CHEMOBYL NUCLEAR DISASTER
After Bhopal disaster, the next remarkable disaster took
place at Chemobyl Nuclear Power Station in the Soviet Union on 26-4-1986 and the losses reported are as under.
The accident led to 31 deaths and 203 have
suffered various degrees of radiation sickness. 14 were in hospital with 80 to
90 per cent burns due to radiation. The 1,35,0100 people around Chemobyl who
were evacuated to safer sites would be monitored for radiation effects. Among this group there will probably be 280
more cancer deaths than normal over the next 70 years. The Soviet scientists
analysed a total population of 74.5 millions in II different regions, the most
heavily populated being the urban areas of southern and eastern Ukraine for long-term impact of the radiation
release. About 5,000 fatal cancer cases
are likely from the release. This must be looked at in comparison with some
9,500,000 cancer cases anticipated from all other causes over the 50- year
period in the same population. And an
additional 1,500 deaths are predicted from thyroid cancer in people consuming
contaminated milk and other foodstuffs.
The
Causes of the Accident :
A detailed report is published in Front-line of October
4-17, 1986 of which a
short abstract describes the causes as under.
The operating personnel at Unit 4 of the Chernobyl nuclear power station in north-western Ukraine 'had virtually invited the disaster. In a series
of unauthorised and impermissible actions, the plant personnel, while trying to
perform a rather banal experiment, tinkered dangerously with the reactor,
triggering the calamity. The accident spread radioactivity and panic all over Europe and revived the worst fears about nuclear energy.
The Chernobyl reactor was an advanced system with a number of
computerised monitoring and control mechanisms. These provided for the regular
monitoring of the various parameters of the reactor, including temperature,
pressure and water flow rate. There were batteries and diesel generators for
back up power in case the main source failed. A system of light and sound
alarms in the control room would alert the plant operators to danger. Plant conditions were printed out
automatically. There were a variety of mechanisms, including an emergency core
cooling system, to bring the reactor under control in case it showed signs of
instability. .
The cause of the tragedy was not the absence of
safety systems. The accident was due to the fact that most of these protective
systems were turned off by the plant operators as they conducted a fatal
experiment on April 25-26,1986.
The unit had an electrical capacity of 1000 MW
and a thermal power rating of 3200 MW.
The experiment was to be conducted at 25% power. Therefore the turbo
generator No. 7 and the emergency core cooling system (ECCS) were switched off.
The aim was to prevent a spurious triggering of the ECCS during the turbine test
but it eliminated one key safety device. However the test was postponed till
late in the night and the power reduction was resumed at 11 am. on April 25, in preparation for the test.
But the operators found it difficult to maintain
the reactor at 700-1000 MW thermal power, the optimum level for the proposed
experiment, because of the error in switching off the local automatic control
of the reactor, the power level fell to I percent. By I am. on April 26 they
managed to stabilise it at around 6 percent (200 MW thermal). At the same time
they
After sustained operation of a reactor, xenon gas
builds up, absorbing neutrons needed in the chain reaction and slowing down the
rate of fission.
To counteract the xenon poisoning, the operators
pulled out control rods from the core, leaving only six to eight in place instead of the stipulated minimum of
30. This perhaps was the major blunder which set the stage for the reactor
running out of control.
The operators then began to introduce further
disturbances in the reactor. They engaged all the eight pumps instead of the
normal six, with two as reserve, to cool the reactor. This dramatically increased the water flow in
the reactor. More water meant less steam and the water-steam balance in the
reactor core was upset and steam pressure in the drums where steam is collected
began to fall. This would have in the
normal course led to .an automatic shut-down of the reactor, but the staff
blocked the emergency protection signals from the steam drums. Thus one more
safety device was put out in an effort to press on with the tests.
By 1:22:30 am. on April 26, the state of the reactor called for a
shut-down and cancellation of the experiment.
But the test began. At 1:28:04 turbo generator No. 8 was cut off from the steam.
A minute earlier the operator had abruptly reduced the flow of make-up water,
thus increasing the water temperature in the reactor. The switching off of both the turbo
generators would have triggered an automatic shutdown but the operators blocked
the reactor protection system, relying on shut-down signal from two turbo
generators. This error was committed
with the intention of repeating the experiment a second time.
Meanwhile as the turbine ran down, the water flow
in the reactor began to fall sharply since four of the eight cooling pumps were
running only on the residual energy of
the turbine. Because of the water steam imbalance and the positive void
coefficient the power of the reactor began to surge rapidly. Within 56 seconds,
the shift manager realised that something had terribly gone wrong, and at 1:23:40 am. called for a complete shut-down of the reactor,
and pressed the scram button to let the control rods down into the core, but
unfortunately the rods did not reach the bottom, and an operator cut their
controls hoping they would fall further on their own. But instead they got stuck, probably due to
the thermal distortion already set in 'at the core as a result of the high
temperature. The effort to shut down the
reactor was too late. Within three
seconds, power output surged from 200 MW (thermal) to more than 530 MW
(thermal). By l;24 there was a huge
explosion, followed by a second one., The Soviet investigators theorised that
the initial blast that blew the reactor structure could have been caused by the
steam explosion. As the power output surged there was intense steam formation
around the fuel which in turn reduced the ability to remove heat from the fuel
elements and the resulting explosion wrecked the building.
The second explosion could have due to the
formation of Hydrogen. The steam in the
reactor would have reacted with the zirconium enclosing the fuel elements and
the graphite moderator of the reactor, generating large quantities of Hydrogen,
which exploded when it mixed with air. The two explosions sent hot radioactive
fragments high into the atmosphere. The debris from the explosions eventually
set off about 30 fires in the plant.
Thus the issue posed by Chernobyl is the fallibility of modern technology. The disaster at Chernobyl and that of the US
Challenger raise a number of questions about man-machine interface, the
complexity in the management of modern technological systems,, the pressures of
their routine operation, the strains on large bureaucratic organisations involved
in running advanced technological systems, the external economic and political
pressures on them and the like.
Preventive
Measures:
The preventive measures seem to be (1) The
written and approved procedure to carry out any new experiment (2) No isolation
or bypassing of safety devices .(3) Presence of the most experienced senior
officers in the plant instead of leaving it in the hands of operators (4) HAZOP
and Consequence Analysis of new experiment on such radiation system and (5)
Following safe close down procedure in case of any abnormality.
No comments:
Post a Comment