Application of Reactor Antineutrinos:Neutrinos for Peace

F. Suekanea

aRCνS, Tohoku University, Sendai, Japan

Abstract

In nuclear reactors, 239Pu are produced along with burn-up of nuclear fuel. 239Pu is subject of safeguard controlssince it is an explosive component of nuclear weapon. International Atomic Energy Agency (IAEA) is watchingundeclared operation of reactors to prevent illegal production and removal of 239Pu. In operating reactors, a hugenumbers of anti electron neutrinos (ν̄e) are produced. Neutrino flux is approximately proportional to the operatingpower of reactor in short term and long term decrease of the neutrino flux per thermal power is proportional to theamount of 239Pu produced. Thus rector ν̄e’s carry direct and real time information useful for the safeguard purposes.Since ν̄e can not be hidden, it could be an ideal medium to monitor the reactor operation. IAEA seeks for noveltechnologies which enhance their ability and reactor neutrino monitoring is listed as one of such candidates. Currentlyneutrino physicists are performing R&D of small reactor neutrino detectors to use specifically for the safeguard usein response to the IAEA interest. In this proceedings of the neutrino2012 conference, possibilities of such reactorneutrinos application and current world-wide R&D status are described.

Keywords: Neutrino, Applied Antineutrino, Reactor, Reactor neutrino, Safeguard, Plutonium, Monitor, Burn-up,IAEA, Novel technology

1. Introduction

In operating nuclear reactors, 239Pu are continuouslyproduced from 238U after successive β-decays and ther-mal neutron absorption. 239Pu is a fissile element andsubject of nuclear safeguard controls since it is an ex-plosive component of nuclear weapons. 8 kg of 239Pu isenough to make one nuclear bomb. That amount can beproduced within 3 days in regular nuclear reactor whosethermal energy is 3 GW. In order to prevent undeclaredproduction and removal of 239Pu from nuclear core, In-ternational Atomic Energy Agency (IAEA) [1] moni-tors illegal actions of reactor operators by using surveil-lance cameras, seals, containment verification, destruc-tive and non-destructive analysis, environmental sam-pling, etc. In order to improve such abilities, IAEAseeks for novel technologies and reactor neutrino moni-toring is one of such technology candidates.

A huge number of anti electron neutrinos (ν̄e) are gen-

erated in operating nuclear reactors. The reactor neutri-nos carry real time information of operating power andfuel components and can not be hidden by any practicalmeans. Thus measurement of reactor neutrinos could bean ideal method of reactor monitoring.

Neutrinos are extremely difficult to detect and easilyswamped by backgrounds. However, there have beenmany successful reactor neutrino experiments with sizesranging from a fraction of ton to kilo tons and distancesfrom reactors, from 10 m to hundreds of km. Neutrinophysicists are now performing R&D of reactor neutrinodetectors for use specifically to the safeguard purposesin response to the interest of IAEA.

There have been three workshops on safeguard ap-plication of neutrinos held by IAEA since 2003 [2].In the workshops, IAEA staffs and neutrino physicistsexchanged information of safeguard needs and possi-bilities of reactor neutrino monitoring and both par-

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Nuclear Physics B (Proc. Suppl.) 235–236 (2013) 33–38

0920-5632/$ – see front matter © 2013 Elsevier B.V. All rights reserved.

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http://dx.doi.org/10.1016/j.nuclphysbps.2013.03.008

Figure 1: Time evolution of concentrations of fissile elements in anuclear fuel. The fission rate is proportional to the product of fissioncross section and the concentration.

ties have agreed to proceed to study this option. Theneutrino physicists hold Applied AntiNeutrino PhysicsWorkshop annually since 2004 [3] with attendance fromIAEA.

2. Reactor Neutrino and Plutonium

There are 4 main fissile elements in nuclear fuel,namely, 235U, 239Pu, 238U and 241Pu. They perform fis-sion reaction and releases about 200 MeV of thermalenergy after absorbing a neutron. The fission productsare generally neutron rich nuclei which perform succes-sive β-decays until they become stable nuclei. One ν̄eis produced in each β-decay. This is called reactor neu-trinos. About 6 × 1020ν̄e are produced per second ina regular power reactor. Average number of neutrinosproduced per thermal energy release and their energyspectra depend on the fissile elements and this propertycan be used to measure 239Pu by reactor neutrinos.

If 238U absorbs a thermal neutron, it turns to 239U.239U then performs successive β-decays and 239Pu isproduced. Fig. 1 shows development of fuel compo-nents in time for a hypothetical light water reactor core.The fission rate is proportional to the fission cross sec-tion and the concentration. For equilibrium reactors,more than 80% of energy, and thus ν̄e, is generated from235U and 239Pu only.

Usually ν̄e is detected by inverse β-decay (IBD) re-action with a free proton in organic scintillator or watertarget in neutrino detectors.

ν̄e + p→ e+ + n (1)

The neutrino event rate (Nν) depends on the target ma-

0 1 2 3 4 5 6 7 8 9 10

E (MeV)

Ev

ent

Ra

te

(arb

itra

ry u

nit

)

235U

239Pu

Figure 2: Neutrino energy spectra per fission for 2 main components.

terial in detector, but is roughly,

Nν[/day] ∼ 500Pth[GW]M[kg]

(L[m])2 , (2)

where Pth is thermal power of reactor, M is target massof the detector and L is the distance between reactor coreand the detector. Thus there are reasonable number ofneutrino events if we use ∼ton scale detector at a dis-tance of a few tens of meter from GW reactor. Detec-tion efficiency has to be multiplied to the equation-(2) tocalculate the number of neutrinos we can detect. Gen-erally very strict event selections have to be applied toreduce backgrounds sufficiently and the detection effi-ciency is small. An important purpose of the currentneutrino detector R&D is to develop techniques to effi-ciently reduce the backgrounds without losing neutrinosignals so much.

Fig.-2 shows energy spectra per fission, of the neu-trino signals originated from the two main fissile ele-ments: 235U and 239Pu. The number of neutrino eventsper fission from 239Pu is roughly 65% of that from 235U.In order to make discussions simple, the small contri-bution of 238U and 241Pu will be ignored below. Thereactor power at time t is expressed as,

Pth(t) = qU fU(t) + qPu fPu(t) (3)

where fX(t) is fission rate of fissile element X. qX is en-ergy release per fission. It is nearly same for all fissileelements; qU ≈ qPu ≈ 200 [MeV/fission] and the totalfission rate is constant in reactors operating with con-stant power. Neutrino detection rate is,

Nν(t) = DU fU(t) + DPu fPu(t) (4)

where, DX is a function of neutrino production rate perfission, the IBD cross section, distance between reactor

F. Suekane / Nuclear Physics B (Proc. Suppl.) 235–236 (2013) 33–3834

Figure 3: SONGS1 experiment.

and detector, detection efficiencies etc. In principle, DXcan be calibrated and calculated precisely. 1-DPu/DUrepresents difference of neutrino rate per fission of 239Puand 235U and known to be ∼ 0.35. For reactors operat-ing with constant thermal power, the increase of 239Pufission rate is directly associated with the decrease ofneutrino detection rate:

d fPu

dt∼ − 1

0.35DU

dNνdt

(5)

The concentration of the 239Pu in reactors can be cal-culated from the fission rate by using the fission crosssection of the element and neutron flux or initial 235Uconcentration in the core. These relations show that thereactor neutrino monitoring could be a good indicatorof Pu breeding in reactor.

3. Possibilities of ν̄e for safeguard applications

Table 1 summarizes possibilities of safeguard use ofthe reactor neutrinos. Most of them are described inreference [4] in detail.

3.1. Reactor ON/OFF detection, real time measure-ments of operation power and 239Pu concentration.

SONGS1 experiment successfully demonstratedthese possibilities [5]. The SONGS1 group constructeda Gd-loaded liquid scintillator detector with target massof 0.64 ton and deployed it in tendon gallery of SanOnofre Nuclear Generation Station, which locates 25mfrom the reactor core (fig. 3) and 30 m.w.e. overburden.

Fig. 4 shows neutrino event rate around fuel ex-change period. SONGS1 has demonstrated that reac-

Application(1) Detection of reactor ON/OFF(2) Real time measurement of operation power

and burn-up(3) 239Pu measurement in operating reactor fuel(4) Detection of hidden reactor(5) Detection of nuclear explosion(6) Deterrence

Table 1: Possibility of neutrino monitoring.

Figure 4: Result of SONGS1 experiment [5]. Horizontal axis in-dicates the date. For vertical axes, left unit shows then neutrinoevents/day and right unit shows reactor power in %.

tor ON/OFF can be detected within 5 hours with 99.9%C.L. The stability of thermal power can be measuredwithin 8% every day or 3% every week. The differenceof neutrino rates between the end of the cycle and thestart of the next cycle in the fig.4 indicate amount of239Pu extracted from the reactor. It is estimated that re-moval 70 kg of 239Pu can be detected with 95% C.L.

3.2. Detection of hidden reactors.

KamLAND experiment, which uses 1,000 ton of liq-uid scintillator at 2,700 m.w.e., has been observing ν̄ecoming from several reactors whose average baseline isapproximately 180 km. If we could construct hundredsof kilo tons of detector, it would be possible to detect re-actor neutrinos coming from 1,000 km away. One inter-esting idea is that the distance from the neutrino sourcecan be measured with single detector by observing neu-trino oscillation pattern in energy spectrum. The energydependence of neutrino oscillation is

POscillation(Eν) = 1 − sin2 2θ sin2(Δm2L/4Eν),

where θ is a neutrino mixing angle (sin2 2θ ∼ 0.85),Δm2(∼ 8 × 10−5eV2) is a mass-squared difference. Thefrequency in 1/Eν distribution represents the distance

F. Suekane / Nuclear Physics B (Proc. Suppl.) 235–236 (2013) 33–38 35

to the reactor. Fig. 5 shows an example of oscillationpattern which was measured by KamLAND. However,

Figure 5: KamLAND neutrino oscillation result. KamLAND mea-sures neutrinos from multiple reactors and an average baseline is ∼180 km.

this is possible only if we can construct a huge neutrinodetector with a good energy resolution at very low back-ground environment.

3.3. Detection of nuclear explosion

The fission process in nuclear explosion is similar tothat of power reactors. Neutrinos from nuclear explo-sion arrive within a few second (although explosion it-self completes within micro seconds, neutrinos are pro-duced in β-decays of fission products with typical life-time of ∼ seconds [4] ) and it will be immune frombackgrounds and only a few events are enough to iden-tify nuclear explosion. However, one serious problem isgenerally the neutrino flux is too small to be detected.It can be understood from a simple calculation that theenergy release of fission of 8 kg of 239Pu is ∼ 6× 1014 J,which is equivalent to the energy produced within only2.3 days in 3 GWth reactor. If fissioning efficiency istaken into account, the expected neutrino flux becomesmuch less than that. Thus detection of nuclear explosionfrom distance 100 km or more requires at least hundredsof kilo ton’s detector.

3.4. Deterrence

Estimation of the DX factors in equation-(5) requiresa special knowledge of neutrino detection technique andof information such as target material and mass, detec-tion efficiency etc., and is very difficult for people notinvolved in the constrution. Thus the reactor operatorcan not know the sensitivity of the system and an ex-istence of a neutrino detector around the reactor woulddeter intension of the reactor operator to perform unde-clared operation.

4. Current status of world R&D activities

Several R&D’s are going on world wide. They aimat, above ground operation, safety, mobility, low cost,small footprint, etc. Some of up-to-date activities arebriefly described in this section.

4.1. SONGSThe SONGS1 experiment [5] finished successfully

and the group has been proceeding to the next steps asdescribed below.

4.1.1. CANDU Deployment [6]CANDU (CANada Deuterium Uranium) reactor is a

pressurized heavy water reactor. In CANDU rectors,online refueling is possible and the fuel can be removedwithout stopping its operation. The neutrino flux willbecome stable after 200 days from operation start. It isa good analogue to study safeguards for future ”Bulk-Process” reactors. The SONGS group upgraded theSONGS1detector by enlarging the target mass, from0.6 t to 3.6 t and improved light correction (fig.-6).The detector will be deployed at 75 m from the coreof 2.2 GWth Point-Lepreau Generating Station later in2012.

Figure 6: CANDU detector and deployment.

4.1.2. Segmented Scintillator Detector [7]Another activity of SONGS group is to develop a seg-

mented detector which uses plastic or liquid scintilla-tors (fig.-7). Each module is covered by ZnS:Ag/6LiFscreens, which work to identify the neutron and reducethe background by two orders of magnitude and thehit pattern reduces the background for another two or-der of magnitude. The enhanced background rejectionwill allow above-ground operation. four cells prototype(∼ 55kg active mass) has been build and deployed in antendon gallery 25 m from the SONGS reactor core.

F. Suekane / Nuclear Physics B (Proc. Suppl.) 235–236 (2013) 33–3836

Figure 7: SONGS2 detector and deployment.

Figure 8: KASKA detector. (a) Neutrino Detector. (b) Site.

4.2. Nucifer [8]Nucifer group has constructed a neutrino detector

with 0.85m3 Gd loaded liquid scintillator. The LS iscontained in a cylindrical stainless steel tank viewed by16 8-inch PMTs from the top (fig.-8). The detector isdeployed at 7m from Osiris research reactor, located atand operated by Saclay, with power of 70MWth. Thelevel of the detector location is shallow; 5 m.w.e. Thegroup is now taking data. They are planning to upgradethe liquid scintillator and γ-ray shielding.

4.3. KASKA Prototype [9]A Gd-loaded liquid scintillator detector was devel-

oped as a prototype detector for the reactor θ13 projectin Japan called KASKA (KAShiwazaki-KAriwa nuclearpower station). The group tried to detect neutrinos fromJOYO fast research reactor with 140 MWth power atabove ground 24 m from the reactor core. The KASKAprototype detector is now being remodeled by makinguse of stable LS, double layered structure and pulseshape neutron/gamma background separation capabili-ties (fig.-9).

4.4. PANDA [10]PANDA (Plastic Anti-Neutrino Detector Array) lead

by Tokyo University uses segmented plastic scintillator

Figure 9: KASKA prototype detector. (a) Schematic. (b) Picture.

modules with Gd-containing sheets in between. Thesize of one unit of plastic scintillator quadratic prismis 10 cm × 10 cm × 100 cm(10 kg) viewed by twophoto multipliers from both side. It is noncombustibleabove ground mobile detector installed on a van. 360 kg(6 ×6 arrays) prototype detector was deployed by theOhi reactor unit-2 (3.4 GWth) for two months in 2011-2012. The distance from the reactor core was 36 m. Thegroup marginally observed the reactor ON/OFF differ-ence and demonstrated satisfactory unmanned field op-eration (fig.-10).

Figure 10: PANDA detector and deployment.

4.5. ANGRA [11]

Angra group is building 1 ton Gd loaded waterC̆erenkov detector (fig.-11). Neutron does not producelight in water and it is expected that fast neutron back-ground can be reduced much. The detector will beplaced just outside the reactor building; 25 m from the

F. Suekane / Nuclear Physics B (Proc. Suppl.) 235–236 (2013) 33–38 37

Figure 11: ANGRA (a) detector and (b) experimental container.

reactor core of Angra nuclear power station with ther-mal energy of 4 GWth. Now the target vessel is underconstruction and it will be deployed to the reactor sitein the end of 2012.

4.6. DANSS [12]

DANSS group is constructing a plastic scintillatorbased reactor neutrino detector with dimensions of10mm × 40mm × 1, 000mm. Each scintillator stripis surrounded by a Gd-containing light-reflecting coat-ing. The detector will be placed 10-20m under the 4thcore of Kalinin nuclear power plant, whose power is3.05 GWth. The target sensitivity is to measure ther-

Figure 12: DANSS: (a) Detector design. (b) Prototype.

mal power with precision 2% per day or better, 239Pumeasurement with accuracy ∼ 5% per day, etc. In 2011,two section have been mounted. In 2012, the detectorwill be complete and in 2013, shielding house will beconstructed at reactor building hall and the data takingis planned to start in 2014.

5. Summary

Reactor neutrinos carry direct and real time infor-mation of reactor operation and plutonium breeding in

reactor core. Since neutrinos can not be hidden, theycould be an ideal medium to monitor reactor operation.There are a number of R&D’s going on in world widein order to demonstrate the capability of the neutrinomonitoring in response to the interest of IAEA.

Acknowledgments

The author thanks Drs., M. Minowa, N. Bowden,D. Reyna, J. Anjos, A. Burnstein, T. Lasserre, V. Sinevand M. Fallot for providing up-to-date information ofeach project.

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