How Long Can Nuclear Reactors Run Without Human Interference?

Nuclear reactors can run for long periods of time without human interference, but they are not designed to do so. In the event of a natural disaster or a power outage, backup systems would kick in to keep the reactor stable until the fuel runs dry. However, if the backup power is also compromised, the reactor would eventually heat up and either explode or melt through the reactor chamber.

“The unleashed power of the atom has changed everything save our modes of thinking, and we thus drift toward unparalleled catastrophes.”-Albert Einstein

On April 26, 1986, the world experienced what is still its worst civil nuclear disaster: the Chernobyl accident in what is now Ukraine. (Smaller-scale accidents had happened earlier—Three Mile Island in 1979, Windscale in 1957, Kyshtym in 1957—but Chernobyl was on a different scale entirely.) So bad was the disaster that the half-million Soviet soldiers were dispatched to help contain the radiation. Chernobyl, to its credit, was not a small nuclear power plant. It was one of the largest nuclear power plants of its time and had the best engineers working to maintain it. Even with engineers actively running the plant, Chernobyl’s reactor 4 went out of control and exploded within seconds of the failed safety test. How long would a nuclear reactor function properly before it failed if humans were removed from the equation?

Working Of A Nuclear Power Plant

In our explanation, we will only be considering the working of a thermal nuclear reactor. Thermal nuclear reactors can broadly be classified into three main categories. The Containment Structure houses the main nuclear reactor and the steam generator. It is a building that is always made from reinforced concrete to ensure that radiation is always contained within the structure.

The Nuclear Reactor is where nuclear fission takes place. Nuclear fission is a nuclear process (not a chemical one)—the nuclei of heavy atoms such as uranium-235 split apart when struck by a neutron, releasing huge amounts of heat and more neutrons that sustain the chain reaction. This heat is transferred to water, which circulates within the reactor and carries heat to the steam generator. Even though the water in the reactor reaches well over 300°C, it does not boil. This is because it is held under very high pressure—about 155–160 bar, roughly 155 times atmospheric pressure—which keeps it as a liquid instead of letting it flash into steam.

The Steam Generator is responsible for taking away incoming heat from the nuclear reactor. The steam generator also contains water, but it must be remembered that water from the reactor and the water present within the steam generator never mix. This is because water from the nuclear reactor is radioactive and never leaves the nuclear reactor. Water in the steam generator is converted to steam, which is carried to the generators.

The Electrical Generators are responsible for the generation of electricity. This is possible when steam is brought in from the steam generator. Steam entering the generator comes in at very high speeds. This helps move the turbine of the generator, which converts the mechanical energy from the turbine into electricity. Once the steam passes through the turbine of the generator, it is sent to a condenser.

A Condenser is usually a system of metallic pipes that come into contact with the steam exiting a generator. Its duty is to cool the steam down, so it shifts back into the water; the cooled down water is then sent back to the steam generator. After cooling down the steam, the water in the condenser carries away the heat to the cooling tower. A Cooling Tower handles the cooling of the incoming water from the condenser. This is usually done with the help of large mechanical fans present within the tower. However, even with the fans, a certain amount of water will evaporate over time. Another property of the cooling tower is to provide a constant fresh supply of cool water, when needed, from the reservoir.

How Long Does A Nuclear Power Plant Actually Last?

Before we picture a plant with nobody minding it, it helps to know how long one is meant to run with people in charge. That is a different question from the one in our title, and it is the thing most people are really asking when they type "how long does a nuclear power plant last" into a search bar.

In the United States, the Nuclear Regulatory Commission (NRC) issues the first operating licence for a commercial reactor for up to 40 years. That figure was never a verdict on how long the hardware would physically survive; it was a financial and administrative number written into the original Atomic Energy Act. As that first licence runs out, an operator can apply for a licence renewal that adds another 20 years, taking the plant to 60 years of operation. Almost every operating reactor in the US has now done exactly that.

The story does not even stop at 60. The NRC also reviews applications for a subsequent licence renewal, a further 20-year extension that can keep a reactor running for up to 80 years. So a well-maintained plant can legally generate power for the better part of a century. When a reactor is finally retired, the deciding factor is usually money or aging components rather than a sudden safety cliff. Oregon's Trojan plant, pictured above, is a good example: it shut down after roughly 16 years not because it was unsafe to operate, but because cracks in its steam-generator tubing made continued repairs uneconomic. Once retired, a plant is decommissioned, a careful, decades-long process of removing the fuel and dealing with the radioactive waste left behind. That is also why the question of an unmanned reactor is so different: an attended plant can run for 80 years, but cut the power and the people, and the clock shrinks to days.

Learning From Man-Made Errors And Natural Disasters

To understand the magnitude of the disastrous effects of an unmanned nuclear reactor, let’s take a look back at history. Let’s take a look at the two biggest accidents of all time – Chernobyl and Fukushima.

The Chernobyl Disaster spanned two days from April 25-26 in 1986. It is considered the biggest nuclear disaster of the former Soviet Union. It occurred in the small town of Pripyat, in modern-day Ukraine.  The disaster occurred for two main reasons. The first one is that the higher officials had directed the engineers to switch off the safety systems before investigating a power blackout late one night. The second cause is that the reactor core had design flaws, and was arranged in a position, not in line with the safety checklist provided to the engineers. These two factors combined together led to uncontrolled nuclear fission, which resulted in the reactor’s core heating up and causing a catastrophic blast.

The blast and its effects were so deadly that the city of Pripyat was immediately evacuated. Irradiated dust from the blast spread as far as Sweden. To combat the effects of the radiation, the Soviet Union mobilised some 500,000 “liquidators” (military and civilian) to build the original concrete-and-steel “Sarcophagus”—a hastily built shelter with walls roughly 3–4 metres thick at its base—to contain the radiation. (A larger New Safe Confinement arch was slid over the failing sarcophagus in 2016.) If you think Chernobyl was bad, however, the Fukushima Disaster has actually been given a higher disaster impact rating by the World Nuclear Association.

The Fukushima Daiichi Disaster is the full name for the disaster that occurred on the east coast of northern Japan, in the Fukushima prefecture. The accident began on March 11, 2011. A magnitude 9.0–9.1 undersea earthquake off the Pacific coast of Tōhoku—the most powerful earthquake ever recorded in Japan—triggered an enormous tsunami. The earthquake itself caused the external power lines to the plant to fail, but the reactor cores at Fukushima Daiichi automatically shut down (SCRAMmed) as designed, and backup diesel generators kicked in to keep cooling water circulating.

About 50 minutes after the earthquake, however, a tsunami roughly 13–14 metres high arrived—far higher than the plant’s 5.7-metre seawall was designed for—and overtopped the defences. Sea water flooded the basement rooms housing the backup diesel generators, knocking out almost all emergency power. With no way to remove decay heat, three of the six reactor cores melted down over the following days. Molten fuel and reactor materials (“corium”) breached the bottom of the pressure vessels and pooled in the primary containments at units 1, 2, and 3.

Can You Just Shut A Nuclear Reactor Down, And How Long Does It Take?

A lot of readers arrive here imagining that a reactor is impossible to switch off, like a fire you cannot put out. The truth is more reassuring. Stopping the chain reaction is fast and reliable; the catch is the heat that lingers afterwards.

Every power reactor can be shut down on demand. The emergency stop is the SCRAM: the rapid insertion of the neutron-absorbing control rods into the core. According to the NRC, a scram drops those rods in just 2 to 4 seconds, which is enough to push the chain reaction below a self-sustaining level almost instantly. It can be triggered automatically by sensors or manually by an operator, and at Fukushima the three running reactors all SCRAMmed by themselves the moment the earthquake struck.

So why does a reactor still need looking after for so long? Because pulling the rods stops fresh fissions, but the fuel is now packed with radioactive fission products that keep decaying and releasing what engineers call decay heat. The World Nuclear Association puts it in numbers: right after a SCRAM the core still produces about 7% of its full power as heat, falling to roughly 1% after two hours, 0.5% after a day, and 0.2% after a week. For a typical large reactor that runs at around 3,000 megawatts of thermal power, even that 1% is roughly 30 megawatts of heat with nowhere to go, so cooling water must keep circulating for days. Stopping the reaction is the easy part; carrying away the leftover heat is the job that actually needs people, pumps, and power.

Can A Nuclear Reactor Explode Like A Nuclear Bomb?

This is the fear lurking behind a lot of "what if humans disappeared" searches, so it is worth stating plainly: a commercial reactor cannot detonate like a nuclear bomb. It is physically impossible, not just unlikely.

The reason comes down to the fuel and the design. As the US Department of Energy explains, reactor fuel is enriched to only about 3 to 5% uranium-235, while a nuclear weapon needs uranium enriched to over 90%. A bomb is built to dump all of its energy in the blink of an eye; a reactor is engineered to do the opposite, releasing heat slowly and steadily for years. You simply cannot coax a bomb-style chain reaction out of fuel that dilute, no matter what goes wrong.

That does not mean reactors are explosion-proof, and this is where the popular picture gets muddled. The blasts at Chernobyl and Fukushima were real, but they were not nuclear detonations. Chernobyl's reactor 4 was destroyed by a steam explosion (and the fire that followed) when its power surged out of control. At Fukushima, the hydrogen given off as overheated fuel cladding reacted with steam built up and ignited, producing the hydrogen explosions that tore the tops off the reactor buildings. Both were chemical and pressure events driven by runaway heat, the same decay-heat problem we just met, rather than a mushroom-cloud fission blast. An abandoned reactor that loses its cooling can melt down, breach its vessel, and scatter radioactive material, which is plenty dangerous on its own, but it will never go off like a weapon.

THE ANSWER…..

Now that we have a clearer understanding of man-made errors and natural disasters, let’s try constructing a modern-day scenario of what might happen to nuclear reactors without any human presence in the modern day.

Today’s state-of-the-art control systems ensure that human error is held to a bare minimum. Even without humans, nuclear power plants have automated protocols like SCRAM, which can shut down the reactor completely. The control systems are updated to the point that, as long as mainline and backup power is present, it will keep the reactor in a stable state until the fuel runs dry.

However, in the case of natural disasters, the first thing that would happen is that the main power line would be shut down. If the backup power source is not compromised, then it would kick in as soon the main line fails. The cooling system remains online even if the main power line fails, due to the help of the back-up. Either the backup power runs out of juice or the backup power is also compromised (as in the case of Fukushima) due to a severe natural disaster. This would lead to the reactor core heating up, at which point two possible situations occur. Either the reactor design is weak, which leads to an explosion, like what happened at Chernobyl, or the fuel burns through the reactor and penetrates to the bedrock.

The amount of time an unmanned nuclear reactor would last given the above scenarios would be a week, This time frame has been concluded upon after studying the timeline of failure of Fukushima and Chernobyl.

FUN FACT: Read about the elephant’s foot, the nuclear fuel deposit at the bottom of the Chernobyl plant!