Nuclear reactors

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 in the form of small pellets. These are stacked in watertight metal sheaths assembled in assemblies. Placed in a steel tank filled with water, these assemblies form the heart of the reactor. They are the seat of the chain reaction, which brings them to high temperature. The water in the tank heats up on contact (over 300 ° C). It is kept under pressure, which prevents it from boiling, and circulates in a closed circuit called the primary circuit.

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secondary circuit to produce steam

The water in the primary circuit transmits its heat to the water circulating in another closed circuit: the secondary circuit. This heat exchange takes place through a steam generator. In contact with the tubes through which the water from the primary circuit passes, the water from the secondary circuit heats up in turn and turns into steam. This steam turns the turbine driving the alternator which produces electricity. After passing through the turbine, the steam is cooled, transformed back into 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, it must be cooled. This is the purpose of a third circuit independent of the other two, the cooling circuit. Its function is to condense the steam leaving the turbine. To do this, a condenser is installed, a device made up of thousands of tubes in which circulates cold water taken from an external source: river or sea. In contact with these tubes, the vapor condenses to turn into water. As for the condenser water, it is rejected, slightly heated, at the source from which it comes. If the flow of the river is too low, or if we want to limit its heating, we use cooling towers, or air coolers. The heated water coming from the condenser, distributed at the base of the tower, is cooled by the air flow which rises in the tower. Most of this water returns to the condenser, a small part evaporates into the atmosphere, causing these white plumes characteristic of nuclear power plants.

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2) The pressurized water reactor EPR European

This project for a new Franco-German reactor does not present any major technological breakthrough compared to the PWR, it just brings significant elements of progress. It must meet the safety objectives set by the French Safety Authority, the DSIN, and the German Safety Authority, with their technical support from the IPSN (Institute for Nuclear Protection and Safety) and the GRS, its German counterpart. . This adaptation of common safety rules encourages the emergence of international references. The project, in order to be able to meet specifications extended to several European utilities, incorporates three ambitions:

- comply with the safety objectives defined in a harmonized manner at the international level. Safety must be significantly improved from the design stage, in particular by reducing the probability of core melt by a factor of 10, limiting the radiological consequences of accidents, and simplifying operation.

- 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.

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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, thanks to a containment system, it changes to plasma 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 suggests, is the first material element encountered beyond plasma. The energy which appears in the form of kinetic energy of the neutrons is, for its part, converted into heat in the tritium blanket, an element beyond the first wall, but nevertheless inside the vacuum chamber. The vacuum chamber is the component that closes the space where the fusion reaction takes place. First wall, cover and vacuum chamber are obviously cooled by a heat extraction system. The heat is used to produce steam and power a conventional turbine and alternator assembly that produces electricity.

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

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