Reactor Monitoring with Antineutrinos - A Progress Report

Adam Bernstein

Lawrence Livermore National Laboratory, 7000 East Ave., Livermore, CA 94306

Abstract

The Reactor Safeguards regime is the name given to a set of protocols and technologies used to monitor the con-sumption and production of fissile materials in nuclear reactors. The Safeguards regime is administered by the In-ternational Atomic Energy Agency (IAEA), and is an essential component of the global Treaty on Nuclear Nonpro-liferation, recently renewed by its 189 remaining signators. (The 190th, North Korea, withdrew from the Treaty in2003). Beginning in Russia in the 1980s, a number of researchers worldwide have experimentally demonstrated thepotential of cubic meter scale antineutrino detectors for non-intrusive real-time monitoring of fissile inventories andpower output of reactors. The detectors built so far have operated tens of meters from a reactor core, outside of thecontainment dome, largely unattended and with remote data acquisition for an entire 1.5 year reactor cycle, and haveachieved levels of sensitivity to fissile content of potential interest for the IAEA safeguards regime. In this article, Iwill describe the unique advantages of antineutrino detectors for cooperative monitoring, consider the prospects andbenefits of increasing the range of detectability for small reactors, and provide a partial survey of ongoing globalresearch aimed at improving near-field and far field monitoring and discovery of nuclear reactors.

Keywords: antineutrino, reactor monitoring, applied physics, safeguards, burnup

1. Introduction

Antineutrino detection holds promise as a means formonitoring power levels and fissile content of nuclearreactors. Such monitoring may be useful for the Inter-national Atomic Energy Agency’s (IAEA) reactor safe-guards regime. The reactor safeguards regime is thename given to a set of protocols and technologies usedto track the flow of fissile materials into and from nu-clear reactor sites. As part of its charter under theTreaty on the Non-Proliferation of Nuclear Weapons,the IAEA is responsible for implementing this regime,which applies to all non-nuclear weapons states withnuclear power infrastructure. In this article, we brieflysummarize the goals of the safeguards regime, describehow information on reactor operations can be derivedfrom antineutrino measurements, survey ongoing activ-ities in this field, and consider the possibility of remotedetection of small reactors.

2. Reactor Safeguards

The IAEA safeguards regime is implemented at civilnuclear fuel cycle facilities in non-nuclear-weaponsstates. The goal of the IAEA reactor safeguards regimeis to detect diversion of fissile materials from civil nu-clear reactors in non-nuclear-weapons states. In practi-cal terms, this involves tracking the influx of fresh fuelonto the reactor site, its placement in the core, the dis-charge of spent fuel into cooling ponds near the reac-tor, and, in some cases, its shipment to a downstreamfacility for storage or for reprocessing 1. Currently,the IAEA uses nuclear material accountancy, as well ascontainment and surveillance (CS) techniques to verifythe quantities of fuel used in and discharged from re-actors. Nuclear material accountancy refers to a quan-titative and independent check of fuel inventories, per-

1Reprocessing refers to the extraction of residual Pu and U fromspent nuclear fuel for reuse.

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formed by the Agency. At reactors, the predominantmaterial accountancy method is item accountancy, orcounting of items (fresh and spent fuel assemblies androds) considered to contain known quantities of fissilematerial. The presence and integrity of radioactive spentfuel assemblies and rods in cooling ponds at the reactoris also checked by Cherenkov light measurements andother methods. CS techniques, such as videocamerasand seals on the reactor head, are also used [1].By contrast, antineutrino-based safeguards offer a

form of near-real-time and nondestructive bulk accoun-tancy. Bulk accountancy methods provide estimates ofthe total fissile mass without relying on assumptionsabout the mass contents of premeasured items. Exam-ples include coincidence neutron counting, mass spec-troscopy and chemical analyses. As such, antineutrinobased methods are complementary to the existing safe-guards regime, since they provide independent quanti-tative information about fissile material inventories aslong as the reactor is operational.Among other uses, this information can provide in-

dependent confirmation that the fuel inventory through-out the reactor cycle is consistent with operator dec-larations. In principle, the inventory estimate derivedfrom the antineutrino rate can also be used to check forshipper-receiver differences, both for fresh fuel taken inby the operator and for spent fuel sent to downstreamreprocessing or storage facilities. A simulation of thereactor fuel evolution, using standard codes, would al-low estimates to be made of the discharged fissile iso-topics at end of cycle, and thereby confirm the correct-ness of operator declarations of both spent and freshfuel. Reactor simulation codes typically accurately pre-dict fissile masses at the 1 − 3% level [2]. Using fissionrate information versus cycle day taken from the code,one can also estimate the antineutrino flux and spec-trum throughout the cycle. The consistency of the reac-tor simulation, and hence of the core fissile inventoriescould then be checked by a comparison of the measuredantineutrino flux and spectrum to the prediction derivedfrom the reactor simulation code. A recent study hasshown that replacement of ∼ 80 kg of Pu with the equiv-alent fissile worth of Uranium can be detected with 95%confidence and a 5% false positive level with 90 days ofa data from a detector capable of detecting 2000 eventsper day [3]. This core-wide uncertainty can be appor-tioned among assemblies through the precision reactorsimulation. While the ultimate sensitivity to fissile con-tent of the technique remains to be determined, it is im-portant to emphasize that the IAEA currently does notmake independent estimates of the final fissile inven-tories in the reactor, relying instead on operator decla-

rations. Antineutrino based measurement systems maythereby provide the IAEA with an inventory estimatebeyond its current capability, one that does not dependon intrusive monitoring techniques such as in-core in-strumentation, direct thermal power measurements, ornon-destructive assay of fresh or spent fuel.

3. Monitoring Power and Fissile Content

Fission reactors emit huge numbers of antineutrinos,approximately 1021 per second. Despite the small cross-section (∼ 10−43cm2), the high flux of antineutrinosfrom reactors means that cubic meter scale detectorsat tens of meters standoff can record hundreds or thou-sands of antineutrino events per day, after accountingfor detection efficiency. The antineutrino rate and spec-trum are both accessible with proven detector technol-ogy, and may be useful for the measurement of twoquantities of interest for reactor safeguards: the reactorspower and fissile inventory throughout its cycle.As a typical power reactor core proceeds through

its irradiation cycle, the mass of each fissile isotopevaries in time. Neutrons fission uranium and plutoniumthroughout the cycle, while the competing process ofneutron capture on 238U produces plutonium. As a con-sequence of this variation in mass, the relative fissionrates of the isotopes also vary significantly throughoutthe reactor cycle, even when constant power is main-tained. This change in fissile content induces a system-atic shift in the antineutrino flux over the course of thecycle, known as the burnup effect. Burnup, measuredin Gigawatt days per ton of heavy metal, is a commonmeasure of the amount of neutron exposure and hencelevel of uranium consumption and net plutonium pro-duction in the core.Following Klimov [6], the PWR core antineutrino

count rate Nν̄(t) at time t in the fuel cycle can be ex-pressed as a product of two time-dependent factors:

Nν̄(t) = Pth(t) · γ(1 + k(t)). (1)

Pth(t) is the reactor thermal power. The term (1 + k(t))depends on the changing fissile isotopic content of thecore, embodied in the parameter k(t). γ is a constantrelated to the detector mass, efficiency, and standoff dis-tance. Thus an antineutrino rate measurement aloneconstrains the combination of thermal power and fissileisotopic mass. For this case, thermal power informationprovided by the reactor operator would be required toallow a constraint on the masses of fissioning isotopes.However, as described in [4], a direct measurement of

A. Bernstein / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 101–109102

the antineutrino spectrum would provide sufficient in-formation to simultaneously constrain both power andfissile isotopic content.

4. The State-of-the-Art

4.1. Rovno

Russian physicists appear to have been first world-wide to recognize and exploit antineutrino detection asa tool for reactor monitoring [5]. Multiple basic andapplied experiments were conducted over the course ofmany years, beginning in 1982, at the Rovno AtomicEnergy Station in Kuznetsovsk, Ukraine. The worksummarized here is from an experimental deploymentthat focused specifically on measurements relevant forsafeguards applications [6].The Rovno detector described in that article con-

sisted of 1050 liters of Gd-doped organic liquid scin-tillator, viewed through light guides by 84 photomulti-plier tubes. This is perhaps the first applied antineutrinophysics detector. A 510 liter central volume was used asthe primary target, with a 540 liter surrounding volume,separated by a light reflecting surface, employed asshield against external gammas, and as a capture volumefor gamma-rays emitted respectively by the positron an-nihilation and neutron capture. The deployed locationof the detector was 18 meters vertically below the re-actor core, providing substantial overburden for screen-ing of muons. An active muon rejection system wasapparently not used in this deployment. The antineu-trino source was a Russian VVER-440 pressurized wa-ter reactor, loaded with LEU fuel, with a nominal powerof 440 MWe. The gross average daily antineutrino-likeevent rate was 909 ± 6 per day, with a reactor-off back-ground rate of 149 ± 4 events per day (i.e. a net an-tineutrino rate of about 760 ± 7 per day). The intrin-sic efficiency of the Rovno detector was approximately30%. The Rovno deployment clearly demonstrated sen-sitivity to both the power and the isotopic content of thecore, based on rate and spectral measurements. Figure 1reveals the expected variation in the antineutrino countrate due to the burnup effect.The 6% change in rate is well matched to a predic-

tion based on a model of the fuel isotopic evolution inthe core, combined with the theoretically predicted an-tineutrino emission spectra for each isotope.The absolute precision on the reconstructed thermal

power of the reactor is 2%, with the largest uncertain-ties arising from imperfect knowledge of the detectionefficiency and detector volume. For comparison, di-rect thermal power measurements by the most accurate

Figure 1: The points with error bars show the relative antineutrinocount rate versus full power day (effective day) for the Rovno experi-ment. An approximate 6% change in the measured antineutrino countrate is due to the change in the fissile isotopic mixture in the coreacross the 300 day cycle. The dashed line is a predicted flux based ona reactor simulation.

Figure 2: The ratio of beginning-of-cycle to end-of-cycle antineutrinospectra, as measured at the Rovno reactor. The points with error barsare the data, the curve is a prediction based on a simulation of thereactor core evolution.

methods used by reactor operators range have precisionranging from 0.5-1.5% depending on the method.The change in the antineutrino spectrum over the

course of the fuel cycle is also visible in the Rovno data.Figure 2 shows the ratio of the reconstructed antineu-trino energy spectra at the beginning and end of the fuelcycle. The variation in spectra is most pronounced atthe highest energies, consistent with predictions, and iscaused by the net consumption of 521 kg of fissile ma-terial (both plutonium and uranium) over the course ofthe fuel cycle. While not directly quoting an uncertaintyon this value, derived from the antineutrino measure-ment, the Rovno group independently estimated fuelconsumption from the reactors thermal power records,and found a value of 525 ± 14 kg, close to the valuederived from the antineutrino data.

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Figure 3: A cut away diagram of the SONGS1 detector, showing thescale and major subsystems.

4.2. San Onofre

The SONGS deployments in the United States wereperformed independently of the earlier Russian experi-ments, for the express purpose of demonstrating the fea-sibility of antineutrino detection in the context of IAEAsafeguards. In a report [7] following a 2003 expertsmeeting at IAEA headquarters in Vienna , the IAEA re-quested study of the feasibility of confirmation of theabsence of unrecorded production of fissile material indeclared reactors with antineutrino detectors.The SONGS1 detector was built in response to this

request, and sought to demonstrate that antineutrinodetectors could meet other anticipated IAEA require-ments. In addition to simulating the antineutrino sig-nal and confirming its sensitivity to thermal power andfissile content, the SONGS detector demonstrated sta-ble long-term unattended operation, using a simple, lowchannel count detector design, non-intrusiveness to re-actor site operations at a commercial power plant forseveral years, and remote and automatic collection ofantineutrino data and detector state of health informa-tion.The SONGS1 detector at the San Onofre Unit 2 Nu-

clear Reactor began operating in 2002, with the fulldetector volume operational continuously from 2006through summer 2008. SONGS1 has an approximatelycubic meter central target, containing 0.64 tons ofgadolinium (Gd) loaded liquid scintillator contained infour stainless steel cells, each read by two Photomulti-plier tubes (PMTs). As seen in Figure 3, a six-sided wa-ter polyethylene shield of average 0.5 meter thickness isused for passive shielding of neutrons and gamma-rays,and a 5-sided muon detector for tagging and vetoingmuon-related backgrounds. A total of 24 PMTs wereused to read out both the muon veto and main detector.The detector was deployed in the Unit 2 tendon

gallery, an annular room that lies directly under thecontainment dome, and which allows access to steelreinforcement cables that extend through the contain-ment structure. The gallery is 25 meters from reac-tor core center. Many commercial reactors have ten-don galleries. They are well suited for deployment ofan antineutrino detector because the large, vacant spaceis rarely accessed by plant personnel, and because ofthe muon-screening effect of approximate 10 mwe earthand concrete overburden. (The SONGS overburden is10 mwe: other sites may differ but are of the same depthscale .) At the SONGS location, background muon ratesare reduced by a factor of approximately 5 compared toabove-ground backgrounds.The measured antineutrino rate (signal plus back-

ground) in the SONGS detector was 564±13 events perday at full reactor power, with a measured backgroundrate of 105±9 events per day at zero reactor power. Theintrinsic efficiency of the SONGS detector is approxi-mately 10%.SONGS1 demonstrated sensitivity to the operational

status, power and fissile content of the reactor. Figure4 shows a short-term excursion of the reactor power asreflected in the antineutrino rate. The upper plot is thehourly antineutrino count rate plotted versus hour. Asdescribed in [8], a cumulative statistical test statistic,known as the Sequential Probabilistic Ratio Test, is usedto determine to a desired level of confidence the opera-tional status of the reactor. The evolution of this statisticis shown in the lower plot. In this example, confirma-tion of a change in the reactor operational status is de-termined within 4 hours.Figure 5 is a histogram of the weekly antineutrino

rate, normalized to the average weekly rate for the prior4 weeks. This metric provides a relative estimate of thereactor power on a weekly basis, accurate to 3%. Thislimitation is imposed only by the counting statistics fordata accumulated over a week. Further precision can beobtained with a larger or more efficient detector.Figure 6 shows the long term change in the antineu-

trino rate measured in SONGS1 over the course of afull reactor cycle. The total change in the antineutrinorate is approximately 12% over the entire cycle, and thischange is predicted by a detailed simulation of the coreisotopic evolution. The measured change in rate can beused to roughly estimate the sensitivity to changes infissile content. In a SONGS refueling outage, approxi-mately 250 kg of 239Pu and 241Pu is removed and 1500kg of 235U is added in fresh fuel. This gives rise tothe measured 12% change in antineutrino rate. At theone standard deviation level in this prototype, this cor-responds to the ability to detect a reduction in total Pu

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Figure 4: (a): the hourly number of antineutrino-like events, plottedversus hour, through a reactor outage. (b) the value of the test statisticplotted versus hour over the same time range. In both plots, the ver-tical line indicates the hour in which the reactor shutdown occurred.The dashed lines in (b) are the 99% confidence level values of the teststatistic, known as the Sequential Probabilistic Ratio Test. For thisdata set, these values are obtained only during the appropriate period(on or off), meaning that no false positives or negatives occurred.

Figure 5: A histogram of the weekly detected antineutrino rate in theSONGS detector, divided by the average of this quantity for the priorfour weeks. This relative metric gives a 3% accurate measurementof the reactor thermal power, limited only by counting statistics. Thepurpose of the normalization is to reduce the effect of burnup on theantineutrino rate to well below one percent, in order to reveal the pro-portionality to reactor power.

content of 40 kg and a simultaneous increase of 235Ucontent of about 250 kg, using only the antineutrinorate. Further improvement in this sensitivity can be ex-pected from a higher statistics measurement (increaseddetector size or efficiency) or through spectral analysis.As described in the introduction, a study [3] based onthe SONGS data, coupled with a simulation performed

Figure 6: The daily background subtracted antineutrino rate over a600 day cycle of the SONGS Unit 2 reactor. Data from two succes-sive cycles were combined to create this plot. The solid line showsthe rate change predicted by a simulation of the reactor isotopic evo-lution. In [9], the response of the detector response was shown to bestable in a relative sense to better than 1% over the entire data-takingperiod, meaning that the measured rate change is not due to changesin detector response.

using the ORIGEN deterministic reactor code, demon-strated sensitivity to removal of ten partially burn as-semblies, containing ∼ 80 kg of plutonium, and their re-placement with ten fresh assemblies containing no plu-tonium. To achieve this sensitivity in the SONGS con-figuration would require a background subtracted countrate of approximately 2000 events per day, and a back-ground not exceeding 25% of the signal. These signaland background rates have been achieved in a numberof detectors, including the Rovno and Bugey [10] de-tectors.SONGS1 also addressed important practical consid-

erations relevant to safeguards deployments. SONGS1collected and analyzed data continuously, remotely andin real time, using a local computer and telephone mo-dem uplink to a laboratory in Northern California. Thedetector location was completely removed from dailyreactor operations, and no maintenance of the detectorby site personnel was required. Only occasional detec-tor maintenance was required by LLNL and SNL de-ployment team, even for this non-optimized prototype.

5. Ongoing Research Worldwide

There are currently many efforts underway around theworld to explore the potential of antineutrino based re-actor safeguards. The evolution of these efforts is sum-marized in the agendas of annual Applied Antineutrino

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Physics (AAP) Workshops 2 3 4 5 6.At present, work is funded by a variety of national

agencies acting independently, though there is frequentcommunication between the physicists involved at theAAP meetings. Below we describe some of the world-wide activities in this burgeoning field.

5.1. RussiaAs mentioned above, the concept of using antineutri-

nos to monitor reactor was first proposed by Mikaelyan,and the Rovno experiment [6] was among the first todemonstrate the correlation between the reactor antineu-trino flux, thermal power, and fuel burnup. Severalmembers of the original Rovno group continue to de-velop antineutrino detection technology, e.g. develop-ing new Gd liquid scintillator using the LAB solvent.They now propose to build an improved cubic meterscale detector specifically for reactor safeguards , andto deploy it at a reactor in Russia.

5.2. U.S.A.A collaboration between the Sandia National Labo-

ratories (SNL) and the Lawrence Livermore NationalLaboratory (LLNL) has been developing antineutrinodetectors for reactor safeguards since about 2000. Thefocus is on demonstrating the feasibility of antineutrinobased monitoring to both the physics and safeguardscommunities. This involves developing detectors thatare simple to construct, operate, and maintain, and thatare sufficiently robust and utilize materials suitable fora commercial reactor environment, while maintaining auseful sensitivity to reactor operating parameters.The LLNL-SNL group is now investigating the use of

Gd-doped water as an antineutrino detection medium.This method should be largely insensitive to the corre-lated background produced by cosmogenic fast neutronsthat recoil from a proton and then capture. A 250 litertank of purified water containing 0.1% Gd by weightwas built to test this concept. An above-ground calibra-tion with neutrons has clearly demonstrated the requiredsensitivity to neutrons, and to correlated events [15].The detector has also been deployed in the SONGS ten-don gallery. Data analysis in unshielded and passivelyshielded configurations is underway.The neutral current coherent scatter process,

ν̄ + A → ν̄ + A (2)

2Neutrino Sciences 2005, http://www.phys.hawaii.edu/ sdye/hnsc.html3AAP 2006, http://neutrinos.llnl.gov/workshop/aap2006.html4AAP 2007, http://www.apc.univ-paris7.fr/AAP2007/index.phtml5AAP 2009, http://indico.cern.ch/conferenceDisplay.py?confId=504986AAP 2010, http://www.awa.tohoku.ac.jp/AAP2010

also holds promise for reactor monitoring since it hascross-section several orders of magnitude higher thanthat for inverse beta decay. This could eventually yieldsignificantly smaller monitoring detectors, though theweakness and non-specific nature of the underlying nu-clear recoil make detection a major challenge. To ex-plore this process, LLNL is investigating the potentialof dual phase argon detectors for coherent scatter detec-tion [11]. The SNL group is exploring the use of highpurity germanium detectors for the same purpose.

5.3. France5.3.1. Double ChoozThe Double Chooz collaboration plans to use the

Double Chooz near detector ( about 400 meters from thetwo Chooz reactors), for a precision non-proliferationmeasurement [12]. The Double Chooz detectors willrepresent the state-of-the-art in antineutrino detection,and will be able to make a benchmark measurement ofthe antineutrino energy spectrum emitted by a commer-cial PWR. The MURE Monte Carlo project is being ledby Double Chooz to improve the reactor simulationsused to predict reactor fission rates and the measure-ments of the antineutrino energy spectrum emitted bythe important fissioning isotopes. This work is neces-sary for the physics goals of Double Chooz, and mayalso improve the precision with which the fuel evolu-tion of a reactor can be predicted.

5.3.2. NuciferThe Double Chooz near detector design is too com-

plex and costly for widespread safeguards use. There-fore, the Double Chooz groups in CEASaclay, IN2P3-Subatech, and APC plan to apply the technology devel-oped for Double Chooz, in particular detector simula-tion capabilities and high flash-point liquid scintillator,to the development of a compact antineutrino detectorfor safeguards named Nucifer [13]. The emphasis ofthis design will be on maintaining high detection effi-ciency, good energy resolution and background rejec-tion. Nucifer will be commissioned against research re-actors in France, including the 70 MWt OSIRIS reac-tor at Saclay, and possibly the ILL reactor in Grenoble.This last location is particularly interesting as the fuelused in the ILL is 97% 235U. Following the commis-sioning phase, Nucifer will be deployed against a com-mercial PWR, where it is planned to measure reactorfuel evolution using the antineutrino energy spectrum.

5.4. BrazilAn effort to develop a compact antineutrino detector

for reactor safeguards is also underway in Brazil, at the

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Angra dos Rios Nuclear Power Plant . This may also bea precursor to a second generation theta-13 experiment.Several deployment sites near the larger of the two re-actors at Angra have been negotiated with the plantoperator and detector design is well underway, basedon the Gd-doped water Cerenkov detection technology.The Brazilian work is particularly interesting since athird reactor is being built at Angra, at which spacemay be reserved specifically for an antineutrino mon-itoring detector, and because of the regional safeguardspresence (ABBAC, the Agencia Brasileiro-Argentina deContabilidade e Controle de Materials Nucleares) in ad-dition to the IAEA. Regional agencies such as ABACCoften take the lead in the development and testing ofnew safeguards technologies.

5.5. JapanA prototype detector for the KASKA theta-13 exper-

iment has been deployed at the Joyo fast research reac-tor in Japan [14]. The KASKA prototype is significantfor three reasons: it was the first deployment againsta research reactor, it was likely the first antineutrinodetector to be placed above ground without associatedoverburden, and the first attempt to deploy a safeguardsdemonstration instrument in a country that is subject toIAEA safeguards.A large Japanese collaboration (Tohoku Univ., Ni-

igata Univ., Tokyo Metropolitan Univ., Tokyo Inst. ofTech., Kobe Univ., Tohoku Gakuin Univ., KEK, andthe Hiroshima Inst. of Tech., Miyagi Univ. of Edu-cation) developed a prototype detector to measure theantineutrino flux from Joyo Fast Reactor during 2006-2007. The detector comprised a 0.9 cubic meter volumeof Gadolinium doped liquid scintillator contained by aUV transparent acrylic sphere, with 16 eight inch pho-tomultiplier tubes (PMTs) for photon detection. Figure7 shows a picture of the detector.The Joyo Fast Reactor is an experimental reactor used

to obtain various data for the development of fast reactortechnologies, which is operated by JAEA (Japan AtomicEnergy Agency). The fuel is Uranium-Plutonium Mix-ture oxide with approx. 18 wt% of enriched U and ap-proximately 23-30wt% of Pu. The maximum thermalpower is 140MW.The detector was placed just outside the reactor con-

tainment at approximately 25 m from the reactor core.The detector was shielded by 6mm lead sheets, 5cmthick paraffin blocks and also made use of several largeplastic scintillators to veto the cosmic-ray muons. Dueto a much higher than expected background, an initialexperiment was unable to distinguish between reactoron and off periods. However, a number of important

Figure 7: The prototype KASKA detector.

lessons were learned for future above ground deploy-ments; it will be necessary to employ improved shield-ing and cosmic ray veto to decrease cosmic muon back-ground; it will be necessary to improve pulse shape dis-crimination to reject the fast neutron background and byreducing dead time in the electronics this should leadto a reduced detector chamber footprint. The KASKAteam is confident that by utilizing these improvements,the performance of the neutrino detector should improvesignificantly and neutrino detection for power monitor-ing at small power reactors should be possible by aboveground deployment.

6. Future prospects

The research described above concerns cooperativemonitoring of reactors at standoff distances within about100 meters of a reactor core. It is also of interest tomake note of the event rates and likely backgrounds invery large detectors arising from the presence of a small(∼ 10MWt) reactor at several hundred kilometer stand-off. For example, a hypothetical 1 Megaton detector,possibly composed of gadolinium doped water or liq-uid scintillator, would detect 16 events per year at 400km standoff from a 10 MWt reactor. To achieve the re-quired sensitivity would require approximate 100-foldimprovements in background suppression relative to the

A. Bernstein / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 101–109 107

state-of-the-art 1000 ton KamLAND scintillator detec-tor [16]. Real antineutrino backgrounds would also haveto be minimized by deploying in largely reactor freezones. Considering current power reactor deployments,this is likely for the most part to be practical only in theglobal South.A one megaton mass represents a 20 fold increase

in size relative to the largest neutrino detector everbuilt, the 50 kiloton Super-Kamiokande [17] detector.No antineutrino detectors have been built larger thanKamLAND, nor neutrino detectors larger than Super-Kamiokande. However, instruments at the 100 kilotonto megaton scale are now being proposed and developedto study a wide range of fundamental physics topics[18]. Gadolinium doping of water is the most impor-tant breakthrough necessary to achieve the large scalesrequired for remote reactor monitoring.The ability to exclude the presence of a small reac-

tor in a large geographical region may hold interest forfuture treaties, particularly for those large swaths of theworld where no reactors are now thought to exist.In summary, the distinguishing features of far-field

applications are:

1. Detector sizes are tens of kilotons for tens of kilo-meter reactor standoff distances, and 1 megaton for400 kilometer distances;

2. Event rates of the order of a few per month for thesmall reactors of likely interest, even in very largedetectors;

3. Neutrino oscillations must be taken into account;4. Detector related backgrounds, and real antineu-trino backgrounds from other reactors play a moreimportant role.

While there appears insufficient motivation for de-velopment of these detectors for nonproliferation today,detectors on the required scale are now being designedwithin the fundamental physics community in order tostudy proton decay, supernova bursts, antineutrinos aris-ing from the Earth’s crust and mantle, and other motiva-tions. More detail on long range detection prospects fornonproliferation may be found in [19]. Another detailedstudy of sensitivity, taking in account realistic back-grounds in liquid scintillator detectors, may be foundin [20].

7. Conclusions

Beginning with pioneering efforts at Rovno, and con-tinuing with a series of experiments in the US at the

SONGS site, researchers have demonstrated that prac-tical detectors may be built that are capable of moni-toring or constraining the power and fissile content ofoperating nuclear reactors. Current sensitivity has beendemonstrated to changes in fissile content within the en-tire core at the ∼ 80 kg level with 3 months of data.This capability appears to hold some promise for theIAEA reactor safeguards regime, which is concernedwith tracking fissile material flows into and from nu-clear plants. A wide range of activities are ongoingworldwide to advance the technological base and ap-plication space for this new safeguards approach. Asidefrom technology development, the IAEA has begun torecognize the potential benefits of this technology forreactor safeguards regime, and in 2010 convened an AdHoc Working Group to consider how and whether an-tineutrino based safeguards might complement the ex-isting safeguards regime. Continued close interactionbetween the physics and nonproliferation communitiesis necessary for such analysis to succeed.With a much longer timeline, remote monitoring for

the presence or absence of reactors may one day be pos-sible. It would require routine operation of 100 kilotonto Megaton scale inverse beta detectors. Such detectorsare under consideration or active development withinthe fundamental physics community, but remain at leasta decade from realization.

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