The accident at Chernobyl Unit 4, on 26 April 1986, did not occur during normal operation of the reactor. It happened during a test designed to assess the reactor's safety margin in a particular set of circumstances. The test, which had to be performed at less than full reactor power, was scheduled to coincide with a routine shut-down of the reactor. |
The Test
The sequence of events which follows has been compiled following a review of a large number of reports and it represents what we consider to be the most likely sequence of events. |
April 25: Prelude | |
01:06 |
The scheduled shutdown of the reactor started. Gradual lowering of the power level began . |
03:47 |
Lowering of reactor power halted at 1600 MW(t). |
14:00 |
The emergency core cooling system (ECCS) was isolated (part of the test procedure) to prevent it from interrupting the test later. The fact that the ECCS was isolated did not contribute to the accident; however, had it been available it might have reduced the impact slightly. The power was due to be lowered further; however, the controller of the electricity grid in Kiev requested the reactor operator to keep supplying electricity to enable demand to be met. Consequently, the reactor power level was maintained at 1600 MW(t) and the experiment was delayed. Without this delay, the test would have been conducted during `day shift'. |
23:10 |
Power reduction recommenced. |
24:00 |
Shift change. |
April 26: Preparation for the test | |
00:05 |
Power level had been decreased to 720 MW(t) and continued to be reduced. |
00:28 |
Power level was now 500 MW(t). |
00:32 |
In response, the operator retracted a number of control rods in an attempt to restore the power level. |
01:00 |
The reactor power had risen to 200 MW(t). |
01:03 |
An additional pump was switched into the left hand cooling circuit in order to increase the water flow to the core (part of the test procedure). |
01:07 |
An additional pump was switched into the right hand cooling circuit (part of the test procedure). |
01:15 |
Automatic trip systems to the steam separator were deactivated by the operator to permit continued operation of the reactor. |
01:18 |
Operator increased feed water flow in an attempt to address the problems in the cooling system. |
01:19 |
Some manual control rods withdrawn to increase power and raise the temperature and pressure in the steam separator. |
01:21:40 |
Feed water flow rate reduced to below normal by the operator to stabilise steam separator water level, decreasing heat removal from the core. |
01:22:10 |
Spontaneous generation of steam in the core began. |
01:22:45 |
Indications received by the operator, although abnormal, gave the appearance that the reactor was stable. |
The Test | |
01:23:04 |
Turbine feed valves closed to start turbine coasting. This was the beginning of the actual test. |
01:23:10 |
Automatic control rods withdrawn from the core. An approximately 10 second withdrawal was the normal response to compensate for a decrease in the reactivity following the closing of the turbine feed valves. |
01:23:21 |
Steam generation increased to a point where, owing to the reactor's positive void coefficient, a further increase of steam generation would lead to a rapid increase in power. |
01:23:35 |
Steam in the core begins to increase uncontrollably. |
01:23:40 |
The emergency button (AZ-5) was pressed by the operator. Control rods started to enter the core. |
01:23:44 |
Reactor power rose to a peak of about 100 times the design value. |
01:23:45 |
Fuel pellets started to shatter, reacting with the cooling water to produce a pulse of high pressure in the fuel channels. |
01:23:49 |
Fuel channels ruptured. |
01:24 |
Two explosions occurred. One was a steam explosion; the other resulted from the expansion of fuel vapor. |
The explosions lifted the pile cap, allowing the entry of air. The air reacted with the graphite moderator blocks to form carbon monoxide. This flammable gas ignited and a reactor fire resulted. Results
Chernobyl - Positive Void Coefficient
A reactor is said to have a positive void coefficent if excess steam voids lead to increased power generation, and a negative void coefficient if excess steam voids leads to a decrease in power. The coefficient is simply a measure of the speed of change of state of the reactor. |
These factors have reduced the positive void coefficient from +4.5Beta [Greek symbol] to +0.7Beta [Greek symbol], eliminating the possibility of power excursion. Beta [Greek symbol] is the delayed neutron fraction, which is neutrons emitted with a measurable time delay. The next consideration was to reduce the time taken to shut the reactor down and eliminate the positive void reactivity. Improvements include:
RBMK Modifications
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RBMK - Control Rod Redesign One of the post-accident changes to the RBMK was the redesign of the control rods. 179 of 211 control rods are inserted into the core from the top. To improve their effectiveness, they are equipped with "riders" fixed to their bottom end but with a gap between the rider and the bottom tip of the control rod. Approximately 1.0m water columns remained under and above it. When the control rod is in its uppermost position, the rider is in the control rod cooling tube within the fuelled region of the core. The rider being made substantially of graphite, is almost transparent to neutrons, while water, which would occupy the tube otherwise, plays as an absorber. When the reactor is "poisoned" with Xenon and with partially inserted control rods, the major part of the power is produced within the lower region of the core. This means that when the rod started to move down from its uppermost position, the rider removed water from the lower part, causing an increase in reactivity and hence in power. It was calculated that a surge of reactivity after the emergency shut down button had been pressed could reach +2Beta[Greek symbol]eff. To counter this problem the control rod design has been changed with the rider tie length increased to prevent water columns in the lower part of the core. RBMK light-water graphite reactor The Soviet designed RBMK is a pressurised water reactor with individual fuel channels and using ordinary water as its coolant and graphite as its moderator. It is very different from most other power reactor designs as it was intended and used for both plutonium and power production. The combination of graphite moderator and water coolant is found in no other power reactors. The design characteristics of the reactorwere shown, in the Chernobyl accident, to cause instability when low power. This was due primarily to control rod design and a positive void coefficient. A number of significant design changes have now been made to address these problems. |
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