"cavelamb himself" wrote in message
...
Test plan

During the daytime of April 25 1986, reactor 4 at [show location on an
interactive map] 51°23'22.39?N, 30°05'56.93?E was scheduled to be shut
down for maintenance. A decision was made to test the ability of the
reactor's turbine generator to generate sufficient electricity to power
the reactor's safety systems (in particular, the water pumps), in the
event of a loss of external electric power. A RBMK-1000 reactor requires
water to be continuously circulated through the core, as long as the
nuclear fuel is present.

Chernobyl's reactors had a pair of backup diesel generators, but because
there was a 40-second delay before they could attain full speed, the
reactor was going to be used to spin up the reactor's turbine generator.
Once at full speed, the turbine would be disconnected from the reactor and
allowed to spin under its own rotational momentum. The aim of the test was
to determine whether the turbines in the rundown phase could power the
pumps while the generators were starting up. The test was previously
successfully carried out on another unit (with all safety provisions
active) with negative results - the turbines did not generate sufficient
power, but because additional improvements were made to reactor four's
turbines, there was a need for another test.

Conditions prior to the accident

As conditions to run this test were prepared during the daytime of April
25, and the reactor electricity output had been gradually reduced to 50%,
a regional power station unexpectedly went offline. The Kiev grid
controller requested that the further reduction of output be postponed, as
electricity was needed to satisfy the evening peak demand. The plant
director agreed and postponed the test to comply. The ill-advised safety
test was then left to be run by the night shift of the plant, a skeleton
crew who would be working Reactor 4 that night and the early part of the
next morning. This reactor crew had had little or no experience in nuclear
power plants, many had been drafted in from coal powered plants and
another had had a little experience with nuclear submarine power
plants.[4]

At 11:00 p.m., April 25, the grid controller allowed the reactor shut-down
to continue. The power output of reactor 4 was to be reduced from its
nominal 3.2 GW thermal to 0.7-1.0 GW thermal in order to conduct the test
at the prescribed lower level of power.[5] However, the new crew were
unaware of the prior postponement of the reactor slowdown, and followed
the original test protocol, which meant that the power level was decreased
too rapidly. In this situation, the reactor produced more of the nuclear
poison product xenon-135 (the xenon production rate:xenon loss rate ratio
initially goes higher during a reactor power down), which dropped the
power output to 30 MW thermal (approximately 5% of what was expected). The
operators believed that the rapid fall in output was due to a malfunction
in one of the automatic power regulators, not because of reactor
poisoning. In order to increase the reactivity of the underpowered reactor
(caused unknowingly by neutron absorption of excess xenon-135), automatic
control rods were pulled out of the reactor beyond what is allowed under
safety regulations.

Despite this breach, the reactor's power only increased to 200MW, still
less than a third of the minimum required for the experiment. Despite
this, the crew's management chose to continue the experiment. As part of
the experiment, at 1:05 a.m. on April 26 the water pumps that were to be
driven by the turbine generator were turned on; increasing the water flow
beyond what is specified by safety regulations. The water flow increased
at 1:19 a.m. - since water also absorbs neutrons, this further increase in
the water flow necessitated the removal of the manual control rods,
producing a very precarious operating situation where coolant and
xenon-135 was substituting some of the role of the control rods of the
reactor.

Fatal experiment

At 1:23:04 the experiment began. The unstable state of the reactor was not
reflected in any way on the control panel, and it did not appear that
anyone in the reactor crew was fully aware of any danger. The steam to the
turbines was shut off and, as the momentum of the turbine generator drove
the water pumps, the water flow rate decreased, decreasing the absorption
of neutrons by the coolant. The turbine was disconnected from the reactor,
increasing the level of steam in the reactor core. As the coolant heated,
pockets of steam formed voids in the coolant lines. Due to the RBMK
reactor-type's large positive void coefficient, the steam bubbles
increased the power of the reactor rapidly, and the reactor operation
became progressively less stable and more dangerous. As the reaction
continued, the excess xenon-135 was burnt up, increasing the number of
neutrons available for fission. The prior removal of manual and automatic
control rods had no substitute, leading to a runaway reaction.

At 1:23:40 the operators pressed the AZ-5 ("Rapid Emergency Defense 5")
button that ordered a "SCRAM" - a shutdown of the reactor, fully inserting
all control rods, including the manual control rods that had been
incautiously withdrawn earlier. It is unclear whether it was done as an
emergency measure, or simply as a routine method of shutting down the
reactor upon the completion of an experiment (the reactor was scheduled to
be shut down for routine maintenance). It is usually suggested that the
SCRAM was ordered as a response to the unexpected rapid power increase. On
the other hand, Anatoly Dyatlov, deputy chief engineer at the nuclear
station at the time of the accident, writes in his book:

Prior to 01:23:40, systems of centralized control . didn't register
any parameter changes that could justify the SCRAM. Commission . gathered
and analyzed large amount of materials and, as stated in its report,
failed to determine the reason why the SCRAM was ordered. There was no
need to look for the reason. The reactor was simply being shut down upon
the completion of the experiment.[6]

The slow speed of the control rod insertion mechanism (18-20 seconds to
complete), and the flawed rod design which initially reduces the amount of
coolant present, meant that the SCRAM actually increased the reaction
rate. At this point an energy spike occurred and some of the fuel rods
began to fracture, placing fragments of the fuel rods in line with the
control rod columns. The rods became stuck after being inserted only
one-third of the way, and were therefore unable to stop the reaction. At
this point nothing could be done to stop the disaster. By 1:23:47 the
reactor jumped to around 30 GW, ten times the normal operational output.
The fuel rods began to melt and the steam pressure rapidly increased,
causing a large steam explosion. Generated steam traveled vertically along
the rod channels in the reactor, displacing and destroying the reactor
lid, rupturing the coolant tubes and then blowing a hole in the roof.[7]
After part of the roof blew off, the inrush of oxygen, combined with the
extremely high temperature of the reactor fuel and graphite moderator,
sparked a graphite fire. This fire greatly contributed to the spread of
radioactive material and the contamination of outlying areas.

http://en.wikipedia.org/wiki/Chernob...r#The_accident


History channel had a program about the accident in which some of the
workers
that survived the accident gave thier account of what happened. Thank god we
don't opperate our plants like the russians. As i recall from the program
the person
in charge had no training in the nuclear field at all and got his job thru
political
connections. Everybody interviewed claimed his lack knoweledge was the major
cause of the situation they found themselves in. Its worth watching if they
air the
program in the future. Very detailed sequence of events and why they
happened.

Best Regards
Tom.