The reactor core
consists of rods containing uranium oxide pellets or mixed uranium and
plutonium oxide pellets (MOX fuel), located in fuel assemblies, contained
in a steel vessel. When fissioned, the uranium 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 in
the form of heat. The primary water enters the core from below at a
temperature of about 285°C, flows up along the fuel rods and exits
at the top at a temperature of about 320°C.
At the beginning of an operating cycle, the core represents a considerable
reserve of energy, which gradually decreases during the cycle as the
fissile nuclei disappear. The chain reaction, and hence the reactor
power, is controlled by:
• the rod control cluster assemblies
which penetrate the core and contain elements capturing the neutrons.
These enable the reactor to be started and stopped and its power level
to be adjusted to the quantity of electricity to be produced. Falling
of the clusters under the effects of gravity triggers automatic reactor
trip;
• varying the boron (also an absorber
of neutrons) content in the primary system water. The high initial reactivity
is offset by the boron - in the form of boric acid - dissolved in the
primary system water, since boron has neutron absorbing properties. Its
concentration in the water is adjusted during the cycle according to the
gradual depletion of the fissile material in the fuel.
The operating cycle ends when the boron concentration approaches zero.
An extension is however possible, if the temperature and possibly the
power level are brought below their nominal values. At the end of the
campaign, the reactor core is unloaded for renewal of part of the fuel.
EDF uses two types of fuels in its pressurised water reactors:
• a uranium oxide (UO2) fuel initially
enriched with U-235. Most of this fuel is manufactured by FBFC, a subsidiary
of Framatome and COGEMA. However, with a view to diversifying its supplies
EDF has, since 1980, been obtaining fuel from several foreign fuel manufacturers.
Initial U-235 uranium enrichment for UO2 fuel using natural uranium is
limited to 4.2%;
• fuels made from a mixture of plutonium
and depleted uranium oxides (MOX). MOX fuel is produced by the COGEMA
MELOX plant at Marcoule. An initial plutonium content, limited by regulation
to an average of 7.08% per fuel assembly, provides an energy equivalence
with 3.25% U-235 enriched UO2 fuel. This fuel can be used in the CP1 and
CP2 series 900 MWe reactors where provision is made in the authorisation
decrees for MOX fuelling. Twenty reactors out of twenty-eight are concerned.
Fuel management is different in the various reactor series. It can in
particular be characterised by:
• the nature of the fuel used and its
initial fissile content;
• the maximum degree of fuel depletion
at removal from the reactor, characterising the quantity of energy extracted
per ton of material (expressed in GWd/t);
• the length of the burnup cycle (generally
given in months),
• the number of new fuel assemblies
loaded at each reactor refuelling outage (1/3 or 1/4 of the total number
of assemblies);
• the reactor operating mode, with
or without major power variation, characterising the stresses to which
the fuel is subjected.
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