Neutron FluxMeasurement

10-11

210-14

10-12

10-13

10-9

10-10

10-4

10-5

10-6

10-8

10-7

1.5

10-3

10-2

10-1

1

2x10

2x105

2x104

2x102

2x103

2x1010

2x109

2x108

2x106

2x107

3x1014

2x1014

2x1013

2x1011

2x1012

Reactor Power(Fraction of F.P.)

Thermal NeutronFlux (n/cm 2/sec)

Ion Chambers of RRSControls above 5x10 -7 F.P.

In Core DetectorsControls at about 15% F.P.

InitialStartupRange

LowPowerRange

PowerRange

Log vs. Linear Meters

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

.000000001

.0000001

.0000001

.000001

.00001

.0001

.0001

.001

.01

.1

1.0

Linear Scale

LogScale

Four Power Scales

FP

FP

1.1.01.001.0001

.00001.000001

.0000001

10-7 10-6 10-5 10-4 10-3 10-2 10-1 100

-7 -6 -5 -4 -3 -2 -1 0

10010

Decades

1.1.01.001.0001

.00001 %FP

Startup Instrumentation

Reactor Power After Shutdown1.00E-03

1.00E-04

1.00E-05

1.00E-06

1.00E-07

1.00E-080 5 10 15 20 25 30 35 40

Days

Fra

ction o

f F

P

Boron Tri-fluoride Detectors

BF3 GAS

n

α+→+ 710 LiBn

More about Start-up Instrumentation! He-3 detectors can also be used

� More sensitive to neutrons� Smaller current pulse

! Detector come in a number of sizes! Normally detectors are external to the

core! Installed only when needed

Even More About Start-up Instrumentation! Typically installed in spare ion chamber

holes! Provisions for installing right into the

core� New cores � Extremely long shutdowns

! Detector Burnout! Damped response

Fission Chambers

! Enriched Uranium lines the walls of a chamber

! Inert fill gas! Neutrons cause fissions in the U235! Ionizations from the fission products are

detected! Can incorporate U-238 to breed more

active

Run up Instrumentation

Ion Chambers

Typical Ion Chamber Locations

Ion Chamber Circuits

Gamma Discrimination

! Minimizing the effect of gamma! Small detectors

� Minimize gamma energy deposited! Lead Shielding

� Lead absorbs gamma but is relatively transparent to neutrons

Ion Chamber Accuracy.

! Moderator Level� Changes in moderator level affect the

spectrum ! Loss of high voltage! Low reactor power levels! High Voltage Drift

Gas Detector Curves

(1) (2)(3)

(4)

(5)(6)

Uns

atur

ated

Reg

ion

(Rec

ombi

natio

n)

Satu

rate

d R

egio

n(Io

n C

ham

bers

)

Con

tinuo

usD

isch

arge

Reg

ion

Gei

ger-

Mue

ller

Reg

ion

Prop

ortio

nal R

egio

n

Lim

ited

Prop

ortio

nal

Reg

ion

Detector Bias Voltage

Rel

ativ

e Pu

lse

Size

1

102

106

104

108

1010

1012

1014

Gamma

Alpha

Beta

Under Load Instrumentation

In-Core Detectors

Platinum Emitter Inconel Sheath

Current Meter

InsulatorMgO Powder

n

Major Reactions

! Neutron capture and later beta decay! Neutron capture followed by a gamma

and the gamma releases a Compton or photo electron

! Gamma from external source releases an a Compton or photo electron

Neutron Beta reaction

Neutron Gamma Reaction

Response of Detectors

! (n, Β) response is delayed after a change in neutron population

! (n, γ) response is prompt following a change in neutron population

! External γ is prompt for fission gammas! External γ is delayed for fission products

Platinum Detectors

! Beta gives 3% of signal! Neutron capture gammas 60%! External gammas 40%

Inconel Detectors

! Negligible (n,Β)! Almost all signal from (n,γ) followed by

photo or Compton electron! Over prompt response

Vanadium

! Almost all response is (n,Β)! Response is delayed

Detector Location

Overlapping Ranges

Reactor Power

NeutronFlux

(n/cm2 s)

100%

109

1013

10120

1014

1011

1010

108

107

20% 40% 80%60%

106Startup Instrumentation

(scintillation)0 - 10-6 FP

Running Instrumentation(Linear In-Core Flux Detection)

0.15 - 1.5 FP

Runup Instrumentation(Log Rate External Ion Chambers)

10-7 - 1.5 FP

A

B

C

In-Core Detector Accuracy

! Fuelling or reactivity device movement! Start-up of the reactor! Long term exposure! Moderator poison load

For You To Do

! Read pp. 60-80! Answer Questions pp. 86-88, #28-38