Expectation to Nuclear Emulsion Technique for New Application in

Nuclear Engineering

T. Iguchi, K. Morishima, T. Naka, J. Kawarabayashi, K. Watanabe

Nagoya Univ.

Neutron Measurement in Nuclear Engineering

• New type of neutron detectors are required in nuclear engineering.

　 For instance,– High resolution neutron imager J-PARC neutron diffraction spectrometer– Standard neutron detectors for a few tens of keV Standardization of a neutron field– Neutron distributions or absolute total flux from spent fuel rods sub-criticality measurement

Neutron detectors appropriate for these applications have not yet developed !!

Standardization of 24 keV neutron fieldISO8529-1: “Reference neutron radiations

-Characteristics and methods of production-”

Unestablished region

Established Energy points

Thermal neutrons

2.5 MeV144 keV

565 keV 5.0 MeV

14.8 MeV

NeutronProduction

0.01 eVNeutronEnergy

RI source +Graphite pile 　　　

7Li （ p,n ）7Be

D(d,n)3He T(d,n)4He

1 keV 10 keV 100 keV 1 MeV 10 MeV

several 10 keV

45Sc(p,n)Reactor+Fe filter

24 keV neutron: One of the reference radiations for the response of neutron-measuring devices as a function of neutron energy.

@ NMIJ (National Metrology Institute of Japan) in AIST

IMPORTANT: from the viewpoint of radiation protection.

Measurements of 24 keV neutrons

3He proportional counterRecoil-proton proportional counter

Measurement Method

Spectrometry

Flux Determination

Neutron Monitor

BF3 or 3He proportional counter 24 keV≦Recoil-proton counter > 24 keVBF3 or 3He proportional counter

Characterization

0

0.05

0.1

0.15

0.2

0.25

0.3

0 200 400 600 800 1000 1200エネルギー(keV)

カウ

ント/時

間(1/

sec)

Fe/ AlFe/Al/ Ti

24 keV

Cou

nts/

sec

Deposited Energy (keV)

Thermal peak due to 3He(n,p)T

Neutron induced reaction base counter

(3He(n,p), 10B(n,) reaction)Difficulty in separating

from thermal peak

Recoil-proton counterOrganic Scintillator:

n- discriminationGas-filled type: Low sensitivity

Spectra obtained from He-3 covered by Cd

Problem

Neutron Flux: ~500 n/cm2/s (Reactor “YAYOI”&Fe filter)~5 n/cm2/s (45Sc(p,n)@AIST)

For (safety and) effective store of fuel assemblies in nuclear power plants（ spent fuel rod pit)

Accident example ；Two unused fuel assembles (PWR) causes a

nuclear criticality in a water (non-boric) pool.

To avoid nuclear criticality outside reactor cores,1.The fuel rods should be distributed at a distance.2.Appropriate absorber will be placed around the fuel rods.3.Experimental measurement of burn-up of the fuel rods, not calculatio

n.

http://www.tepco.co.jp/~fukushima1-np/b42307b.html

Control of Nuclear Criticality

Sub-criticality should be estimated based on experimental measurement.

Closed packed

Sub-criticality MeasurementSub-criticality of fuel rod depends on a spacial distribution of nuclides composition (U,Pu,TRU,FP) inside of the fuel rod.

However, non-destructive measurement of the spacial distribution of nuclides composition is impossible.

Conventional methods for Sub-criticality Measurement are based on total amount and fluctuation of neutrons.

ex. γ/ ｎ method 、 neutron source multiplication method, noise analysis method

Problems on neutron measurement for fuel assembly

Spacial distribution of neutron flux is estimated by an operating history of a nuclear reactor.

According to IAEA, the operating histry is NOT admissible as “experimental data”.

• Neutron measurement under the condition of high -ray background

（ /n=105 ～ 106 ）

• Criticality is not uniform along axial direction4 x fission chamber (for n detection)

2 x high purity Ge detector (for g detection)

2 x 20 x ionization chamber (for g detection)

●●●●●●●●●●●●●●●●●●●

●●●●

●●●●●●●●●●●●●●●●●●●

●●●●

fuel assembly

Idea of Neutron Detection by Nuclear Emulsion Technique

• Nuclear emulsion has not widely applied to nuclear engineering excepting personal dosimeter

• Complex readout of nuclear tracks is bottle neck for application.

We propose a novel neutron detector by nuclear emulsion.

Track of recoil proton should have information about neutron energy, orientation of neutron and spacial distribution of neutron source.

Automatic readout system for nuclear emulsion developed by Prof. Niwa is key technique for application of nuclear emulsion.

Characterization of keV Neutron Field using Multilayer Nuclear Emulsions

Multilayer nuclear emulsions

Tracks of recoil-protons

・ Neutron Spectrum・ Directional Distribution・ Neutron Yield

Reconstruction

Good discrimination between neutrons and gamma-rays& High Sensitivity (large volume & solid state)

Requirements for keV Neutron determinationHigh sensitivity & n-gamma discrimination

Neutron Characterization from a Fuel Assembly by Nuclear Emulsions

Raping the fuel assembly by the emulsion films

After several hours, track information is read out and neutron energy and absolute total flux are reconstructed.

Gamma and neutron discrimination by track darkness (dE/dx deference)

⇒Applicable for high gamma-ray field

4.2m

21cm

Requirements for Nuclear Emulsions

To apply the nuclear emulsion for these applications, we need to evaluate

• sensitivity for ~10 keV neutron(range of 10keV proton is order of 10-7m) • neutron/gamma discrimination characteristics(n/g ratio is more than 106).

Neutron Irradiation Test of OPERA film at Yayoi Reactor

stacked emulsion films (5 OPERA) were irradiated by fission neutrons at Yayoi reactor (Univ. of TOKYO)

Fast neutron reactor : max output 2kW

Stacked OPERA film

125 mm

100 mm

Cut into 25x25mm squareLight shieldedVacuum packing

Neutron Irradiated Direction and Recorded Tracks

１． perpendicular ２． horizontal

Tracks of Recoil Proton in OPERA FilmDate:20071213 　 reactor power:2000 WDistance form column: 3840mm 　 hour ： ~ ２

BR04 pl02 perpendicular BR09 pl02 horizontal

(Details see poster “Test of neutron monitoring” by Mr. Morishima)

Summary• We expect that the nuclear emulsion may show

good characteristics as a neutron detector and be applicable for– ~10keV neutron detector– sub-criticality monitor of fuel assembly.

by turning the software of automatic readout system and use of NIT to select the neutron tracks.

• Study of emulsion as a neutron detector is now launched…

超高速自動飛跡読み取り装置（ S － UTS ）・ 1972 年より世界に先駆けて開発 ： TS （ Track Selector ）・現在の世界最高速システム　＝　毎時 50平方センチ

飛跡認識画像処理専用プロセッサ

原子核乾板・高空間分解能で、全ての荷電粒子飛跡を１本１本記録（最小電離粒子）

乳剤の機械塗布　　Ｆｕｊｉフィルム社製　「 OPERA film 」　　　　　　　　　　　　　　　　　　　　　　　　均質、取り扱い容易表面保護層：１ミクロン乳剤：４４ミクロン

支持体：２０５ミクロン（ TAC ）

乳剤

乾板断面図（電顕写真）10ミクロン

乾板断面図（電顕写真）

原子核乾板で捕らえた反跳原子核の飛跡BR04 pl02垂直照射200712132000 W射出口から 3840mm約２時間

原子核乾板で捕らえた反跳原子核の飛跡BR09 pl02水平照射200712132000 W射出口から 3840mm約２時間

Tracks Penetrating Two Films

Pulse Height ： number of penetrated filmVolume Pulse Height ： sum of scattered light

MIP

Recoil Proton （？）

Distribution of PH +VPH