∙ adjustment of the concentration of boron (neutron absorbing element) in the reactor coolant system water during the cycle according to the gradual depletion of the fissile elements in the fuel; ∙ the presence of neutron-absorbing elements in the fuel rods which, at the beginning of the cycle, compensate the excess core reactivity after partial renewal of the fuel. At the end of the cycle, the reactor core is unloaded so that some of the fuel can be replaced. EDF uses two types of nuclear fuel in its PWRs: ∙ uranium oxide (UO2) based fuels enriched with uranium-235 to a maximum of 4.2% by mass. These fuels are fabricated in several French and foreign plants, by Framatome and Westinghouse; ∙ fuels consisting of a mixture of depleted uranium oxide and plutonium oxide (MOX). MOX fuel is produced by Orano’s Melox plant. The maximum authorised plutonium content is currently set at 9.08% (average per fuel assembly) giving an energy performance equivalent to UO2 fuel enriched to 3.7% uranium-235. This fuel can be used in the twenty-four 900 Megawatts electric (MWe) reactors, for which the Creation Authorisation Decrees (DACs) authorise the use of plutonium fuel. EDF is currently preparing to introduce MOX fuel into a few 1,300 MWe reactors. 1.4 THE PRIMARY SYSTEM AND THE SECONDARY SYSTEMS The primary system and the secondary systems transport the energy given off by the core in the form of heat to a turbine generator set which produces electricity. The reactor coolant (primary) system comprises cooling loops, of which there are three for a 900 MWe reactor and four for the 1,300 MWe, 1,450 MWe or 1,650 MWe EPR type reactors. The role of the reactor coolant system is to extract the heat given off by the core by means of circulating pressurised “primary water” or “reactor coolant”. Each loop, connected to the reactor vessel containing the core, comprises a circulating pump, called the “reactor coolant pump” and a SG. The reactor coolant, heated to more than 300°C, is maintained at a pressure of 155 bars by the pressuriser, to prevent boiling. The primary system is entirely situated within the containment. The primary system coolant transfers its heat to the water of the secondary systems in the SGs. The SGs are heat exchangers which contain from 3,500 to 6,000 tubes, depending on the model, through which the primary reactor coolant water circulates. These tubes are immersed in the secondary system water, which thus boils without coming into contact with the reactor coolant. Each secondary system consists primarily of a closed loop through which water passes, in the form of liquid in one part and in the form of steam in the other. The steam produced in the SGs is partially expanded in a high-pressure turbine and then passes through moisture separator-reheaters before entering the lowpressure turbines for final expansion, from which it passes to the condenser. Once condensed, the water is then sent to the SGs by the extraction pumps, followed by the feedwater pumps after passing through the reheaters. 1.5 THE SECONDARY SYSTEM COOLING SYSTEM The function of the secondary system cooling system is to condense the steam exiting the turbine. To do this, it has a condenser comprising a heat exchanger containing thousands of tubes through which cold water from outside (sea or river) circulates. On contact with these tubes, the steam condenses and can be returned in liquid form to the SGs (see point 1.4). The water in the cooling system heats up in the condenser and is then either discharged into the environment (once-through circuit) or, if the river discharge is too low or the heating too great for the sensitivity of the environment, is cooled in a Cooling Tower (closed or semi-closed circuit). Channel head Control rod drive mechanisms Reactor core Reactor pressure vessel Pressuriser Steam discharge Moisture separators Feedwater ring Bundle wrapper Tube bundle Tube support plate Steam Generator Primary pumps Core Instrumentation Vessel closure head A steam generator and a main primary system of a 1,300 MWe reactor 292 ASN Report on the state of nuclear safety and radiation protection in France in 2023 • 10 • The EDF Nuclear Power Plants
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