- 79 - Below is a schematic diagram of a pressurised water reactor: Core and fuel management The reactor core consists of rods containing uranium oxide pellets or mixed uranium and plutonium oxides (fuel referred to as MOX) contained in metal tubes, referred to as the cladding, grouped in fuel assemblies. As a result of fission, the uranium or plutonium nuclei emit neutrons, which in turn produce further fissions: this is known as the chain reaction. These nuclear fissions release a large amount of energy as heat. The primary system water enters the core from the bottom of the reactor vessel at a temperature of about 285°C, flows up along the fuel rods and exits through the top at a temperature of about 320°C. At the beginning of the operating cycle, the core has a very large reserve of energy. This gradually falls during the cycle, as the fissile nuclei disappear. The rate of the chain reaction, and hence the reactor power, is controlled by: inserting control rod assemblies containing elements that absorb neutrons, to varying depths in the core. These enable the reactor to be started and stopped and its power level to be adjusted to the electrical power to be produced. Dropping of the control rod assemblies under the effects of gravity triggers automatic reactor trip; the concentration of boron (absorbing neutrons) in the primary system water is adjusted during operation as the fissile material in the fuel becomes depleted. At the end of the cycle, the reactor core is unloaded for replacement of part of the fuel. EDF uses two types of fuels in its pressurised water reactors: uranium oxide based fuels (UO2) with uranium 235 enrichment to a maximum of 4.5%. These fuels are fabricated in several plants in France and abroad, which belong to the fuel suppliers AREVA and WESTINGHOUSE; fuels consisting of a mixture of depleted uranium oxides and plutonium (MOX). The MOX fuel is produced by the AREVA MELOX plant. The initial plutonium content is limited to 8.65% (average per fuel assembly) and provides an energy equivalence with UO2 fuel initially enriched to 3.7% Uranium 235. This fuel can be used in the 900 MWe reactors for which the decree authorising their creation (the DAC) provides for the use of MOX. There are twenty-two reactors authorized to use MOX.
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