The entire scenario surrounding the events which took place on April
26, 1986 at the Chernobyl Nuclear Power Plant were the result of a safety
experiment gone awry. All power reactors produce significant amounts of
heat even when not in use ( due to the residual radiation with the materials
surrounding the fuel). As a result, when a reactor is in its shutdown
state, there is still a need to employ cooling systems to remove this heat
to keep the fuel from melting. Normally, the electric power necessary to
keep these systems running is drawn from the offsite power grid. However,
if there is a loss of offsite power with a shutdown, it is customary to
employ large diesel generators to supply the needed electricity to keep
the required emergency core cooling systems (ECCS) in operation.
In the United States, the time required to start one of these diesel
generators is on the order of 6 seconds. At Chernobyl, this time was on
the order of 3 minutes. In an effort to supply the necessary power to run
the safety systems in the 3 minute window between reactor shutdown and
diesel generator start, Chernobyl engineers thought that useful electric
power could be extracted from the turbine while it was in the process of
coasting to a halt. It was during a safety experiment designed to test
this theory that events were allowed to unfold which would ultimately lead
to the Chernobyl accident. It is the purpose of this section to outline,
in as simple language as is possible, the events which unfolded on April
Chernobyl Time Line
||Plant Power decreasing, signifying the start of the reactor shutdown.
||Emergency Core Cooling System (ECCS) is disconnected.|
||Power reduction resumed.
- Automated regulation system is shut off.
- Power decreased to 30 Mwt.
||Increased power to 200 Mwt by withdrawing rods.
- Operating Reactivity Margin (ORM) decreases to below 30 rods.
- No station managers approval for operation with less than 30 rods.
||Two additional recirculation pumps started - all 8 pumps running.|
||Increased feedwater flow to steam drums.
- Decreased void.
- All but 6 rods withdrawn.
- Emergency Protection Signals blocked by operators.
||Feedwater flow to steam drums decreased to very low value - 30 secs later
reactor inlet temperature begins to rise.
- Control room printout of core reactivity showed the excess reactivity
- required immediate shutdown - warning ignored and test initiated.
||Turbine feed valves closed.|
- Recirculation pumps begin to coastdown with the turbine.
- Reactor flow decreases.
||Emergency Scram initiated by button AZ-5. Button pushed because:|
- Power went up.
- Test was over.
- Operator saw rods moving on core map.
||Power increasing rapidly due to positive void coefficient.|
- Doppler decreases reactivity addition.
- Fuel fragments, causes rapid voiding.
- Power and steam drum pressure increase rapidly.
||Explosion occurs followed by a second explosion a few seconds later.|
The Chernobyl accident reminds us of the continuing importance of safe
design in both concept and implementation; of operational controls, of
competence and motivation of plant management and operating staff to operate
in strict compliance with controls; and of backup features of defense
against potential accidents.
Although a large nuclear power-plant accident somewhere in the United States
is unlikely because of design and operational features, we cannot relax
the care and vigilance that have made it so.
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