A nuclear reactor is a device in which a chain reaction can be initiated, moderate and controlled (contrary to a atomic bomb, for which the chain reaction occurs in a fraction of a second).

The applications of the nuclear reactors include/understand primarily:

  • production of heat, which itself will feed another use (production of vapor for a mechanical work or the electrical production, production of fresh water by desalination (see Nuclear plant,…)
  • production of plutonium of civil use (Combustible MOX) or soldier (atomic bomb). According to the terms of the Treaty of non-proliferation, the military production of plutonium must be realized in dedicated installations distinct from the civil installations.
  • production of free neutrons or radioactive isotopes, used for research and in medicine.

The principal applications are the electrical production and the propulsion of ships, soldiers (Sous-marins nuclear power S, Porte-avions,…) or civilians (Ice-breaker in particular).

Since the years 1950, many nuclear reactors function in the world on the principle of the Nuclear fission to produce electricity. During these 50 last years, different technologies and civil reactor families was developed.

In parallel, of research relate to engines which would function on the principle of the nuclear Fusion. There exists in the world two main roads of research:

History

The first nuclear reactor is built with the the United States in 1942, at the University of Chicago, by Enrico Fermi and Leó Szilárd. It consists of a stacking of 6 tons metal Uranium, 34 tons of Oxyde of uranium and 400 tons of Graphite, this is why it bears the name of Atomic pile. Its power is only of 0,5 Watt, but its Divergence made it possible to consolidate the theory on the mechanisms of fission; this engine was also used as pilot installation to produce the engines intended for the production of the Plutonium necessary to the atomic bomb developed within the framework of the Projet Manhattan.

In France, the first engine of test was built by Lew Kowarski and Frederic Joliot-Curie in the center of studies of Fontenay-Aux-Roses (Hauts-de-Seine) of the Commissariat à l'Energie Atomique (ECA). This atomic pile, called the Pile Zoe, launched its first process of Chain nuclear reaction in 1948. The purpose of this engine was to place France in the group of the nuclear powers as a manufacturer of plutonium for the atomic bomb.

Dimensioned Russian, the first engines RBMK were built to produce military plutonium. The startup of the engine of Obninsk in 1954 provides electricity with a power of 5 MW. It can be regarded as the first nuclear engine in the world, because it is the first designed from a generating point of view. Its exploitation will last 48 years.

In 1956, the G1 engine is started at the research center of the ECA of Marcoule, it acts of the first French engine to produce not only plutonium but also of the electricity. It initiated the French die then Natural Uranium Graphite-gas (UNGG), replaced today by the technology of American origin to ordinary Eau under Pression (REFERENCE MARK).

Operation of an engine

Nuclear fission

The neutrons and the protons of the core of an atom are connected by very large forces, which can act only at one limited distance. The very heavy atomic nuclei such as uranium or plutonium contain protons enormously and must sometimes attract an additional neutron to guarantee the stability of the core.

If one of these atoms very heavy (for example uranium-235 or plutonium-239) aspires a neutron, it recovers occasion of energy consequently. This energy transforms it in a very unstable state (U-236 or Pu-240) then it is divided very quickly into releasing two or three free neutrons, which are available for other fissions of core: it is the principle of the chain reaction.

Energy coming from fission

The new cores resulting from division, called Fission product, have a more important binding energy per nucleon than the old heavy atoms. The difference in binding energy is partially transformed into kinetic energy of the fission products. Those give this energy in the form of heat by shocks on surrounding material. This heat is evacuated using a cooling agent and can for example be used for the heating or the electrical production.

Thermal neutrons and regulator

The slower one neutron is, the more the probability that it is collected by a U235-Kern atom is large. This is why one slows down the fast neutrons coming from the reaction of fission by a moderating . A regulator is a material which contains many very light atomic nuclei, almost as light as a neutron. The neutrons are then slowed down by the shocks on these light atomic nuclei until the speed of these cores of the regulator. According to the theory of the Brownian Movement, the speed of the cores of the regulator is defined by its temperature. One thus speaks about Thermalization of the neutrons rather than of deceleration of the neutrons.

An engine which uses thermal neutrons to carry out nuclear fission is called Thermal reactor. On the contrary, a fast engine uses for the fission of the neutrons which were not slowed down (from where the denomination Fast reactor).

Piloting of the chain reaction

The piloting of a nuclear reactor rests on the maintenance of a critical mass of Fuel nuclear in the middle of the engine. To allow a better output of the engine, one carries out a Thermalization of the neutrons using a moderating . And to evacuate the thermal energy produced by the chain reaction, a coolant is used. In the case of an engine REFERENCE MARK, water is used at the same time of coolant and regulator.

So that the chain reaction does not develop indéfiniement, it must be controlled. For that, one uses a material absorbing the neutrons. For example, the Cadmium, Gadolinium and the Boron. Starting from chemical compositions of these elements, one manufactures for example the control rods of the nuclear reactor. The engine can be controlled by the introduction or the withdrawal of these bars in the heart. The chain reaction is maintained according to the following principle: by surrounding fissile material of reflectors of neutrons, one supports fission, which decreases the quantity necessary to the release of the reaction; on the other hand, the presence of a neutron absorber to the contrary effect.

The description of the behavior of the heart rests on the Neutronique. The most important parameter of an engine is its reactivity, it is expressed in Pcm and makes it possible to control that an engine does not carry out a Empoisonnement with the xenon.

The Xenon and the Samarium are the two principal fission products emitted by the disintegration of the fissile cores. They are present as from the moment or there is a nuclear reaction. It is said that they poison the heart because their presence tends to choke the chain reaction.

For the people charged to control the engine, the principal concern is to control the effects of these poisons, in particular during the variations of power. The variations of the negative reactivity brought by Xenon and Samarium are then followed with interest because they cause an axial imbalance and sometimes, one can osbserver an azimuth imbalance of nuclear flow.

By considering that the fuel load is cylindrical, that the bunches of control operate vertically top to the bottom and that the coolant warms up by reassembling the fuel pins one can " imager" these imbalances:

  1. the axial imbalance of flow (Dpax or axial offset) is the difference in flow noted between the bottom and the top of the engine. Bunches fitting by the top of the engine, flow with thus always tendency to be more important in bottom of the heart. The burnup is thus exerted gradually upwards heart. If flow became more important in top than in bottom of the heart, there would be on the one hand an irregular wear of the heart of fuel and on the other hand a risk of partly high boiling of the heart. Indeed, water being hotter in top of the heart, it is probable to reach the conditions of saturation of water.

  2. azimuth imbalance (DPAzn) represents the image of flow " sight of the dessus" heart. Flow observed must be circular (thus regular) since the engine is cylindrical. If flow is not circular then that signile that the nuclear power is not uniform on a unit of section of the heart. That is thus synonymous with hot spots (or of localized overpower) which can cause a localized boiling leading to overheating (by the effect of warming) and lead to the fusion of fuel.

In all the cases, the Technical specifications of Exploitation prohibit these operations and thus prescribe an action to be taken as the fall of the power for example or the stop. If the dynamics of the phenomenon is important, of protections initiate the automatic stop of the engine.

To correct axial imbalance, the operators act on 3 parameters:

  • boron concentration of the primary education circuit (dilution/borication) to compensate for the variations of the poisons and thus to maintain the quantity of antiréactif necessary to the maintenance of criticality.
  • the effect temperature (margin of approximately + 0,8°C) to play on the favorisation or not of the chain reaction (dilation of the regulator).
  • the position of the bunches of control of the power to adjust the nuclear power of the engine to that of the turbine generator set.

After-heat

Even if the engine is shut down, the activity of the continuous fission products to produce heat. The power of this after-heat corresponds approximately to 5% of the nominal thermal power and disappears in space from a few days.

To be able to evacuate the after-heat in the event of urgency, the nuclear plants preserve a cooling system permanently. If such a system did not function, the increase in the temperature could lead to a Core fusion of the nuclear reactor. Nevertheless, of the particular procedures of control allow to avoid this risk. The Nuclear accidents most usually worked on simulator, by the drivers of section, are the Accident of criticality and the Core fusion as well as the total loss of cooling.

Types of engines

Nuclear dies

See also: nuclear Die

There exist various types of engines (which define thus nuclear Filière S ):

  • according to the nature of the combustible :

    • oxide of natural uranium, more or less enriched,
    • mixture of oxides uranium-plutonium,
    • Thorium, etc

  • according to the nature of the Coolant :

    • pressurized water,
    • ebullient water,
    • gas,
    • molten metal (sodium)
    • or salts molten.

Civil nuclear industry classifies the nuclear reactors by generations, corresponding each one to technological changes.

The Cycle of nuclear fuel is defined by the three parameters related to the type of engine (Fuel nuclear, regulator, Caloporteur). One employs the expression die of the pressurized water reactors (about the engines), by implicitly including the phases upstream and downstream of the cycle. The expression Cycle of nuclear fuel evokes all the phases explicitly.

Generations of engines

See also: Generation of nuclear reactor

One distinguishes 4 generations from engines.

  • the first gathers the engines built before 1970 (in France die UNGG).
  • the second indicates the engines built between 1970 and 1998 and currently in service, primarily of the die REFERENCE MARK (PWR).
  • the third is that of the engines derived from the precedents, conceived to replace them as from 2010: European EPR, American ESBWR, ATMEA 1 européano-Japanese.
  • the fourth indicates the engines which could enter in service by 2030.

Nuclear reactors in France

See also: List of the nuclear reactors in France

In France, several reactor families were successively developed:

  • 9 engines graphite-gases (UNGG), built with Marcoule, Chinon, Bugey and the St. Lawrence, now displaced.
  • 1 engine heavy gas-water (PHWR) built with Brennilis (power station), in phase of dismantling.
  • 58 pressurized water reactors (REFERENCE MARK), to see the List of the nuclear reactors in France.
  • 2 fast reactors and coolant sodium Phoenix with Marcoule (experimental reactor under operation) and Super-Phenix (stopped in 1997).

The engine with nuclear fission currently in project in France is it:

  • European pressurized Engine (EPR), or European Pressurized toilets Reactor (EPR). EDF wishes to establish this engine of 1600 MW on the site of Flamanville (Handle) in complement of the first two units of 1300 MW which are there already.

Nuclear reactors with the the United States

See also: List of the nuclear reactors of the United States

The the United States have a nuclear park of a hundred engines. Compared to the French nuclear park, the characteristics are rather different:

  • the average power is lower than that of the French nuclear park,
  • the park is more heterogeneous,
  • the park is older,
  • It contributes only to approximately 20% of the requirements in electricity for the United States, against 80% in France (and of the percentages often higher in the European Union).

The United States has a need more urgent for replacement of their park, in terms of times, but less in terms of proportion of supply of electricity.

See too

External bond

  • Engines of generation IV, number of Yellow and the Red (2004)

  • ECA: The operation of a nuclear reactor

  • the PBMR: Discussion with the chairman of PBMR (PTY) Ltd. (N°106 Fusion)

Simple: Nuclear reactor

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