Chapter 2

Nuclear Reactors

Introduction
This chapter provides an introduction to the RBMK reactor units at the
Chernobyl nuclear power plant (NPP), which is necessary before chapter 3
which details the events which led up to the explosion and the subsequent
radioactive releases into the atmosphere. Major components of a unit are
illustrated: turbine hall, central reactor hall, control room and the dosime-
try measurement (i.e. monitoring) recording laboratory after the accident
which has had to replace the previous system, now destroyed, in the control
room. For further reading on the nuclear fuel cycle, reactor operation and
types of nuclear reactor, the book by Patterson1 is recommended.

2.1 Chain reaction

The nuclear chain reaction in 235 U is the basis for electricity generation
using nuclear reactors, as well as for nuclear weapons such as the atomic
bomb at Hiroshima, although a 239 Pu chain reaction is also possible such
as in the atomic bomb at Nagasaki.
In a mass of uranium there are always a few stray neutrons, produced
either by spontaneous fission or by cosmic rays. If one of these stray neu-
trons produces fission of a 235 U nucleus, then, as well as the two fission
products, two or three high energy neutrons will also be produced. Prompt
neutrons are those that emerge at the instant of fission and the probabil-
ity of this occurring is better than 99:1. However, there is a slight chance
that a neutron will not emerge until some seconds later, this is a delayed
neutron.
The three possibilities open to a high energy fission neutron are that it
reaches the surface of the material and escapes; it strikes another nucleus
and is absorbed without any breakdown of the nucleus or it strikes another
nucleus and causes this nucleus to rupture. This third possibility, of induced

12
 Reactor operation 13

fission, depends on the energy of the neutron and on the nucleus it strikes.
Occasionally a fast neutron, fresh from an earlier fission, will rupture a
nucleus, indeed only a fast neutron can rupture a nucleus of 238 U. However,
if the neutron hits several nuclei, one after the other, giving up some of its
energy at each collision, it soon slows down and becomes a slow neutron,
often termed a thermal neutron.
A thermal neutron takes much longer to traverse a nucleus than a
fast neutron and is thus much more likely to cause fission in a nucleus of
235
U. This radionuclide has three fewer neutrons than 238 U and in naturally
occurring uranium this nuclide with a mass number of 235 forms only 0.7%
of the ore.
If there are enough 235 U nuclei close together then the neutrons can
induce more and more fissions, releasing more and more neutrons. This is
termed a chain reaction, which, when out of control as in an atomic bomb,
will cause a nuclear explosion, but when controlled can form the basis for
nuclear reactor design.
Reactor fuel contains enriched uranium, by which it is meant that
there is more than 0.7% of 235 U but even a small increase, of say 2–3%,
can make a marked difference, provided that there are sufficient thermal
neutrons. To ensure this, the core of the reactor contains not only the
uranium fuel but also a moderator . This is a material with light nuclei,
such as hydrogen or carbon, which are used in the form of water or graphite.

2.2 Reactor operation

Reactor fuel is sealed in casings termed cladding, which confines the fission
products which are produced. Assemblies of sealed fuel are termed fuel
elements and are interspersed with moderators, and also with neutron ab-
sorbers such as boron, to control the reaction. These are the control rods
which in the Chernobyl accident were not inserted quickly enough to stop
the explosion.
The region in which the chain reaction occurs is termed the core of the
reactor. When the reactor becomes critical with the establishment of a self-
sustaining chain reaction, each neutron lost by causing fission is replaced
by exactly one neutron, prompt or thermal, which does likewise. The
dependence of the chain reaction on thermal neutrons permits a gradual
adjustment of the reaction rate.
Removing an absorber, that is, a control rod, out of a stable chain
reaction, is called adding reactivity, the neutron density increases and the
rate of the chain reaction increases. Inserting an absorber is termed adding
negative reactivity and produces a reverse effect.
However, before removing the control rods sufficient for the reactor to
become critical, precautions must be taken against gamma-radiation and
14 Nuclear Reactors

neutrons pouring out of the core, since they can, for example, depending on
their energy, travel through metres of concrete. The reactor must therefore
be surrounded by enough concrete or other protective material to cut down
the radiation level outside. In many types of reactor installations this is
achieved by a specially designed containment building, such as at Three
Mile Island, where the presence of a containment building limited the effects
of that disaster. For the RBMK-1000 reactors at Chernobyl there was no
such containment building and the biological shield of 2000 tonnes on the
top of the reactor space was blown out of position and came to rest at an
angle of 15◦ to the vertical.
Normal start-up and shutdown of a reactor are both lengthy processes
and may take many hours. If, however, it is necessary to stop the chain re-
action, for instance in the event of a malfunction, the emergency shutdown
is termed a scram. At Chernobyl the emergency scram button was pressed
on the orders of the shift foreman at 01:23:40 hours on 26 April 1986 but
by then this had no effect and could not stop the explosion which occurred
at 01:23:44 hours, due to the rapid increase in power which is estimated to
have been 100 times full power.
If an operating reactor is left to itself its reaction rate will gradu-
ally fall, in part because of the build-up of fission products which absorb
neutrons. One of the most effective fission produced neutron absorbers is
135
Xenon and the phenomenon is termed xenon poisoning. 135 Xe is pro-
duced by the decay of 135 Tellurium and 135 Iodine which are generated for
several hours after start-up. 135 Xe nuclei which fail to capture a neutron
undergo beta decay into 135 Caesium. If the chain reaction rate remains
constant then the average concentration of 135 Xe in the core also remains
constant. The half-life of 135 Xe is 6.7 hours.
Nuclear fuel eventually has to be replaced because the 235 U reduces
as fission occurs. The replacement procedure is termed refuelling and the
spent fuel , is then stored. Currently some of the highest radiation dose
rates inside the Sarcophagus are in the room where the spent fuel still
remains. There are many different fission products which occur, including
those of plutonium radionuclides with mass numbers 239, 240 and 241: it
was 239 Pu which was the fissile material for the atomic bomb dropped on
Nagasaki. However, not all fission products are solids, some are gaseous
such as xenon and krypton, all of which escaped into the atmosphere at
Chernobyl.
Complete fission of all the nucleii in one kilogram of 235 U would release
energy totalling one million kilowatt-days. The nuclear fuel in the core
is so arranged that the heat is given off gradually enough to keep the
temperatures manageable. The amount of heat given off per unit volume
in a reactor core is termed the power density.
Heat is removed from the reactor by pumping a heat-absorbing fluid
through the core past the hot fuel elements. This fluid is the coolant and
 RBMK-1000 nuclear power units at Chernobyl 15

may be a gas such as air or carbon dioxide, or a liquid such as water.

The cooling system can be open-ended or designed as one or more closed
circuits. A closed circuit can be pressurized. For electricity generation the
heat released can generate steam to run turbines, as with the Chernobyl
power station.
There are several different reactor types, see table 2.1, of which the
Chernobyl RBMKs are of a pressure tube design in which the fuel elements
lie in vertical pressure tubes filled with light water, as distinct from heavy
water, and are surrounded by a graphite moderator.

Table 2.1. Reactor types.

Gas cooled power reactors e.g. Magnox reactors and advanced gas cooled
reactors
Light water reactors e.g. Pressurized water reactors boiling water
reactors
Heavy water reactors e.g. CANDU (Canadian-deuterium-uranium)
reactors
Fast breeder reactors e.g. Liquid metal FBRs

2.3 RBMK-1000 nuclear power units at Chernobyl

2.3.1 History of RBMK reactors
The history of RBMK type reactors in the Soviet Union had been, until
1986, very successful. After the early development of the system, the USSR
went directly to full scale 1000 MW(e) units. The first RBMK-1000 was put
into service at Leningrad in 1974. The Leningrad, Kursk and Chernobyl
power stations each have four units built in pairs, each unit supplying two
500 MW(e) turbogenerators. The first two (of four) units are operating at
Smolensk and two more were being constructed at Chernobyl at the time
of the accident.
The first of the two larger, 1500 MW(e), versions of these reactors was
put into service at Ignalina, Lithuania, in 1984. Its physical size is similar
to that of the RBMK-1000 but it has a fuel power density 50% higher.
In safety terms, there had, before 1986, been practical demonstration
that the design can handle significant faults. For example, at Kursk nuclear
power station in January 1980, a total loss of station internal load occurred
that was sustained satisfactorily, and there have been a number of feedwater
system transients. None of these presented severe plant safety problems.
Electricity production for the period 1981–85 at the Chernobyl NPP was
106.6 × 109 kilowatt-hours.
16 Nuclear Reactors

Figure 2.1. Map of the site of the power plant in relation to the city of Kiev
and the Kiev reservoir2 . (Courtesy: USSR KGAE.)

2.3.2 Location of the Chernobyl NPP site

The Chernobyl NPP is situated in the eastern part of a large region known
as the Belarussian–Ukranian woodlands, beside the 200–300 m wide river
Pripyat which flows into the Dnieper. Figure 2.1 is a map of the imme-
diate area surrounding the NPP including the 30 km exclusion zone. The
NPP’s cooling pond is linked to the Kiev reservoir. Minsk, the capital of
Belarus with a population of 1.3 million, is 320 km from the NPP, and
Kiev, the capital of Ukraine with a population of 2.5 million, is 146 km
from the NPP. The regional centre is the 12th century town of Chernobyl
with a population of 12 500 in 1986, situated 15 km south-east of the NPP.
Nearer to the NPP, only 3 km distant, is the town of Pripyat where 45 000
power plant workers and their families lived. The population of all Belarus
is 10 million, including 2.3 million children, and of Ukraine is 60 million
including 10.8 million children under the age of 15 years.
Figure 2.2 is a map of a wider area and shows the capital cities of
Ukraine, Belarus, Poland, Austria, Hungary, Yugoslavia and Romania.
One of the earliest concerns was the possibility of contamination of the
river Dnieper, all its tributaries, and eventually the Black Sea.
 RBMK-1000 nuclear power units at Chernobyl 17

Figure 2.2. Map of Kiev and the Pripyat marshes in relation to Belarus, Poland,
Austria, Hungary, Yugoslavia, Romania and the Black Sea.

2.3.3 Construction plans

Construction was planned for three stages with each stage comprising two
RBMK-1000 units. The first stage of Units No. 1 and No. 2 was con-
structed between 1970 and 1977, and the second comprising Units No. 3
and No. 4 was completed in late 1983. It was Unit No. 4 which exploded.
In 1981 work was begun on the construction of two more units also using
RBMK-1000s, at a site 1.5 km to the south-east of the existing site. They
were almost completed for commissioning when the accident occurred but
were immediately abandoned and have been left to rust.

2.3.4 Overall view, central reactor hall and turbine hall

Plate I is an aerial view of the NPP taken five months after the accident.
The cooling pond is in the background, the tall chimney in the centre is a
ventilation stack and was contaminated from top to bottom, the shattered
reactor Unit No. 4 is clearly seen and just in front of it, the long white
building houses the turbine hall on which damage to the roof is shown.
The yellow painted turbines can just be seen through the hole in the roof.
18 Nuclear Reactors

It was on this roof that many of the firemen who died received their high
radiation doses. All the forests in this photograph were contaminated and
had to be cut down.

Figure 2.3. Turbine hall, November 1982. (Courtesy: TASS.)

Figure 2.3 is a photograph of the turbine hall in 1982 taken at a cele-

bration of the 60th anniversary of the USSR, and figure 2.4 is the central
reactor hall of Unit No. 1 in June 1986. The small squares in its centre
are the covers to the heads of the fuel rods: these covers were reported to
have been blown into the sky at least 1 km when the explosion took place.

2.3.5 Design features

The design features of RBMK nuclear reactors, of which there were four
originally operational at the Chernobyl NPP, are well described by INSAG3
and by UNSCEAR4 .
A cross-section view of a typical unit at Chernobyl NPP is seen in figure
2.5. Each reactor in a pair supplies steam to two 500 MW(e) turbines. The
 RBMK-1000 nuclear power units at Chernobyl 19

Figure 2.4. Central reactor hall of Unit No. 1, June 1986. (Courtesy: TASS.)

Figure 2.5. Cross-sectional view of the RBMK reactor nuclear power unit2 .
(Courtesy: USSR KGAE.)
20 Nuclear Reactors

two reactors, together with their multiple forced circulation circuits, are
located in separate blocks, between which are installed auxiliary systems,
and the turbine generator room, figure 2.3, is common to two reactor units.
It houses four turbogenerators and associated systems.
An RBMK-1000 reactor is a graphite-moderated light-water cooled
system with uranium dioxide (UO2 ) fuel in 1661 individual vertical chan-
nels. The geometrical arrangement of the core consists of graphite blocks
250 mm × 250 mm, 600 mm in height, stacked together to form a cylindri-
cal configuration 12 m in diameter and 7 m high. The mass of the graphite
moderator is 1700 tonnes. It is located in a leaktight cavity formed by a
cylindrical shroud, the bottom support cover and the upper steel cover. In
the accident the bottom cover dropped 4 m leaving a gap through which
molten fuel could travel. This was not initially realized and the search for
the missing fuel took a considerable time. Working in the nearest room
to the reactor it took 18 months to drill through the adjoining wall. Oil
industry engineers were the drillers and the work was completed in October
1988. To their surprise the reactor room was empty. The next approach
was to use, because of the high dose rates, a remote controlled device which
consisted of a child’s toy tank costing 15 roubles, to which a camera was
strapped: this was also unsuccessful in locating the fuel. Eventually it was
found that this 4 m gap was present and nuclear fuel masses, the lava, were
finally located.
Each graphite block has a central hole which provides the space for
the fuel channels, thus forming a lattice pitch of 250 mm. Fuel and control
rods channels penetrate the lower and upper steel structures and connect
to two cooling systems below and above the core. The drives of the control
rods are located above the core below the operating floor shield structure.
The fuel, in the form of UO2 pellets, is sheathed with a zirconium–
niobium alloy. A total of 18 fuel pins, approximately 3.5 m in length are
arranged in a cylindrical cluster of which two fit on top of each other into
each fuel channel. Fuel replacement is done by a refuelling machine located
above the core. One to two two fuel channels can be refuelled each day.
The coolant system consists of two loops and the coolant enters the
fuel channels from the bottom at a temperature of 270◦ C, heats up along its
upward passage and partially evaporates. The wet steam of each channel is
fed to steam drums, see figure 2.5, of which there are two for each cooling
loop.
The separated dry steam, with a moisture content of less than 0.1%,
is supplied via two steam pipes to two turbines, while the water, after
mixing with the turbine condensate, is fed through 12 downcomers to the
headers of the main circulation pumps. The condensate from the turbines
enters the separators as feedwater, thereby sub-cooling the water at the
main circulation pump inlet. The circulation pumps supply the coolant to
headers which distribute it to the individual fuel channels of the core.
 RBMK-1000 nuclear power units at Chernobyl 21

The coolant flow of each fuel channel can be independently regulated

by an individual valve in order to compensate for variations in the power
distribution. The flow rate through the core is controlled by circulation
pumps. In each loop four pumps are provided, of which one is normally on
standby during full power operation.
From the fission reaction approximately 95% of the energy is trans-
ferred directly to the coolant. 5% is absorbed within the graphite moder-
ator and mostly transferred to the coolant. The latter part of the fission
energy is transferred to the coolant channels by conduction leading to a
maximum temperature within the graphite of approximately 700◦ C. A gas
mixture of helium and nitrogen enhances the gap conductance between the
graphite blocks and provides chemical control of the graphite and pressure
tubes. The control and protection system in the RBMK reactors has the
basic functions listed in table 2.2.

Table 2.2. Basic functions of the RBMK control and protection system3 .
• Regulation of the reactor power and reactor period in the range 8 × 10−12
to 1.2 times full power
• Manual regulation of the power distribution to compensate for changes in
reactivity due to burnup and other effects
• Automatic stabilization of the radial-azimuthal power distribution
• Controlled power reduction to safe levels when certain plant parameters
exceed preset limits
• Emergency shutdown under accident conditions

The system includes 48 measuring devices. These are 24 ionization

chambers placed in the reflector region which are used to drive three banks
of automatic regulation rods and 24 fission chambers which are in-core
detectors located in the central openings of the fuel assemblies which are
used to drive the local automatic controllers. There are 211 absorbing rods
in the core which are functionally grouped, table 2.3.
When the reactor is started up, the 24 emergency protection rods are
the first to be raised to the upper cut-off switches. The speed of the control
rods is 0.4 m per second. When a control rod is disconnected from its drive,
which is necessary in the case of a power loss, the speed is about 0.4 m
per second driven by free fall. Flow resistance precludes a higher velocity.
The highest level of emergency is Level 5, which results in the insertion of
all the rods (except the 24 shortened absorber rods) into the core up to
the lower cut-off switches. The over-power trip set point is set at present
power plus 10% of nominal power. The system includes the measurements
and subsystems listed in table 2.4. For a full description of the safety
systems including the emergency core cooling system, see INSAG3 , and for
22 Nuclear Reactors

Table 2.3. Functional grouping of the 211 absorbing rods3 .

• 24 shortened
absorbing rods
• 24 auto-control rods 12 local auto-control (LAC):
regulation rods in 12 zones.
12 average power control:
3 banks of 4 rods per bank.
• 139 manual rods and 24 emergency control:
24 emergency rods uniformly selected.
24 local emergency protection (LEP):
2 rods per zone.
115 manual control.

a summary of the IAEA’s co-operative programme for consolidating the

technical basis for further upgrading the safety of RBMKs see the 1996
paper by Lederman5 .

Table 2.4. Measurements and subsystems for the RBMK reactor3 .

• Flow rates in all the fuel channels and the control channels: 1661 plus
223 points.
• The temperatures of the graphite core and metal structures: 46 plus 381
points.
• A system of monitoring the main components of the forced circulation
system, such as the drum separators, the main circulation pumps and the
suction and pressure headers.
• A system for monitoring the power distribution: 130 radial plus 84 axial.

2.3.6 Control room

Major areas of a nuclear power unit, besides the reactor, include the reactor
central hall, the turbine hall, and the control room of the unit, which were
similar for all four units. The control room is shown before and after the
accident in figures 2.6 and 2.7.

2.3.7 Principal specifications

The principal specifications4 are given in table 2.5.
 RBMK-1000 nuclear power units at Chernobyl 23

Figure 2.6. Control room of Unit No. 1, December 1987. (Photograph:

R F Mould.)

Figure 2.7. Damaged control room of Unit No. 4, June 1998. (Photograph:
R F Mould.)
24 Nuclear Reactors

Table 2.5. Principal specifications of the Chernobyl Unit No. 4 reactor.

Thermal power 3200 MW
Fuel enrichment 2.0%
Mass of uranium in fuel assembly 114.7 kg
Fuel burn-up 20 MW d/kg
Maximum design channel power 3250 kW
Isotopic composition of unloaded fuel
235
U 4.5 kg/t
236
U 2.4 kg/t
239
Pu 2.6 kg/t
240
Pu 1.8 kg/t
241
Pu 0.5 kg/t

Figure 2.8. Radiation monitoring inside the Sarcophagus, October 1986.

(Courtesy: V Zufarov.)
 Measures to improve the safety of RBMK plants 25

Figure 2.9. Measurement recording laboratory, June 1998. (Photograph:

R F Mould.)

2.3.8 Measurement recording laboratory

Before the accident all the dosimetric and temperature monitoring infor-
mation required by the power plant operators of Unit No. 4 would have
been available in the control room. However, after the accident all the
measuring systems were destroyed and had to be replaced.
The initial monitoring was performed manually, as there was no al-
ternative, figure 2.8, but eventually bore holes were made for three types
of sensors: those for the measurement of neutron dose rates, gamma dose
rates and temperature. The laboratory in which these measurements are
recorded is shown in figure 2.9 and seen to be rather basic. In June 1998
the maximum gamma dose rate was 4000 roentgen/hour: recorded in the
spent fuel storage pond. The maximum temperature in June 1998 was
recorded as 40◦ C. In 1986 the temperatures were some 300–400◦ C.

2.4 Measures to improve the safety of RBMK plants

Since the accident, several organizational and technical measures have been
developed and implemented to improve the safety of operating RBMK
plants. These have been reported to INSAG and table 2.6 summarizes
the aims of these measures6 .
26 Nuclear Reactors

Table 2.6. Aims of improvement measures for RBMK plants6 .

• Reducing the positive steam (void) coefficient of reactivity and the effect
on reactivity of complete voiding of the core. This has been provided by
the installation of additional fixed absorbers, up to 90, into the core, and
through the introduction of the use of fuel with 2.4% 235 U enrichment.
• Improving the speed of the scram system. The speed of insertion of control
and safety rods has been increased with the time for full insertion into
the core reduced from 18 s to 12 s.
• Introducing new computational codes for the operational reactivity mar-
gin (ORM) with numeric indication of the ORM in the control room. This
has been increased to between 43 and 48 control rods, depending on the
reactor.
• Precluding the possibility of bypassing the emergency protection system
while the reactor is at power, through an operating limit requirement and
the introduction of a two key system for the bypass action.
• Avoiding modes of operation leading to reduction of the departure from
nuclear boiling margin for the coolant at reactor inlet. This addresses the
question of adequate subcooling at the core inlet. Operating instructions
have been updated to take into account lessons learned from the accident
and among the new provisions is one which now sets a lower limit of 700
MW (th) for steady operation of an RBMK reactor.

Figure 2.10. The Chernobyl NPP was named after V I Lenin and his bust
remains in front of the administration building in 1998. One of Lenin’s quotes is
‘Russia is communism and electrification’. (Photograph: R F Mould.)