page_title: RBMK
section_title: Cooling and steam circuits
hierarchical_section_title: RBMK / Reactor design and performance / Cooling and steam circuits
caption_attribution_description: English: Illustrated are the channells in the core, steam separators (red), circulation pumps (yellow) and pipe networks.
context_page_description: The RBMK is a class of graphite-moderated nuclear power reactor designed and built by the Soviet Union. The name refers to its unusual design where, instead of a large steel pressure vessel surrounding the entire core, each fuel assembly is enclosed in an individual 8 cm diameter pipe which allows the flow of cooling water around the fuel.
  The RBMK is an early Generation II reactor and the oldest commercial reactor design still in wide operation. Certain aspects of the RBMK reactor design, such as the active removal of decay heat, the positive void coefficient properties, the 4.5 m graphite displacer ends of the control rods and instability at low power levels, contributed to the 1986 Chernobyl disaster, in which an RBMK experienced a very large reactivity excursion, leading to a steam and hydrogen explosion, a large fire and subsequent meltdown. Radioactivity was released over a large portion of Europe. 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.
context_section_description: The reactor has two independent cooling circuits, each having four main circulating pumps (three operating, one standby). The cooling water is fed to the reactor through lower water lines to a common pressure header (one for each cooling circuit), which is split to 22 group distribution headers, each feeding 38–41 pressure channels through the core, where the feedwater boils. The mixture of steam and water is led by the upper steam lines, one for each pressure channel, from the reactor top to the steam separators, pairs of thick horizontal drums located in side compartments above the reactor top; each has 2.8 m (9 ft 2 in) diameter, 31 m (101 ft 8 in) length, wall thickness of 10 cm (3.9 in), and weighs 240 t (260 short tons). Steam, with steam quality of about 15%, is taken from the top of the separators by two steam collectors per separator, combined, and led to two turbogenerators in the turbine hall, then to condensers, reheated to 165 °C (329 °F), and pumped by the condensate pumps to deaerators, where remains of gaseous phase and corrosion-inducing gases are removed. The resulting feedwater is led to the steam separators by feedwater pumps and mixed with water from them at their outlets. From the bottom of the steam separators, the feedwater is led by 12 downpipes (from each separator) to the suction headers of the main circulation pumps, and back into the reactor. There is an ion exchange system included in the loop to remove impurities from the feedwater.
  The turbine consists of one high-pressure rotor and four low-pressure ones. Five low-pressure separators-preheaters are used to heat steam with fresh steam before being fed to the next stage of the turbine. The uncondensed steam is fed into a condenser, mixed with condensate from the separators, fed by the first-stage condensate pump to a chemical purifier, then by a second-stage condensate pump to four deaerators where dissolved and entrained gases are removed; deaerators also serve as storage tanks for feedwater. From the deaerators, the water is pumped through filters and into the bottom parts of the steam separator drums.
  The main circulating pumps have the capacity of 5,500–12,000 m³/h and are powered by 6 kV electric motors. The normal coolant flow is 8000 m³/h per pump; this is throttled down by control valves to 6000–7000 m³/h when the reactor power is below 500 MWt. Each pump has a flow control valve and a backflow preventing check valve on the outlet, and shutoff valves on both inlet and outlet. Each of the pressure channels in the core has its own flow control valve so that the temperature distribution in the reactor core can be optimized. Each channel has a ball type flow meter.
  The nominal coolant flow through the reactor is 46,000–48,000 m³/h. The steam flow at full power is 5,440–5,600 t (6,000–6,170 short tons)/h.
  The nominal temperature of the cooling water at the inlet of the reactor is about 265–270 °C (509–518 °F) and the outlet temperature 284 °C (543 °F), at pressure in the drum separator of 6.9 megapascals (69 bar; 1,000 psi). The pressure and the inlet temperature determine the height at which the boiling begins in the reactor; if the coolant temperature is not sufficiently below its boiling point at the system pressure, the boiling starts at the very bottom part of the reactor instead of its higher parts. With few absorbers in the reactor core, such as during the Chernobyl accident, the positive void coefficient of the reactor makes the reactor very sensitive to the feedwater temperature. Bubbles of boiling water lead to increased power, which in turn increases the formation of bubbles. After 1986 absorbers were introduced in the fuel assembly, permanently assuring a negative void coefficient at the cost of higher enrichment requirements of the uranium fuel.
  If the coolant temperature is too close to its boiling point, cavitation can occur in the pumps and their operation can become erratic or even stop entirely. The feedwater temperature is dependent on the steam production; the steam phase portion is led to the turbines and condensers 
caption: Circulation system of the RBMK illustrating the Steam separators (red), Pumps (yellow) and pipe network.
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