Inside a reactor

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04/19/2011

A nuclear power plant produces electricity by using the energy released by a nuclear fission reaction. While the operating principle is relatively simple, the main issue lies in ensuring the highest level of safety and security.

Loading the reactor. Chooz B1, nuclear power plant in France, November 1995
© Areva / Joly Emmanuel

Nuclear Fission

A fissile atom is one whose nucleus can be split through collision with a neutron (elementary particle with no electrical charge).
The best-known fissile atoms are uranium 235 and plutonium 239. The two fragments obtained, known as fission products, are usually radioactive. This reaction results in a huge release of energy and increase in heat in the material.

The principle of a nuclear reactor involves recovering this energy in the form of heat in order to convert it into electricity.

In a nuclear reactor, the chain reaction is controlled.

Each fission also produces two or three neutrons, which in turn produce new fissions and release other neutrons and so on. This is called a chain reaction. In a nuclear reactor, the chain reaction is controlled. Only one neutron is allowed to propagate and continue the reaction, the others are trapped. Thus one fission gives rise to one fission, not two or three. The amount of heat released in the uranium mass is therefore completely controlled¹. In practice, absorbents such as boron, indium, and cadmium absorb excess neutrons and control the chain reaction. Mobile control rods are used - these can be inserted or extracted from the reactor core. If necessary, they will automatically fall into the fuel and thus stop the chain reaction almost instantly.

A Large Boiler

A nuclear reactor operates exactly the same way as a domestic boiler - although one that is
13 meters high! The heat produced by nuclear fission is recovered by a heat-transfer fluid (this conveys heat) and used to make steam. The steam drives a turbine, which in turn drives an alternator that produces electricity.

A reactor has 4 main components:

   • A fuel where the fission is produced

   • A heat-transfer fluid that conveys heat outside the reactor

   • A regulator (except for fast neutron reactors) that slows down the neutrons to increase their chances of fissioning a uranium atom

   • Control rods to control the chain reaction.

Each of these components, particularly the first 3, has variants (in terms of type, operation, etc.). For example:

   • The heat-transfer fluid can be used directly to drive the turbine. In this case, it is water converted into steam using the heat emitted by nuclear fission. This is the case, for example, in the boiling water reactor (BWR) sector.

   • Another variant is where the heat-transfer fluid transfers its heat to another fluid via a steam generator. This second fluid, in this case water, is converted into steam and drives the turbine. This is the case, for example, in the pressurized water reactor (PWR) sector.

There are 6 main sectors currently active worldwide, but pressurized water reactors (PWRs) produce about 60% of nuclear electricity worldwide.

Producing electricity from nuclear energy does not involve any carbon emissions.

True and (a little) False. Nuclear power plants do not emit any greenhouse gases (such as CO2)- contrary to electrical power plants that burn fossil fuels. Thus, replacing a conventional coal-powered plant producing 1000 megawatts of electricity with a nuclear power plant producing equivalent levels of power would reduce the amount of carbon emissions discharged into the atmosphere by 7 million tons per year².

However, if we consider the entire nuclear cycle, from uranium extraction to the dismantling of the plants, things change slightly. Mining activity, transporting the ore, and manufacturing the fuel are all industrial activities that generate greenhouse gases.

Reactor Safety and Security: Defense In-depth

Security and safety measures must include the following on a continuous basis:

   • Monitoring the chain reaction and consequently the power produced.

   • Cooling the fuel to evacuate residual power, i.e. the heat released by the radioactivity of the fission products, including after the chain reaction has stopped.

   • Containing radioactive material.

A reactor's safety and security is based on the principle of defense in depth. This principle states that the entire scheme is considered vulnerable, so it needs to be relayed or protected with other systems. Each line of defense reduces the probability and/or severity of an accident occurring.

So, for example, a number of physical barriers are used to isolate radioactive products from the surrounding environment. They are:

• Sealed metal tubes containing the rods containing the fuel pellets

• The reactor tank, made of very thick steel

• The containment enclosure surrounding the buildings is made of very thick concrete walls that are sometimes double³

There are three physical barriers isolating radioactive products from the surrounding environment.

Several events in nuclear history show the role of successive barriers to contain radioactivity.

   • In 1979, in the United States, the Three Mile Island accident involved the meltdown of part of the core of the PWR reactor. However, the incident did not have any effect outside the site. The contamination was contained inside the containment enclosure, protecting local residents and the surrounding environment.

   • In 1986 in the Ukraine, the Chernobyl accident saw the explosion of an RBMK type reactor (i.e. one with no containment enclosure), causing large-scale human, material, and environmental damage. This was due to human error as the safety and security procedures had been bypassed - the automatic safety systems were deactivated. Operators lost control and the chain reaction surged, causing a steam explosion inside the power plant and a fire. The graphite in the reactor core burned for about ten days before the fire was brought under control, discharging a significant amount of artificial radioactive products, which then drifted over hundreds of square kilometers, affecting many countries. 32 people died within the first 3 months of the accident and many more (several thousand) were exposed to extremely high levels of radiation.

   • In Japan on March 11, 2011, a powerful earthquake and ensuing tsunami claimed numerous victims and caused a series of serious accidents in the Fukushima nuclear power plant. These exceptional geophysical events caused a power loss followed by backup generator failures. Due to the loss of their cooling circuit, the reactors suffered explosions and heat-related damage, releasing radioactive material into the atmosphere. Employees also suffered severe irradiation.

Nuclear incidents take place regularly.

True. As in any other industry, a number of accidents take place in the nuclear industry. And in the nuclear industry, any incident or deviation must be declared. Each case is analyzed and corrective measures are taken. There is an international scale according to which even the most minor incidents and accidents are ranked. This is the INES scale - the International Nuclear Event Scale, which has 7 levels. Level 0 refers to deviations that have no safety consequences. Levels 1 to 3 refer to incidents and level 4 and up refer to accidents.

In 2009 in France the following were declared:

   • 941 deviations (level 0)
   • 130 level 1 incidents
   • 3 level 2 incidents
   • 0 level 3 incidents and above

The Three Mile Island accident was ranked at level 5 and the Chernobyl accident at level 7 on the INES scale.The Fukushima accident was ranked at level 7 on April 12, 2011.