Vol 123, No 5 (2018)

The RBMK design-stage service life was assumed to be 30 years. Ten power-generating units have now exceeded this limit. Service life extension of nuclear power plants is a common practice in countries using nuclear energy. This approach was also applied to power-generating units with RBMK-1000 with servicelife extension to 45 years or more. To maintain electricity production in NPPs with RBMK, a technology for restoring the service-life characteristics was developed and implemented in the No. 1 unit of the Leningradskaya NPP. The results of the adoption of this technology in combination with experimental and computational validation of safety made it possible to extend the operation of the No. 1 unit of the Leningradskaya NPP in the power-generation mode since December 2013. Similar work was subsequently successfully completed in other power-generating units with RBMK.

The results of computational and experimental studies performed at NIKIET to validate the serviceability of the actuating mechanisms of the safety-and-control system in terms of assuring the passability of the absorbing working rods in the case of channel bending under conditions of RMMK-1000 graphite stack distortion are presented. The results of this work were used for the power-generating units with RBMK-1000. They can be used for other types of reactors, in which the working absorbing rods of the SCS move in channels, as well as in other areas of industry where the passability of solid elements is important.

The neutron-physical characteristics of RBMK at the Kursk, Leningradskaya, and Smolensk NPPs for 2015–2017, as determined from calculations of more than 6000 actual states at nominal power, are presented. The results of computational modeling of shutdown regimes upon actuation of the shutdown systems of the emergency protection and fast power reduction are presented. It is shown that the physical characteristics, efficiency, and fast response of the shutdown systems of the emergency protection and fast power reduction ensure the nuclear safety of RBMK. The effect of repair of the graphite stack with a sharp graphite blocks on the neutron-physical characteristics of the reactor is determined. It is shown that the removal of some of the graphite during the repair of 500–600 graphite columns results in a reduction of the steam coefficient of reactivity and partial or complete removal of additional absorbers from the core while maintaining the neutronic characteristics and nuclear safety of RBMK.

A procedure for evaluating the technical conditions and residual service life of the RBMK-1000 graphite masonry and a program of research have been developed in order to evaluate the serviceability and the possibility of service life extension of the RBMK graphite stack at the final stage of operation. The serviceability criteria for the graphite stack, periodicity and methods of controlling them, which, in combination with predictive calculations have made it possible to validate the possibility of safe operation of the RBMK-1000 graphite stack above the assigned service life allowed to substantiate the possibility of safe operation of the graphite stack RBMK-1000 over a designated period of time, were determined.

The economic aspects of the R&D complex to support RBMK operation under conditions of graphite distortion, development and adoption of the restoration technology for the service characteristics of the graphite stack are examined. The results of a technical-economic analysis of the restoration of the graphite stack of RBMK power-generating units, performed on the basis of a modeling scenario, are presented. Comparative economic indices of electricity production, proceeds, and investment resources are presented for the studied scenarios. It is shown that the restoration of the graphite stack in NPP with RBMK is economically efficient.

In providing justification for the operational safety of RBMK-1000 power units with graphite stack distortion, earthquake resistance tests were performed on a full-scale model of RBMK graphite stack under seismic platform conditions. The tests were performed for three mutually perpendicular axes of the model successively for seismic loads corresponding to magnitudes 6, 7 and 8 on the MSK-64 scale. The tests confirmed the earthquake resistance of the graphite stack for prescribed loads. The results were used to verify the FEMGR code (Kurchatov Institute) and the LEGAK-DF finite-element software system (RFYaTs – VNIIEF) for purposes of computational validation of the earthquake resistance of the post-repair restored RMBK-1000 core.