How an RBMK reactor works

There is no vessel. The core is a stack of graphite blocks 11.8 meters in diameter and 7 tall, about 1,700 tonnes, crossed from top to bottom by 1,661 pressure tubes of zirconium-niobium alloy, 88 mm in diameter with 4 mm walls. Each tube holds two bundles of 18 uranium oxide rods, one above the other, adding up to 7 meters of fuel. The graphite moderates; the water flowing inside the tubes only cools.

The stack rests on a lower biological shield and is covered by an upper one, a 2,000-tonne steel disc filled with serpentine, and is kept in a nitrogen-helium atmosphere so that the graphite, at up to 700 °C, does not burn. As in a CANDU, the reactor is refueled on power: a machine above the central hall couples to a channel, depressurizes it, swaps the bundle and moves on.

Water enters from below. Eight main circulation pumps, four per half, push it at about 270 °C to group distribution headers, and from each header an individual pipe runs to each channel. Inside the channel the water rises, heats up along the 7 meters of fuel and boils: at the outlet about 14% of the mass is steam, at 70 bar and 284 °C.

In that it resembles a BWR, with one decisive difference. In a BWR the water is both moderator and coolant, so bubbles slow fewer neutrons and reduce power. In the RBMK the moderator is the graphite: bubbles only remove water, which was absorbing neutrons, and power rises. The design is explained in section 04.

The water-steam mixture rises from each channel through its own pipe to four separators, two per half: horizontal steel drums 30 meters long and 2.3 in diameter, on the highest level of the building. There the water settles downward and the steam, now dry, leaves through the top toward two turbines of 500 MWe each. There is no steam generator: it is a single circuit.

As in the BWR, that steam passed through the core and carries nitrogen-16, so the turbine hall is a controlled area. The condensate returns as feedwater to the drums, mixes with the separated water and flows down the downcomers to the pumps, which send it back to the headers. Two independent loops, one for each half of the core.

The RBMK has a positive void coefficient: more steam in the channels, more power, more steam. At full power with a fresh core the effect was moderate; with burned fuel, at low power and with almost all rods withdrawn, it became large and unstable. The 211 control rods entered from above at 0.4 m/s, about 18 seconds to the bottom, and carried a 4.5-meter graphite displacer at the tip: with the rod out of the core, the displacer occupied the center of the channel and left a 1.25 m column of water below it. When the rod was inserted, that graphite pushed out the water at the bottom before the absorber arrived, and for a few seconds reactivity in the lower part of the core rose instead of falling.

On April 26, 1986, at Chernobyl unit 4, a low-power test with the reactor in that state ended with the emergency shutdown button being pressed. The simultaneous insertion of the rods sent power surging in the lower region, the water in the channels flashed to steam, the tubes burst and two explosions destroyed the reactor. The IAEA's INSAG-7 report attributed the accident to the design more than to the operators. Afterwards enrichment was raised from 2.0 to 2.4% and later to 2.8%, some 80 fixed absorbers were installed in fuel channels, the minimum operating reactivity margin was increased, the rods were redesigned so the displacer left no water beneath, the shutdown was shortened to about 12 seconds and a fast shutdown system of 24 rods in 2.5 seconds was added. The void coefficient went from about +5 β to under +1 β.

A PWR or a BWR encloses the reactor in a concrete-and-steel containment building able to hold the rupture of the largest pipe in the circuit. The RBMK has none: the reactor sits in a conventional industrial hall with a light roof. The design logic was that, with more than a thousand independent tubes, the maximum credible accident was the rupture of one or two, and for that an accident localization system under the reactor was enough: leak-tight compartments and bubbler pools where the steam from a leak would condense.

That system did not protect the upper part of the reactor, which was exactly what blew off in 1986: the 2,000-tonne upper shield lifted, the roof of the hall vanished and the core was left open to the air. The ventilation stack, shared by each pair of units, discharged the filtered air from the halls; the one at Chernobyl 3 and 4 became the symbol of the accident, and was dismantled in 2013 to erect the new confinement over the sarcophagus.

Seventeen RBMKs were built, all in the Soviet Union. Lithuania closed the two RBMK-1500s at Ignalina in 2004 and 2009 as a condition of joining the European Union; the three remaining Chernobyl units stopped in 2000; Leningrad 1 and 2 and Kursk 1 and 2 closed between 2018 and 2024. Seven remain in operation, all in Russia: Leningrad 3 and 4, Kursk 3 and 4 and Smolensk 1, 2 and 3, with the post-1986 modifications and 2.8% fuel with erbium as burnable poison.

They run on licenses extended to 45 years, with staggered closures planned through about 2035 as the VVER-1200s of Leningrad II and Kursk II enter service. The aging problem specific to an RBMK is the graphite: irradiation deforms and cracks the blocks and bows the channels, and at Leningrad 1 the stack had to be cut and realigned in 2013 to keep operating. It is the last graphite-moderated, water-cooled power reactor still in service.