The accident
Why this disaster happened?
(source : canteach.candu.org)
The irony of 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 seen.
The purpose of this test was to know how the power plant will react in the event of a power outage. A power outage at a power plant may seem paradoxical, but in the worst case, it could happen. So, the Soviets thought using the kinetic energy of the turbine and alternator to generate power for a few seconds at startup of diesel generators. These generators start in about 30 seconds, but during those 30 seconds we must keep the pumps 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, those people are involved in the triggering of the disaster:
- The reactor No. 4 at Chernobyl was the one that worked best of all RBMK 1000 reactors. This corporate image has made operators too confident.
- The test was scheduled before a complete shutdown of the reactor for maintenance. If it fails, they would have to wait a year before they can start again. This fact maintained high pressure on technicians.
- This test, which had already been done before, was seen by technicians as a simple electrical test. They probably did not give enough attention to possible effects on the reactor.
How the accident took place
The events have, in fact, begun 24 hours earlier and ends 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. Reducing power begins. This is a normal situation.
April 25 - 13h05mn:
The power reactor reaches 50%. A turbine is stopped and all the steam is sent to the turbine remaining. We are always in a planned test.
April 25 - 14h00mn:
The reactor continues to operate at 50% of its power for 10 hours because of an unexpected demand for energy.
April 25 - 23h10mn:
The permission to continue the test is given.
April 26 - 00h28mn:
Resuming the decrease in power, the operator makes an error that will reduce power at 1%, almost a shutdown of the reactor. This causes the core filled with water and also produces xenon (a neutron absorber) making it impossible to reach the power requirements for the test.
April 26 - 01h00mn 01h20mn:
The operator manages to raise 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 safety of low water levels 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 and he wanted to keep the engine working 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 flow and the power supply pumps causes a gradual increase in the vapor in the core, which leads to increased power. The operator presses the emergency stop button. But because of 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 reached 100 times its rated power, fuel breaks and excessive pressure steam breaks pressure tubes. The pressure in the reactor core blows the 1,000 tons shielding, breaking the rest of pressure tubes attached thereto.