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all withdrawn from the reactor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

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)(

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

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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