lawrence livermore and sandia national laboratories nathaniel bowden advanced detectors group...
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Lawrence Livermore and
Sandia National Laboratories
Nathaniel BowdenAdvanced Detectors Group
Lawrence Livermore National Laboratory
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory in part under Contract W-7405-Eng-48 and in part under Contract DE-AC52-07NA27344.Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company,
for the United States Department of Energy under contract DE-AC04-94AL85000
Status of Recent Detector Deployment(s) at SONGS
December 14, 2007
2
Introduction
Since 2003 a small detector based on Gd loaded liquid scintillator has been deployed at a commercial plant in the US (SONGS)
This relatively simple and non-invasive design has demonstrated remote and unattended monitoring of:• reactor state (power level, trips)• reactor fuel evolution (burnup)
Recently, we have been investigating several paths to more deployable detectors• Use of doped water Cerenkov detectors instead of scintillator• Use of less flammable and combustible, more robust, plastic
scintillator
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Constant(Geometry,Detector EfficiencyDetector mass)
• ~ 6 Antineutrinos are produced by each fission:
• Antineutrinos interact so weakly that they cannot be shielded, but small detectors have useful interaction rates
• 0.64 ton detector, 24.5 m from 3.46 GW reactor core
• 3800 events/day for a 100% efficient detector
• Rate is sensitive to the isotopic composition of the core• e.g. for a PLWR, antineutrino rate change of about 10% through a 500 day
PLWR fuel cycle, caused by Pu ingrowth
thPN
thPkN )1( Fuel composition dependentSum over fissioning isotopes, Integral over energy dependent cross section, energy spectrum, detector efficiency
Reactors Produce Antineutrinos in Large Quantities
4
The Antineutrino Production Rate varies with Fissioning Isotope: PLWR Example
Days into Cycle0 150 300 450 600F
ract
ion
of
To
tal
Fis
sio
ns
0.0
0.2
0.4
0.6
0.8
1.0235U239Pu238U241Pu
The fuel of a PLWR evolves under irradiation: 235U is consumed and
239Pu is produced
Energy (MeV)
The energy spectrum and integral rate produced by each fissioning isotope is different
5
Prediction for a PLWR
Rea
cto
r P
ow
er (
%)
-20
0
20
40
60
80
100
Date
06/2005 10/2005 02/2006 06/2006 10/2006
Det
ecte
d A
nti
neu
trin
os
per
day
0
100
200
300
400
500
Predicted rate Reported power
Cycle 13Outage
Cycle 14Cycle 13
thPkN )1(
Non-neutrino background
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LLNL/Sandia Antineutrino Detector “SONGS1” (2004-2006)
Detector system is…
• ~1 m3 Gd doped liquid scintillator readout by 8x 8” PMT
• 6-sided water shield
• 5-sided active muon veto
see NIM A 572 (2007) 985
7
Tendon gallery is ideal location
• Rarely accessed for plant operation
• As close to reactor as you can get while being outside containment
• Provides ~20 mwe overburden
3.4 GWth => ~ 1021 / s
In tendon gallery ~1017 / s per m2
Around 3800 interactions expected per day (~ 10-2 / s)
SONGS Unit 2 Tendon Gallery
~25 m
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Short Term monitoring – Reactor Scram
With a one hour integration time, sudden power changes can be seen
In this case, a scram is “detected” via SPRT with 99.9% confidence after 5 hours
Manuscript accepted by JAP
9
Relative Power Monitoring Precision
Weekly average 3% relative uncertaintyin thermal power estimate (normalized to 30 day avg.)
Daily average 8 % relative uncertaintyin thermal power estimate (normalized to 30 day avg.)
Manuscript accepted by JAP
10
Rea
cto
r P
ow
er (
%)
-20
0
20
40
60
80
100
Date
06/2005 10/2005 02/2006 06/2006 10/2006
Det
ecte
d A
nti
neu
trin
os
per
day
0
100
200
300
400
500
Predicted rate Reported powerObserved rate, 30 day average
Cycle 14Cycle 13outage
Cycle 13
SONGS1 Fuel Burnup Measurement Removal of 250 kg
239Pu, replacement with 1.5 tons of fresh 235U fuel
11
SONGS1 was very successful, but….
The liquid scintillator used is somewhat flammable, rather combustible, can spill
LS must be transported as a hazardous material, and is transferred onsite into the detector
With the SONGS1 run completed, we are leveraging the installed infrastructure to investigate several paths to more deployable detectors• Use of doped water Cerenkov detectors instead of scintillator• Use of less flammable and combustible, more robust, plastic
scintillator
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Solid, non-flammable, less combustible, Plastic detector
Replace half of liquid scintillator with plastic scintillator (PS):• Must retain neutron capture capability, ideally on Gd - commercial
neutron capture PS not suitable/available (e.g. Boron loaded BC-454)
• Final design: 2 cm slabs of BC-408 PS, interleaved with mylar sheets coated in Gd loaded paint
13
Such a design is a trade off:
Reactor Operator/ Safeguards Agency
Reduction in combustible inventory of ~ 40%
No leakage or flammable vapour concerns
No transportation of hazardous material required
Preassembled
Physics
XX Lower neutron capture efficiency on Gd
(LS: 80% / 20% Gd/H
PS: 60% / 40% Gd/H)
XX ~ 10% fewer protons/cc
XX Dead material in main volume
14
Design Optimization: Gd loading/PS thickness
Use a Geant4 simulation to explore the effect on neutron capture of varying:• Plastic slab thickness• Gd loading
Use 2 cm thickness, 20 mg/cm2 loading
0
20
40
60
80
100
0.01
0.1
1
10
100
0.40.6
0.81.01.21.41.61.82.0
% c
aptu
res
on G
d
Gd
area
l den
sity
(mg/
cm2 )
Plastic pitch (cm)
0 20 40 60 80 100
Capture fraction, 2cm pitch
Gd areal density (mg/cm2)
0.01 0.1 1 10 100
% c
aptu
res
0
20
40
60
80
100
GdPVT
15
Design Optimization: Optical Modeling
Investigate several readout configurations to optimise position uniformity
y=100
x (mm)
-400 -200 0 200 400
z (m
m)
-300
-200
-100
0
100
200
300
16 18 20 22 24
y=0
x (mm)
-400 -200 0 200 400
z (m
m)
-300
-200
-100
0
100
200
300
16
18
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22
24
y=100
x (mm)
-300 -200 -100 0 100 200 300
z (m
m)
-400
-200
0
200
400
10 12 14 16 18 20 22
y=0
x (mm)
-300 -200 -100 0 100 200 300
z (m
m)
-400
-200
0
200
40010 12 14 16 18 20 22
16
Construction
17
Installation at SONGS
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Initial Plastic Data
Response to AmBe neutron source
Response to background at
SONGS
Energy
The plastic detector responds to neutrons in the expected fashion: neutron captures on Gd are observed, as well as correlated (gamma,neutron) events from an AmBe neutron source
Inter-event time
Correlated events
19
Deployment Status
The plastic detector were successfully inserted into the SONGS Unit 2 Tendon Gallery during a two week campaign in August• The removal of liquid scintillator reduced the
combustible inventory in the gallery by almost 40% Neutron captures and correlated events are observed We use a scheduled reactor outage beginning Nov. 27
to observe the detector antineutrino sensitivity……
STOP PRESS!
20
Interevent Time (s)0 50 100 150 200
Co
un
ts
500
750
1000
Plastic detector outage data
Interevent Time (s)0 50 100 150 200
Co
un
ts
500
750
1000
Reactor OnReactor Off
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Pow
er (
%)
-50
0
50
100
Date
Sep 17 Oct 01 Oct 15 Oct 29 Nov 12 Nov 26
Cou
nts
per
day
0
50
100
150
200
250
300
PredictedObservedReactor Power
Plastic detector outage data
22
Conclusion
A robust antineutrino detector based on a large volume of commercial plastic scintillator has been designed, constructed and deployed
This device has several important advantages over the liquid scintillator that it replaces in a commercial reactor environment:• Non-flammable, non-hazardous, and no possibility of
liquid spillage• Near complete preassembly is relatively simple
The device clearly observes reactor antineutrinos, i.e. can monitor reactor state
Forthcoming work will focus on detector stability and calibration, with a view to observing fuel burnup
23
24
Test of compact steel shielding
Low density shielding is the bulk of the detector volume Replace 60cm water shield with 10 cm steel and measure:• Change in gamma bkg - should be unchanged• Change in correlated bkg (antineutrino like) due to:
Neutrons not attenuated by the steel Neutrons produced in the steel by cosmic ray muons
25
Steel installation in Jan ‘07
26
Steel results
We compare detector halves near and far from steel wall
Near
Before
Near After
Near Ratio
Far
Before
Far
AfterFar
ratio
e+/gamma events /day
149,500 150,500 1.0 166,000 166,000 1.0
Neutron events/day
5,900 7,100 1.2 6,200 6,200 1.0
Correlated events/day
280 360 1.3 280 280 1.0
Correlated bkg events/day
60 140 2.3 70 70 1.0
As expected, gamma ray background is unchanged, but more neutrons get through, producing more correlated background
27
Unscheduled SONGS Unit 2 outage
Unit 2 went down for one week in late October for unscheduled maintenance• Coincidently, wildfires came near the plant a few days
later!
28
Antineutrino Detection
We use the same antineutrino detection technique used to first detect (anti)neutrinos:
e + p e+ + n• inverse beta-decay produces a pair of correlated events in the detector
– very effective background suppression Gd loaded into liquid scintillator captures the resulting neutron after a
relatively short time
Positron• Immediate• 1- 8 MeV (incl 511 keV s)
Neutron• Delayed (= 28 s)• ~ 8 MeV gamma shower(200 s and 2.2 MeV for H capture)
n
ep
~ 8 MeV
511 keV
511 keV e+
Gd
~ 30 s
prompt signal + n capture on Gd
29
Acknowledgements and Project Team
Nathaniel Bowden (PI)
Adam Bernstein
Steven Dazeley
Bob Svoboda
David Reyna (PI)
Lorraine Sadler
Jim Lund
Many thanks to the San Onofre Nuclear Generating Station
Alex Misner
Prof. Todd Palmer
Lawrence Livermore National Laboratory