Vol 121, No 1 (2016)

The year 2016 marks the 30th anniversary of the accident in the No. 4 unit of the Chernobyl NPP and the 5th anniversary of the accident at the Fukushima NPP in Japan. The Chernobyl accident substantially changed the public’s attitude toward severe accidents at NPPs, promoting on the one hand modernization and improvement of safety systems and on the other hand a higher safety culture by harmonization of standards and regulations, and adoption of deeply echeloned protection and a scientifically validated approach to the analysis of accidents. The Fukushima accident revealed a new class of accidents – extreme actions caused by naturally occurring cataclysms and/or different external actions, which actually are identical to loss of a large part or all of the means for controlling an accident. The present article examines the main lessons on accidents in terms of understanding the phenomenology of the events occurring during severe accidents and confirms the importance of the deterministic component of the safety concept. At the same time, it emphasizes that special attention must be given to hydrogen safety, systems for prolonged removal of residual heat, emergency readiness, and emergency response under the extreme conditions of severe accidents. The computational means used to develop accident counter-measures must be properly set to take account of the design features of the reactor facility, the containment of the NPP, and the adjoining territories in order to determine the time required to implement the procedures for controlling accidents and mitigating the consequences.

The transition to new design and circuitry requires careful validation of the efficacy of the new solutions. For the sodium-water BN-1200 macro-modular steam generator, the damage is determined by the failure of the structural material in the zone of a water leak into the sodium and the amount of water entering the coolant. The parameter that ultimately determines the magnitude of the damage is the time interval between the start of a small leak and the actuation of the steam generator’s protection system. Acoustic methods are effective for decreasing the lag of the indication system. The design is based on the use of two-frequency ranges – vibro-acoustic (0.1–40 kHz) and ultrasonic (40–500 kHz) – of high-sensitivity acoustic sensors, and fast software-hardware methods of analyzing acoustic signals in real-time. The presence of thermal insulation and protective jackets on the sodium equipment and pipelines strongly affects the character of the outflow and burning of the sodium and eliminates dispersal. Analysis shows that there are strong grounds for reducing the conservatism (safety margins) in studying emergency regimes associated with sodium leaks.

The materials science aspects of increasing the operational safety of advanced VVER reactors are described. The requirements of the materials for increasing the operational safety of the reactor vessel are formulated. The use of 15Kh2NMFA class 1 steel and 15Kh2MFA-A mod A and mod B steels for fabricating reactor vessels with enhanced operational reliability is analyzed. The results of investigations of the radiation and thermal embrittlement of the improved steels 15Kh2MFA-A mod A and mod B are presented.

A mathematical model of diffusion of calcium hydroxide in the system concrete barrier–ambient environment was developed to determine the influence of corrosion processes of the first and second kinds, engendered by water and radionuclides, on the lifetime of a concrete barrier of a decommissioned commercial uranium-graphite reactor. The CaOH distribution over the thickness of the barrier in a single crack was determined. Calculations were performed to determine the CaOH concentration over the barrier thickness and to analyze the effect of corrosion and the use of S-3 superplasticizer in concrete on the lifetime of the barrier. The superplasticizer can extend the operating lifetime of the concrete barrier to 800 years.

The transition of a lead-cooled reactor initially fueled with enriched uranium into an equilibrium operating regime is examined. Regenerated fuel is used consistently upon closure of the fuel cycle. The defi ciency of recycled fissile material is compensated by enriched uranium. The time to the equilibrium regime is 45 years. The breeding ratio reaches 1 in approximately 38 years. The neutron-physical characteristics were calculated using the multigroup diffusion code Arktika and the PRIZMA spectral code, which employs the Monte Carlo method. The RISK code was used to calculate the evolution of the nuclide composition. The change in the nuclide composition in the external fuel cycle was calculated after each microrun.