Nuclear reactors

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The different types of nuclear reactors: operating principle.

Keywords: reactor, nuclear, operation explanation, PWR, EPR, ITER, hot fusion.

Introduction

The first generation of reactors include reactors developed in 50-70 years in particular, those of the gas graphite natural uranium sector (GCR) in France and die "Magnox" in the UK.

La second generation (70-90 years) sees the deployment of water reactors (the reactors pressurized water for France and boiling water as in Germany and Japan) which constitute today more than 85% of the power plants in the world, but also water reactors Russian design (VVER 1000) and Canadian heavy water reactors of the Candu.

La third generation is ready to be built, taking over from the second reactor generation, whether theEPR (European Pressurized water Reactor) reactor or SWR to 1000 boiling water models proposed by Framatome ANP (a subsidiary of Areva and Siemens) or the AP 1000 reactor designed by Westinghouse.

La fourth generation, The first industrial applications could intervene 2040 the horizon, is being studied.

1) The pressurized water reactors (PWR)

Primary circuit: to extract heat

Uranium, slightly "enriched" in its variety - or "isotope" - 235, is packaged as small pellets. These are stacked in sealed metallic sheaths joined together in assemblies. Placed in a steel tank filled with water, these assemblies form the core of the reactor. They are the seat of the chain reaction, which brings them to high temperature. The water in the tank heats up when in contact (more than 300 ° C). It is kept under pressure, which prevents it from boiling, and circulates in a closed circuit called primary circuit.

secondary circuit to produce steam

The primary system water transfers its heat to the water circulating in another closed circuit: the secondary circuit. This heat exchange is effected via a steam generator. In contact with the tubes through which water from the primary circuit, the secondary circuit water heats up in turn and becomes steam. This steam spins the turbine driving the generator that produces electricity. After passing through the turbine, the steam is cooled, converted back to water and returned to the steam generator for a new cycle.

Cooling system: to condense the steam and dissipate the heat

For the system to operate continuously, must ensure its cooling. It is the goal of a third independent circuit of the other two, the cooling circuit. Its function is to condense the steam exiting the turbine. For this is arranged a condenser unit consisting of thousands of tubes in which cold water taken from an external source. River or sea On contact with these tubes, the steam condenses to turn into water. As for the condenser water, it is rejected, slightly heated, the source from which it came. If the river flow is too low, or if one wants to reduce its heating, use of cooling towers or air coolers. The heated water from the condenser, distributed at the base of the tower, is cooled by the air stream that rises in the tower. Most of this water is returned to the condenser, a small part evaporates into the atmosphere, causing these white plumes characteristics of nuclear power plants.

2) The pressurized water reactor EPR European

This draft new Franco-German reactor presents no major technological break from the EPR, it just brings significant progress elements. It must meet the safety objectives set by the French safety authority DSIN, and the Authority of German security, with technical support IPSN (Institute of Protection and Nuclear Safety) and GRS, his German counterpart . This common security rules for adaptation encourages the emergence of international references. The project, in order to meet specifications expanded several European utilities, includes three goals:

- meet safety objectives in a harmonized way internationally. Security must be improved significantly from the design, including the reduction of a factor 10 the probability of fusion of the heart by limiting the radiological consequences of accidents, and simplify operations

- maintaining competitiveness, notably by increasing the availability and service life of the main components

- reduce releases and waste generated during normal operation, and seek a strong ability to recycle plutonium.

A little more powerful (1600 MW) That the second generation of reactors (of 900 1450 in MW) EPR also benefit from the latest advances in research in the field of security reduces the risk that a serious accident occurs. Especially because its security systems will be strengthened and that the EPR will have a giant "ashtray". This new device placed under the heart of the reactor, cooled by a supply independent water and prevent the corium (mixture of fuel and materials), formed in a hypothetical accidental fusion of the heart of a nuclear reactor, s 'escape.

The EPR will also have a better heat conversion efficiency into electricity. It will be more economical with a gain of about 10% on the price per kWh: the use of a "heart 100% MOX" will extract more energy from the same amount of material and recycle plutonium.

3) The experimental thermonuclear fusion reactor ITER

The deuterium-tritium fuel mixture is injected into a chamber where, by means of a confinement system, it passes into a plasma state and burns. In doing so, the reactor produces ash (helium atoms) and energy in the form of fast particles or radiation. The energy produced in the form of particles and radiation is absorbed in a particular component, the "first wall", which, as the name implies, is the first material element encountered beyond the plasma. The energy which appears as the kinetic energy of the neutrons is converted into heat in the tritigene cover, an element beyond the first wall but nevertheless inside the vacuum chamber. The vacuum chamber is the component which closes the space where the fusion reaction takes place. First wall, cover and vacuum chamber are of course cooled by a heat extraction system. Heat is used to produce steam and power a conventional turbine and alternator generating electricity.

source: Origin: Embassy of France in Germany - 4 pages - 4 / 11 / 2004

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