The RBMK-1000 is a graphite moderated pressure tube type reactor, using slightly enriched (2% 235U) uranium dioxide fuel. It is a boiling light-water reactor, with direct steam feed to the turbines, without an intervening heat-exchanger. Water pumped to the bottom of the fuel channels boils as it progresses up the pressure tubes, producing steam which feeds two 500 MW(e) turbines. The water acts as a coolant and also provides the steam used to drive the turbines. The vertical pressure tubes contain the zirconium-alloy clad uranium dioxide fuel around which the cooling water flows. A specially designed refuelling machine allows fuel bundles to be changed without shutting down the reactor, a design interesting for the extraction of fresh plutonium for military use.

The moderator, whose function is to slow down neutrons to make them more efficient in producing fission in the fuel, is constructed of graphite. A mixture of nitrogen and helium is circulated between the graphite blocks largely to prevent oxidation of the graphite and to improve the transmission of the heat produced by neutron interactions in the graphite, from the moderator to the fuel channel. The core itself is about 7 m high and about 12m in diameter. There are four main coolant circulating pumps, one of which is always on standby. The reactivity or power of the reactor is controlled by raising or lowering 211 control rods, which, when lowered, absorb neutrons and reduce the fission rate. The power output of this reactor is 3200 MWth or 1000 MW(e). Various safety systems, such as an emergency core cooling system and the requirement for an absolute minimal insertion of 30 control rods, were incorporated into the reactor design and operation. Figure II.1 is a sketch of the RBMK reactor.

The most important characteristic of the RBMK reactor is that it possesses a ‘positive void coefficient’. This means that if the power increases or the flow of water decreases, there is increased steam production in the fuel channels, so that the neutrons that would have been absorbed by the denser

water will now produce increased fission in the fuel. However, as the power increases, so does the temperature of the fuel, and this has the effect of reducing the neutron flux (negative fuel coefficient). The net effect of these two opposing characteristics varies with the power level. At the high power level of normal operation, the temperature effect predominates, so that power excursions leading to excessive overheating of the fuel do not occur. However, at a lower power output of less than 20% of the maximum, the positive void coefficient effect is dominant and the reactor becomes unstable and prone to sudden power surges. This was a major factor in the develop­ment of the accident.

Another safety deficiency was the design of the control rods: the boron carbide absorbing section of the rods was preceded by a 4.5 m long pure graphite displacer. The reason for this displacer was to prevent water from replacing boron carbide when the rods were in the high position. Indeed the neutron absorbing character of water would have lessened the effect of the boron carbide. However, the graphite displacer was not long enough to occupy the full length of the channel. Thus, the fall of a control rod initially in its highest position first replaced water by graphite in the lower part of the channel, with a subsequent increase of the reactivity. It is only after the rod is significantly engaged in the core that the absorbant becomes effective. Furthermore, the rods’ insertion was rather slow, taking about 20 s.