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The accident
Why this disaster happened?
(source : canteach.candu.org)
The irony of the history is that the accident started with a test designed to improve the safety of the reactor and ended with the worst nuclear disaster that we have known.
The purpose of this test was to know how the power plant will react in the event of a power outage. A power outage in a power plant may seem paradoxical but, in the worst case, it could happen. So, the Soviets thought use the kinetic energy of the turbine and alternator to generate power during the few seconds needed to the startup of diesel generators. These generators get started in about 30 seconds, but during those 30 seconds the pumps must be working so that the core remains covered with water. So, it was necessary to know whether the kinetic energy of the turbine would be sufficient to supply the main pumps.
The same test was done previously on the reactor No. 3 with no effect on the operation of the reactor, but the voltage had dropped too quickly. It was therefore necessary to redo the test on the reactor No. 4 after modifying the electrical installation.
The principle of the test was to reduce the power of the reactor below 50%. At this power, a single turbine was sufficient to absorb the steam produced. Then, the arrival of steam would be disconnected from the turbine, and inertial energy from the turbine would be used for the operation of pumps for a short time.
Other parameters, human, are involved in the triggering of the disaster:
How the accident took place
The events have, in fact, begun 24 hours earlier and have ended by the explosion of the reactor on April 26.
Below, a summary of the operations that will be detailed later:
April 25 - 01h00mn:
The reactor is operating at full power. The power reduction begins. This is a normal situation.
April 25 - 13h05mn:
The reactor power reaches 50%. A turbine is stopped and all the steam is sent to the remaining turbine. We are always in a planned test.
April 25 - 14h00mn:
The reactor continues to operate at 50% of its power for more than 9 hours because of an unexpected demand for energy.
April 25 - 23h10mn:
The permission to continue the test is given.
April 26 - 00h28mn:
By resuming the power reduction, the operator makes an error that causes the power to drop at 1%, almost a shutdown of the reactor. This causes the filling the core with water and also produces an emanation of xenon (a neutron absorber) making impossible to reach the power requirements for the test.
April 26 - 01h00mn à 01h20mn:
The operator manages to raise the power at 7%. He tries to control the reactor, causing sudden changes in flow and temperature.
April 26 - 01h20mn:
The operator turns off the reactor automatic shutdown, the security of water low level and the security of loss of the two turbines. The technician feared that a stop should abort the test. A repetition of the test was planned, he wanted to keep the reactor ready in such an eventuality.
April 26 - 01h23mn04s:
The technician stops the remaining turbine to start the test.
April 26 - 01h23mn40s:
The power begins to increase. The reduction in the water cooling flow due to the slow down of the pumps cause a gradual increase of the vapor rate in the core, which leads to increased power.
The operator presses the emergency stop button. But due to the design of the control rods and stopping rods, it is exactly the opposite which occurs. The power rises sharply instead of falling.
April 26 - 01h23mn44s:
The power of the reactor reach 100 times its rated power, the fuel disintegrates and the excessive steam pressure breaks the pressure tubes. The pressure in the reactor core blows the 1,000 tons of the upper slab , breaking the rest of the pressure tubes attached thereto.