1. Status of Recent Detector Deployment(s) at SONGS December 14, 2007
Nathaniel Bowden
Advanced 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

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

3. Reactors Produce Antineutrinos in Large Quantities
~ 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
Fission produces antineutrino, which leave reacto, and carry information about the content of the reactor core
Constant
(Geometry,
Detector Efficiency
Detector mass)
Fuel composition dependent
Sum over fissioning isotopes, Integral over energy dependent cross section, energy spectrum, detector efficiency

4. The Antineutrino Production Rate varies with Fissioning Isotope: PLWR Example
The fuel of a PLWR evolves under irradiation: 235U is consumed and
239Pu is produced
The energy spectrum and integral rate produced by each fissioning isotope is different
Plot of fission antineutrino spectrum
Rates from inverse beta decay
Evolution of spectrum with equation
Energy (MeV)

5. Prediction for a PLWR
Non-neutrino background

6. LLNL/Sandia Antineutrino Detector “SONGS1” (2004-2006)
Detector system is…
~1 m3 Gd doped liquid scintillator readout by x 8” PMT
6-sided water shield
5-sided active muon veto
VERY SIMPLE!
see NIM A 572 (2007) 985

7. SONGS Unit 2 Tendon Gallery
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 n / s
In tendon gallery ~1017 n / s per m2
Around 3800 interactions expected per day (~ 10-2 / s)
~25 m

8. 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
Daily average
8 % relative uncertainty in thermal power estimate
(normalized to 30 day avg.)
Weekly average
3% relative uncertainty in thermal power estimate
(normalized to 30 day avg.)
Manuscript accepted by JAP

10. 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

12. 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
X Lower neutron capture efficiency on Gd
(LS: 80% / 20% Gd/H
PS: 60% / 40% Gd/H)
X ~ 10% fewer protons/cc
X 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

15. Design Optimization: Optical Modeling
Investigate several readout configurations to optimise position uniformity

16. Construction

17. Installation at SONGS

18. Initial Plastic Data PRELIMINARY
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
Response to AmBe neutron source
Correlated events
PRELIMINARY
Response to background at
SONGS
Inter-event time
Energy

19. STOP PRESS! 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……

20. Plastic detector outage data
PRELIMINARY

21. Plastic detector outage data
PRELIMINARY

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

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 Near Ratio ratio 1.0
We compare detector halves near and far from steel wall
Near
Before
Near After
Near Ratio
Far
After
ratio
e+/gamma events /day
149,500
150,500
1.0
166,000
Neutron events/day
5,900
7,100
1.2
6,200
Correlated events/day
280
360
1.3
Correlated bkg events/day 60
140
2.3 70
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:
ne + p g 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 n e p
 ~ 8 MeV
511 keV e+ Gd
t ~ 30 ms
Positron
Immediate
1- 8 MeV (incl 511 keV gs)
Neutron
Delayed (t = 28 ms)
~ 8 MeV gamma shower
(200 ms and 2.2 MeV for H capture)
prompt signal + n capture on Gd

29. Acknowledgements and Project Team
Lawrence Livermore National Laboratory
Alex Misner
Prof. Todd Palmer
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