This EOE article is adapted from an information paper published by the World Nuclear Association (WNA). WNA information papers are frequently updated, so for greater detail or more up to date numbers, please see the latest version on WNA website (link at end of article).
The Soviet designed RBMK (reactor bolshoy moshchnosty kanalny - high-power channel reactor) is a pressurized water reactor (PWR) with individual fuel channels that uses ordinary water as its coolant and graphite as its moderator. It is very different from most other nuclear power reactor designs as it derived from a design principally intended for plutonium production and was intended and used for both plutonium and power production.
The combination of graphite moderator and water coolant is found in no other nuclear power reactors. The design characteristics of the reactor mean that it is unstable at low power levels, as was shown in the Chernobyl, Ukraine 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 pressurized water that 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 nuclear 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 in unit 4 at the Chernobyl, Ukraine nuclear facility, the primary concern became reducing the positive void coefficient. As a result, all operating RBMK reactors in the former Soviet Union had the following changes implemented to improve operating safety:
- To improve the operational reactivity margin, the effective number of manual control rods was increased from 30 to 45;
- The installation of 80 additional absorbers in the core to inhibit operation at low power; and
- An increase in fuel enrichment from 2% to 2.4% to maintain fuel burnup with an increase in neutron absorption (i.e., less reliance on cooling water for this function).
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, or the neutrons emitted from each fission with a measurable time delay.
The next consideration was to reduce the time taken to shut the nuclear reactor down and eliminate the positive void reactivity. Improvements include:
- Scram (rapid shut down) rod insertion time cut from 18 to 12 seconds;
- The redesign of control rods;
- The installation of a fast scram system; and
- Precautions against unauthorized access to emergency safety systems.
Redesign of the control rods
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 remain 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, that would occupy the tube otherwise, acts as an absorber. When the reactor is "poisoned" with xenon and with partially inserted control rods, the majority of the power is produced within the lower region of the core. This means that when the rod starts to move down from its uppermost position, the rider removes water from the lower part, causing an increase in reactivity and hence in power.
In addition to the safety changes, it was decided that RBMKs should be modified more fundamentally. Unit 1 at the Chernobyl, Ukraine nuclear facility was relicensed for operation in October 1995, following extensive maintenance that included the removal of some fuel channels to evaluate the metal and some backfitting as follows. The modification process consisted of:
- Replacement of the fuel channels at all units except Smolensk-3;
- Replacement of the group distribution headers and addition of check valves;
- Improvements to the emergency core cooling systems;
- Improvements of the reactor cavity over-pressure protection systems; and
- Replacement of the process computer, SKALA.
Positive void coefficient
Positive void coefficient is a term often associated with the RBMK reactors—the type involved in the Chernobyl, Ukraine disaster. Reactors that have a positive void coefficient can be unstable at low power and are prone to experiencing rapid, uncontrollable power increases. While nuclear power reactors other than the RBMK type also 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:
- the water is a more efficient coolant than steam; and
- the water acts as a moderator (enhancing the fission reaction) and neutron absorber (hindering it), while steam is barely capable of providing these functions.
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, of course, is a safety feature.
Most of the world's operating nuclear 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.
However, in some reactor designs, 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 nuclear chain reaction. This enhances power production which causes additional heating. The additional heat then 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 that at unit 4 at the Chernobyl facility in 1986 where the power peaked at several hundred times the normal full rating. The coolant vaporized and reacted with the zirconium cladding to produce hydrogen that then exploded and destroyed the reactor 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 nuclear 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 against the runaway effects of the positive void coefficient.
RBMK Plants Globally
There are currently 13 operating RBMKs in the world, in Russia and Lithuania, and one more under construction in Russia (Kursk-5) due for commercial operation in 2003. The nuclear reactors have a net generating capacity of 12,545 MWe. All operating RBMKs began operation between 1973 (Leningrad-1) and 1990 (Smolensk-3). There are currently three distinct generations of reactors, with significant differences with respect to their safety design features:
- The four first-generation units are Leningrad-1 and -2, and Kursk-1 and -2. They were designed and brought on line in the early-to-mid 1970s, before new standards on the design and construction of nuclear power plants (OPB-82) were introduced in the Soviet Union;
- Second-generation RBMKs, brought on line since the late 1970s and early 1980s, include Leningrad-3 and -4; Kursk-3, and -4; Ignalina-1; and Smolensk-1 and -2. Ignalina-2 contains safety features beyond those of other second-generation units. These units conform to updated standards issued in 1982; and
- After the Chernobyl accident, Soviet safety standards were revised again (OPB-88). One RBMK (Smolensk-3) has been built to these third-generation standards. Additional design changes now are being incorporated in the construction of Kursk-5.
The 'operating till' dates in the following table detail the scheduled shut down for these plants. However, Russia has adopted a policy of extending plant life, aiming to add a further 10 years to their operation. Lithuania, on the other hand, is likely to close Ignalina early as a condition for entry into the European Union. Russia's long-term plans include 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.
|Location||Unit||Commercial Operation||Unit net capacity (MWe)||Status|
|Ignalina||1||1984||1,250||Operating till May 2004|
|2||1987||1,250||Operating till August 2018|
|Kursk||1||1977||700||Operating till October 2007|
|2||1979||700||Operating to August 2009|
|3||1984||950||Operating till March 2014|
|4||1986||950||Operating till February 2016|
|5||-||950||Due for commercial operation 2003|
|Leningrad||1||1974||950||Operating till November 2004|
|2||1976||950||Operating till February 2006|
|3||1980||950||Operating till June 2010|
|4||1981||950||Operating till August 2011|
|Smolensk||1||1983||950||Operating till December 2013|
|2||1990||950||Operating till July 2015|
|3||1990||950||Operating till July 2023|