Chernobyl - 15 Years Later
(April 26, 2001)
By: Canadian Nuclear Association




Overview
On April 26, 1986 a serious accident occurred in unit 4 of the Chernobyl
nuclear power station near Kiev in what is now the Ukraine. The Chernobyl 4
reactor was destroyed and large quantities of radioactive material were
released to the environment.
Thirty-one of the operating staff were killed by radiation exposure or fire
at the time of the accident and an estimated 600,000 clean- up workers
received significant doses of radiation. Several hundred thousand people
were relocated because of concern about radioactive contamination. Year's
later about 1,800 young people who had been exposed as infants to the
radio iodine released in the accident were diagnosed with thyroid cancer, ten
of which died. Beyond these terrible casualties, scientific studies have not
identified any major public health impact due to radiation exposure from the
accident.
In Canada, this accident could never have happened because of the CANDU
Technology of the Canadian operated reactors, the excellent regulatory
system in Canada under the Canadian Nuclear Safety Commission and Canada's
world class expertise in engineering and safety.
The accident was a result of a flawed and inadequate design, inappropriate
operation, lack of an effective regulatory system and an operating
organization that did not give proper regard to safety. The design
(designated RBMK) would never have been approved in Canada or any other
western nation nor would the test that led to the accident. The destroyed
Chernobyl 4 reactor was sealed with a concrete cover, referred to as the
"sarcophagus", which has contained the remains of the reactor for the past
fifteen years. Plans have been developed to repair and rebuild this
structure.
Since they were a major source of electricity for the Ukraine, the other
Chernobyl units continued to operate, after some basic changes to improve
their safety. Unit 2 was shut down in 1991, unit 1 in 1997 and unit 3 in
December 2000.Over the years since the accident several extensive scientific
reviews have been conducted. As a result there is now a good understanding
of the causes of the accident and of its consequences. These are summarized
in the following pages.
The RBMK reactor
The reactors at the Chernobyl nuclear power station were of the RBMK type, a
design developed in the former USSR and used only in Russia, Ukraine and
Lithuania. The slightly enriched uranium fuel is contained in vertical
pressure tubes placed inside a large block of graphite, which serves as the
moderator (material to slow down the neutrons emitted in the fission
process). Heat produced in the fuel by fission is transferred to pressurized
water, which is pumped vertically through the pressure tubes, turning to
steam at the top.
As some of the heat produced by the fuel is developed in the moderator, the
graphite of the RBMK design operates at a high temperature, about 700 C. To
keep the graphite from burning it is encased in a metal container, which is
filled with inert gas. The piping below the reactor is inside "leak- tight
boxes" connected to a large pool of water, called a "bubbler pond". If a
pipe containing reactor coolant broke the concept was that the steam would
be forced into the pond where it and any radioactive material it contained
would be trapped in the water inside the box.
RBMK reactors do not have containment, i. e., a structure surrounding the
entire plant to prevent the release to the atmosphere of any radioactive
material that might escape from the reactor, such as the containment
buildings of CANDU and other nuclear power plants.
The RBMK reactors do have combined control and shutoff rods, i. e., metal
rods containing neutron absorbing material that will stop the fission
reaction, which are driven into the reactor core from the top. However, as
determined by Canadian analyses, the design of these rods ended up creating
the power surge that led to the destruction of the reactor.
The shutoff rods are cooled by water, which also absorbs neutrons. To
enhance their effect the bottom of the shutoff rod had a graphite extension.
Just prior to the accident, because of the abnormal operation, most of the
reactor power was being generated near the bottom of the core. As the rods
were driven in, the graphite extension displaced the rod cooling water,
enhancing the reactivity and, hence, causing a large, fast rise of power.
This power surge, which lasted less than five seconds, created a steam
explosion in the fuel channels, which erupted through the top of the
reactor.
There was 18 RBMK reactors built in the former Eastern Bloc countries, 14 of
which are still operating: 11 in Russia, 2 in Lithuania, and 1 in Kurdistan.
All have had their shutdown systems modified in light of the Chernobyl 4
accident.
The Accident
Ironically, the accident at the Chernobyl unit 4 reactor on April 26, 1986
began with a test to demonstrate a safety feature. The operators wished to
determine the length of time the reactor cooling water would continue to
flow when steam to the turbine was cut off and all on- site electrical power
was lost.
The sequence of events began on April 25 when the power of the reactor was
lowered to 50 per cent of full power, Because of demands on the local
electrical system the reactor was kept at this level for nine hours,
resulting in the test being deferred until the overnight hours. A little
past midnight, early April 26, the reactor power was decreased to about 20
per cent but, due to an operating error, continued to decrease to about 1
per cent. The characteristics of the RBMK reactor would have led it to
shutting itself down completely which was not desired. In an attempt to
raise the power the operator withdrew all of the control and shut- off rods.
The operators then proceeded with the test. One of their actions was to
reduce the feed water flow to the reactor cooling system. They then closed
the turbine valves. This led to increased boiling of the coolant in the fuel
channels. Because RBM reactors have a positive void coefficient 1 the power
increased.
Noting the increasing power, an operator pressed the emergency button to
insert the shut- off rods into the core. However, because of an unusual
aspect of the RBMK design, the initial result was an increase in reactivity,
rather than a decrease, leading to even more rapid rise in power. 2 Within
seconds the power rose to about 100 times normal full power, peaking there
only because the reactor destroyed itself.
1 "Positive void coefficient" means that an increase in the boiling
of the coolant (increasing the voids) would make the reactor more reactive,
i. e.,would cause a rapid increase in power.
2 See report 910580 PA "Chernobyl - A Canadian Perspective" from
Atomic Energy of Canada Limited, revised in 1991.
Fuel melted, reacted with the coolant, producing a steam explosion in the
fuel channels, many of which ruptured. The explosion lifted the heavy cap on
top of the reactor allowing air to enter and react with the hot graphite,
resulting in a major fire.
Some of the very radioactive fuel was ejected along with some mildly
radioactive graphite. Large quantities of volatile radioactive fission
products, such as caesium and iodine were also released. Over the days after
the accident helicopters were used to drop tonnes of boron, dolomite, sand,
clay and lead on to the burning reactor to extinguish the blaze and limit
the release of radioactive material. Subsequently a concrete cover was built
over the destroyed Chernobyl 4 reactor. This structure, referred to as the
"sarcophagus", has contained the remains of the reactor for the past fifteen
years.
The consequences
(1) Early consequences
Because the Chernobyl 4 reactor did not have containment (as do Canadian
CANDU reactors and all other power reactors outside of the former USSR and
associated states) the radioactive material released by the accident entered
the atmosphere. The heavier particles were deposited relatively close to the
station but the lighter material was carried by the wind over part of the
Ukraine, Belarus, Russia and northern Europe.
The early casualties of those on site at the time of the accident and
fireman from Pripyat included 26 who died of high radiation dose. Four
others were killed by fire or falling masonry and one other died
subsequently of a heart attack, for a total of 31 direct deaths.
An estimated 600,000 workers (called "liquidators") were involved in the
subsequent clean- up. Some of these received significant radiation dose,
averaging about 100 mSv. 3 There are no recorded deaths due to these
exposures. Within 48 hours after the accident those living within 10 km of
the Chernobyl station, including the town of Pripyat, were evacuated. A week
later about 116,000 people living within a 30 km radius of the plant were
relocated. Over the following year a further 210,000 people were resettled
into less contaminated areas. Caesium 137 contaminated about 3,900 square
kilometers to a level exceeding 5 curies per square kilometer.
3 For comparison, in most of Canada each person receives 2 mSv each
year from natural sources.
(2) Later effects
Over the years there have been several international scientific studies of
the radiological and other consequences of the 1986 accident at Chernobyl
unit 4.
The first major study was the International Chernobyl Project, coordinated
by the International Atomic Energy Agency (IAEA). That study, which involved
over 200 experts from around the world (including Canada), assessed the
situation in the three Soviet republics of: Ukraine SSR, Byelorussia SSR
(now Belarus) and the Russian Soviet Federated Republic. (It did not examine
the people evacuated from near the plant nor the emergency workers.) The
report of the Project, entitled "An Assessment of the Radiological
Consequences and an Evaluation of Protective Measures" was presented at a
special meeting held in Vienna, Austria in May 1991.
Among the conclusions of that study were the following:
There were no health disorders that could be directly attributed to
radiation exposure. There was no indication of an increase in the incidence
of leukemia or cancers. The accident had substantial negative psychological
consequences in terms of anxiety and stress due to continuing and high
levels of uncertainty, relocation and other measures.
Further studies were conducted by the World Health Organisation (WHO) in
1995. These generally confirmed the earlier work but identified about 700
cases of thyroid cancer among children exposed as infants to the radioiodine
emitted by the accident.
The most recent authoritative study is included in the year 2000 report of
the United Nations Scientific Committee on the Effects of Atomic Radiation
(UNSCEAR). Again the broad observations of the earlier studies were
confirmed. However, at the time of the study (1999) about 1,800 cases of
thyroid cancer among young people had been identified, of which 10 were
fatal. The report states:
"Apart from this [thyroid cancer] increase, there is no evidence of a major
public health impact attributable to radiation exposure 14 years after the
accident. There is no scientific evidence of increases in overall cancer
incidence or mortality or in non- malignant disorders that could be related
to radiation exposure."
Various groups have made extravagant claims of the death toll of the
Chernobyl accident based on an inappropriate application of a hypothesis,
used for radiation protection purposes, that any amount of radiation is
potentially lethal. The UNSCEAR report makes it clear that such claims are
not supported by scientific assessments.
Chernobyl today
The destroyed Chernobyl 4 reactor is enclosed by the large concrete
"sarcophagus" which has contained the remains of the plant for 15 years. It
was built hastily after the accident and is now in need of repair or
reconstruction. Over $700 million (US) has been contributed by several
countries including Canada towards an International Shelter Implementation
Plan for that purpose. The remedial work is currently underway.
In March 2001 a $36 million (US) project was begun to construct a
radioactive waste management facility to treat and store waste from all of
the Chernobyl plants.
Because of the need for electrical power in the Ukraine, the other three
units at Chernobyl were re- started following the accident, after
improvements were made to their safety systems. Unit 2 was shut down in 1991
due to a fire in its turbine hall.
Unit 1 was shut down in 1997 and the last one, unit 3 was finally shut down
in December 2000.