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 field

ISO8529 1: “Reference neutron radiations -Characteristics and methods of production ”

24 keV neutron

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

IMPORTANT

: from the viewpoint of radiation protection.

@

NMIJ

(N ational M etrology I nstitute of J apan ) in AIST Neutron Energy 0.01 eV 1 keV 10 keV 100 keV 1 MeV 10 MeV Neutron Production RI source +Graphite pile

45 Sc(p,n) Reactor+Fe filter

7 Li （ p,n ） 7 Be D(d,n) 3 He T(d,n) 4 He

Unestablished region

several 10 keV

Established Energy points Thermal neutrons 144 keV 2.5 MeV 565 keV 5.0 MeV 14.8 MeV

Measurements of 24 keV neutrons

Problem

Measurement Method Spectrometry 3 He proportional counter Recoil-proton proportional counter Flux Determination BF 3 or 3 He proportional counter Recoil-proton counter > 24 keV ≦ 24 keV Neutron Monitor BF 3 or 3 He proportional counter 0.3

24 keV

Neutron induced reaction base counter

( 3 He(n,p), 10 B(n, a ) reaction) Difficulty in separating from thermal peak 0.25

0.2

0.15

Thermal peak due to 3 He(n,p)T Fe/Al Fe/Al/Ti 0.1

Recoil-proton counter

Organic Scintillator

: n g discrimination

Gas-filled type

: Low sensitivity 0.05

0 0 200 400 600 800 1000 1200 Neutron Flux: ~500 n/cm 2 /s (Reactor “YAYOI”&Fe filter) ~5 n/cm 2 /s ( 45 Sc(p,n)@AIST) Spectra obtained from He-3 covered by Cd

Control of Nuclear Criticality

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

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.

Closed packed

3.Experimental measurement of burn-up of the fuel rods, not calculation

.

Sub-criticality should be estimated based on experimental measurement

.

Sub-criticality Measurement

Sub-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

• Criticality is not uniform along axial direction fuel assembly

Spacial distribution of neutron flux

operating history of a nuclear reactor.

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

4 x fission chamber (for n detection) 2 x high purity Ge detector (for g detection) • Neutron measurement under the condition of high g -ray background （ g /n=10 5 ～ 10 6 ） ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● 2 x 20 x ionization chamber (for g detection)

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.

Automatic readout system for nuclear emulsion developed by Prof. Niwa

is key technique for application of nuclear emulsion. Track of recoil proton should have information about neutron energy, orientation of neutron and spacial distribution of neutron source.

We propose a novel neutron detector by nuclear emulsion.

Characterization of keV Neutron Field using Multilayer Nuclear Emulsions

Requirements for keV Neutron determination

High sensitivity & n-gamma discrimination Multilayer nuclear emulsions Tracks of recoil-protons Reconstruction ・ Neutron Spectrum ・ Directional Distribution ・ Neutron Yield

Good discrimination between neutrons and gamma-rays & High Sensitivity

(large volume & solid state)

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) 4.2m

⇒ Applicable for high gamma-ray field 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 -7 m) • neutron/gamma discrimination characteristics (n/g ratio is more than 10 6 ).

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

Cut into 25x25mm square Light shielded Vacuum packing 100 mm 125 mm

Neutron Irradiated Direction and Recorded Tracks １． perpendicular ２． horizontal

Tracks of Recoil Proton in OPERA Film Date:20071213 reactor power:2000 W Distance 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 ： ・現在の世界最高速システム ＝ TS 毎時 （

50

Track Selector 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） 乾板断面図 （電顕写真） 乾板断面図 （電顕写真） 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

10

ミクロン 支持体：２０５ミクロン （ TAC ） 乳剤

原子核乾板で捕らえた反跳原子核の飛跡 BR04 pl02 垂直照射 20071213 2000 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 20071213 2000 W 射出口から 3840mm 約２時間

Tracks Penetrating Two Films

MIP Distribution of PH +VPH Recoil Proton （？） Pulse Height ： number of penetrated film Volume Pulse Height ： sum of scattered light