RU2410772C1 - Method of determining integrity of casing of irradiated fuel elements - Google Patents

Abstract

FIELD: physics, nuclear. ^ SUBSTANCE: invention relates to nuclear engineering and can be used in monitoring the state of fuel cells after their irradiation in a nuclear reactor. The fuel cell is placed in an adjustment device, the local section of the gas collector is heated to temperature which ensures removal of caesium condensate. The fuel cell is then cooled. Gamma-radiation intensity of the radioactive Kr-85 isotope is measured in the region of the gas collector and then compared to radiation intensity of a standard fuel cell, from which the state of monitored fuel cell is determined. ^ EFFECT: high reliability of determining integrity of fuel cell casing. ^ 4 cl, 4 dwg, 1 tbl

Description

The invention relates to the field of nuclear engineering and can be used to monitor the state of fuel elements after irradiating them in a nuclear reactor.

A known method for determining the integrity of the fuel rods, based on the puncture of the protective shells with subsequent manometric and mass spectrometric determination of the amount and composition of the released gas, the main components of which are helium, xenon and krypton [Sulaberidze B.Sh., Samsonov B.V., Kirillovich A. P. et al. Investigation of the yield of gaseous fission products under the cladding of fuel elements with compact uranium dioxide: preprint: NIIAR-26 (541). - Dimitrovgrad 1982].

Violation of the tightness of highly radioactive objects and the complexity of technical performance are the main disadvantages of this method, which is widely used both in our country and abroad.

The closest analogue that coincides with the claimed technical solution for the largest number of essential features is a method for determining the integrity of the shells of irradiated fuel elements, which consists in placing a fuel element in an alignment device, determining the intensity of gamma radiation of the krypton radioactive isotope (Kr-85) in the gas collection area and comparing this intensities with the radiation intensity of the reference fuel element, on the basis of which the state of the cladding of the fuel element is determined [Gorsky V.V. The role of non-destructive testing in solving the problem of increasing the burnup of nuclear fuel in PWR reactors // Nuclear Technology Abroad, 1983. No. 1. 11-11-19.].

Low sensitivity and accuracy are the main disadvantages of this method. A large error in measuring the Kr-85 radiation intensity is associated with its low specific activity and the presence of background gamma-ray emitters - volatile isotopes of cesium Cs-134 and Cs-137 in gas collectors.

The low specific activity of the gaseous Kr-85 isotope is due to its low concentration in the free volume of the fuel rod and the low yield of gamma rays with an energy of 514 keV per beta decay event (0.4%). Since the cesium isotopes on the walls of the gas collector are in a condensed state, and the output of gamma radiation with an energy of 605, 661 and 796 keV is about 90%, their activity per unit length of the gas collector is significantly (10-100 times) higher. The high load of the gamma spectrometer, the large Compton pedestal from the emission of cesium isotopes not only impair the accuracy of determining the peak area of the total absorption of gamma rays of the Kr-85 isotope with an energy of 514 keV, but it is often impossible to measure it at all.

The claimed technical solution allows to eliminate these disadvantages and ensures greater reliability of determining the integrity of the cladding of fuel rods due to the increased accuracy of measuring the intensity of gamma radiation of the Kr-85 isotope in the gas collector.

The above disadvantages are eliminated in the method for determining the integrity of the cladding of irradiated fuel elements, including the placement of a fuel element in the alignment device, measuring the gamma radiation intensity of the Kr-85 radioactive isotope in the gas collection area and comparing it with the radiation intensity of the reference fuel element, based on which the state of the controlled fuel element is judged, they are heated the controlled portion of the gas collector to a temperature that ensures the removal of cesium condensate, then it is cooled and the gamma radiation intensity of the Kr-85 isotope is measured this area with its subsequent comparison with the reference.

The controlled section of the gas collector is heated to a fixed temperature that does not reach the phase transition points of the fuel cladding material, then it is cooled below the melting temperature of krypton.

The studies showed that heating to a temperature of 400-600 ° C leads to almost complete purification from cesium of the fuel rod section of the fuel, on which the radiation intensity of the Kr-85 isotope is measured (Fig. 1). For domestic thermal and fast neutron fuel rods, the optimum temperature is 600 ° C, which is significantly lower than the phase transition points of the E110 zirconium alloy (zirconium - 1% niobium) and stainless steel (ChS-68).

In fuel rods with low burnup or a small GPA exit from fuel, a significant increase in the accuracy of measuring the radiation intensity of the Kr-85 isotope is achieved by cooling the gas collector below the melting point of krypton. The physical properties of this inert gas are such (see table) that at liquid nitrogen temperature (-195.8 ° C), krypton in the solid phase rapidly condensates on the walls of the cooled section of the fuel rod gas collector.

Table Melting point, ° С Boiling point, ° С The amount of gas in 1 dm ³ condensate, l -157.3 -153.4 644

The error in measuring the area of the total absorption peak located on the Compton background of the instrumental gamma spectrum is determined from the expression

,

,

where C is the relative standard deviation of the photopeak area;

A is the gamma line count rate;

B is the counting rate of the interfering background under the peak;

t is the measurement time.

Therefore, both a decrease in the background level B and an increase in the count rate of the useful signal A leads to an increase in the sensitivity and accuracy of measuring the intensity of gamma radiation of the Kr-85 isotope in the fuel rod gas collector.

Figure 1 presents the scans of the intensity of gamma radiation Cs-137 with an energy of 661 keV along the length of the fuel rod to (13) and after (14) heating its portion of 170 mm in length.

Figure 2. illustrates a decrease in the Compton pedestal, and, consequently, a decrease in the spectrometer load, under the peak of the total absorption of Kr-85 gamma radiation after cleaning the gas collector from cesium. On figa shows the spectrum of gamma emitters at around 80 mm from the bottom of the fuel rod to heat, on figb - after heating.

In figure 3. gamma-ray spectra are shown in the region of the gas collector before (a) and after (b) local cooling of the fuel element of the BOR-60 reactor with a burnup of ~ 1% t.a.

The device with which the inventive method is implemented is shown in figure 4, where:

1 - fuel core of a fuel rod;

2 - a protective sheath of a fuel rod;

3 - end caps;

4 - adjusting device;

5 - collimator;

6 - gamma spectrometer;

7 - gas collector of a fuel rod;

8 - heating device;

9 - krypton condensate;

10 - housing of the cooling device;

11 - tightness nodes;

12 - refrigerant (liquid nitrogen).

Install the fuel core 1 in the protective shell 2, sealed with end caps 3, vertically in the alignment device 4 opposite the collimator 5 of the gamma spectrometer 6, viewing the portion of the gas collector below the end cap. A gas collector is a free volume designed to compensate for the pressure of a gas engine leaving the fuel core during operation of fuel rods. Along with xenon and krypton, volatile products migrate to the gas collector, in particular the Cs-134 and Cs-137 isotopes.

Before measuring the intensity of gamma radiation of the Kr-85 isotope using a removable heating unit 8, the gas collector 7 is heated to a temperature below the phase transition points of the cladding material of the fuel element (Fig. 4a), to clean its cesium isotopes, then the fuel element is placed in a cooling device and cooled ( figb) to a temperature below the melting of krypton - (-157.3 ° C). Since radioactive products ( ⁵⁴ Mn, ⁵⁸ Co, ⁶⁰ Co, etc.), which contribute to the Compton background, are activated by a neutron in a massive blank, the condensation and measurement of the gamma radiation count rate of ⁸⁵ Kr are carried out at a certain distance (~ 100 mm) from the end of the fuel rod gas collector.

The temperature increase in the local area of the gas collector can be achieved using ohmic, induction, microwave, etc. heating up. To obtain Kr-85 condensate (9), a cooling device is used, consisting of a housing 10, two tightness assemblies 11 and a refrigerant 12, for example liquid nitrogen. The tightness nodes of white vacuum rubber withstand hundreds of freeze-thaw cycles of the gas collector section. When using a cooling device with a housing made of durable foam to preserve Kr-85 condensate, 200 ml of liquid nitrogen is sufficient for the duration of the control measurements. The use of a special Dewar vessel will significantly reduce the flow of refrigerant by one measurement.

Claims (3)

1. A method for determining the integrity of the cladding of irradiated fuel elements, including the placement of a fuel element in the alignment device, measuring the intensity of gamma radiation of the Kr-85 radioactive isotope in the gas collection area and comparing it with the radiation intensity of a reference fuel element, based on which the state of the controlled fuel element is judged, characterized in that the controlled section of the gas collector is first heated to a temperature that ensures the removal of cesium condensate, then it is cooled and the gamma radiation intensity of the Kr-85 isotope is measured on this locally stretch. 2. The method according to claim 1, characterized in that it is heated to a temperature not reaching the points of phase transitions of the fuel cladding material. 3. The method according to claim 1, characterized in that before the measurements for condensation of the gaseous Kr-85 isotope, the controlled portion of the fuel rod gas collector is cooled below the melting temperature of krypton.

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