reactivity compensation and control in and ex-core...
TRANSCRIPT
Reactivity compensation and
control
In and ex-core detectors
Course on Operation of Nuclear Reactors
4th lecture
Dr. Szabolcs Czifrus
associate professor
Budapest University of Technology and Economics
Institute of Nuclear Techniques (BME NTI)
Reactivity compensation and
control
Definition of excess reactivity
• = the maximum amount of reactivity that can be
released in the reactor
• It can be released if the neutron absorbing materials are
all withdrawn from the reactor
• However, it depends on the state of the reactor
• It can be changed by modifying certain physical
parameters
• can be released at all parameters being
nominal if all of the neutron absorbents are withrawn
• is the reactivity that can be released by changing
the parameters of the reactor
excess
nominalexcess,
hidden
rtntt ,,
Burnup cycle
• Burnup: decrease of the amount of fissile
material, increase of the amount of fission
products
• Burnup cycle: the period between two
consecutive refuellings
• Effective operational time
•
0
)(
)(
P
dttP
TT
eff
operatingrealeff TT ,
Excess reactivity during the burnup
cycle
• During startup, the coolant is warmed up to
almost operational temperature
• First using pumps, then with electric heaters up
to around 260
• In this period the reactivity decreases some 4%
• During power increase from 0 to 100 %,
reactivity further decreases approx. 1,5 %
C0
(%)15
0P
pcmP
• In order for Xe poisoning to achieve saturation (equilibrium value), 50 – 70 hours elapse
• This decreases excess reactivity by 2.5-3%
• Sm poisoning further reduces that by 0.6-0.7 %
• Alltogether, the reactor looses approximately half of the initial axcess reactivity in the first half month
• Later, in every month decreases around 1% every month
excess
Excess reactivity during the burnup
cycle
Reactivity compensation and control
• In the =0 case, reactor power is constant
• Therefore, must be reduced
• Tools:
1. Burnable poisons
2. Control rods
3. Application of boric acid
dissolved in coolant
The B-10 isotope has the high absorption cross
section
BB 1110
barn400010
excess
Application of burnable poisons
• Installed into the fuel
• Materials with high absorption cross section
• Not controllable
• Only for compensation
• Influences locally
• Can have an effect on the spatial distribution on
the power density
• Can be used to reduce unevenness
• Must be compatible with the fuel material
• Boron, gadolinium can be used
• in Al matrix, borosilicate, gadolinium-oxide 32OGdCB4
Control assemblies, rods
• Movable components
• Their number is limited due to technological
reasons
• The main goal is to control reactivity and
therefore control reactor power
• They have an influence on the spatial
distribution of neutron flux
• In the case of PWRs, they are usually cylindrical
rods with height being the same or a bit smaller
than that of reactor core
Possible solutions for the construction of
PWR control rod groups
P
4
4
4
4
2 2
22P P
P
P
P
P P5
5
55
5
5
55
11
1 11 3
3
3
3
3
31
3
31
1
1
1, 2, 3, 4, 5: teljes hosszúságúszabályozó rúdnyalábok csoportjánaksorszáma;P: részhosszúságú szabályozórudak;
szabályozórudak szabályozó rúdnyalábok leállító rúdnyalábok
Arrangement of control rods in a PWR
Control rods
Control rod groups
Shutdown rods
Full length control rods
Partial length control rods
Control rod of the BWR
Control rod
Control rod
Fuel assembly
Assembly wall
Decreasing enrichmentWater position (no fuel)
Reactivity worth
• Differential: reactivity worth of 1 cm of control
rod
• It depends on the value of thermal neutron flux
at the given location
• Integral: total negative reactivity of the part of a
control rod being inside the reactor core
• Total: rod worth, the worth of the fully inserted
control rod
Boric acid
• Main task is to compensate slow changes
• Solubility of boric acid is about 100 g/kg
• Maximum: 40 g/kg for different reasons
• Increases moderator reactivity coefficient
• Can even make it positive!
• Chemical problems, corrosion
• Reactivity change 15 to 500 times slower than
with control rods
• Critical boric acid concentration: that is able to
decrease excess reactivity to zero
B10 barna 4000 eVEn 025,0
Trends
• Longer fuel cycle
• 1.5 years instead of 1
• Higher power density
• Higher coolant temparature
• Higher enrichment
• Boric acid concentration cannot be increased
further to compensate for the large amount of
starting excess reactivity
• Therefore, burnable poisons must be used
• Boron can also be enriched in B-10
In and ex core detectors
Reactor core surveillance and
monitoring
The measured parameters
• Measurement of physical parameters is
cruical for the safe operation of the reactor
• The following must be measured:
– Neutron flux at different locations, inside the
core and outside
– Temperature of the coolant at as many places
as possible
– Flow rate of the coolant
– Pressure of the coolant
Neutron flux measurements with ex-
core detectors
Range
term, ncm–2s–1 P, %P0
Startup 0,1–105 10–10–10–4
Intermedier 104–1010 10–5–10
Power 108–1,21011 3–110
Why ex-core measurements?
Advantages
Disadvantages
Different ex-core detectors
should be used in different
neutron flux ranges
Startup range
Intermedier range
Power range
Neutron flux, n/cm2s
Startup range detectors
• In the startup range: gamma dose rate is very
high compared to neutron flux
• Neutrons should be measured in very high
background
• Electronic signal discrimination is extremely
important
• The more the difference between neutron signal
and gamma signal, the better the signal-to-noise
ratio
• Therefore, usually fission chambers are applied
• Pulse mode is absolutely necessary!
• Can be used up to 105 to 106 counts/s
1 – mutual electrode; 2 -3He own electrode; 3 –
detector wall; 4 - 4He-own
electrode; 5 – insulator
Construction of compensated ion chambers
One part is sensitive to n+gamma
One part is sensitive to gamma only
They operate in current (integral) mode
Neutron converter material can be boron or He-3
Detector type Neutron
meas.
range,
ncm-2s–1
Neutron
sensitivity,
A/ncm–2s–1
Gamma
sensitivity,
A/Gyh–1
Max.
neutron-
fluence,
ncm–2
Max.
gamma
irradiation
limit
Gy
VVER–440
Russian
KNK–15 0,1–105 - - - -
KNK–4 104–1010 10–13 3,9410–4 - -
KNK–3 - 3,310–15 2,310–3 - -
PHOTONIS
French
CFUG08 0,2–71010 810–13 3,410–8 21019 109
CFUH08 0,2–21012 10–14 3,410–8 21019 109
CFUK08 0,3–1010 610–13 2,110–8 21019 109
CFUL01 1–1010 210–13 710–9 21019 109
CFUL08 1–1010 210–13 710–9 21019 109
CFUM11 10–1011 10–14 10–9 21019 109
CFUM18 10–1011 10–14 10–9 21019 109
Some detector types
Positioning of ex-core detectors
Usually positioned either between the vessel (RPV)
and bioshield or
in some protecting tubes inside the bioshield
Maximum of thermal neutron flux is ~1010 n/cm2s
3 to 4 orders of magnitude less than inside the core
Positioning of ex-core detectors
Ex-core neutron
detectorEx-core neutron
detector
Ex-core neutron detectors are only sensitive to thermal neutrons
Therefore, it is important that some thermalizing material should cover the detector
This can be the bioshield itself (hydrogen in concrete!)
Ex-core detector weight function
Definition:
The value of the weight
function tells the
contribution of a fission
neutron starting from
one fuel pin to the
detector signal
It can be absolute or
relative
Ex-core neutron
detector
Ex-core detector weight function –
calculation model
Weight function is important and can be determined by calculations
MCNP is a good Monte Carlo code to calculate the function
Ex-core detector weight function
Very quick decrease
towards center, away
from detector
Approx. 1 order of
magnitude decrease in
fuel assemblies
The ex-core detectors
only detect neutrons that
come from external
fuel assemblies
Axial sensitivity – vertical weight
function
Reactor core
Upper boundary of reactor core
Lower boundary of reactor core
Axial weight function is
extremely important if the
reactor is large axially
The axial xenon oscillations
should be detected and controlled
Only way to detect is by placing
several detectors vertically
Question: where do the detected
neutrons come from
In-core detectors
•Advantages, disadvantages•Much higher flux
•Harsh environment
•Burning out
•But: flux mapping inside core
•Necessary to use them
•Different types:•Miniature fission chambers
•SPNDs
•Aeroball system
•Temperature
measurements
The aeroball system
Miniature fission chambers
In BWRs, miniature fission chambers are used
as in-core detectors
They should be placed inside fuel assemblies
Guide tubes inside fuel assemblies are
present for that purpose
Burn-up of fissile material can be significant
SPND = Self-powered Neutron
Detector
They are intended for monitoring of local value of neutron flux density
(power density) in core of nuclear power reactors.
SPND is a source, in which the measured current is generated due to
kinetic energy of charged particles born during interaction of reactor
neutrons with neutron sensitive element of SPND.
SPND consists of emitter, collector, insulator (separating them) and
communication line. As an emitter there are used substances
emitting charged particles at interaction with neutrons. Passing
through the insulator and gathering at the collector, these particles
create a potential difference between the emitter and collector. The
second electrode of SPND (collector) is usually earthed. From the
point of view of electricity, SPND is a power source - the primary
current of charged particles is subject to measurement. Voltage is
determined by resistance of the load and tends to increase with its
growth.
Construction of SPND
1 Emitter,
2 Insulator,
3 Collector,
4 Communication line,
5 Cover,
6 Cable insulation,
7 Current lead,
8 Background wire,
9 Sealed input,
10 Power pins
Temperature measurements
• In NPPs temperature measurements are crucial
• Most temperature sensors are either thermocouples or
resistance thermometers
• Resistance thermometers are based on the fact that
resistance of metals (eg. platinum) depends on
temperature
• Thermocouples operate on the principles that a circuit
made by connecting two dissimilar metals produces a
measurable voltage (emf-electromotive force) when a
temperature gradient is imposed between one end and
the other.
Construction and operation of thermocouplesThermocouples are inexpensive, small, rugged and accurate
Thomson effect: EMF due to the contact of two dissimilar metals
Peltier effect: temperature gradients along conductors in a circuit generate an EMF
Thermoelectric power is only a function of temperature
Voltage or EMF produced depends on:Types of materials usedTemperature difference between the measuring junction and the reference junction