A nuclear reactor is an industrial facility that uses nuclear fission to produce electricity.
Fissile nuclear material replaces fossil fuel (coal, petroleum) used in conventional power plants. When a neutron collides with a nucleus of fissile material, the latter breaks up, freeing other neutrons and energy in the form of heat. The free neutrons will then collide with other nuclei and so on: the reaction is self-sustaining, and is known as a chain reaction.
The heat released during the chain reaction is used to produce steam. Just as in conventional power plants, steam drives a turbine and its alternator to produce electricity.
Operation is based on three independent systems filled with water that govern heat exchanges:
- The reactor coolant system is a closed loop which ensures transmission of heat released in the reactor core (where the fuel is located and the chain reaction occurs) to the steam generators, which transform the heat into steam.
- The secondary system is a closed loop which brings steam produced in the steam generators to the turbine generator which produces electricity. The steam is transformed back into water in the condenser.
- The cooling system supplies the condenser with cold water from a river or the sea.
The reactor core, where the chain reaction producing heat takes place, consists of fuel assemblies. Each fuel assembly has 264 metal tubes containing fissile material (fuel rods), 24 tubes for a rod assembly that controls the chain reaction, and an instrumentation tube. The fuel rods, which are approximately four meters high, are made of zirconium alloy tubes, or cladding.
Inside the rods, the fissile material takes the form of small cylinders (pellets) that are approximately 8 mm in diameter and 1.4 cm long and composed of uranium dioxide (UO2) or a mixture of uranium and plutonium oxides ((U, Pu)O2), which make up the nuclear fuel. The fuel is partially renewed during scheduled reactor outages, which occur every 12 to 18 months. The fuel remains in the vessel for approximately five years before it is discharged and stored in the spent fuel pool in the fuel building.
The core is placed inside a carbon steel reactor vessel which has a stainless steel liner and a head that is removed for refueling operations. In normal operation, the reactor vessel is filled with water at a pressure of 155 bar.
The safety of nuclear power plants is based on the principle of defense in depth: multiple layers of protection, or lines of defense, already present in the design of the facility, make the risk of an accident with serious consequences outside the nuclear power plant extremely small.
Each safeguard, which is assumed to be vulnerable, must have an independent backup. One of the major safety objectives of nuclear facilities is thus to contain radioactivity in all circumstances.
In a pressurized water reactor (PWR), taking into account the concept of defense in depth implies the existence of three barriers to contain radioactive products in the reactor core:
- The cladding around the fuel rods retains the radioactive products created in the fuel pellets. poor heat removal or mechanical stress that is too great may result in cladding failure, and more or less significant damage to the fuel pellets.
- The reactor coolant system: the fuel rods are constantly recooled by the reactor coolant which circulates in a closed circuit between the core and steam generator loops. The reactor coolant system constitutes a second containment that prevents the spread of radioactive products in the fuel if the cladding fails.
- Containment building: it consists of a concrete building housing the reactor coolant system.
Before and after use, nuclear fuel is stored in the spent fuel pool in the fuel building located next to the reactor building. Its main role is to store used fuel until its residual power is sufficiently low and it can be definitively removed from the site. During plant outages, new or already radiated fuel is placed here prior to loading in the reactor.
The spent fuel pools have a volume on the order of 1,000 m3 and a depth of 12 meters. The fuel assemblies are located at the bottom of the pool and covered with approximately eight meters of water. A system equipped with pumps and heat exchangers keeps the temperature of the pool below 50°C. It is commonly accepted that in the event of loss of pool cooling, the temperature of the pool will increase to boiling and the water level will decrease, potentially until the assemblies are uncovered after several days. To avoid uncovery of the assemblies, the procedure is to compensate water lost through evaporation with a water makeup, after opening the building to remove steam. The procedure is implemented until a cooling means is found.
IRSN carries out R&D programs to understand the behavior of the first containment barrier as well as on spent fuel pools in the various accident situations that could occur in a PWR.