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This case has been updated. Compatible choices are kept; revisit the evidence before drawing your conclusion.
01 · The briefing
Begin with the question.
On the night of 25 to 26 April 1986, the crew at Chernobyl Unit 4, a reactor in the Soviet Union, was preparing a test before a routine shutdown. The test asked how long the turbines would keep spinning, and supply power to the main circulating pumps, after a loss of the main electrical supply. The same test had been tried the previous year, but the turbine power ran down too rapidly, so new voltage regulator designs were to be tested.
The test was meant to run at a reactor power of 700 MW(th). The operators could not reach that level, and they changed the test conditions on the spot. Several explanations have been offered since. One points to operator error. Another points to the reactor's design, including its control rods and a tendency of the core to become more reactive as steam forms. A third combines procedural lapses with design faults.
This investigation uses the IAEA's 1992 INSAG-7 report, which revised an earlier account, and the World Nuclear Association's summary. It separates what the record shows about the test, the reactor's physics and the operators' decisions, and it says where the evidence is still debated.
The timeline
During physical start-up tests at Ignalina Unit 1 and Chernobyl Unit 4, the positive scram effect of control-rod insertion is found experimentally.
The crew begins preparing for a test before a routine shutdown of Unit 4.
At 00:28 the reactor power falls to about 30 MW(th) after control passes from local to global power control.
At 00:43:27 a turbogenerator trip signal is blocked. INSAG-7 corrects the earlier time of 01:23:04 given in INSAG-1.
By 01:00 the operating reactivity margin is below the required minimum, according to INSAG-7.
The test starts at about 200 MW(th), well below the 700 MW(th) in the procedure, and the reactor is destroyed in a power surge during the shutdown attempt.
The IAEA's INSAG-7 report revises the account given in INSAG-1.
A researched reconstruction for learning. These reasoning steps simplify a complex record; no score can capture the full history.
02 · The evidence
What does the record show?
Read each exhibit before settling on an explanation. The source links below every exhibit lead to the record behind it.
Exhibits reviewed 0 of 4
The test departed from its written conditions
INSAG-7 states that the test was started at about 200 MW(th), well below the 700 MW(th) in the test procedure. It says the procedure was altered on an ad hoc basis because the operators could not reach the intended power, and that reactor conditions left by earlier low-power operation made the reactor highly unstable when the test began. This establishes that the test departed from its written conditions. It does not by itself show which change made the reactor unstable.
Source record s1www-pub.iaea.org
The positive scram effect
INSAG-7 notes that the emergency protection rods took 18 seconds to insert from the upper limit stop switches, and that as a rod moved down from its upper stop, it displaced water from the lower part of the channel, inserting positive reactivity in the lower core. It says the effect was known to the design engineers before the accident. It also says the scram just before the sharp power rise may well have been the decisive contributory factor. That is a judgement about the most likely sequence, not a measured fact.
Source record s1www-pub.iaea.org
Revisions to the operator-error account
INSAG-7 revises several claims made in INSAG-1. It says the drop in power to 30 MW(th) at 00:28 was called operator error in the earlier account, but recent reports suggest there was no operator error as such. It gives the blocking of the turbogenerator trip as 00:43:27, not 01:23:04, and says the block followed operational and test procedures. It also states that the minimum operating reactivity margin was violated by 01:00. The review establishes what its authors concluded on these points; it does not establish every reason for each operator decision.
Source record s1www-pub.iaea.org
The reactor's positive void coefficient
WNA's summary describes the RBMK's positive void coefficient, in which more steam bubbles increase reactivity. It says that at the time of the accident the fuel burn-up, control-rod configuration and power level produced a positive void coefficient large enough to dominate the reactor's power coefficient. It also says the operators removed too many control rods, lowering the operating margin. WNA is a secondary summary, and it notes that the operators' actions could be read as a symptom of the safety culture of the time.
Source record s2world-nuclear.org
03 · Your reasoning
Follow the strongest explanation.
Choose an interpretation, then compare it with the evidence. You can revise your choice. The goal is understanding, not a perfect score.
04 · The findings
The chain behind the outcome.
Your notebook
Case report
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The best-supported account has three parts. First, the crew started the test at about 200 MW(th), well below the 700 MW(th) in the procedure, and changed the conditions on the spot because the reactor would not reach the intended power. INSAG-7 says the reactor's condition after earlier low-power running made it highly unstable when the test began. Second, the reactor's design allowed a positive void coefficient, which WNA describes as dominant at that time, and the control-rod design gave a positive scram effect, in which inserting the rods raised power in the lower core. INSAG-7 says the scram just before the sharp power rise may well have been the decisive contributory factor, and it says most analyses link the severity of the accident to design defects. Third, the operators removed too many control rods, which lowered the operating reactivity margin, and blocked or disabled some protective functions. INSAG-7 says the blocking of one trip followed the procedures and does not support blaming operators for it. The 1992 review lessened the weight the earlier account gave to operator actions, but it still treats the human factor as a major element and calls some critical operator actions most ill judged. The sources agree that design and operator actions both contributed; how much weight each deserves is the main point of dispute.
Contributing causes and conditions
- The reactor was run at very low power after earlier half-power operation, leaving conditions that made it unstable and prevented the intended test power from being reached.
- Control rods were withdrawn far enough to push the operating reactivity margin below the required minimum.
- A positive void coefficient meant that the formation of steam increased the core's power.
- Descending scram rods displaced water from the lower channels and raised power in the lower core, and full insertion took 18 seconds
- Protective trips were blocked, and the reactor was brought to a state that the procedures had not specified.
OutcomeChernobyl accident (1986)
Take the method with you
- When a process is pushed outside its written conditions, check whether the procedure, the equipment or the people changed first.
- Distinguish a physical mechanism that the evidence supports from a judgement about how much it mattered.
- Be cautious with an early official account once a later review has revised its key facts.
You can read findings at any time. Exploring every reasoning step marks this case as complete in your notebook.
The open record
Sources & further reading
Read the original record and its scope before relying on a conclusion. Source references support the exhibit summaries; Faultory’s reasoning prompts are editorial reconstructions.
- s1
- s2World Nuclear Association — Chernobyl accident
world-nuclear.org
Written and curated for Faultory by Yunus Emre Vurgun, with AI-assisted research and drafting. Sources and interpretations can need correction. Read our editorial standards or report an issue.
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