Nuclear energy harnesses an extraordinary power, which is not without its dangers. It is based on nuclear fission, which generates large doses of radioactivity. However, like the shark behind the glass in the aquarium, this danger is well controlled thanks to a number of mechanisms. It is the safest source of electricity in the world.
In the wake of the Chernobyl and Fukushima accidents, nuclear energy appears to many to be very dangerous. Yet it is in fact one of the safest sources of electricity. Here, we’ll look in detail at the source of nuclear hazards, radioactivity, and how nuclear power plants are designed to produce safely.
Part 1. Nuclear risks
To fully understand nuclear safety, we need to understand the risks involved. Here we look at
- Radioactivity
- The release of radioactive elements when the reactor melts down
- Hydrogen diffusion in the plant
- The notion of risk and the notion of danger
1.1. Radioactivity
Radioactive! It’s a frightening word, associated in the imagination with devastated and dead zones, atomic bombs, uncontrolled mutations and other joyous events. And yet, we are literally immersed in it. To understand this, we need to return to the notion of radioactivity and recall the orders of magnitude.
Obviously, the nuclear reaction produces radioactivity that is toxic to humans:
Nevertheless, the dose of radioactivity we receive from living near a nuclear power plant is trivial, and we can safely walk through nuclear waste storage areas. To understand this, we need to recall a few orders of magnitude:
- Living near a nuclear power plant, you receive 0.01µS
- Eating a 150g banana, you receive ~0.12µSv.
- Having an X-ray of your lumbar spine, you receive 1.9 mSv.
- The only level above which we can observe an increase in long-term pathologies is 100mS/year
Note that nuclear technologies, using radiation, are presented by the FAO as a means of improving agricultural practices, notably through varietal innovation.
To find out more, see our article on radioactivity.
1.2. reactor meltdown and diffusion of radioactive elements
If the reactor is not cooled down sufficiently, the core can melt: the fuel is consumed and rises in temperature to the point of melting. The combustion releases radioactive gases: xenon, krypton, iodine, cesium and tellurium. This is the main problem with serious nuclear accidents: they create plumes of radioactive particles that can contaminate vast areas. It’s even more serious if it starts raining: the radioactive gases will be picked up by the water and seriously contaminate the area overflown. This is what happened in the north-west zone of Fukushima.
Iodine has been implicated in the development of thyroid cancer, notably in Chernobyl. However, it is short-lived. Cesium, on the other hand, is a long-lasting source of radioactivity.
1.3. Hydrogen diffusion in the power plant
For power plants using zirconium-clad fuel (the most common type), there is a risk of hydrogen production at temperatures above 1,200°C. This is because zirconium reacts with zirconium to produce hydrogen. This metal reacts with heat and oxidizes, releasing dihydrogen. This gas, which is particularly difficult to contain, can easily escape outside the intended pathways, especially if the pressure is too high and the equipment fails. In the case of the Fukushima Daiichi accident, this caused the roofs of three buildings to explode.
1.4. The difference between risk and danger
An important principle in all safety matters is the distinction between hazard and risk. A hazard is an abstract notion: a shark is dangerous, uranium is dangerous, bleach is dangerous.
Risk, on the other hand, is a concrete notion. So, in principle, you’re not in a risky situation if you look at a shark in an aquarium, if you visit a nuclear power plant in operation or if you have a pack of bleach at home.
Part 2. Notable nuclear events
Nuclear events are classified according to their severity on the INES scale. There are accidents and incidents.
Note that there are fewer and fewer problems:
2.1. The INES scale
Nuclear events are classified according to their severity on the INES scale from 0 to 7.
Events with a severity of 0 to 3 are referred to as simple “incidents”. These range from a deviation from normal operation, with no safety implications (level 0), to an anomaly, where authorized operation is not respected (level 1), to a serious incident, where a very small release is emitted and an accident is narrowly avoided (level 3)
Events of severity 4 to 7 are referred to as “accidents”. They involve the loss of defenses and contamination. From level 5 upwards, they involve serious damage to the reactor or radiological barriers. At level 7, they involve a major release with considerable health and environmental impact.
2.2. Major nuclear accidents
There have been a few dozennuclear accidents. Those that have had the greatest impact on the general public are:
- Three Miles Island. This accident, on March 28, 1979, caused the destruction of a nuclear reactor and the release of radioactive steam, which nevertheless seems to have had no health consequences. It was a crucial event for nuclear safety, which completely revised its approaches afterwards.
- Chernobyl. On April 26, 1989, the Chernobyl nuclear power plant undertook a test to see whether the emergency cooling circuits could operate with the residual energy produced by the reactor in the event of an emergency shutdown. Following a number of malfunctions, the test results in the reactor melting down. Radiation kills 28 of the employees on site and causes around 4,000 thyroid cancers in the region. Large areas became “dead zones”, off-limits to all visitors, and a large arch was built around the plant in 2017.
- Fukushima. On March 11, 2011, the most powerful earthquake in Japan’s history shook the coast, 180km from the Fukushima Daiichi nuclear power plant. Less than an hour later, a tsunami rendered almost all the emergency batteries and generators unusable. Deprived of monitoring tools, operators were unable to respond properly to the loss of cooling circuits in the days that followed. The three reactors in operation before the earthquake melted down. This caused significant radioactive releases, in the form of a plume, but also through cooling water run-off into the ocean. This was the second accident to reach level 7 on the INES scale. However, the radioactivity caused no casualties.
To find out more, read our article on nuclear accidents.
2.3. Nuclear incidents in France
In France, there have been two minor nuclear accidents (level 4): one in 1969 and one in 1980. Incidents are, however, commonplace. Every year, there are around a thousand level 0 events, around a hundred level 1 events and a few level 2 events. In French history, there have been 5 level 3 incidents.
These are listed by the French nuclear safety agency (ASN) and, from level 1 upwards, made public.
Part 3. The main threats to nuclear power plants
A study of nuclear accidents reveals a number of recurring safety risks:
- Loss of cooling circuits, leading to fuel meltdown.
- Valve malfunction.
- Loss of seals.
3.1. Loss of cooling circuit
This is the most serious risk: loss of cooling circuits. This is the cause of major accidents: the water no longer cools the nuclear fuel properly, causing it to melt and release hydrogen and radioactive gases.
3.2. Valve malfunction
One of the main functions of a nuclear power plant is to manage liquid (cooling) and gaseous flows, to limit pressure. Valve systems are used to manage liquid and gas flows. A malfunction can have serious consequences, as we saw with the Fukushima accident.
3.3. Leakage losses
Finally, another recurring problem is leakage. These are the weak points of flow management systems, and pressure can cause them to lose their effectiveness. This is also where hydrogen, a tiny molecule, is most likely to pass through.
3.4. Stress corrosion
A recurring problem, particularly observed in French nuclear power plants recently, is stress corrosion.
It’s a little-known phenomenon.
3.5. Nuclear power and war
One might ask whether there isn’t a risk of war near a nuclear power plant.
However, we need to put this into perspective:
- It can be very dangerous for both sides: the radioactive plume will move in the direction of the wind. It can be blown back to the guilty country and, if it rains, penalize it even more.
- Hydropower, for example, is also very sensitive to this problem, since it is possible to blow up a dam and flood the area below. This is a much more viable tactic militarily, as it is more predictable. In fact, when Russia attacked Ukraine, it hit the dam near Kryvyi Rih on September 14, 2022, endangering a town of 630,000 inhabitants.
This risk became a reality during the war in Ukraine. The Chernobyl power plant was invaded by Russian troops in the early days of the invasion of Ukraine. There was even a Russian bombing raid on the Zaporijia power plant on March 4, 2022. This did not cause any problems for the plant’s operation.
Part 4. The end of nuclear life
4.1. The question of nuclear waste
Nuclear waste is defined by the fact that it is not intended to be reused (which, in France, excludes used uranium). There are different types of waste, defined according to their intensity and the longevity of their radioactivity. Broadly speaking, they can be divided into low-level waste, medium-level waste and high-level waste. Low-level waste is very bulky, but not very radioactive. They are destined for surface storage. High-level waste is very small in volume, but highly active. They will be stored in deep geological disposal facilities when they are built.
4.2. Dismantling nuclear power plants
The dismantling of nuclear power plants may seem a long way off, given that we haven’t had any high-profile cases. Nevertheless, it’s an operation with no real technical hurdles, and several reactors have already been dismantled. It’s a long (around 15 years) and costly operation (around 500 million euros).
To find out more:
- IRSN, dossier on nuclear facilities: https://www.irsn.fr/FR/connaissances/Installations_nucleaires/Pages/Home.aspx
- Ex: Nuclear risk and its management
- On radioactive exposure in the event of an accident: https://www.irsn.fr/FR/connaissances/Installations_nucleaires/Les-accidents-nucleaires/accident-fukushima-2011/deroulement-accident-fukushima-2011/Pages/3-contamination-environnement-japon-suite-accident-fukushima-daiichi.aspx
- Tristan Kamin’s Twitter feed on the distinction between danger and risk: https://twitter.com/TristanKamin/status/1592660332291698689
- The distinction between risk and danger explained in 1’14: https://twitter.com/Dr_Keefer/status/1585474965922521088
- The explanation in the form of a Twitter feed: https://twitter.com/Dr_Keefer/status/1585652850062987264