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

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

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

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)

Prediction for a PLWR Non-neutrino background

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 VERY SIMPLE! see NIM A 572 (2007) 985

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

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

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

SONGS1 Fuel Burnup Measurement Removal of 250 kg 239Pu, replacement with 1.5 tons of fresh 235U fuel

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

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

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

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

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

Construction

Installation at SONGS

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

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……

Plastic detector outage data PRELIMINARY

Plastic detector outage data PRELIMINARY

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

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

Steel installation in Jan ‘07

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

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!

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

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