In general, nuclear plants work very much like any ordinary power plant. Fuel is consumed to produce heat, which is exploited for boiling water. The steam from the boiling water sets a turbine into rotary motion, and electricity is generated. The difference between a nuclear power plant and a "conventional", fossil-fuel power station is the source of energy used for boiling the water. There are considerable advantages in using nuclear power over conventional sources: in this type of power plant, fueling is required only once in many years, and there is no emission of pollutants to the environment. In contrast, there is a considerable risk of radioactive pollution in cases of failures in the power plant. In this article I will review the general process in which electricity is generated in nuclear reactors while putting an emphasis on the inherent risks. Finally, I will try to explain the occurrences which led to the major failures in the Japanese nuclear power plants following the tsunami disaster of March 2011.

In nuclear power reactors, the energy arises from nuclear fission in reactive matter, which can be either uranium or plutonium. For the sake of simplicity, I will use uranium in the subsequent descriptions, but these are also true for plutonium. In the process of nuclear fission, a neutron hits a uranium atom in a manner that splits it into two different radioactive elements. Additional neutrons and photons are also emitted in the process. Since the mass of the particle products is smaller than that of the original uranium atom, a great amount of energy is released in the process. A number of neutrons are emitted in each fission reaction, and each such neutron can hit another uranium atom causing it to split. For this reason the whole process can take place very quickly, leading to the release of enormous amounts of energy in a short time interval. Such a process takes place upon the detonation of an atomic bomb. Luckily, this process can be regulated and controlled in nuclear reactors.

Nuclear fission of a uranium atom (neutrons are indicated by small blue circles).

The reactive material in nuclear reactors is stored in fuel rods. When a uranium atom undergoes fission, about 80 percent of the energy is released by the two resulting atoms. These atoms remain trapped in the fuel rod. The remainder of the energy is released by means of photons and neutrons. In order to control the whole process special neutron-absorbing control rods are employed between the uranium rods. Increasing the number of control rods leads to the absorption of more neutrons, and the rate of fission decreases.

This process takes place in the core of the reactor, which must be insulated from its surrounding due to the nuclear reactions taking place inside it, giving rise to radioactive products. Thus, the core is enveloped by several protective layers that completely block in the radiation.

Immense heat is an important product of the fission process. This heat is carried away from the reactor using a special cooling system and is then used to generate electricity. In some reactors the water passing through the core is also the source for the steam used to propel the turbine. In other reactors different systems are employed to carry the heat away from the core into an external water reservoir that is used for steam generation. This is done in order to keep the reservoir  water free of radioactive materials. To summarize, no matter how it's done, the fuel rods in a nuclear reactor are constantly being cooled by a sophisticated cooling system.

In contrast to other types of power plants, a nuclear plant cannot be shut down. When the control rods are completely immersed in the core, the process of nuclear fission comes to a halt. However, the heat output in the core does not go down to zero but to seven percent of the normal output. The reason for this is that the fuel rods contain radioactive material that builds up during the reactor's activity. These materials keep decaying, emitting radiation of different types. This radiation is mostly absorbed within the fuel rods, heating them up in the process. The heating rate gradually declines with time, since the radioactive materials keep breaking down, until a steady state is reached. However, until this happens, the rods must be actively cooled down despite the reactor's inactivity.

A failure in a nuclear reactor's cooling system can lead to several devastating outcomes:

(1)   Generation of steam from water inside the core. If the cooling system employs water flowing through the core, or if the water is flowing but cannot cool down the core at a rate that is faster than that of heat generation, the temperature of the water will rise quickly and the water will turn into steam, elevating the pressure inside the core. The greatest concern is to keep the core envelope, which can be damaged by the immense pressure building up in the core, intact. Breakdown of the envelope would lead to uncontrollable release of radioactive material to the surrounding.

(2)   The fuel rods are covered by a protective layer. When the temperature reaches 1200 degrees centigrade, oxidation of the protective material starts taking place. This process is accompanied by release of hydrogen gas, which is highly explosive and can damage the core envelope.

(3)   Meltdown of the fuel rods. This is the most extreme scenario. When the rods reach a temperature of 2600 degrees centigrade, they melt, and great amounts of radioactive materials are released. In addition, the extremely hot liquid that forms can break through the bottom of the envelope, leading to a severe environmental crisis.

An important point is that the risks described above involve environmental radioactive contamination. A common misconception is that the main risk is an accidental nuclear explosion. However, as explained earlier, the rate of the nuclear fission reactions is highly regulated by the control rods, and there is no real danger of an explosion.

Aside from the necessity to constantly cool the reactor and the active fuel rods, an additional requirement is to keep cooling used fuel rods that have been removed from the core. To this end such rods are usually kept inside large water reservoirs in the vicinity of the nuclear plant. Failure to cool down these rods can also lead to elevated temperatures and the release of hydrogen gas from their oxidized protective layer, or in more radical cases to their melting and the release of radioactive material.

When radioactive materials are released into the atmosphere, the radiation level in the area increases, sometimes for years since some of these agents take a very long time to decay. The severity level of such a nuclear disaster is determined by the level of deviation of the radiation level in the vicinity of the nuclear plant from levels permitted by the International Atomic Energy Agency. Seven different levels have been defined, where 0 is normal everyday level and 7 is an extreme scenario. So far in history, only the Chernobyl disaster of 1986 was classified as a level 7 event. The scale is designed so that each level is defined as 10 times more severe than the level below it.

Based on the above explanations, let's try and understand the events that occurred in the nuclear accident that took place in Japan in March 2011. Importantly, at the time of writing these lines, the picture is yet to be completely clear. This event is currently classified as level 3 or 4 – a severe event with local ramifications only, but it is still unclear how things may unfold.

During the earthquake in Japan, the nuclear fission processes in the nuclear plants were automatically halted, as part of a properly functioning emergency system. As a result of the earthquake the electricity supply to the different plants was stopped, among them the Fukushima plant which contains several adjacent nuclear reactors. In such cases emergency generators are activated. These are protected behind a wall that is designed to withstand tsunami waves. However, the waves that hit the Japanese coastline during this disaster were much more powerful than anticipated, and the generators were hit and disabled. An additional backup system was also activated and batteries supplied electricity to the cooling systems for eight more hours. When these ran out, the cooling of the reactor was terminated, and the control rods began to heat up as explained above.

The reactor workers put great efforts into keeping the rods cool and prevent their melting, but the rate of heat generation in the rods exceeded the cooling rate. The rise in temperature in the reactor core led to growing pressure due to the generation of steam. Engineers decided to release some of the steam in order to avoid damage to the core envelope. Since the steam comes into contact with the reactor core it carries with it radioactive materials that are released into the atmosphere.

Additionally, hydrogen gas began forming due to oxidation of the protective sheath of the rods. Inside the reactor core this gas mixes up with steam and does not explode, but once it was released to the atmosphere explosions ensued. It is still unknown whether or not these explosions damaged the core envelope but it is well known that such explosions spread out the radioactive materials very efficiently, and a rise in the level of radiation was recorded following each explosion.

Aside from the failures in the reactors themselves, additional problems arose in the storage areas where used rods began heating up. Currently, the two main dilemmas are whether any of the core envelopes were damaged, and whether some of the rods began to melt. The answer to these questions, together with the incoming information concerning the cooling efforts, will shed light on the severity of the nuclear disaster that occurred in Japan.

To summarize, the March 2011 earthquake in Japan and its consequences brought the potential risks of nuclear energy to public attention. Still, we must remember that this is still considered the most environment-friendly technology for the generation of electricity. Nuclear plants are designed to withstand some of the most powerful forces of nature, but apparently the earthquake that hit Japan was so powerful that all emergency and backup systems failed. Yet, it's important to mention that as of this moment it seems that the core envelopes of all the reactors are still intact, preventing this event from becoming a most severe nuclear disaster with extensive environmental implications.

The core of a Swiss nuclear reactor, with the fuel rods inside.

Yaron Gross
Department of Condensed Matter Physics
Weizmann Institute of Science

A note to the reader
If you find these explanations insufficiently clear or if you have further questions on this topic, please write about this in our forum, and we will relate to your comments. Your suggestions and constructive criticism are always welcome.