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A condition for safe operation of a BBER reactor is that a temperature margin with respect to the fracture toughhess of the reactor vessel must be maintained. In the course of operation this margin, i.e. the difference between the admissable critical embrittlement temperature (Tc admiss) and the actual temperature (Tr decreases as a result of radiation embrittlement, increasing the probability of brittle fracture. For this reason, a condition for safe operation of the vessel is Tcadmiss > T c.The degree of embrittlement of materials depends on many factors: the quality of the steel, determined by the content of impurities (copper, phosphorus) and alloying elements; the temperature-time regime of operation; the neutron spectrum and fluence; and, so on.According to current computational standards where the coefficient Af depends on the type and brand of steel, the impurity content, and the irradiation temperature. The mechanism of radiation embrittlement is based on the production of defects, redistribution of impurities, specifically, migration of impurities to sinks, and the appearance of conditions for brittle fracture, both transcrystalline and along grain boundaries. These processes are accelerated in the presence of radiation defects. For this reason, not only the decrease in the impurity concentration but also an increase in the irradiation temperature (intensification of the thermal annealing of defects) decrease radiation embrittlement (the coefficient Af decreases).Together with a thermal effect, there is also an effect due to radiation 3' annealing. This effect has been discovered in diamond, confirmed in graphite, and should be observed in other materials [2]. The irradiation conditions as well as the relative changes in the hardness and the values of AT F as a function of the metal thickness are presented in Tables 1 -3 for 15Kh2MFA steel (main metal), 10KhMFT steel (seam metal), and 25Kh3HM steel, which were used to fabricate the vessels of the decommissioned reactors in the first two power-generating units of the Novovoronezh nuclear power plant and an experimental prototype reactor. These results are presented graphically in Fig. 1.The flux density of ~,-rays (~v) was calculated for an energy of 1 MeV. At the center and boundary of the reactor core, ~v was calculated for an equivalent homogeneous reactor with the same power and contact of the elements as for a real heterogenerous reactor. The decrease of the T-radiation along the path from the core boundary up to the irradiation location was calculated using the formulas for a fiat source taking account of absorption and accumulation in alternating layers of water and iron for a specific reactor. The additional contribution to T-radiation as a result of activation of iron by thermal neutrons was estimated for the interior surface of the vessel.Examination of Fig. la shows that the changes in the displacement of the critical embrittlement temperature (AT F) in a direction along the thickness of the vessel are completely different for different cases ...

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