RU2301463C1 - Method for checking core of shipboard water-cooled nuclear power plant for condition - Google Patents

Abstract

FIELD: nuclear power engineering; shipboard nuclear power plants.

SUBSTANCE: proposed method for checking core of shipboard water-cooled nuclear power plant for condition includes measurement of reference-radionuclide specific beta-activity in primary coolant samples taken from running reactor followed by evaluation of core condition. Sample for evaluating core condition is taken downstream of ion-exchange filter under any operating conditions of reactor. Used as reference radionuclide is tritium. Such method makes it possible to check reactor core for condition under any of its operating conditions, including shutdown condition.

EFFECT: reduced dose rate suffered by attending personnel under poor tightness condition of cans.

2 cl

Description

The invention relates to the field of nuclear engineering and can be used to monitor the state of the active zone of a ship's nuclear power plant with a water coolant.

A known method of controlling the tightness of the shells of fuel elements (fuel elements), based on the registration of delayed neutrons emitted mainly by short-lived isotopes of bromine and iodine. A significant disadvantage of this control method is that the measurement of delayed neutrons is difficult due to the presence of a large background created by the induced activity of ¹⁷ N [1].

Closest to the claimed method for monitoring the state of the reactor core is radiochemical control of primary water. The basis of radiochemical control is the determination of the specific total beta activity of iodine radionuclides in water samples of the first circuit [2]. Radiochemical control is carried out by sampling through a sampling system to an ion exchange filter with their subsequent analysis. For the purpose of comparability of the analysis results, the specific total beta activity of iodine radionuclides is measured two hours after sampling and is linearly converted to the rated (100%) reactor power. With values of the total beta activity of iodine radionuclides exceeding the limit value (3.7 × 10 ⁸ Bq / kg) for any core energy production, the operation of the reactor installation is not allowed, with the exception of circumstances due to the special situation. Sampling is carried out under the following conditions:

- the power level of the reactor should be constant in the range from 40 to 60% of the nominal power value;

- the reactor operating time at a selected constant power level, within the above limits, up to the time of sampling should be at least 24 hours.

The objective of the invention is to create a method that allows you to control the state of the active zone of the reactor at any power levels and operating modes of the reactor, depending on the energy production of the active zone.

The technical result achieved during the implementation of the invention is the ability to control the state of the active zone under any operating conditions of the reactor installation, including the standby mode, as well as reducing dose loads on maintenance personnel in the conditions of leaking fuel claddings.

To achieve the specified technical result, it is proposed to use a method based on radiometric measurement of tritium activity in samples of the primary coolant. The main source of tritium formation in the ship nuclear reactor is nuclear fission. In the fuel of light-water reactors, one act of fission of ²³⁵ U, ²³⁸ U, ²³⁹ Pu produces (0.85; 2; 2) × 10 ^-4 tritium atoms, respectively [3]. As a result of diffusion through the cladding of fuel rods, as well as cracks and microcracks in the claddings of tritium, fuel can enter the coolant from the fuel. The output of tritium from fuel rods with a sheath made of corrosion-resistant steel is 1%, and from a zirconium alloy - 0.1% of the total amount under the sheath [4]. Thus, the output of tritium from fuel elements can be represented in the form of two processes: a constant diffusion output and a random one associated with their depressurization. The contribution of other sources to the formation of tritium activity in the coolant is incommensurably small; therefore, under conditions of emergency depressurization of the claddings of fuel elements, they can be neglected.

A distinctive feature of the proposed method is that a sample for radiometric analysis to determine tritium can be taken through a sampling system both before and after the ion-exchange filter, because tritium is not adsorbed on ion exchangers and surfaces. This distinguishing feature is significant from the point of view of dose loads on maintenance personnel in the conditions of leakage of fuel cladding, especially in emergency situations when the sampling of the coolant to the filter is impossible due to the extremely high radioactivity of γ-emitting radionuclides. Therefore, in emergency situations, well-known methods of operational control of the tightness of the cladding of fuel rods are unacceptable. In contrast to the method using the specific total beta activity of iodine radionuclides, the proposed method for determining the tritium activity in a coolant sample does not require preliminary long-term operation of the reactor at a stationary power level, holding the sample for two hours and converting the measurement result to rated power. In addition, when using this method of assessing the state of the active zone, strict conditions are not required when sampling the coolant, because A coolant sample is taken after a standard primary circuit filter, i.e. after sorption of γ-emitting radionuclides on ion exchangers, which ensures the creation and maintenance of a favorable radiation environment during all technological operations for monitoring the state of the active zone.

According to the proposed method, the state of the active zone during the campaign is evaluated as follows:

- the active zone is sealed when the following conditions are met:

- the active zone is leaking when the following conditions are met:

Where

And _az is the activity of tritium formed in the core at the time of measurement, Bq;

And _Ik is the measured activity of tritium in the water of the primary circuit, Bq;

G is the rate of formation of tritium in the active zone, Bq / MW · h;

Q - energy production, MW · h;

To _o - the coefficient of tritium output through the cladding of fuel rods - for fuel rods made of corrosion-resistant steel is 0.01, and for fuel rods with a cladding of zirconium alloy - 0.001 of the total amount under the cladding.

The method is as follows. After sampling the primary coolant in any reactor operating conditions, including the standstill, an aliquot of the sample is taken for radiometric analysis to measure tritium activity. The measured value of tritium activity, taking into account the dilution that occurs due to drainage of the primary coolant and subsequent replenishment, is compared with the amount of tritium activity formed in the core at the time of measurement taking into account the output coefficient. When condition (1) is fulfilled, the active zone is recognized as tight, and when condition (2) is fulfilled, when the obtained value is greater than the value of tritium activity formed in the active zone at the time of measurement, taking into account the output coefficient, the active zone is recognized as leaky.

For measurements of tritium activity in a primary coolant, for example, a radiometric method using a liquid scintillator can be used.

Information sources

1. Ovchinnikov V.F., Golubev L.I., Dobrynin V.D. and other. Operational modes of water-cooled power nuclear reactors. M., Atomizdat, 1977, 280 p.

2. OST 95 10002-95 Water quality standards for the first and third circuits of the reactor installation of vessels of the Department of Maritime Transport of the Russian Federation with water-cooled reactors of the KLT-40 type. Moscow, 1995, 17 pp.

3. Ionizing radiation: sources and biological effects. Doc. UNSCEAR for 1982, New York: UN, Vol. 1, 1982.

4. Production and emission of tritium from nuclear facilities, and the resulting problem. in: Processing of Behavior of Tritium in the Environment: San Francisco, October 16-20, 1978. Vienna: IAEA, 1979, p.105-123.

Claims (2)

1. A method for monitoring the state of the active zone of a ship’s nuclear power plant with a water coolant, comprising measuring the specific beta activity of the reference radionuclide in the primary coolant samples at a working reactor, followed by an assessment of the state of the active zone, characterized in that a sample is taken to assess the state of the active zone after ion-exchange filter in any operating mode of the reactor, and tritium is used as a reference radionuclide. 2. The method according to claim 1, characterized in that the state of the active zone during the campaign at each particular moment in time is evaluated by fulfilling the following conditions: the core is sealed when the condition

the core is leaking when the condition

Where

And _az is the activity of tritium formed in the core at the time of measurement, Bq; A _Ik is the measured activity of tritium in the water of the primary circuit, Bq; G is the rate of formation of tritium in the active zone, Bq / MW · h; Q - energy production, MW · h; To _o - the output coefficient of tritium through the cladding of fuel rods, for fuel rods made of corrosion-resistant steel is 0.01, and for fuel rods with a cladding of zirconium alloy - 0.001 of the total amount under the cladding.