Just in case you guys were interested (sorry about the formatting)...:
Chernobyl – Twenty Years On.
April 26 th 2006 marks the 20 th Anniversary of the worlds most significant Nuclear Event at
Chernobyl NPP in the Ukraine. More than any other, this event has shaped the way we operate
Nuclear Power Stations and the Nuclear Safety principles that support this operation. As a direct
result of the Chernobyl accident, WANO - the World Association of Nuclear
Operators, was formed
in 1989. One of the fundamental processes set up by WANO was the sharing of
Operating
Experience. This Event Brief has been prepared to remind everyone of the key points of this event
twenty years ago. It should be used to support discussions of the significance of this to our industry
today and our role in maintaining and influencing Nuclear Safety and a strong Safety Culture.
Chernobyl NPP
Chernobyl Nuclear Power Station is located about 60 miles north of Kiev in the Ukraine. Four
reactors were built at Chernobyl, with Unit No. 4 becoming operational in 1984.
Chernobyl was an RBMK type reactor, the
core of which was built up from graphite
blocks perforated by 1661 vertical channels,
each containing a pressure tube. In effect
the RBMK is a hybrid design of graphite
moderator and light water cooling. Unlike
AGRs and PWRs, the RBMK design does
not have a separate Primary Reactor
Coolant system and Secondary Steam side
system. Fuel is cooled by water passing
through the channels in the core, producing
a steam water mixture. Four steam drum
separators situated above the reactor
separate the steam and water. The dry steam is passed to one of two 500 MW (electrical) turbine
generators. The steam is then condensed and returned back to the steam drum separators, from
where it is pumped into the bottom of the reactor and then back through the core.
The Event
Operators were preparing to undertake a test to show that in the event of a loss of power, the kinetic
energy of the running down turbine generator would continue to supply power to the auxiliary
systems. This was the second time this test had been attempted.
Preparations for the test started on 25 th April with the reactor power being gradually reduced. As the
start of the test was nearing, a request to delay the turbine run down was received from the Grid due
to the demands for electricity at that time. The test was delayed for more than nine hours.
In the early hours of April 26th , permission was given to start the test. Due to Xenon build up and the
water/steam flow conditions in the core, the operators struggled to set the reactor power to the level
required for the test (between 700 and 1000 Megawatts thermal). Some automatic control systems
had been switched to manual and the Emergency Core Cooling System had been disconnected for
the test. Reactor power was stabilised at 200 MW. Despite the reactor power being well outside
the required test power range, with control rods largely withdrawn and the operators unable
to increase power any further, it was decided to continue with the test.
As the test progressed, the operators had to continually adjust the coolant flow rate through the
core, which in turn affected temperature and the production of steam. Due to the design of the
reactor, at this low power these changes in flow rate and water/steam levels also affected the core
reactivity, which initially began to fall (the water absorbs neutrons reducing reactivity). To keep the
reactor from tripping, some protection systems were bypassed and control rods were
removed further. Warning alarms were received that the control rods were outside the limits
allowed for operation. The test continued.
As the turbine ran down, the amount of cooling water being
provided to the reactor decreased and steam production
increased at a rapid rate. The reactivity and reactor power
suddenly began to increase. This resulted in more steam
being produced and hence more power. This situation
rapidly became uncontrollable and attempts were made to
shut the reactor down.
The control rods could not be inserted in time. Fuel
overheated and fuel channels began to rupture. A first
explosion, thought to be caused by steam pressure and
chemical reaction with the exposed fuel, blew the 1,000-tonne
sealing cap on the reactor off the core. This was
followed by a second explosion which threw burning fuel
and graphite from the core. Air rushed into the core causing
the graphite
moderator to burst into flames. A number of
other fires broke out across the plant. Most were
extinguished by that morning but the graphite continued to
burn for another nine days releasing large amounts of
activity to the environment.
Some of the Consequences
œ Thirty-one people died as an immediate result of the accident. Thousands more involved in the
emergency and recovery operations received high radiation doses following the event.
œ The effects were felt all over the world. Contamination was detected in almost every country in
the Northern Hemisphere including North America and Japan.
œ According to the World Health Organisation, a total of about 12 x10 18 bequerels of radioactivity
was released. This is 400 times more radioactivity than the atomic bomb dropped on Hiroshima.
œ The long-term health effects of the accident are not possible to predict precisely, but in the
‘Chernobyl Forum Report’ published in 2005, scientists said that the public health effects have
not been nearly as substantial as had been first estimated.
œ International efforts are still underway to stabilise the containment structure (Sarcophagus) built
around the destroyed Unit to contain radioactive material from the event.
Key Items to Consider
œ This event more than any other has shaped our industry, the way we operate Nuclear Power
Stations and the way the public perceives Nuclear Power.
œ What can you do to influence Nuclear Safety and demonstrate a strong Safety Culture?
œ What barriers are in place and tools available to prevent us from entering error likely situations?
œ How can we apply the lessons learnt from an event 20 years ago, to prevent events from
happening in the future?
Further information can be found in ‘Chernobyl Fact File’ published by NucNet, and INPO SER 34-86
which can both be found on the BE
Operating Experience Intranet page.
Information Sources: 1.‘Chernobyl Fact File’ published by NucNet, Feb 2006 2. BE
AGR Systems and
Operations training material