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(May 2007)
The Soviet designed RBMK (high-power channel reactor - reactor bolshoy moshchnosty kanalny) is a pressurised water reactor with individual fuel channels and using ordinary water as its coolant and graphite as its moderator. It is very different from most other power reactor designs as it derived from a design principally for plutonium production and was intended and used in Russia for both plutonium and power production.
The combination of graphite moderator and water coolant is found in no other power reactors. The design characteristics of the reactor mean that it is unstable at low power levels, and this was shown in the Chernobyl accident. The instability was due primarily to control rod design and a positive void coefficient. A number of significant design changes have now been made to address these problems.
Features of the RBMK
Fuel: Pellets of slightly-enriched uranium oxide are enclosed in a zircaloy tube 3.65m long, forming a fuel rod. A set of 18 fuel rods is arranged cylindrically in a carriage to form a fuel assembly. Two of these end on end occupy each pressure tube.
Pressure tubes: Within the reactor each fuel assembly is positioned in its own vertical pressure tube or channel about 7 m long.. Each channel is individually cooled by pressurised water which is allowed to boil in the tube and emerges at about 290°C.
Refuelling: When fuel channels are isolated, these fuel assemblies can be lifted into and out of the reactor, allowing fuel replenishment while the reactor is in operation.
Graphite moderator: A series of graphite blocks surround, and hence separate, the pressure tubes. They act as a moderator to slow down the neutrons released during fission so that a continuous fission chain reaction can be maintained. Conductance of heat between the blocks is enhanced by a mixture of helium and nitrogen gas.
Control rods: Boron carbide control rods absorb neutrons to control the rate of fission. A few short rods, inserted upwards from the bottom of the core, even the distribution of power across the reactor. The main control rods are inserted from the top down and provide automatic, manual or emergency control. The automatic rods are regulated by feedback from in-core detectors. If there is a deviation from normal operating parameters (e.g. increased reactor power level), the rods can be dropped into the core to reduce or stop reactor activity. A number of rods normally remain in the core during operation.
Coolant: Two separate water coolant systems each with four pumps circulate water through the pressure tubes to remove most of the heat from fission. There is also an emergency core cooling system which will come into operation if either coolant circuit is interrupted.
Steam separator: Steam from the heated coolant is fed to turbines to produce electricity in the generator. The steam is then condensed and fed back into the circulating coolant.
Containment: There is no secure containment in the sense accepted in the West. The reactor core is located in a concrete lined cavity that acts as a radiation shield. The upper shield or pile cap above the core, is made of steel and supports the fuel assemblies. The steam separators of the coolant systems are housed in their own concrete shields.
Immediate Safety Changes
After the accident at Chernobyl unit 4, the primary concern was to reduce the positive void coefficient. All operating RBMK reactors, in the former Soviet Union therefore, had the following changes implemented to improve operating safety:
These factors have reduced the positive void coefficient from +4.5 beta to +0.7 beta, eliminating the possibility of power excursion. Beta is the delayed neutron fraction, which is neutrons emitted from each fission with a measurable time delay.
The next consideration was to reduce the time taken to shut the reactor down and eliminate the positive void reactivity. Improvements include:
Many - 179 of 211 - control rods are inserted into the core from the top. To improve their effectiveness, they are equipped with "riders" fixed to their bottom end but with a gap between the rider and the bottom tip of the control rod. Approximately 1.0m water columns remained under and above it. When the control rod is in its uppermost position, the rider is in the control rod cooling tube within the fuelled region of the core. The rider being made substantially of graphite, is almost transparent to neutrons, while water, which would occupy the tube otherwise, plays as an absorber. When the reactor is "poisoned" with xenon and with partially inserted control rods, the major part of the power is produced within the lower region of the core. This means that when the rod started to move down from its uppermost position, the rider removed water from the lower part, causing an increase in reactivity and hence in power.
Longer-term modifications
In addition to the safety changes, RBMKs were modified more fundamentally. Chernobyl unit 1 was relicensed for operation in October 1995, following extensive maintenance which included the removal of some fuel channels to evaluate the metal and some backfitting as follows. The modification process consisted of:
Positive void coefficient
Positive void coefficient is a term often associated with the RBMK reactors, the type involved in the Chernobyl disaster. Reactors that have a positive void coefficient can be unstable at low power and are prone to experiencing a rapid, uncontrollable power increase. While some reactors other than the RBMK type have positive void coefficients, they incorporate design features to prevent such instability from occurring.
In a water cooled reactor, steam may accumulate to form pockets, known as voids. If excess steam is produced, creating more voids than normal, the operation of the reactor is disturbed, because
A reactor is said to have a positive void coefficient if excess steam voids lead to increased power generation, and a negative void coefficient if excess steam voids leads to a decrease in power. The quantum of the coefficient is simply a measure of the speed of change of state of the reactor.
When the void coefficient is strongly positive, the power can increase very rapidly because any power increment that occurs leads to increased steam generation, which in turn leads to a further increase in power. Such positive feedback processes are, therefore, very difficult to control.
When the void coefficient is negative, excess steam generation will tend to shut down the reactor. This is, of course, is a safety feature.
Most of the world's operating power reactors have negative void coefficients. In those reactors where same water circuit acts as both moderator and coolant, excess steam generation reduces the slowing of neutrons necessary to sustain the nuclear chain reaction. This leads to a reduction in power.
In some reactor designs however, the moderator and coolant are in separate circuits, or are of different materials. In these reactors, excess steam reduces the cooling of the reactor, but as the moderator remains intact the nuclear chain reaction continues.
In some of these reactors, most notably the RBMK, the neutron absorbing properties of the cooling water are a significant factor in the operating characteristics. In such cases, the reduction in neutron absorption as a result of steam production, and the consequent presence of extra free neutrons, enhances the chain reaction. This increases power production which causes additional heating. The additional heat raises the temperature in the cooling circuit and more steam is produced. More steam means less cooling and less neutron absorption, and the problem gets worse.
This positive feedback can occur very rapidly, potentially leading to a serious accident such as at Chernobyl unit 4 in 1986 where the power peaked at several hundred times the normal full rating. The coolant vapourised and reacted with the zirconium cladding to produce hydrogen which then exploded and destroyed the core, dispersing about 5% of it to the outside environment.
In order to avoid problems with positive void coefficient there are two approaches. Either the reactor characteristics can be altered to reduce the positive void coefficient, or systems can be provided that will shut the reactor down very quickly if an increase in power is detected. Since the Chernobyl disaster, the RBMK reactor design has been altered and units have been equipped to protect them against the runaway effects of the positive void coefficient.
There are currently 12 operating RBMKs in the world, in Russia and Lithuania. One more is under construction in Russia (Kursk-5), the completion of which is unlikely. All operating RBMKs began operation between 1973 (Leningrad-1) and 1990 (Smolensk-3). There are currently three distinct generations of reactors having significant differences with respect to their safety design features:
The 'operating till' dates in the following table are the scheduled shut down for these plants, with 15 year life extensions in some cases. Lithuania, on the other hand, closed Ignalina-1 early as a condition for entry into the European Union and will close unit 2 in 2009.
Russia's long-term plans earlier included the possibility of replacing the Leningrad units, at the end of their extended service life, by new MKER-1000 units. These are a modification of the RBMK design. The main differences are in the spacing of the graphite lattice in the core and the incorporation of passive safety systems.
In 2006 Rosatom said it was considering lifetime extensions and uprating of its eleven operating RBMK reactors. Following significant design modifications made after the Chernobyl accident, as well as extensive refurbishment including replacement of fuel channels, a 45-year lifetime is seen as realistic for the 1000 MWe units. In 2005 they provided 48% of Russia's nuclear-generated electricity. The R&D Institute of Power Engineering is preparing plans for 5% uprating of them.