RBMK reactors
The Chernobyl’s family of reactors
Cold War and the Iron Curtain led after 1947 to the development of separate reactor designs in Western countries and in the Soviet Union. Espionage and secret service exploits have far outweighed the lack of industrial contacts!
RBMK reactors
Simplified section of a 1000 MW RBMK reactor. Fuel assemblies are contained in pressurized tubes inside which the cooling fluid circulates. 1700 pressure tubes are placed vertically in a stack of graphite bricks which acts as a moderator. Above the reactor, a reloading machine enables the continuous fuel unloading and loading nto the pressure tubes. During the accident, the graphite bricks burned for 10 days, prolonging the release of radioactivity.
© IRSN ©
At the end of World War II, Americans developed reactors at Hanford to produce plutonium for their atomic bombs. Such reactors were abandoned in the West for the production of electricity, but continued in Russia. The Soviets developed the RBMK system, enriched uranium reactors, the design of which stems from their first atomic prototypes. The RBMK reactor became tragically famous with the Chernobyl accident.
These reactors are graphite moderated and water cooled. The fuel is an uranium oxide enriched at the 2 to 2.6% level in uranium-235. The core is bulky, 20 times that of a PWR reactor. No pressure vessel, but a large number of vertical tubes, called “pressure tubes” containing the fuel. There is no containment enclosure. The Chernobyl reactor contained 190 tons of enriched uranium oxide distributed in 1681 “pressure tubes”.
Each pressure tube houses a uranium dioxide fuel assembly around which cooling water circulates at a pressure of approximately 70 atmospheres. The water acts as a coolant and provides the steam directly used to drive the turbines (there is no steam generator). Fuel loading and unloading takes place continuously, without the need to shut down the reactor.
The characteristics of the reactor (boiling water, graphite as a moderator, slightly enriched fuel, absorbents, etc.) generates an instability at certain operating regimes to be avoided, particularly at low power. The temperature coefficient then becomes positive : an increase in temperature causes a new increase in temperature and so on. The initial effect is thus amplified. This destabilizing effect then makes it difficult to control the reactor. To fight it, one should rely play the control rods. Since the 1986 Tchernobyl accident, the number of control rods in the core have increased and the insertion speed of these rods, which was too slow, has improved.
This functional fragility contributed to the Chernobyl accident, but was not the primary reason. RBMK reactors had safety systems that could have played their role, but some had been deliberately disabled for testing.
At the time of the Chernobyl accident, 16 RBMK reactors were in operation. After the accident, between 1991 and 2000, all Chernobyl RBMK reactors were permanently shut down. Two 1500 MWe reactors at Ignalina in Lithuania were also closed (the last at the end of December 2009), their closure having been required for the country entry into the European Union. On this date, there remained 11 RBMK reactors of 1 GWe, in operation, 4 at the Leningrad power plant, 3 at the Smolensk power plant and 4 at the Kursk power plant. 1 GWe
RBMK Smolensk Power Plant
Turbine hall of the Smolensk power plant. The 3 units of the power plant are among the 11 RBMK units, still in service in Russia, which have been considerably modernized since the Chernobyl era. Their safety in particular has been completely reviewed. Based on these improvements, the operation of RBMKs has been extended.
© M.Chouha ©
The 11 remaining Russian units have undergone major modernizations aimed at improving their safety. As a result of these improvements, the safety level of RBMKs has been significantly increased. Their operation was foreseen to last another fifteen years. Unit No. 3 of the Smolensk power plant, equipped with the last RBMK-1000 commissioned in 1990, should be operated until 2034.