam generator. As will be shown below, the use of this particular isotope as a reference is due to a unique combination of its properties, which are manifested in the proposed method and ensure its implementation.

The essence of the claimed method is as follows. Operational control of the tightness of the heat exchange surface of the steam generator is carried out by constantly monitoring the activity index of the ⁴¹ Ar isotope. For this, a measuring capacitance is used in the gas system of the first circuit. The tank has sensors for measuring temperature and argon pressure. In addition, spectrometric sensors are placed on the outside of the vessel to measure the activity of the ⁴¹ Ar isotope contained therein.

(Бк), температуру $$T_{изм}^k$$

(К) и давление $$P_{изм}^k$$

(Па) аргона в емкости. Регистрируют измеренные величины. По этим величинам рассчитывают объемную активность изотопа ⁴¹Ar, приведенную к нормальным условиям (P₀=1,01·10⁵ Па, T=293 К):When the reactor installation is in stationary mode, that is, at a constant power level and constant coolant flow rate, a certain amount of protective gas of the primary circuit - argon - is cleaned of vapors and aerosols and pumped through the measuring tank. The activity of the ⁴¹ Ar isotope is measured. $$A_{\mathrm{and}sm}^k$$

(Bq), temperature $$T_{\mathrm{and}sm}^k$$

(K) and pressure $$P_{\mathrm{and}sm}^k$$

(Pa) argon in a vessel. Record the measured values. The volumetric activity of the ⁴¹ Ar isotope reduced to normal conditions (P ₀ = 1.01 · 10 ⁵ Pa, T = 293 K) is calculated from these values:

where k is the serial number of the measurement, V _EI is the volume of the measuring capacity, m ³ .

Volumetric activity a _{k is} used as a reference indicator for monitoring the tightness of the heat exchange surface of the steam generator.

Measurements and calculations of the reduced activity a _k continue to be carried out with a given frequency. Comparing the obtained data with the previous ones, the establishment of a stationary activity value a _stat is fixed.

They continue to repeat measurements and calculations, comparing the newly obtained values of a _k with the stationary value of a _stat .

The conclusion about the leakage of the heat exchange surface of the steam generator is taken in the case when the calculated reduced volumetric activity of the ⁴¹ Ar isotope exceeds its stationary value by some predetermined value determined by the error threshold.

The industrial applicability of the claimed method and the achievement of a technical result can be confirmed by the following arguments.

During the operation of the reactor installation, the primary coolant contains inert gases dissolved in it: argon from the gas system and gaseous fission products entering the coolant as a result of fission of uranium contained as an impurity in the coolant in the steel of the core elements and in the surface contamination of the cladding of the fuel elements with the fuel composition .

Gaseous fission products can also enter the coolant from the fuel elements when a through defect of the shells occurs. These dissolved inert gases circulate along the installation circuit, are activated in the core, their isotopes through diffusion exit through the coolant mirror into the gas system. The gaseous products of argon activation are ³⁷ Ar, ³⁹ Ar, and ⁴¹ Ar isotopes, and the gaseous fission products are krypton and xenon isotopes. In the stationary mode of operation of the installation, that is, at a constant power level and a constant flow of coolant in its volume and in the gas system of the primary circuit, the equilibrium concentrations of most of the radioactive isotopes of these elements are established.

If we apply a chamber computational model to the indicated processes, then the balance of activity of the ith inert gas isotope in the coolant and in the gas system can be described by the system of equations

, $$A_i^{гс}\left(0\right)=A_{0i}^{гс}$$

,with zero initial conditions $$A_i^{tn}\left(0\right)=A_{0i}^{tn}$$

, $$A_i^{g\mathrm{from}}\left(0\right)=A_{0i}^{g\mathrm{from}}$$

,

- активность i-го изотопа в теплоносителе, Бк; $$A_i^{гс}$$

- активность i-го изотопа в газовой системе, Бк; $$q_i^{тн}$$

- скорость поступления i-го изотопа в теплоноситель, Бк/с; λ_i - постоянная распада, с^-1; ν - относительная скорость дегазации (постоянная дегазации) теплоносителя, с^-1, равная отношению скорости выхода газа из теплоносителя в газовую систему к его содержанию в теплоносителе; ς - относительная скорость утечки газа из газовой системы, с^-1.Where $$A_i^{tn}$$

- activity of the i-th isotope in the coolant, Bq; $$A_i^{g\mathrm{from}}$$

- activity of the i-th isotope in the gas system, Bq; $$q_i^{tn}$$

- the rate of arrival of the i-th isotope in the coolant, Bq / s; λ _i is the decay constant, s ^-1 ; ν is the relative rate of degassing (constant degassing) of the coolant, s ^-1 , equal to the ratio of the rate of gas exit from the coolant to the gas system to its content in the coolant; ς is the relative rate of gas leakage from the gas system, s ^-1 .

High leak tightness requirements are imposed on gas systems of the first circuit, limiting leakage to a level of no more than 10 ^-12 s ^-1 . When calculating the activity of the considered isotopes in the gas system, the leak from it can be neglected and then the solution of the system of equations (2) for the activity of the ith isotope in the gas system takes the form:

Thus, in a stationary mode of operation after a certain period of time, the equilibrium values of the activity of most isotopes in the gas system are established.

The equilibrium is disturbed during sparging, when a through microdefect of the heat exchange surface of the steam generator occurs. The steam-water mixture from the second circuit penetrates into the liquid metal coolant, steam bubbles form, i.e. an internal interface develops. Through it, the inert gases contained in the coolant diffuse into the internal volume of the vapor bubbles, circulate along with them along the contour and, when the bubbles rise to the coolant mirror, enter the gas system. As a result, the activity of the gaseous products of activation and fission in the gas system of the primary circuit increases.

If we assume that the value of ν is directly proportional to the surface of the gas exit from the liquid metal, then the relative degassing rate of the coolant under bubbling conditions can be described by the following expression

where: S _zerk - the area of the coolant mirror (interface) m ² ; S _barb is the area of the interface formed by the bubbles, m ² ; ν _o - constant degassing of the coolant in the absence of sparging, s ^-1 ; ν _barb - constant degassing of the coolant during sparging, s ^-1 .

Under conditions of dynamic equilibrium, the activity of the ith isotope of inert gas in the gas system can be expressed by the formula

при барботировании будет больше, чем в отсутствие барботирования, если скорость поступления изотопа в теплоноситель $$q_i^{тн}$$

постоянна. Причем, чем меньше период полураспада изотопа, тем заметнее будет увеличение его активности в газовой системе при барботировании теплоносителя. Как показывают расчеты, для реакторов с тяжелым жидкометаллическим теплоносителем ν_o имеется величина порядка 10^-6 с^-1, а ν_барб - от 10^-6 до 10^-4 с^-1. Оптимальная для диагностирования величина постоянной распада реперного изотопа λ_i находится в интервале от 10^-5 до 10^-4 с^-1. Вне этого интервала либо не достигается нужная чувствительность измерений, либо не хватает запаса времени для их проведения.Since under any conditions ν _barb > ν _o , then the value $$A_i^{g\mathrm{from}}$$

when bubbling will be greater than in the absence of bubbling, if the rate of isotope entry into the coolant $$q_i^{tn}$$

constant. Moreover, the shorter the half-life of the isotope, the more noticeable will be an increase in its activity in the gas system during bubbling of the coolant. As calculations show, for reactors with heavy liquid metal coolant ν _o there is a value of the order of 10 ^-6 s ^-1 , and ν _barb - from 10 ^-6 to 10 ^-4 s ^-1 . Optimal for diagnosis, the value of the constant decay of the reference isotope λ _i is in the range from 10 ^-5 to 10 ^-4 s ^-1 . Outside of this interval, either the desired measurement sensitivity is not achieved, or there is not enough time to carry them out.

This condition is satisfied by the ⁴¹ Ar isotope, for which λ = 1.05 · 10 ^-4 s ^-1 . It is not a fission product, and in the case of depressurization of fuel elements, the growth of the protective gas activity will occur due to the contribution of other isotopes, in particular xenon and krypton, but not ⁴¹ Ar. Therefore, the measurement of activity of exactly ⁴¹ Ar allows avoiding a false conclusion about the leakage of the steam generator during depressurization of fuel elements.

The ⁴¹ Ar isotope, along with the optimal value of the decay constant and the fact that it is not a fission product, is characterized by another property necessary for the implementation of the proposed method that other gaseous products of argon activation do not possess, namely, the presence of a pronounced (above 0.5 MeV) gamma radiation lines. This feature allows you to use the gamma spectrometric method to measure its activity, which is highly reliable and sensitive and does not require the placement of sensors directly inside the gas system of the primary circuit. The gamma radiation energy of the ⁴¹ Ar isotope is 1.29 MeV, which allows the use of conventional industrial sensors-spectrometers and place them from the outer side surface of the measuring capacitance. To increase the reliability of measurements and ensure maintainability, the installation of two or more sensors is desirable. At the same time, argon must be supplied to the tank after it has been preliminarily cleared of vapors and aerosols so that they do not accumulate in the tank and do not create spurious background radiation.

When determining the calculated stationary value of the volumetric activity of ⁴¹ Ar in the measuring capacity, it should be brought to normal conditions, since during measurements, there may be differences in the pressure and temperature of the gas in the tank and, therefore, its density may vary. Therefore, normalization (calculated reduction to normal conditions) reduces the measurement error. A value of the order of 10% corresponding to the measurement error can be taken as the threshold excess value of the reduced stationary activity. If this threshold value is exceeded, a decision is made on the presence of a leak in the steam generator.

As an example of a specific application of the method, we can consider its implementation in a stationary reactor operating with a lead coolant, at a constant level of thermal power of 600 MW and a constant coolant flow rate of 3.84-10 ⁴ kg / s. In this case, the rate of ⁴¹ Ar isotope activity entering the coolant can be estimated at 1.47 · 10 ⁷ Bq / s.

The volume of the gas system of the primary circuit of a reactor installation of this type reaches 300 m ³ . The stationary value of the volumetric activity of the ⁴¹ Ar isotope corresponding to the specified operating mode in the gas system, determined by formula (2), is 5.82 · 10 ⁶ Bq / m ³ . The corresponding stationary activity of this isotope in the coolant is 1.38 · 10 ¹¹ Bq.

The area of the coolant mirror is estimated at 130 m ² . In the absence of bubbling, the relative rate of argon exit from the coolant, determined using the Fick law and the Einstein-Stokes formula for the diffusion coefficients in a liquid, is 1.34 · 10 ^-6 s ^-1 .

Calculations show that as a result of leakage in one tube of the steam generator, a steam-water mixture with a mass flow rate of about 1.16 · 10 ^-4 kg / s (10 kg / day), which is 5.7 · 10 ^-10 of the flow rate, will enter the lead coolant steam generator. In the first circuit, a bubble mode of two-phase flow will occur.

The effective radius of the vapor bubble R _eff corresponding to the flow regime in the first circuit will be 0.5 mm, and the bubble lifetime τ will be approximately equal to the average circulation time of the coolant along the primary circuit t _c and will reach 250 s, the specific vapor volume υ (for average temperature and lead pressure in the primary circuit) will be 0.485 m ³ / kg, and the volumetric flow rate of the leak g - 5.62 · 10 ^-5 m ³ / s.

The interfacial surface of the bubbles, determined by the formula (4), is 83 m ² . The relative degassing rate, determined by the formula (3), as a result of a steam generator leak, will increase to 2.17 · 10 ^-6 s ^-1 .

A method for diagnosing a leak in a heat exchange surface of a steam generator of a reactor plant with lead coolant includes the following operations.

When the reactor installation is operated with lead coolant in the above constant power mode, a certain amount of protective argon gas is cleaned of vapors and aerosols and then sent to a measuring tank of 10 l. Cold traps and filters can be used for cleaning, and activity monitoring should be carried out using a standard system for monitoring the tightness of the shells of fuel elements, which is equipped with gamma-spectrometric equipment. In this case, two or more gamma-ray spectrometers, consisting, for example, of a GC-1518 germanium detector and a Canberra type DSA-1000 analyzer, are placed in special slots on the outer surface of the measuring capacitance.

At a frequency of one hour, ⁴¹ Ar activity is measured in the measuring vessel, and the measurement time is about 600 s. At the same time, the temperature and pressure of the gas in the measuring vessel are measured. In this case, the gas temperature is taken equal to 50 ° C, pressure - 1.05 · 10 ⁵ PA.

According to the readings of spectrometers, pressure sensors and gas temperature in the measuring tank, knowing its volume, calculate the average volumetric activity of the ⁴¹ Ar isotope in the measuring tank, reduced to normal conditions. The described procedure is repeated until the stationary value of the volumetric activity of ⁴¹ Ar is established, which in this case will be 6.11 · 10 ⁶ Bq / n-m ³ .

Then, with the given periodicity, the measurements described above and the calculation of the volumetric activity of ⁴¹ Ar reduced to normal conditions are continued.

The obtained values of volumetric activity are compared with a stationary value, taking into account the possible measurement error.

The limiting relative error can, for example, be chosen equal to 10%.

The measurement error in this case is 6.11 · 10 ⁵ Bq / n-m ³ . If the measured value of the volumetric activity of ⁴¹ Ar exceeds 6.72 · 10 ⁶ Bq / n-m ³ , then the leakage of the heat exchange surface of the steam generator (steam generator leak) is considered established.

The change in the volumetric activity of the ⁴¹ Ar isotope in the gas system as a result of the occurrence of a steam generator leak, calculated by the formula (2), is shown in the figure. If the indicated steam generator leak occurred at a time corresponding to three hours on the graph, then the next measurement (after an hour) can establish the fact of the steam generator leak. Moreover, the gas content in the coolant is ~ 1.3 · 10 ^-5 , which confirms the higher sensitivity of the proposed method in comparison with known, for example, neutron-noise, in which the sensitivity threshold corresponds to a gas content of 10 ^-4 .

Claims (1)

A method for monitoring the tightness of the heat exchange surface of a steam generator of a reactor plant with a heavy liquid metal coolant, in which measurements are made, the gas content in the liquid metal coolant circuit is calculated, and a change in this indicator caused by bubbling of the coolant with a steam-water mixture is used to diagnose leakage, characterized in that during stationary operation of the reactor installation in which argon is used to compensate for the expansion of the coolant, argon is cleaned from vapors and aerosols, they are pumped through a measuring vessel, in which its temperature and pressure are measured, as well as spectrometric measurements of the activity of its component ⁴¹ Ar, the volumetric activity ⁴¹ Ar reduced to normal conditions, selected as an indicator of gas content, is calculated, its stationary value is determined, and the loss of tightness of the heat exchange surface of the steam generator is diagnosed by exceeding the reduced activity ⁴¹ Ar of its stationary value.