The 12 The 12 self-powered detectors fingers contain each 6 self-powered detectors
C. Aeroball system for monitoring power distribution in the reactor core. The fundamental function of that movable nuclear fixed
XX. 2 : Axial distribution of the neutron flux
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THERMAL-HYDRAULIC IN NUCLEAR REACTOR
As indicated in the following figure, insertion of the control rods modifies As indicated in the following figure, insertion of the control rods modifies the shape of the axial flux distribution curve by moving the position of the shape of the axial flux distribution curve by moving the position of maximum flux clearly into the lower half of the core. This movement is maximum flux clearly into the lower half of the core. This movement is accompanied by an increase in the value of the maximum flux in the accompanied by an increase in the value of the maximum flux in the
same region.
same region.
It is therefore necessary, as we will see later on, to verify that the thermal It is therefore necessary, as we will see later on, to verify that the thermal
flux at this point does not exceed certain limiting values.
flux at this point does not exceed certain limiting values.
The situation also makes it necessary to operate at normal rated reactor The situation also makes it necessary to operate at normal rated reactor power with minimum insertion of the control rods in the core, which power with minimum insertion of the control rods in the core, which allows more homogeneous fuel burn-up & ensures a greater negative allows more homogeneous fuel burn-up & ensures a greater negative
reactivity reserve in case of reactor trip.
reactivity reserve in case of reactor trip.
Nevertheless, the fast reactivity variations, related to rapid changes in the Nevertheless, the fast reactivity variations, related to rapid changes in the power level must be compensated for by the RCCA. That means a power level must be compensated for by the RCCA. That means a
compromise must be looked for & a certain insertion is acceptable.
compromise must be looked for & a certain insertion is acceptable.
THERMAL-HYDRAULIC IN NUCLEAR REACTOR
To be able to keep the control rod clusters at the position indicated in the To be able to keep the control rod clusters at the position indicated in the figure, which is in the so-called ô reference ằ zone, these must be figure, which is in the so-called ô reference ằ zone, these must be
another means of reactor control available soluble boron.
another means of reactor control available soluble boron.
This element is present in the form of boric acid, diluted in the reactor This element is present in the form of boric acid, diluted in the reactor coolant. Thus, slow reactivity variations, in particular those related to coolant. Thus, slow reactivity variations, in particular those related to progressive burn-up of the fuel, shift over from cold shutdown to hot- progressive burn-up of the fuel, shift over from cold shutdown to hot- shutdown, & scheduled slow reactor power changes are compensated shutdown, & scheduled slow reactor power changes are compensated
for by causing the boron concentration to vary correspondinly.
for by causing the boron concentration to vary correspondinly.
This adjustment is quite slow (some 10 minutes is necessary to change This adjustment is quite slow (some 10 minutes is necessary to change the boron concentration but homogeneous i.e. does not influence the the boron concentration but homogeneous i.e. does not influence the
power distribution.
power distribution.
This drawback relates to the quantity of liquid effluents produced at each This drawback relates to the quantity of liquid effluents produced at each
change in the boron concentration of the reactor coolant.
change in the boron concentration of the reactor coolant.
Immediately after the first power escalation after reactor fueling, at full Immediately after the first power escalation after reactor fueling, at full power, the boron concentration is approximatively 1000 ppm power, the boron concentration is approximatively 1000 ppm according to the fuel management strategy and the fuel loading according to the fuel management strategy and the fuel loading
pattern.
pattern. 329329
THERMAL-HYDRAULIC IN NUCLEAR REACTOR
Normal fuel burn-up reduces the concentration by 3 to 4 ppm per day.
Normal fuel burn-up reduces the concentration by 3 to 4 ppm per day.
Figure XX.3: Axial neutron flux distribution with RCCA insertion.
Figure XX.3: Axial neutron flux distribution with RCCA insertion.
THERMAL-HYDRAULIC IN NUCLEAR REACTOR
c) The Xenon effect c) The Xenon effect
Some of the fission products formed strongly capture free neutrons & are Some of the fission products formed strongly capture free neutrons & are known as ô poisons ằ. This particularly the case of the Xenon 135, known as ô poisons ằ. This particularly the case of the Xenon 135, which can cause considerable variations in the core reactivity within which can cause considerable variations in the core reactivity within the space of few hours, thus leading to difficulties in controlling the the space of few hours, thus leading to difficulties in controlling the reactor power (overall effect) & in controlling the power distribution reactor power (overall effect) & in controlling the power distribution
(local effect).
(local effect).
The Xenon effect is responsible for the following phenomena:
The Xenon effect is responsible for the following phenomena:
- Start-up: If one starts the reactor from a situation where no Xenon is - Start-up: If one starts the reactor from a situation where no Xenon is present (after a shutdown lasting for several days), the progressive present (after a shutdown lasting for several days), the progressive formation of Xenon 135 makes it necessary to withdraw the RCCA or formation of Xenon 135 makes it necessary to withdraw the RCCA or to reduce the concentration of the soluble boron in order to maintain to reduce the concentration of the soluble boron in order to maintain the desired neutron flux or power level, until the Xenon has attained its the desired neutron flux or power level, until the Xenon has attained its
equilibrium concentration.
equilibrium concentration.
- Power increase: Starting from a situation where the Xenon is in a state - Power increase: Starting from a situation where the Xenon is in a state of equlibrium, it is necessary to temporarily increase the boron of equlibrium, it is necessary to temporarily increase the boron concentration or to insert the control rod clusters, whereas power concentration or to insert the control rod clusters, whereas power
increase is obtained by the opposite movement.
increase is obtained by the opposite movement.
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THERMAL-HYDRAULIC IN NUCLEAR REACTOR
- Power decrease: Starting from a situation in which the Xenon is in - Power decrease: Starting from a situation in which the Xenon is in equilibrium, one obtains an opposite effect from that mentioned just equilibrium, one obtains an opposite effect from that mentioned just above, but one that is much more pronounced. This is called ô the above, but one that is much more pronounced. This is called ô the Xenon peak ằ. After a reactor trip, it can limlit the return to criticality Xenon peak ằ. After a reactor trip, it can limlit the return to criticality
and then the full reactor power for periods of several hours.
and then the full reactor power for periods of several hours.
- Stable power distribution: The production of Xenon depends upon the - Stable power distribution: The production of Xenon depends upon the disappearance of the iodine 135, the local quantity of the Xenon disappearance of the iodine 135, the local quantity of the Xenon depends on the evolution of the flux at the point under consideration.
depends on the evolution of the flux at the point under consideration.
The distribution of Xenon in the core influence the power distribution The distribution of Xenon in the core influence the power distribution
& can cause instabilities in the later.
& can cause instabilities in the later.
The following figure shows the evolution of the negative reactivity due to The following figure shows the evolution of the negative reactivity due to
the Xenon over time, as function of the power level variations).
the Xenon over time, as function of the power level variations).
THERMAL-HYDRAULIC IN NUCLEAR REACTOR
Figure XX.4: Evolution of the Xenon negative effect.
Figure XX.4: Evolution of the Xenon negative effect.
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THERMAL-HYDRAULIC IN NUCLEAR REACTOR
XXI.