Chernobyl: Anatomy of a Disaster Quiz

Answer: Emergency power supply

The test was designed to measure the reactor's capability to survive a critical one-minute period after an emergency shutdown: with the reactor itself shut down it was no longer producing power but the emergency diesel generators needed time to spin up. To bridge this time, the rotational energy of the still spinning main turbine was to be used - a difficult proposition as the decreasing speed would result in decreasing voltages and make maintaining a constant voltage for the reactor systems over this minute very difficult.

The reactor had failed this test before and after improvements to the electrical systems, it was to be repeated that day.

Answer: Control was transferred to the night shift

The experiment was originally to be conducted by the highly experienced day shift who had also specifically prepared for the situation. However the delay in shutdown time meant that the night shift - consisting of relatively inexperienced staff since it was envisioned they would only have to watch and control a shut-down reactor through its cooling period - had taken over and was now responsible for executing a difficult, undocumented procedure requiring the deactivation of several important safety systems.

Answer: It continued to decrease without operator command

One of the nuclear fission products produced in Chernobyl-type reactors is Xenon-135, a short-lived isotope that however tends to absorb neutrons and thus decrease the rate of nuclear reactions. Due to the lengthy half-power state, the concentration of this isotope was higher than normal and thus the setting that should have produced 700 MW of power resulted in under 500 MW, which in turn accelerated the Xenon production.

Answer: He inserted all control rods, bringing the reactor to a near shutdown

A natural reaction of any reactor operator who notices an unexplained and uncommanded behavior is to immediately reduce reactivity and thus danger by inserting control rods. Without the still pending test, this would have been a wise reaction - it might have caused some extra costs and trouble during restart of the reactor, but it would have brought it to a safe state.

However with the test still pending and not being ordered abandoned, the operators needed more power, not less. The action was quickly reverted, but the four minutes of extremely low power output had vastly increased the Xenon-135 concentration.

The actual reason for the operator's action - panic, a mistake or a planned action - never became known: he died in the reactor's explosion.

Answer: A voltage instability in the main generator

The reason for the ultimate failure of the reactor was founded in three main factors: The control rods were extracted beyond the allowable maximum, reducing their emergency effectiveness, the Xenon (which would quickly be destroyed by radiation if the power level were to rise) was the main contributor to lowering the power level and worst of all, any increase in heat with sudden boiling of coolant would also massively increase the reaction rate.

Instead of a stable state, the reactor was at a point where any increase in power would thus quickly accelerate itself with the crew's ability to stop this increase greatly reduced.

The scene was set for the disaster to unfold - the only thing that could have prevented it at this point would have been an immediate and complete shutdown.

Answer: It concluded successfully

In the ultimate irony, the experiment that was to destroy the reactor actually finished as a complete success and the desired proof of concept was delivered. While voltages dropped during the spin-down, they never reached values that would have been critical with a stable reactor core and the diesel generators picked up the load in time. 39 seconds after it had begun, the experiment was registered as a success.

However the damage was done - the slightly reduced amount of coolant led to a higher temperature and the formation of steam voids which in turn increased the reactor output. Automatic control rods were returned to the reactor core to stabilize the output.

Answer: About 20 seconds

Most Western reactors have a gravity-actuated SCRAM mechanism. Control rods are suspended over vertical channels and when needed, they are simply released to freefall into the core without any further technology required to work. The Chernobyl reactor however used a motor-driven SCRAM with an insertion rate of about 40 centimeters per second - or almost 20 seconds to traverse the complete height of the reactor core.

Answer: The control rods

The tip of each neutron-absorbing boron carbide control rod in the reactor was made of graphite, a moderator (which is a substance that increases nuclear reaction). This was intentionally done because otherwise reactions around the tip would have been relatively hard to control. Normally, with only a small number of control rods moving at the same time (normal reactor control is achieved by primarily varying the number of rods pulled and inserted, not the distance so that reaction rates would be the same all across the core); this never caused a problem but during the SCRAM procedure, all of them were moving and almost at the same height, causing a horizontal layer of the reactor core to spike in reactivity and overheat. Fuel elements melted and burst and the rods became stuck about one third the way in. With no way to reduce power, steam quickly forming (and increasing power) and Xenon rapidly being converted to other elements (again increasing power), the reactor was now uncontrollable and headed towards the inevitable explosion.

Answer: The sudden generation of superheated steam

The contents of a sealed reactor core are mostly chemically stable even at the highest temperatures and even at maximum reactivity and with no cooling, the distance between the uranium fuel rods is too high to sustain a full nuclear explosion. The cause of the explosion was simply the sudden and uncontrolled boiling off of the entire water supply within the core. With steam taking 1,000 times as much volume as water even at 100°C and much more in superheated state, the already weakened pipes and reactor containment could not resist the intense pressure and burst, exposing the complete reactor core to the outside air and propelling highly radioactive material out of the building.

Answer: Graphite

Not even knowing what they got themselves into, the firemen of Pripyat extinguished the worst remote fires, in particular those that threatened the destruction of the adjacent reactor block #3. Most of them paid the ultimate price for their heroic actions, succumbing to radiation sickness within weeks of the disaster.

However, the high amounts of graphite in the remaining reactor core - pure carbon - continued to burn unmitigated for several weeks and with the heated combustion products, volatile radioactive substances were carried high into the air, polluting wide swathes of Ukraine, Belarus and, to a lesser extent, Central and Western Europe.

Many small factors had come together and several technical flaws, mostly known to but ignored by the creators and operators of the reactor type, combined to cause the worst nuclear disaster of the 20th century - a scale of nuclear pollution that dwarfed even that of 2011's Fukushima disaster by a full order of magnitude.