The RBMK reactor is a direct-cycle boiling-water pressure tube, graphite-mod­erated reactor developed from Russia’s first nuclear power plant, commissioned in 1954. The concept is unique to the former Soviet Union, and it was this type of reactor that was involved in the very serious nuclear accident at Chernobyl in the Ukraine in April 1986. This accident is described in Chapter 5. To aid this later description, the RBMK is covered in rather more detail than the other re­actor concepts.

Figures 2.12 and 2.13 show the main elements of the reactor. The reactor core is 12 m (40 ft) in diameter and 7 m (23 ft) high, and is built up from graphite blocks (A in Figure 2.13) penetrated by vertical channels (B) and con­taining a zirconium alloy (Ar + 2.5% Nb) pressure tube 88 mm (3.5 in) in inter­nal diameter and 4 ^m thick. For a 1000-MW(e) reactor there are 1663 channels. Each channel contains two fuel assemblies each 3640 mm (12 ft) long, held together by a central tie rod suspended from a plug at the top of the channel.

The fuel assemblies consist of 18 pin clusters, each pin in the form of en­riched (2%) uranium dioxide pellets encased in zirconium alloy tubing (13.6 mm in outside diameter. x 0.825 mm thick). The maximum power of any chan­nel is 3.25 MW (thermal).

The fuel is cooled by boiling light water at 70-bar (1000-psia) pressure. The water enters the channel at 270°C, and the “quality" (fraction of the total mass flow that is steam) of the existing steam-water mixture is on average 14% (20% maximum).

Two separate identical coolant loops are provided. Each loop consists of two steam drums (C) (to which the riser pipes from the fuel channels carry the steam-water mixture) and four primary circulating pumps (D) (three are nor­mally operational and one standby).

Fi^^e 2.12: Boiling-light-water, graphite-moderated reactor (RBMK, USSR).

Fi^^e 2.13: Outline diagram of the RBMK: A, Graphite blocks; B, Vertical chan­nels; C, Steam drums; D, Circulating pumps; E, Turbine generators; F, Feed pumps; G, Absorber rods; H, Refueling machine; I, Circulating pump compartment; J, Distribution pipework; K, Surface condenser; L, Pressure supression pools; M, Emergency core cooling.

The dry steam from the steam drums passes to one of two 3000-rpm 500- MW(e) turbine generators (E). The very low pressure steam leaving the con­densers is condensed in tubular condensers, and the condensate is returned to the steam drums via purifying systems and electrically driven feed pumps (F).

About 5% of the energy of the fission is dissipated in the graphite structure as a result of the slowing down of neutrons and of ga^ma heating. This heat is transferred to the fuel channels by conduction and radiation via a series of “pis­ton ring”-type graphite rings that permit good thermal contact between the pressure tube and the graphite blocks while also permitting small dimensional changes. The maximum temperature of the graphite is 700°C. To improve the thermal contact and to prevent graphite oxidation, the graphite structure is en­closed in a thin-walled steel jacket through which a gas (helium-nitrogen ^hr — ture) is slowly circulated.

Perhaps the most important characteristic of the RB^K reactor is that, as originally designed, it has a positive void coefficient. This can be explained in simple terms by recognizing that if the power from the fuel increases or the flow of water decreases (or both), the amount of steam in the fuel channel in­creases and the density of the coolant decreases.

The term positive void coefficient is reactor physicist’s jargon for the fact that reducing coolant density results in an increase in neutron population (light water is a strong absorber of neutrons) and hence in an increase of reactor power. However, as the power increases so too does the fuel temperature, and this has the effect of reducing the neutron population (negativefuel coefficient). The net effect of the positive void coefficient and the negative fuel coefficient clearly depends on the power level.

At normal full-power operating conditions the fuel temperature coefficient dominates and the net effect, termed the power coefficient, is negative. However, below about 20% of full power because of the lower fuel tempera­tures the power coefficient becomes positive. For this reason restrictions were placed on operation below 20% power.

As we shall see in Chapter 5, this fundamental design shortcoming was the critical factor of the accident at Chernobyl.

In short, at lower power an increase in power or a reduction of flow leads to increased boiling and further increases in power and hence to the potential for an unstable situation. As a result, RBMK requires a complex, rapidly responding control system to cope with this positive feedback.

Channels for the control and shutdown rods and for the in-core flux instru­mentation pass through vertical holes in the graphite blocks. Radial flux moni­tors are provided in over 100 channels, and axial flux profiles are monitored in 12 channels.

The system for reactor control and protection uses 211 solid absorber rods (G in Figure 2.13). The rods are divided functionally as follows (Figure 2.14):

• 163 manually operated rods of which 139 are control rods (RR) for radial power shaping and 24 are dedicated to emergency protection (AZ).

• 12 rods for automatic regulation of average power (3 groups of designated AR1, AR2, and AR3, respectively).

• 12 rods for automatic regulation of local power (MR).

• 24 shorted absorber rods (USP) for axial flux profling.

The manual control rods (RR), the automatically operated rods (AR), and the emergency shutdown rods (AZ) are distributed uniformly throughout the core in six groups of 30-36 rods. The control system includes subsystems for local automatic control (MR) and local emergency protection (MZ). All rods except the shortened absorber rods are withdrawn and inserted from above.

Fi^^e 2.14: Diagram of the different control rods, “followers,” and fuel assembly for the RBMK: 1, Shortened absorber rod; 2, Automatic control rod; 3, Fuel assembly; 4, Manual control rod and emergency shutdown rod. 0 followers • absorbers

The emergency shutoff rods are motor-driven at a speed of insertion of 0.4 m/s. Full insertion takes 15-20 seconds. The shorter absorber rods are intro­duced from below the core. The control rod channels are the same diameter as the fuel channels (88 ^m) and are cooled by a separate water circuit. At the end of each rod are a number of articulated elements that do not contain neu­tron-absorbing material. As the rod is withdrawn these “followers” prevent water from occupying the space vacated by the absorber.

The control system is arranged to operate over the following power ranges:

1. From subcritical to 0.5% power, manual operation was used.

2. From 0.5% to 10% power, automatic regulation of overall power was performed

using one of the sets of four rods designated for this purpose (i. e., AR3).

3. From 10% to 100% of the working range, overall automatic regulation was

carried out using control rod groups AR1 and AR2.

4. From 10% to 100/o power, local automatic regulation (fu3R) was also invoked.

The reactor is “tripped” (i. e., switched off completely) only for a specific number of faults, e. g., loss of off-site (station) power, both turbines tripped, loss of three main circulating pumps, 50% loss of feed water, low steam drum water level, and high neutron flux.

For all other faults the reactor power is set back to some lower level consis­tent with the fault’s consequence for the reactor (e. g., on loss of one circulating pump to 80% full power, trip of single turbine to 50% full power).

The RBMK reactors are designed to be refueled at full load, and Figure 2.13 shows the refueling machine (H) operating from the gantry rnnning the length of the charge hall.

The primary coolant system is housed in a series of compartments that act as the containment in the event of an accident. Separate compartments house the primary circulation pumps (I), the coolant inlet headers and distribution pipework (J), and the reactor vault.

Each compartment is designed to withstand a pressure of 4.5 bars and is equipped with sealed electrical and mechanical penetrations and isolation valves on piping. The compartments are connected to one another and to a surface con­denser tunnel (K in Figure 2.13) as well as to two pools of water (“pressure sup­pression pools,” L) to condense the escaping steam and lower the pressure.

The steam drums (C) are housed in separate compartments on either side of the charge hall, but these are not pressure-tight compartments because of the large number of joints in the charge hall floor needed for refueling that provide a leak path between the steam drum compartments and the charge hall.

The RBMK reactor is equipped with an emergency core cooling system (M) that feeds both coolant and consists of

1. a fast-acting flooding system that automatically injects cold water into the damaged part of the reactor from two sets of gas-pressurized tanks holding enough water to cool the core for the first 3 minutes of a major loss of coolant accident. This system is supported with flow from the main feed pumps.

2. an active system of three pumps taking water from the condensate system after the pressurized tanks have emptied. These pumps are driven by three standby diesel generators that can be started within 2-3 minutes.

3. an active recirculating cooling system that consists of six pumps drawing water from the upper suppression pool through heat exchangers feeding the damaged part of the reactor and also driven by the diesels.

The emergency core cooling system is triggered by the coincidence of a high — pressure signal from any of the containment compartments and a low-level sig­nal from the steam drnms.

As a consequence of the accident at Chernobyl a number of modifications have been carried out on other RBMK units. The control rod design has been improved and the rate at which the rods can be inserted into the core has been increased. Automatic shutdown systems have been fitted to prevent the reactor from being operated continuously below 20% full power. The problem of the positive void coefficient has been reduced by fitting fixed neutron absorbers. The main influence of this measure is to alter the balance between absorption of neutrons in fixed absorbers and the variable absorption in the steam-water coolant. To compensate for these measures the enrichment of the fuel has been increased from 2.0% to 2.4% U-235.