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The principle of operation of a crystalline temperature indicator for measuring the maximum temperature is described and examples of its application are given. The operation of the device is based on the expansion of the crystal lattice of diamond or silicon carbide under irradiation in a reactor and a decrease of this expansion depending on the duration and temperature of heating. The results obtained during flights of shuttle models In the tests, the temperature of the heat-shielding coatings intended for installation on the Buran shuttle was tested. Another example of an application is thermometry of the Siemens (Germany) GTX-800 power-generation turbine. In this case, the temperature of the working and guiding blades of different steps of the motor as well as the gas in direct proximity to the blades was measured. The temperature in the space behind a VVER-1000 vessel was measured with the sensor.The basis of experimental methods of measuring temperature is a change of the properties or state of the working substance of the sensor. The diversity of instruments now available for determining temperature competes with the number of physical properties on which the principles of operation are based. However, in science and engineering there arise problems which require changing the range and accuracy of the measurements, the dimensions and other characteristics of the sensors. One method of expanding the possibilities for researchers is to use a crystalline maximum-temperature indicator (CMTI). The operating principle of the sensor is based on the expansion of the crystal lattice of its working substance (diamond or silicon carbide) during irradiation in a reactor at low temperature (~80°C) and a decrease of this expansion on heating.The increase in the volume of the crystal under irradiation can be caused by a change of the crystal lattice parameter, release of new phases, appearance of pores, and other factors. When the expansion is due only to an increase of the lattice parameter and the number of unit cells remains unchanged, the equality ΔV/V = 3Δa/a, where V is the volume of the crystal and a is the crystal lattice parameter, holds. Physically, this relation means that the macroscopic and x-ray density of the material are equal to one another. Thus, if the experiment shows that these two values are the same, then the same number of interstitial atoms as vacancies is present in the volume of the materials (i.e., the crystal is saturated with Frenkel pairs) [1]. The experiments on measuring the density which were performed on diamond samples irradiated at low temperature as well as diamond annealed after irradiation show that they remain equal to one another when the diamond lattice expands by 4-5% (Fig. 1).The macrodensity of a 2-3 mm diamond crystals was measured with a thermal-gradient tube [2] using standards and of powder samples by hydrostatic weighing. The x-ray density was determined from x-ray data.It is known that neutron irradiation can result in the appearance of a wide spectrum of lattice defect...

It is shown that the saturation of the expansion of the diamond crystal lattice during irradiation in a reactor depends not only on the temperature but also on the intensity of the irradiation. Expansion saturation decreases with decreasing irradiation intensity. The temperature influences saturation expansion indirectly via an increase of the effectiveness of the annealing of defects, and the intensity of irradiation stimulates additional annealing by increasing its duration. The same level of the saturation of expansion of the diamond lattice, specifically, 2.75%, can be attained at irradiation temperatures 80 and 300°C if the diamond is irradiated with neutron flux density 10 12 and 10 14 sec -1 ·cm -2 , respectively.One modern idea concerning change in the properties of materials irradiated in a nuclear reactor is based on the zone theory of the formation of defects. Zones with an elevated concentration of defects are formed at the end of the tracks of the first knocked-out atoms. Their size depends on the type of irradiated material. For example, the zone volume in diamond is about 5·10 -21 cm 3 . The displacement zones arising in irradiated material do not overlap initially, so that the number of zones, the number of defects, and the change of the properties, specifically, the expansion of the crystal lattice of diamond, increase at this stage in proportion to the irradiation dose. As the irradiation dose increases, some displacement zones start to overlap and the number of defects increases more slowly, destroying the linearity of the dose dependence of the expansion of the lattice. By this time, the new zones which arise result only in a replacement of old zones by new ones, the concentration of zones does not change, and the expansion of the lattice reaches saturation. Figure 1 shows that the character of the dependence agrees with the zone model: saturation (plateau) appears after the linear section. The dependence, presented in Fig. 1, of the expansion of the diamond lattice in percent on the neutron fluence F is described by the equationwhere 3.65 is the corresponding plateau in the expansion of the diamond lattice (saturation), %; 0.032 characterized the initial slope angle of the (1) dependence. It is assumed that the concentration of defects in any volume excited by displacements is not associated with the time elapsed after their formation. In reality, it does depend on the time, i.e., the "age" of the zone up to the moment of repeated excitation or termination of irradiation, and is determined by thermal annealing. When the structure of the material is photographed instantaneously during irradiation, sections (volumes) which have not been touched by displacements as well as volumes which have transitioned through the state of displacement one or more times can be seen in the photograph. The age of these volumes will be different, for example, the ones that arose at first and remained untouched by secondary excitation up to termination of irradiation will be older than the ones where the displacem...

539.12.04 and I. V. BachuchinThe possible reasons for the signifi cant variance of the experimental data in the investigation of the infl uence of neutron irradiation on the properties of the vessel materials of nuclear reactors are analyzed. It is shown on the basis of our own experimental data and an analysis of foreign experimental data that the dependence of the degree of radiation embrittlement on the neutron fl uence can be nonstandard (nonmonotonic). It is noted that the conventionally studied parameters, viz. the intensity of irradiation and the technological factor, can play only a secondary role when nonmonotonicity is taken into account. It is supposed that the defect structure resulting from neutron bombardment undergoes transformation once or periodically in the direction of loss or restoration of the initial properties. The defective material can be restored, specifi cally, at low dose of irradiation by corpuscular particles, i.e., at some point the degradation by irradiation in a certain dose range even acts as a healing factor.The dependence of the shift ΔT F of the critical temperature of brittleness versus the neutron fl uence F at irradiation temperature 270°C for the seam metal of VVER-440 vessel steel according to strength norms can be described by the function [1]where the coeffi cient of radiation embrittlement is given by the relationwhere P and Cu denote the content of phosphorus and copper (wt.%). Together with this, in contrast to the smooth curve (1), cases of a nonmonotonic change of the dependence have been observed [2][3][4]. In the foreign literature, the expression 'completely atypical embrittlement' is used for such cases [3]. The data on embrittlement of BWR vessel steel, which were obtained at neutron irradiation intensities 10 9 , 2·10 10 , and 7·10 11 sec -1 ·cm -2 , serve as an example of such a dependence [4]. The infl uence of the irradiation intensity can be seen in Fig. 1, but actually its effect does not appear. For example, the reduction of the embrittlement at fl uence 1.5·10 18 cm -2 begins just the same at neutron fl ux density differing by a factor of 10.A similar nonmonotonic shift of the critical temperature of brittleness of the trepan seam metal of a core sample from the decommissioned No. 1 unit of the Novovoronezh NPP is presented in Fig. 2. It was assumed that the nonmonotonicity is explained by the differences in the ratio of the intensity of neutron and γ radiation toward the outer layers of the vessel metal [5,6]. Similar results were observed in core samples from the No. 2 unit of the Novovoronezh NPP and the reactor on the nuclear ice breaker Lenin [5][6][7].

A method of using diamond, which enables verifying activation-method indications obtained with 54 Fe, to determine fast-neutron fluence is described. The procedure is especially effective for investigating the behavior of VVER-100 vessel steel. An advantage of the method is that there are no requirements concerning data on the start and termination of irradiation or stopping of reactor operation. The diamond sensor retains information on the neutron fluence for a long time after irradiation; it is especially effective for irradiation times longer than 3 yr, when information obtained about the neutron flux by means of 54 Mn actually vanished because of the short half-life. The procedure for using diamond is simple and based on determining the expansion of its crystal lattice. Data obtained simultaneously by means of the activation method are used to compensate for the drawbacks of the diamond method.One of the main parameters acting on the properties of materials irradiated in a nuclear reactor is the fast-neutron fluence determined on the basis of nuclear reactions between neutrons and the isotopes of certain elements. The fast-neutron flux can be judged according to the activity of the nuclei which are formed [1].The use of activation methods is irreproachable, but individual factors still limit their use. For example, when 54 Fe is used information about neutron fluxes preceding measurements by three or more years vanishes because of the short halflife of radioactive 54 Mn formed. For this reason, if the flux density of fast neutrons with prolonged irradiation is measured, procedures which are not always reliable must be used to take this into account. Other difficulties arise when niobium, whose half-life is longer, is used.However, neutron fluence can be determined according to the change in other properties of irradiated materials, specifically, the expansion of the crystal lattice of diamond [2]. But even this method is not free of drawbacks. For example, the effect of the irradiation temperature changes the properties of a diamond sensor. In addition, preliminary calibration is required in order to use diamond, specifically, by means of the same activation methods.In this connection, it can be expected that a combination of two methods can reduce the drawbacks to a minimum and maximize the advantages. Such a combination of methods is especially helpful for massive measurements, for example, when studying the properties of control samples of VVER-1000 vessel steel, when the irradiation temperature 300°C is constant [3]. In this case, diamond sensors consisting of 3 mm in diameter and 10 mm long aluminum capsules filled with diamond powder are irradiated simultaneously with 10 × 10 × 57 mm control samples. The sensors are placed in openings located directly in the control samples 5 mm from one end of the capsule [4].The diamond crystal lattice expands under neutron irradiation, and this expansion is functionally related with the neutron fluence. The expansion of the diamond lattice can be measured to a high ...