V. A. Nikolaenko

EFFECT OF GAMMA RAYS ON CRYSTALLINE MATERIALS DURING IRRADIATION IN A REACTOR

The article presents and discusses the results of experiments to determine the effect of gamma rays on the change in the properties of diamond, graphite, and structural steel. The materials were irradiated in a VVER type reactor. For diamonds, the effect on the annealing of defects was investigated. As gamma ray intensity increased, the crystal lattice expansion and defect concentration increased. Graphite lattice expansion and the mechanical properties of structural steel were also examined. Graphite lattice expansion increased with increased neutron flux and decreased irradiation temperature. Changes in the impact toughness of structural steel correlated precisely to the gamma ray flux in the experiments. 6 refs., 3 figs.

Effect of γ-irradiation on defect annealing in diamond

Abstract The results are presented of experiments on annealing in reactor and furnace the diamond pre-irradiated with reactor neutrons. It is established that annealing in a reactor proceeds more intensively than thermal annealing provided the γ-flux was lower in primary irradiation than in the secondary one. The γ-flux ratio varied due to the use of absorbing or amplifying screens and also by varying the reactor types or sample position. The experiment is described on electron annealing of a damaged diamond (electron energy 200 KeV). The volume of an elementary cell (expansion) with the constant measured radiographically was used in all experiments as the standard of defects present. The conclusion is that the contribution of γ-radiation always associated with the reactor neutron radiation to the formation of defect structure in materials is quite significant.

Diamond amorphization in neutron irradiation

The paper presents the results on neutron irradiation of the diamond in a nuclear reactor. It is shown that the neutron irradiation stimulates the diamond transition to the amorphous state. At a temperature below 750°K the time required for the diamond-graphite transition decreases with decreasing irradiation temperature. On the contrary, in irradiation at higher temperatures the time of diamond conversion into the amorphous state increases with decreasing temperature but always remains shorter than in the absence of irradiation.

Disordering of silicon carbide during neutron irradiation

Abstract It has been shown that neutron irradiation of silicon carbide results in : isordering of its crystal lattice. The data are given showing the effect of the irradiation temperature and thermal annealing on disordering. Irradiation to the neutron fluence of 3.2 × 1021 n/cm2 (E > 75 eV) at 120°C causes disordering when about 18% of lattice sites that should be occupied by the atoms of one type are actually occupied by the atoms of another type.

Bombardment of diamond with carbon ions

Abstract Diamond powder and monocrystals were bombarded with 72 MeV carbon ions. The photographs in reflected and transmitted light revealed a layer with a thickness estimated as 7 ±2 μm, where the bombarding ions stopped, as well as a layer with a thickness of about 70 μm, where, due to accidental channelling, some ions had penetrated further into the specimens. The contribution of interstitial atoms to expansion of the diamond crystal lattice was estimated as 1.16±0.30.

Effect of the composition of radiation on the radiation damage to graphite

A statistical analysis is performed of the results on the determination of the critical neutron fluence in MR, SM-2, and BOR-60 with different irradiation temperature. It is shown that the critical neutron fluence depends not only on the irradiation temperature but also, and to an even greater extent, on the radiation composition factor (ratio of the neutron and γ-ray flux densities). Thus the critical neutron fluence for irradiation at 600°C in MR (radiation composition factor 0.13) is 17·1021 cm−2 and in SM-2 (radiation composition factor 0.1) 11·1021 cm−2 at the same temperature. When the same graphite is irradiated in the region of the outer corner of a working block of RBMK, where the radiation composition factor is 0.55, it is expected that the critical neutron fluence will be 31.7·1021 cm−2.In summary, taking account of the effect of γ-radiation introduces substantial corrections: the experimental results obtained in research reactors are found to be at least a factor of 2 too low. This gives hope of substantiating the substantial increase in the service life of the RBMK graphite masonry. 3 figures, 8 references.

Radiation damage in graphite

Abstract The contribution from vacancies to the expansion of graphite lattice along the C axis and to its contraction along the a axis has been calculated on the basis of the dependence of the spacings between the carbon atoms on the bond order. It is found that αvc = 1.32, αva = −0.46. Using the same model the effect of interstitials contained in various complexes on the graphite expansion along the C axis has been estimated. The relationships obtained have been used for the analysis of experimental data on the change in the C and a parameters of graphite irradiated under various conditions. Comparison with other models is also made.

Anomalous expansion of irradiated fine–grained diamond

Abstract The effect of the grain size on the irradiation-induced expansion of diamond has been experimentally shown. Under the similar irradiation conditions (integral neutron flux of 1.4. 1019 n/cm2 E > 75 eV, temperature 130°C) 200 μm and 0.1 μm diamond powers expand up to 0.67 and 1.72% respectively.

EFFECTIVE AVERAGE INTEGRATED γ-RAY FLUX STRIKING RADIATION DEFECTS DURING IRRADIATION IN A REACTOR

The results of experiments on the irradiation of diamond which attest to the fruitfulness of the zone theory of damage to materials are presented. The theory is used to derive an expression for the effective average integrated γ-ray flux striking radiation defects during irradiation in a reactor. It is shown by statistical analysis of the results on the critical neutron fluence for reactor graphite that when the average γ-ray flux is taken into account, other conditions being equal, the statistical error in the estimate of the critical neutron fluence for graphite decreases by a factor of 1.5. This shows that the effective average integrated γ-ray flux is another important parameter of the irradiation conditions, and this expands the physical meaning of the radiation composition factor. 3 figures, 1 table, 9 references.

Effect of the nonuniformity of graphite samples on their shape change under irradiation

It is shown that the use of the initial characteristics of samples makes it possible to decrease substantially the variance and the bias in the determination of the critical neutron fluence for GR-280 RBMK graphite. It is recommended that a graphite model that includes neutron fluence and the initial value of the elastic modulus be used as an approximation for dimensional changes of graphite samples. 2 figures, 2 tables, 3 references.