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RBMK is an acronym for the Russian reaktor bolshoy moshchnosti kanalniy (Russian: Реактор Большой Мощности Канальный) which means "reactor (of) high power (of the) channel (type)", and describes a now obsolete class of graphite-moderated nuclear power reactor which was built only in the Soviet Union. The RBMK reactor was the type involved in the Chernobyl accident. In 2004, several were still operating, but there were no plans to build any more and there is international pressure to close those that remain.

Image:Obninsk npp.jpg
Control room of the world's first nuclear power plant, named the Peaceful Atom, at Obninsk, Russia

The RBMK was the culmination of the Soviet program to produce a water-cooled power reactor based on their graphite-moderated plutonium production military reactors. The first of these, AM-1 ("Атом Мирный", Russian for Atom Mirny, "peaceful atom") produced 5MW (30 MW thermal) and delivered power to Obninsk from 1954 until 1959.

Using light water for cooling and graphite for moderation, it is possible to use natural uranium for fuel. Thus, a large power reactor can be built that requires no separated isotopes, such as enriched uranium or heavy water. Unfortunately, such a configuration is also unstable.


[edit] Design

An RBMK employs long (7 metre) vertical pressure tubes running through a graphite moderator and cooled by water, which is allowed to boil in the core at 290°C, much as in a boiling water reactor. Fuel is low-enriched uranium oxide made up into fuel assemblies 3.5 metres long. With moderation largely due to the fixed graphite, excess boiling simply reduces the cooling and neutron absorption without inhibiting the fission reaction, so the reactor can have a large positive void coefficient, and a positive feedback problem can arise, such as at Chernobyl, which was an RBMK reactor.

Because the water used to remove heat from the core in a light-water reactor absorbs some of the free neutrons normally generated during operation of the reactor, the concentration of the naturally fissionable U-235 isotope in uranium used to fuel light-water reactors must be increased above the level of natural uranium to assist in sustaining the nuclear chain reaction in the reactor core: the remainder of the uranium in the fuel is U-238. Increasing the concentration of U-235 in nuclear fuel uranium above the level that occurs in natural uranium is accomplished through the process of enrichment.

The fuel core for a light water reactor can have up to 3,000 fuel assemblies. An assembly consists of a group of sealed fuel rods, each filled with uranium oxide (UO2) pellets, held in place by end plates and supported by metal spacer-grids to brace the rods and maintain the proper distances between them. The fuel core can be thought of as a reservoir from which heat energy can be extracted through the nuclear chain reaction process. During the operation of the reactor, the concentration of U-235 in the fuel is decreased as those atoms undergo nuclear fission which creates heat energy. Some U-238 atoms are converted to atoms of fissile Pu-239, some of which will, in turn, undergo fission and produce energy. The products created by the nuclear fission reactions are retained within the fuel pellets and these become neutron-absorbing products, also called nuclear poisons, that act to slow the rate of nuclear fission and heat production. As the reactor operation is continued, a point is reached at which the declining concentration of fissile nuclei in the fuel and the increasing concentration of poisons result in lower than optimal heat energy generation. The RBMK has a refueling machine that can change the fuel on-load, while the reactor is still producing power.

[edit] High Positive Void Coefficient

Water acts as both a neutron moderator, and hindrance to reaction. In this reactor design, pressurized water is the coolant, causing problems when changing phase to steam. Steam in the coolant water is a void, a bubble that neither moderates the reactor, or hinders its speed. A reactor's tendency to reduce the effectiveness of its coolant with power output is measured as a void coefficient. A high void coefficient does not automatically make a reactor unsafe. The RBMK design included computer-driven control rods that controlled the reaction speed and, if necessary, stopped the reaction completely.

After the Chernobyl disaster, all RBMKs in operation underwent significant changes, lowering their void coefficients to +0.7 b. This new number precludes the possibility of a low-coolant meltdown.

[edit] Containment

The RBMK design includes several kinds of containment needed for normal operation. There is a sealed metal containment structure filled with inert gases surrounding the reactor to keep oxygen away from the graphite (which is normally at about 700 degrees Celsius). There is also a large amount of shielding to absorb radiation from the reactor core. This includes a concrete slab on the bottom, sand and concrete around the sides, and a large concrete slab on top of the reactor. Much of the reactor's internal machinery is attached to this top slab, including the water pipes.

Initially, the RBMK design focused solely on accident prevention and mitigation, not on containment of severe accidents. However, since the Three Mile Island incident, RBMK design also includes a partial containment structure (not a full containment building) for dealing with emergencies. The pipes underneath the reactor are sealed inside leak-tight boxes filled with a large amount of water. If these pipes leak or burst, the radioactive material is trapped by the water inside these boxes. However, RBMK reactors were designed to allow fuel rods to be changed without shutting down (as in the pressurized heavy water Candu recator), both for refueling and for plutonium production (for nuclear weapons). This required large cranes above the core. As the RBMK reactor is very tall (about 70 metres), the cost and difficulty of building a heavy containment structure prevented building of additional emergency containment structure for pipes on top of the reactor. Unfortunately, in the Chernobyl accident, the pressure rose to levels high enough to blow the top off of the reactor, breaking open these pipes in the process.

[edit] Improvements since the Chernobyl accident

Since the Chernobyl accident, all remaining RBMKs have been retrofitted with a number of updates for safety. The largest of these updates, fixes the RBMK control rod design. Previously the control rods were designed with graphite tips, which when initially inserted into the reactor sped up the reaction, instead of slowing or stopping it. This design flaw caused the first explosion of the Chernobyl accident, when the emergency button was pressed to stop the reactor. The updates are:

  • An increase in fuel enrichment from 2% to 2.4%. This difference improves neutron absorption, reducing the reliance on cooling water for reactor control.
  • Manual control rod count increased from 30 to 45.
  • 80 additional absorbers inhibit operation at low power, where the RBMK design is most dangerous.
  • SCRAM (rapid shut down) sequence reduced from 18 to 12 seconds.
  • Precautions against unauthorised access to emergency safety systems.

[edit] Closures

Of the 13 RBMKs built (and one is still under construction at Kursk), all three surviving reactors at the Chernobyl plant have now been closed and one of the two reactors at Ignalina in Lithuania has shut down with the second due to close by 2010. [1]

[edit] References

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