RBMK
The RBMK is a class of graphite-moderated nuclear power reactor designed and built by the Soviet Union. It is somewhat like a boiling water reactor as water boils in the pressure tubes. It is one of two power reactor types to enter serial production in the Soviet Union during the 1970s, the other being the VVER reactor. The name refers to its design where instead of a large steel pressure vessel surrounding the entire core, the core is surrounded by a cylindrical annular steel tank inside a concrete vault and each fuel assembly is enclosed in an individual 8 cm diameter pipe. The channels also contain the coolant, and are surrounded by graphite.
The RBMK is an early Generation II reactor and the oldest commercial reactor design still in wide operation. Certain aspects of the original RBMK reactor design had several shortcomings, such as the large positive void coefficient, the 'positive scram effect' of the control rods and instability at low power levels—which contributed to the 1986 Chernobyl disaster, in which an RBMK experienced an uncontrolled nuclear chain reaction, leading to a steam and hydrogen explosion, large fire, and subsequent core meltdown. Radioactive material was released over a large portion of northern and southern Europe—including Sweden, where evidence of the nuclear disaster was first registered outside of the Soviet Union, and before the Chernobyl accident was communicated by the Soviet Union to the rest of the world. The disaster prompted worldwide calls for the reactors to be completely decommissioned; however, there is still considerable reliance on RBMK facilities for power in Russia with the aggregate power of operational units at almost 7 GW of installed capacity. Most of the flaws in the design of RBMK-1000 reactors were corrected after the Chernobyl accident and a dozen reactors have since been operating without any serious incidents for over thirty years.
RBMK reactors may be classified as belonging to one of three distinct generations, according to when the particular reactor was built and brought online:
- Generation 1 – during the early-to-mid 1970s, before OPB-82 General Safety Provisions were introduced in the Soviet Union.
- Generation 2 – during the late 1970s and early 1980s, conforming to the OPB-82 standards issued in 1982.
- Generation 3 – post Chernobyl accident in 1986, where Soviet safety standards were revised to OPB-88; only Smolensk-3 was built to these standards.
Lifespan
History
The RBMK was the culmination of the Soviet nuclear power program to produce a water-cooled power reactor with dual-use potential based on their graphite-moderated plutonium production military reactors. The first of these, Obninsk AM-1 generated 5 MW of electricity from 30 MW thermal power, and supplied Obninsk from 1954 until 1959. Subsequent prototypes were the AMB-100 reactor and AMB-200 reactor both at Beloyarsk Nuclear Power Station.By using a minimalist design that used regular water for cooling and graphite for moderation, it was possible to use fuel with a lower enrichment. This allowed for an extraordinarily large and powerful reactor that could be built rapidly, largely out of parts fabricated on-site instead of by specialized factories. The initial 1000 MWe design also left room for development into yet more powerful reactors. For example, the RBMK reactors at the Ignalina Nuclear Power Plant in Lithuania were rated at 1500 MWe each, a very large size for the time and even for the early 21st century. For comparison, the EPR has a net electric nameplate capacity of 1600 MW and is among the most powerful reactor types ever built.
The RBMK-1000's design was finalized in 1968. At that time it was the world's largest nuclear reactor design, surpassing western designs and the VVER in power output and physical size, being 20 times larger by volume than contemporary western reactors. Similarly to CANDU reactors and the Indian IPHWR reactors it could be produced without the specialized industry required by the large and thick-walled reactor pressure vessels such as those used by VVER reactors, thus increasing the number of factories capable of manufacturing RBMK reactor components. No prototypes of the RBMK were built; it was put directly into mass production.
The RBMK was proclaimed by some as the national reactor of the Soviet Union, probably due to nationalism because of its unique design, large size and power output. Meanwhile the VVER design was called the "American reactor" due to the pressurized water design shared with many Western reactors. A top-secret invention patent for the RBMK design was filed by Anatoly Aleksandrov from the Kurchatov Institute of Atomic Energy, who personally took credit for the design of the reactor, with the Soviet patent office. Because a containment building would have needed to be very large and expensive, doubling the cost of each unit, due to the large size of the RBMK, it was originally omitted from the design. It was argued by its designers that the RBMK's strategy of having each fuel assembly in its own channel with flowing cooling water was an acceptable alternative for containment.
The RBMK was mainly designed at the Kurchatov Institute of Atomic Energy and, headed by Anatoly Aleksandrov and Nikolai Dollezhal respectively, from 1964 to 1966. The RBMK was favored over the VVER by the Soviet Union due to its ease of manufacture, due to a lack of a large and thick-walled reactor pressure vessel and relatively complex associated steam generators, and its large power output, which would allow the Soviet government to easily meet their central economic planning targets.
The flaws in the original RBMK design were recognized by others, including from within the Kurchatov Institute before the first units were built, but the orders for construction of the first RBMK units, which were at Leningrad, had already been issued in 1966 by the Soviet government by the time their concerns reached the Central Committee of the Communist Party of the Soviet Union and the Soviet Council of Ministers. This prompted a sudden overhaul of the RBMK. Plutonium production in an RBMK would have been achieved by operating the reactor under special thermal parameters, but this capability was abandoned early on. This was the design that was finalized in 1968. The redesign did not solve further flaws that were not discovered until years later. Construction of the first RBMK, which was at Leningrad Nuclear Power Plant, began in 1970. Leningrad unit 1 opened in 1973.
At Leningrad it was discovered that the RBMK, due to its high positive void coefficient, became harder to control as the uranium fuel was consumed or burned up, becoming unpredictable by the time it was shut down after three years for maintenance. This made controlling the RBMK a very laborious, mentally and physically demanding task requiring the timely adjustment of dozens of parameters every minute, around the clock, constantly wearing out switches such as those used for the control rods and causing operators to sweat. The enrichment percentage was increased to 2.0%, up from 1.8% to alleviate these issues.
The RBMK was considered by some in the Soviet Union to be already obsolete shortly after the commissioning of Chernobyl unit 1. Aleksandrov and Dollezhal did not investigate further or even deeply understand the problems in the RBMK, and the void coefficient was not analyzed in the manuals for the reactor. Engineers at Chernobyl unit 1 had to create solutions to many of the RBMK's flaws such as a lack of protection against no feedwater supply. Leningrad and Chernobyl units 1 both had partial meltdowns that were treated, alongside other nuclear accidents at power plants, as state secrets and so were unknown even to other workers at those same plants.
By 1980 NIKIET realized, after completing a confidential study, that accidents with the RBMK were likely even during normal operation, but no action was taken to correct the RBMK's flaws. Instead, manuals were revised, which was believed to be enough to ensure safe operation as long as they were followed closely. However, the manuals were vague and Soviet power plant staff already had a habit of bending the rules in order to meet economic targets, despite inadequate or malfunctioning equipment. Crucially, it was not made clear that a number of control rods had to stay in the reactor at all times in order to protect against an accident, as loosely articulated by the Operational Reactivity Margin parameter. An ORM chart recorder and display were added to RBMK control rooms after the Chernobyl disaster.
Reactor design and performance
Reactor vessel, moderator and shielding
The reactor pit or vault is made of reinforced concrete and has dimensions 21.6 m × 21.6 m × 25.5 m. It houses the vessel of the reactor, which is annular, made of an inner and outer cylindrical wall and top and bottom metal plates that cover the space between the inner and outer walls, without covering the space surrounded by the vessel. The reactor vessel is an annular steel cylinder with hollow walls and pressurized with nitrogen gas, with an inner diameter and height of 14.52 m × 9.7 m, and a wall thickness of 16 mm.In order to absorb axial thermal expansion loads, it is equipped with two annular bellows compensators, one on the top and another on the bottom, in the spaces between the inner and outer walls. The vessel surrounds the graphite core block stack, which serves as moderator. The graphite stack is kept in a helium-nitrogen mixture, providing an inert atmosphere for the graphite, protecting it from potential fires, and facilitating transfer of excess heat from the graphite to the coolant channels.
The moderator blocks are made of nuclear graphite the dimensions of which are 25 cm × 25 cm on the plane perpendicular to the channels, and with several longitudinal dimensions of between 20 cm and 60 cm depending on the location in the stack. There are holes of 11.4 cm diameter through the longitudinal axis of the blocks for the fuel and control channels. The blocks are stacked, surrounded by the reactor vessel into a cylindrical core with a diameter and height of 14 m × 8 m. The maximum allowed temperature of the graphite is up to 730 °C.
The reactor has an active core region 11.8 m in diameter by 7 m height. There are 1700 tons of graphite blocks in an RBMK-1000 reactor. The pressurized nitrogen in the vessel prevents the escape of the helium-nitrogen mixture used to cool the graphite stack.
The reactor vessel has on its outer side an integral cylindrical annular water tank, a welded structure with 3 cm thick walls, an inner diameter of 16.6 m and an outer diameter of 19 m, internally divided to 16 vertical compartments. The water is supplied to the compartments from the bottom and removed from the top; the water can be used for emergency reactor cooling. The tank contains thermocouples for sensing the water temperature and ion chambers for monitoring the reactor power. The tank, along with an annular sand layer between the outer side of the tank and inner side of the pit, and the relatively thick concrete of the reactor pit serve as lateral biological shields.
File:RBMK reactor from Ignalina ArM.jpg|thumb|Reactor hall of the RBMK-1500 at Ignalina Nuclear Power Plant, Lithuania—the upper biological shield lies several meters below the floor of the reactor hall. There are no channel covers on the fuel channels of the reactor; the control rod drives are below the colored covers.
The top of the reactor is covered by the upper biological shield, also called "Schema E", or, after the explosion of Chernobyl Reactor 4, Elena. The UBS is a cylindrical disc of 3 m × 17 m in size and 2000 tons in weight. It is penetrated by standpipes for fuel and control channel assemblies. The top and bottom are covered with 4 cm thick steel plates, welded to be helium-tight, and additionally joined by structural supports. The space between the plates and pipes is filled with serpentinite, a rock containing significant amounts of bound water. The serpentinite provides the radiation shielding of the biological shield and was applied as a special concrete mixture. The disk is supported on 16 rollers, located on the upper side of the reinforced cylindrical water tank. The structure of the UBS supports the fuel and control channels, the floor above the reactor in the central hall, and the steam-water pipes.
Below the bottom of the reactor core there is the lower biological shield, similar to the UBS, but only 2 m × 14.5 m in size. It is penetrated by the tubes for the lower ends of the pressure channels and carries the weight of the graphite stack and the coolant inlet piping. A steel structure, two heavy plates intersecting in right angle under the center of the LBS and welded to the LBS, supports the LBS and transfers the mechanical load to the building.
Above the UBS, there is a space with upper channel piping and instrumentation and control or control and monitoring cabling. Above that is Assembly 11, made up of the upper shield cover or channel covers. Their top surfaces form part of the floor of the reactor hall and serve as part of the biological shield and for thermal insulation of the reactor space. They consist of serpentinite concrete blocks that cover individual removable steel-graphite plugs, located over the tops of the channels, forming what resembles a circle with a grid pattern. The floor above the reactor is thus known by RBMK plant workers as pyatachok, referring to the five-kopeck coin. There is one cover per plug, and one plug per channel.