The Chernobyl Nuclear Power Plant, now decommissioned, is
located near the city of Prypiat, Ukraine. The V.I. Lenin Nuclear Power
Station as it was known during the Soviet times, consisted of four reactors
of type RBMK-1000, each capable of producing 1000 megawatts of electric
power (3.2 GW of thermal power). Construction of the plant and the
city of Pripyat to house workers and their families began in 1970, with
reactor no. 1 commissioned in 1977. It was the third nuclear power station
in the USSR of RBMK type, and the first ever nuclear power plant on
Ukrainian soil. The completion of the first reactor in 1977 was followed by
reactor no. 2 (1978), no. 3 (1981), and no. 4 (1983). Two more reactors,
nos. 5 and 6, capable of producing 1000 MW each, were under construction at
the time of the accident. Reactor no. 5 was almost complete at the time of
the accident and was scheduled to start operating in the fall of 1986.
However, it has since been abandoned; construction cranes still stand next
to it to this day. Chernobyl 4 was a Soviet RBMK-1000 type reactor rated at
3,200 MW thermal power and 1,000 MW electric output. There were three
similar operating units at the Chernobyl site. The RBMK-1000 is a
graphite-moderated boiling water reactor that contains 1,661 parallel,
vertical pressure tubes loaded with fuel assemblies. Reactor water flow is
provided by six of eight installed main circulation pumps; two pumps are
installed spares. The flow through each of the fuel channels is adjusted
using regulating valves on the inlet of each channel to control heat flux
margins. The water-steam mixture leaving the top of the fuel channels flows
into four horizontal steam drums with moisture separators. The dry steam
drives two 500 MWe turbine generators (numbers 7 and 8). Feedwater is fed
directly to the steam drums, bypassing the reactor, to control water level.
The core is large, 23 feet in height and 39 feet in diameter, so that
regions of the core are loosely coupled to each other neutronically. As a
result, individual regions of the core can be close to criticality. The
reactor exhibits a reactivity increase as water density in the core
decreases (positive void coefficient of reactivity). The fuel temperature
coefficient of reactivity is negative. In normal operation, the overall core
power coefficient is negative at and near full power but becomes positive at
lower power levels. The minimum permitted power level for steady state
operation is 700 MW(th) (22 percent of full power). The station was used for
base-load operation. Since the plant was refueled on line, excess reactivity
was not high. Unless quickly restarted after a shutdown, startup was delayed
due to xenon poisoning. The system for control and protection of the reactor
is based primarily on movement of 211 boron carbide absorber rods in
vertical channels adjacent to the fuel channels. The rods have graphite
followers attached to displace water in the rod channels. However, according
to Soviet descriptions of the RBMK1000, the followers are apparently not
full core length, so that when a rod is fully withdrawn from the core, about
one meter of water remains in the rod channel below the follower. The rod
control system provides automatic control of power level and of the flux
profile in the core. In a normal configuration, the protection system can
quickly reduce power by partial rod insertions; automatic scram is the
ultimate response. The time to fully insert rods for a scram is 20 seconds.
A minimum "operating reactivity margin" is specified. This margin is
referred to by the Soviets as the equivalent of 30 inserted regulating rods.
Control rods are required to be partially inserted into the core during
operation to enhance the initial negative reactivity insertion rate on
scram. In addition, insertion of control rods reduces the positive void
coefficient. The rods are also adjusted to correct spatial power
instabilities in the core. (It is our understanding that the procedural
requirements for 30 equivalent rods minimum reactivity margin is a means of
specifying an overall rod configuration that ensures a certain initial
negative reactivity rate on scram. It also apparently prevents an initial
positive reactivity insertion that can occur when rods enter the core from
the top, displacing water in the rod channels near the bottom of the core.)
During the performance of a turbine-generator coast-down test on April 26,
1986, Chernobyl Unit 4 experienced a severe reactivity excursion that, with
the accompanying pressure surge and fire, destroyed the reactor and breached
the surrounding building. The test procedure had not been adequately
reviewed from a safety standpoint. Management control of the evolution was
not maintained; the test procedure was not followed; several safety
functions were bypassed; and control rods were misoperated. Operators lost
control of the reactor during the performance of the test. The Chernobyl
accident in 1986 was the result of a flawed reactor design that was operated
with inadequately trained personnel and without proper regard for safety.
The April 1986 disaster at the Chernobyl nuclear power plant in the Ukraine
was the product of a flawed Soviet
reactor design coupled with serious mistakes made by the plant
operators in the context of a system where training was minimal. It was a
direct consequence of Cold War isolation and the resulting lack of any
safety culture. Note:
The type of reactor involved in the Chernobyl accident is of an
entirely different design than that from U.S. commercial reactors. It
lacks important safety features that are built into U.S. plants.
Additionally, the societal, institutional, and operational approaches
in the former Soviet Union were quite different than the U.S.
The following major operational events or errors and administrative or
management control breakdowns led to the accident:
On April 25, 1986, at 0100 hours, the operators started reducing power to
perform a test on turbine-generator number 8, prior to a maintenance outage.
The test was intended to determine how long the turbine-generator would
continue to supply power near rated voltage for essential equipment as the
generator coasted down. Similar experiments had been carried out twice
before at Chernobyl. At 1305, turbine-generator number 7 was shut down with
the reactor at 50 percent power. Much of the electric power for the plant,
including four main circulation pumps and two main feed pumps, was now being
provided by turbine-generator number 8. At 1400, the emergency core cooling
system was defeated in accordance with the test program. Evidently this was
to ensure that an inadvertent actuation would not interfere with the test.
The test was then delayed due to a request from the load dispatcher to
continue power generation. Operation of the plant at reduced power, using
only turbine-generator number 8, continued until 2310 on April 25.
(Continued blocking of the emergency core cooling system during this delay
was in violation of operating procedures.) At 2310 the power reduction
resumed. According to the test procedure, the coastdown of the generator was
to be started with the reactor power at 700-1000 MW(th). However, in
switching the automatic control systems from spatial power control to power
level control, the control point had not been properly set. As a result, the
power fell below 30 MW(th). At 0100 on April 26, power was stabilized
briefly at 200 MW(th). Xenon poisoning of the reactor continued to increase.
To compensate, the operators withdrew additional rods (more than allowed by
procedure). A further increase in power to 700-1000 MW(th), as called for in
the test procedure, was hampered by the small excess reactivity available.
At 0103, two additional main circulation pumps were started, so that all
eight were running. (Four pumps were powered by another source to provide
for cooling of the reactor core, while the four circulation pumps powered by
turbine-generator number 8 would coast down as part of the test.) The
coolant flow rate now exceeded the maximum allowed by operating procedures.
The additional flow reduced the steam content of the coolant in the fuel
channels, resulting in lower steam pressure and lower water level in the
steam drums. These circumstances also led to the core inlet temperature of
the coolant (and probably the bulk temperature) being very close to
saturation. Under these conditions, the operators experienced difficulty in
controlling steam drum pressure and water level. To avoid an automatic
shutdown, the operators bypassed the emergency protection signals for
reactor scrams on steam drum pressure and water level. Reactivity continued
to drop slowly due to xenon buildup and decreased voiding, requiring further
control rod withdrawal to maintain power at 200 MW(th). At 0122:30 the
operators noted that the available reactivity margin, related to the number
of rods and their position in the core, had dropped well below the level
requiring immediate shutdown of the reactor (six to eight "equivalent" rods
vice the 30 "equivalent" rods required by procedure). Nevertheless, the
operators continued with the test. The operators also bypassed the reactor
scram that would be called for by the shutdown of the second
turbine-generator (number 8). This was done so that it would be possible to
repeat the test if the first attempt was unsuccessful. At 0123:04 the stop
valves for turbine-generator number 8 were closed, starting the coast-down
test. The reactor was still critical and at power. Four of the eight main
circulation pumps began coasting down as the test started. Conditions had
now been established for a severe transient, as follows:
- Overall management control of the evolution and its integration with
plant operations was not clearly established. The test was directed by
an engineer with expertise in the turbine-generator/electrical area
- The test procedure did not receive an adequate safety review.
- The operators felt a sense of urgency to complete the test. The test
would have been delayed by one year if not performed at that time. The
reasons for the sense of urgency were not well explained in the Soviet
report but may have been caused by outside or management pressures.
Since the evolution occurred over a 24-hour period, more than one
operating shift was involved. The test was conducted early in the
morning and just prior to a Soviet national holiday. These factors may
have influenced performance.
- The power reduction for the test was interrupted at the load
dispatcher's request for nine hours. This changed the initial conditions
in the core from those contemplated in the procedure and may have
contributed to the inability to later attain the power level specified
for the test.
- The operators did not properly adjust the set point on the automatic
power level controller. As a result, the power decreased to 30 MW(th).
- The operators did not follow the test procedure:
- The reactor scram signal for the trip of the second
turbine-generator was bypassed, which violated the test procedure.
This was done to enable repeating the test if necessary. The test
probably would not have led to the accident had the reactor scrammed
at the start of the test.
- Unable to raise the reactor power back to 700 MW(th), the
operators started the test at a lower power level, which violated
test procedure requirements. (The test was started at six percent
power instead of 22 to 31 percent as specified in the procedure.) At
this power level, the reactor was difficult to control. Also, with
very little steam being generated, the eight circulation pumps
produced a flow rate above allowable limits. With the high flow rate
and low power level, the water inlet temperature to the core was
very close to saturation. Under these conditions, an increase in
power caused a much greater increase in steam voids and reactivity
- Other safety systems were also defeated:
- Bypassing of reactor scrams associated with steam separator
pressure and water level enabled operating the reactor despite
- Control rods were withdrawn well beyond safety limits specified by
procedures. This was done to compensate for xenon buildup and
negative reactivity resulting from void suppression in the core.
- The emergency core cooling system was deactivated for over nine
hours while the plant was operating.
- The design of the plant placed a heavy dependence on adherence to
administrative controls and procedures for safe operation. However, the
plant operators did not demonstrate an adequate understanding of the
safety implications of their actions. Their willingness to conduct the
test at a very low power level, with an abnormal (and unauthorized)
control rod configuration and core conditions, and with safety features
bypassed indicates an insufficient understanding of the reactor and its
- The reactor was critical but at a very low power level where it was
unstable and difficult to control.
- The overall coefficient of reactivity was positive with the steam
(void) coefficient being predominant.
- Control rods were near the top of the reactor, in a region of low
reactivity differential worth (low "bite"). It would take several
seconds for the rods to insert appreciable negative reactivity.
Initially, they apparently would insert positive reactivity as the
graphite rod followers displaced water from the lower region of the
As the four circulation pumps coasted down, the lower flow allowed more
steam to form in the core. Because of the strongly positive void
coefficient, the increased steam content in the core initiated a power rise
as the test began. The increased power further increased the steam voids.
The automatically controlled regulating rods began inserting but the power
continued to rise. An operator pushed the reactor scram button at 0123:40 to
scram the remaining shutdown rods but these rods were near the top of the
core and power continued to rise. A loud noise was heard from the reactor.
Noting that the rods had not fully inserted, an operator de- energized the
control rod drives hoping this would facilitate the scram. Two to three
seconds later, operators heard a second loud noise. The core reactivity
exceeded prompt critical, and by Soviet calculation, the power peak exceeded
100 times nominal full power. Energy generated in the fuel by the power
excursion dispersed part of the fuel into minute fragments. The
disintegration of the fuel stopped the chain reaction. A rapid increase in
pressure resulted as disintegrated fuel at very high temperatures contacted
the water. The energy release lifted the 1000-ton reactor cover plate,
severing all the fuel channels. The refueling machine and its crane
collapsed onto the reactor. Upper portions of the reactor building were
destroyed. Hot segments from the core were ejected from the reactor, and the
graphite in the reactor was ignited. Approximately 30 localized fires
started, involving roofing materials and other combustibles. This accident
resulted in the release of about 50 million curies of particulate and iodine
radioactivity, or about three to four percent of total core inventory,
evacuation of 135,000 people from the area within 30 kilometers of the
plant, gross radio- active contamination of the plant site, contamination of
other portions of the evacuated area, and distribution of measurable amounts
of radioactivity over other countries. Approximately 200 personnel were
hospitalized from radiation exposure at the site, and 31 fatalities from
this group have been reported to date.
- The steam void percentage in the core was small but the core inlet
water temperature was near saturation, giving the potential for rapid
voiding over a substantial region in the core.
What has been gained from the Chernobyl disaster?
Leaving aside the verdict of history on its role in melting the Soviet
iron curtain, some very tangible practical benefits have resulted from the
Chernobyl accident . The main ones concern reactor safety, notably in
eastern Europe. (The US Three Mile Island accident in 1979 had a
significant effect on western reactor design and operating procedures.
While that reactor was destroyed, all radioactivity was contained - as
designed - and there were no deaths or injuries.).
While no-one in the West was under any illusion about the safety of early
Soviet reactor designs, some lessons learned have also been applicable to
western plants. Certainly the safety of all Soviet-designed reactors has
improved vastly. This is due largely to the development of a culture of
safety encouraged by increased collaboration between East and West, and
substantial investment in improving the reactors.
Modifications have been made to overcome deficiencies in all the RBMK
reactors still operating. In these, originally the nuclear chain reaction
and power output would increase if cooling water were lost or turned to
steam, in contrast to most Western designs. It was this effect which
caused the uncontrolled power surge that led to the destruction of
All of the RBMK
reactors have now been modified by changes in the control rods,
adding neutron absorbers and consequently increasing the fuel enrichment
from 1.8 to 2.4% U-235, making them very much more stable at low power.
Automatic shut-down mechanisms now operate faster, and other safety
mechanisms have been improved. Automated inspection equipment has also
been installed. A repetition of the 1986 Chernobyl accident is now
virtually impossible, according to a German nuclear safety agency report.
Since 1989 over 1,000 nuclear engineers from the former Soviet Union have
visited Western nuclear power plants and there have been many reciprocal
visits. Over 50 twinning arrangements between East and West nuclear plants
have been put in place. Most of this has been under the auspices of the
World Association of Nuclear Operators, a body formed in 1989 which links
130 operators of nuclear power plants in more than 30 countries. See also
in the Nuclear Power Industry.
Many other international programs were initiated following Chernobyl. The
International Atomic Energy Agency (IAEA) safety review projects for each
particular type of Soviet reactor are noteworthy, bringing together
operators and Western engineers to focus on safety improvements. These
initiatives are backed by funding arrangements. The Nuclear Safety
Assistance Coordination Centre database lists Western aid totalling almost
US$1 billion for more than 700 safety-related projects in former Eastern
Bloc countries. The Nuclear Safety Convention is a more recent outcome.
In 1998 an agreement with the US provided for the establishment of an
international radioecology laboratory inside the exclusion zone.
The 2005 Chernobyl Forum report said that some seven million people are
now receiving or eligible for benefits as "Chernobyl victims", which means
that resources are not targeting the needy few percent of them. Remedying
this presents daunting political problems however.
Report on the Accident at the Chernobyl Nuclear Power Station
- The Official Story
and U.S. Nuclear Industry
The Chernobyl Accident: Updating of INSAG-1