E-Journal of Advanced Maintenance Vol. 11 No. 1 (2019) 46-64
Japan Society of Maintenology

X-band electron LINAC-based compact neutron source for nuclear debris
on-site screening using short-distance neutron resonance transmission
analysis
Yudhitya KUSUMAWATI1,*, Issei OZAWA1, Yuki MITSUYA1, Tomooki SHIBA2 andMitsuru UESAKA1
1

Department of Nuclear Engineering and Management, School of Engineering, The University ofTokyo, Tokyo 113-8654, Japan
2
Integrated Support Center for Nuclear Nonproliferation and Nuclear Security (ISCN) and
Collaborative Laboratories for Advanced Decommissioning Science (CLADS), Japan Atomic Energy
Agency, Ibaraki 319-1184, Japan
ABSTRACTUnaccountable nuclear fuel debris is subject to nuclear safeguards and criticality safety, and their removal is oneof the decommissioning activities of the TEPCO Fukushima Daiichi Nuclear Power Plant (The 1F). In order tosignificantly contribute to efficient debris retrieval, an on-site nuclear debris screening system consisted of
non-destructive analysis using both X-ray and neutron source is being developed. By realizing a compact pulsed
neutron source system from a 3.95 MeV X-band electron LINAC coupled with tungsten and beryllium as
photon and neutron converters respectively, on-site short-distance neutron resonance transmission analysis(NRTA) can be performed to confirm the existence of U/Pu in a nuclear debris sample. Experiment result showsthat neutron TOF energy spectrum up to 70 eV with 4% energy resolution can be measured within only 2.5 m ofneutron flight path through a 3He proportional counter. NRTA experiments using mixed model samples indicatethat multiple resonance peaks in the neutron energy spectrum up to 20 eV, where resonance peaks of major
U/Pu isotopes fall within this range, can be simultaneously observed. The proof-of-principle experiment was
successfully performed. Short-distance NRTA would be an important complementary method for databasecalibration of polychromatic X-ray CT system of nuclear debris, which can provide estimation of U/Pu
distribution data necessary for nuclear debris activity mapping of The 1F reactor core area.
KEYWORDS

ARTICLE INFORMATION

isotope identification; neutron resonance; neutron source; on-sitemeasurement; time of flight; transmission; X-band

Article history:
Received 7 January 2019
Accepted 3 March 20191.

Introduction

Nuclear fuel debris removal is one of the decommissioning activities of the TEPCO Fukushima
Daiichi Nuclear Power Plant (The 1F), which is important for the purpose of nuclear safety andsecurity, and contribution to local people’s life and economic recovery. However, one of the main
problems associated with this activity is the uncertainty of the radioactivity distribution in the target
fuel debris. By analyzing the debris samples recovered from the reactor by a robot, a detailed mappingof the physical and chemical properties of the fuel debris lumps present in the reactor should be able
to be obtained. If this can be realized, it would significantly contribute to efficient debris retrieval andthe reduced exposure of workers to radiation. In particular, the necessary information for application
to metrological control is the distribution of uranium and plutonium content in fuel debris lumps.
Therefore, for this purpose, a method of analyzing the debris and determining its composition on-site
is required. Some established methods on nuclear debris analysis like laser-induced breakdown
spectroscopy (LIBS) which can be used on-site nuclear reactor core [1]. However, as the analyzed
sample would be instantaneously evaporated to induce light-emitting plasma, it would be difficult toobtain the weight information of the debris as well. On the other hand, induction coupling plasma*

Corresponding author, E-mail: yudhitya.k@gmail.com

ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

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Y. Kusumawati, et al./ X-band electron LINAC-based compact neutron sourcefor nuclear debris and spent fuel on-site screening

mass spectrometer (ICP-MS) method that has been established by IRID to analyze Fukushima Daiichi
nuclear debris [2] has minimum destructive effect to the sample, but it is not available for on-site use.
Our research team’s proposal is to realize an on-site non-destructive analysis method for nuclear
debris at The 1F by making use of the mobile, compact-sized, high-frequency X-band electron
LINAC to construct a LINAC-based X-ray and neutron source. The diagram in Fig. 1 shows the flowof on-site screening of nuclear debris activity by utilizing two kinds of measurement technologies
through LINAC-based beam source of X-ray and neutron. Both methods are important andcomplement each other.

Fig. 1. Schematic of on-site nuclear debris screening activity to obtain the data for nuclear debris
activity mapping (reactor pressure vessel map from [3]).

In regards to the inspection methods incorporated for on-site screening system, X-ray CT will be
the one to provide quantitative data of nuclear debris size and imaging necessary for nuclear debris
activity data. Polychromatic X-ray CT method was chosen as it has very good capability ofidentifying material with large gap on their atomic number (Z). It is suitable for detecting uranium andplutonium as both have the highest Z number among other materials contained in the nuclear debris(more explanations regarding this method and experiment result will be explained in later chapter).
Even so, it is difficult to set a proper thresholding in order to get accurate identification of materials
with close Z number [4], which would be the downside of this method in clear differentiation between
uranium and plutonium. The two radioactive elements have very different microscopic cross-section
value where plutonium microscopic cross-section is 200 times larger than uranium microscopic
cross-section. This characteristic would make plutonium to be more reactive and prone to reachcritical condition faster. Therefore, it is necessary to be able to distinguish between uranium andplutonium when handling the screening and analysis of nuclear debris’ contents. For this purpose,
another nuclear debris screening method which can determine the existence of uranium and plutonium
with higher accuracy to complement the X-ray CT imaging result is needed.
Local nuclear waste facility Ningyo-Toge has established several techniques on uranium and47

Y. Kusumawati, et al./ X-band electron LINAC-based compact neutron sourcefor nuclear debris and spent fuel on-site screening

plutonium identification in spent nuclear fuel such as fast neutron direct interrogation and gamma-ray
to determine enrichment [5]. But these methods are mainly applicated to uniform, large quantity
nuclear waste drums, aside from the measurement equipment being heavy and stationary. Therefore, itwould be unsuitable for the purpose of this research, as its approach is the on-site screening.
Measurement targets are the smaller, mixed-elements nuclear debris just to provide a rough estimationof criticality before it is stored in the waste drums. According to the simulation of nuclear debris
formation through slow and rapid cooling performed by Kazakhstan National Nuclear Center (NNC),
the reactor core nuclear debris are expected to be rock-shaped and powder-shaped with average
diameter of 5~10 mm [6].
One of other practical methods for identifying the mixed composition of a nuclear debris sample
is via neutron resonance transmission analysis (NRTA). In NRTA, the neutron energy is measured via
a neutron time-of-flight (TOF) method using a pulsed neutron source, which enables the specificationof the neutron energy to be absorbed by the corresponding nuclide in the sample. As a reference
regarding this method, a research on uranium and other minor actinides identification for Fukushima
nuclear debris using NRTA has been conducted by the cooperation between Japan Atomic Energy
Agency (JAEA) and Joint Research Center (JRC) [7]. Their main subject for identification is therock-like and particle-like mixed-elements nuclear debris, which analysis is more complex andchallenging due to their uncertainties in various aspects. They did an experiment using Geel Electron
Linear Accelerator (GELINA) with 100 MeV energy and flight path of 10-12.5 meters. As the result,they have successfully identified each element contained in a mixed sample that represents nuclear
debris.
It is proved that NRTA could be an effective way to analyze mixed-elements nuclear debris.
However, many existing pulsed neutron sources are large in size and require a long flight path, andtherefore, on-site use near the site of the nuclear reactor is impossible. On the other hand, transportinga sample to the large neutron TOF facility is not possible, since the transportation of fuel debris isstrongly limited by the management in regards to nuclear security. In this case, there is a need for acompact and portable pulsed neutron source that can be used for on-site NRTA in thedecommissioning operations, capable of short-distance TOF within a few meters. Through analytical
calculations, it was determined that 2.5 m of TOF can only measure neutron energies up to 100 eV.
Using this information as reference, we can determine the list of uranium and plutonium isotopes withneutron energy absorption values below 100 eV, where those isotopes will be the identification targets
for this on-site NRTA system. Based on the Japanese Evaluated Nuclear Data Library (JENDL)
reference of neutron reactions cross-section values for each isotope and considering their abundance,
the isotopes subjected to identification would be 238U, 235U, 239Pu, 240Pu, and 242Pu [8].
Figure 2 shows the result of an NRTA Monte Carlo simulation using PHITS with compact
neutron source that produces a total of 108 neutron. The flight path length is set at 2.5 meters, basedon estimation of the distance needed to obtain up to 100 eV neutron energy spectrum. Setting for themeasurement sample is a plate of uranium and plutonium mix containing the aforementioned isotopes
with size of 30×10 mm2 and 0.5 mm thickness. The neutron detector is 3He proportional counter withdimension of 30×10×30 mm3 and 2 atm pressure. This simulation result shows that the positions of
the resonance peaks of plutonium and uranium isotopes occur in different neutron energy ranges andcan be distinguished from each other. According to the plot, the resonance peaks for isotopes of
plutonium are in the 0.1–5 eV range of the neutron energy spectrum, and 6–50 eV for isotopes ofuranium.
Based on this information, our objective for this on-site screening system is to observe theseselected resonance peaks to confirm the existence of uranium and plutonium in a nuclear debris
sample, as pictured in Fig. 3. In this investigation, the demonstration of short-distance TOF
measurement and NRTA using a compact and portable neutron source was conducted for analyzing
nuclear fuel debris. This was realized by combining an X-ray source with photonuclear target. X-band
electron LINAC with a maximum energy of 3.95 MeV was coupled with a tungsten target to produce
X-rays. A beryllium block serves as the photonuclear target, and a 3He proportional counter was used
for neutron detector. For the TOF line at 2.5 m, the measurement of the resonance absorption of theneutrons in the region of 0.1–70 eV, which is the energy region usable for specifying 238U, 235U, 239Pu,
240
Pu, and 242Pu, was demonstrated.

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Y. Kusumawati, et al./ X-band electron LINAC-based compact neutron sourcefor nuclear debris and spent fuel on-site screening

Fig. 2. Plot of an NRTA simulation result for a material with different contents. The resonance peaks
represent the specific isotopes (only 238U, 235U, 239Pu, 240Pu, and 242Pu were used in this simulation).

Fig. 3. Diagram of the short-distance time-of-flight concept for research objective.

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Y. Kusumawati, et al./ X-band electron LINAC-based compact neutron sourcefor nuclear debris and spent fuel on-site screening

2. Short-distance TOF system using X-band electron LINAC-based neutron
sourceA pulsed neutron source is needed to perform neutron TOF measurements. Usually, this kind ofneutron source is produced by a large-sized, high-energy particle linear accelerator. However, byusing the X-band type electron LINAC, which is characterized by a small size with a high frequency,
the size of the neutron source can be greatly reduced, and there is even the potential for mobile use.
Unlike neutron sources based on 2H(d,n)3He (D-D) or 3H(d,n)4He (D-T) fusion reactions, which target
materials are radioactive substances requiring special control, a pulsed neutron source with highmobility can be realized by using beryllium target. A previous study on the utilization of compact 5
MeV electron LINAC to produce pulsed neutron using beryllium has been successfully proven [9],and becomes the reference of the current neutron target. In other related research, 3.95 MeV X-band
electron LINAC has been successfully utilized for on-site non-destructive infrastructure inspection[10]. The neutron source version with additional neutron target has also been developed for moisturedetection in bridge structure [11].
By combining 3.95 MeV X-band LINAC-based X-ray source system with a beryllium target, acompact and portable pulsed neutron source can be assembled. The 3.95 MeV compact neutron source
has a relatively short pulse width of 1–4 µs, which enables TOF measurements at a short distance ofapproximately several meters. Figures 4, 5 and 6 show the schematic of the X-band electron
LINAC-based neutron source, and Table 1 lists its parameters. With an approximate total size of 1 m2
(not including the neutron flight path), it is a very compact pulsed neutron source system with highmobility.

Fig. 4. Schematic of the compact X-band electron LINAC-based neutron source.

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Y. Kusumawati, et al./ X-band electron LINAC-based compact neutron sourcefor nuclear debris and spent fuel on-site screening(a) Neutron TOF measurement setup(b) 3.95 MeV electron LINAC
Fig. 5. Images of the short-distance TOF experiment setup. (a) neutron TOF measurement setup and (b)3.95 MeV electron LINAC.

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Y. Kusumawati, et al./ X-band electron LINAC-based compact neutron sourcefor nuclear debris and spent fuel on-site screening

Fig. 6. Image of the 3.95 MeV electron LINAC with neutron target.
Table 1. Parameters of the 3.95 MeV X-band electron LINAC-based neutron source [12].

Operating frequency
RF source
Input RF power
Length of acceleration tube
Form of acceleration tube
Accelerating cell
Number Cells coupling
Filling time
Shunt impedance
Focusing fashion
Voltage of electron gun
Electron gun type
Electron beam size
Beam energy
Beam current
Pulse width
Pulse frequency
Tungsten target size (ø×thickness)
Neutron intensity (calculation)

3.95 MeV
9.3 GHz
Magnetron
930 kW
500 mmSide coupled structure
Half 1 + full 20
3%0.23 μs
110-130 MΩ/m (Regular part)
RF focusing
20 KV
Triode
5 mm
3.95 MeV
80 μA
2.5 μs220 pulses/s
5×0.5 mm
3.86×107 n/ s

The 100-kilograms photo-neutron target was composed of 230×230×230 mm3 cube of leadwhich mainly serves as gamma ray shielding, filled with boric acid resin layer for neutron shielding52

Y. Kusumawati, et al./ X-band electron LINAC-based compact neutron sourcefor nuclear debris and spent fuel on-site screening

and lead beam collimator. An L-shaped empty space is available inside to put in beryllium, as well as
collimator for X-ray and neutron beam. By using Monte-Carlo code and PHITS, the simulation of thispulsed neutron source shows that the neutron yield is 7.13×106 n/s at the surface of the beryllium.
Figure 7 shows the schematic of the neutron target used in this compact neutron source system [11].
In order to be able to get resonance neutron within the energy of 100 eV, a polyethylene neutron
moderator with 20 mm thickness was put in front of the beryllium inside the lead beam collimator.

Fig. 7. Schematic of the neutron target in compact neutron source.

3.

Experiment on neutron TOF measurement

Experiments on neutron TOF measurements using a compact neutron source system have been
performed and Fig. 8 shows the experimental setup used. The setup is divided into three main
components: neutron source component, signal amplifier component, and digital signal processing
component. The realized configuration of the neutron detector in the neutron source part is shown in
Fig. 9.

Fig. 8. Experimental setup to measure neutron TOF.

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Y. Kusumawati, et al./ X-band electron LINAC-based compact neutron sourcefor nuclear debris and spent fuel on-site screening

Fig. 9. Neutron detector window setup. Boron Carbide was used as housing for 3He proportional counter,with additional boron block for shielding.

In the neutron source component, the X-band electron-LINAC based neutron source produced
pulsed neutrons, which were detected by the 3He proportional counter. Then in the digital signal
processing component, neutron TOF is acquired using a field-programmable gate array (FPGA). The
recorded data is later manually converted to an energy axis and plotted into a neutron energy spectrum
graph. An example of an acquired neutron spectrum for 1-hour measurement is shown in Fig. 10.

Fig. 10. Neutron energy spectrum obtained from 3.95 MeV X-band electron LINAC-based neutron
source.

From the result, the observable pulsed neutron spectrum energy range is up to 70 eV. The settings

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Y. Kusumawati, et al./ X-band electron LINAC-based compact neutron sourcefor nuclear debris and spent fuel on-site screening

in this experiment serve as the default condition and will be the reference for the next TOF
measurement experiments using sample materials.

4.

Neutron Resonance Transmission Experiment

The next experiment involved testing the feasibility of the proposed system for NRTA. ThroughTOF measurement, the identified resonance peak in the experimental data can be used to identify theisotopes contained within the sample material. For safety reasons, instead of nuclear debris, thisexperiment used non-radioactive dummy materials as samples. These dummy samples have neutronenergy absorption value similar to that of 238U and 240Pu. The configuration of the sample and its size
is shown in Fig. 11.

(a) sheet-shaped sample with 0.6 mm thickness (covers whole detector area)(b) Block-shaped sample, with 1.2 mm thickness (smallest sample size)
Fig. 11. Images of neutron detector and sample setup for short-distance NRTA experiment. (a)sheet-shaped sample with 0.6 mm thickness (covers whole detector area) and (b) Block-shaped sample,
with 1.2 mm thickness (smallest sample size).

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Y. Kusumawati, et al./ X-band electron LINAC-based compact neutron sourcefor nuclear debris and spent fuel on-site screening

The materials for the isotope detection experiment are pure metal plate samples of indium andtungsten. They were chosen because their isotopes have relatively low neutron absorption energy witha high cross-section value according to the reference from JENDL. Therefore, the resonance peaks
would be easier to observe within this compact neutron source energy spectrum. Indium has neutron
resonance value close to that of plutonium isotopes between the range of 0.1–5 eV in the neutronenergy spectrum, and tungsten to that of uranium isotopes between the range of 6–50 eV. The
plate-shaped samples with thickness of 0.1 mm each were located in front of the 3He detector, andsince the sample is larger than the detector window, the effective detection area would be 20×80 mm2.
Three plates of indium and tungsten each were stacked alternately together to form a total plate
thickness of 0.6 mm where indium was exposed by neutrons first and tungsten last. The obtained
neutron spectrum for the mixed sample measurement is shown in Fig. 12.
The detected neutron spectrum shows several resonance peaks of different energies that
correspond to different isotope materials that are sufficiently distinguished from each other to be used
for identification of the materials in the sample. The value of the resonance peaks matches thereferences from JENDL for the isotopes 115In (1.37 eV and 3.55 eV) and 186W (14.2 eV), indicatingthat this system is able to detect several different isotopes in a mixed sample simultaneously. Thismeasurement result satisfies the condition where there are distinguishable resonances peak in the
0.1–5 eV range and in the 6–50 eV range, which represents plutonium and uranium isotopes area,
respectively. It also shows that the system will be able to identify whether the detected resonance peakbelongs to plutonium or uranium isotopes in a one-time measurement.

Fig. 12. Neutron energy spectrum obtained from measurement of indium and tungsten mixed platesample with 0.6 mm thickness and 20×80 mm2 detection area. Multiple energy resonance peaks of therespective material in the sample can be observed.

The energy resolution (∆E/E) of a TOF system is related to the time resolution (overall timedispersion divided by flight path lengths), which can be calculated using Eq. (1).

E 2 ET 2  L2
2
E
L 

(1)

In Eq. (1), L is the flight path length, and ∆L is flight path uncertainty, calculated as shown in Eq.
(2), where h is half of the detector’s length.

56

Y. Kusumawati, et al./ X-band electron LINAC-based compact neutron sourcefor nuclear debris and spent fuel on-site screeningL  L2  h 2  L(2)∆T is the TOF uncertainty, which in this case is significantly affected by the LINAC pulse width,and α is 72.3 µs, which is the time of flight of a 1 eV neutron to cover a distance of 1 meter [13].
Table 2 shows the calculated energy resolution for this short-distance TOF system. For the threepoints of neutron energy of interest where the neutron absorptions happen and resonance peaks wereformed, the energy resolution is within 3~10%. Compared to the large TOF facility with a long flightpath and a neutron source pulse width of 1 ns with the energy resolution value of 1% [14], oursystem’s energy resolution seems reasonable, and corresponding isotopes’ resonance peak can still be
distinguished by visual observation of the plot.
As a system that uses neutron TOF measurement, with its compactness and mobility, this X-band
electron LINAC-based neutron source would be the most advantageous and optimum device to be
implemented in an on-site screening system of plutonium and uranium inside nuclear debris.
Table 2. Energy resolution calculation of the compact TOF system with 3He proportional counter.

Neutron energy of interest
(eV)
1.37
3.55
14.20

Pulse width (µs)

L (m)

h (m)

∆T (µs)∆L (m)

∆E/E

2.5
2.5
2.5

2.5
2.5
2.5

0.015
0.015
0.015

2.5
2.5
2.5

3.05×10-2
3.05×10-2
3.05×10-2

3.24%
5.21%
10.42%

To determine whether the resonance peaks can still be observed when the sample is smaller thanthe detection area and closer to solid block rather than sheet samples, measurements using different
sample size and thickness were used. By gradually decreasing the size and increasing the thickness
and perform 1-hour NRTA on each of them, the minimum size and weight of the measurable samples
by using 3.95 MeV X-band electron LINAC-based pulsed neutron source system can be determined
as well. Figure 13 shows the plot of the neutron TOF measurement by using the smallest sample size
possible, which is 20×20 mm2 and 1.2 mm thickness, measured for 2 hours to lessen the fluctuationin the neutron counts especially in the uranium and plutonium isotopes resonance peak area.
In order to let the system to measure smaller fuel debris of a few mm solid, we have to increase
the neutron intensity. For the improvement of neutron source’s intensity, several methods as neutron
target (beryllium) size modification, shorter neutron source’s pulse width, and LINAC power capacity
increase are possible, which contribute to the increase of below ten. If we keep only the compactnessof system, 35 MeV X-band LINAC neutron source with 1 klystron unit of 8×1011 n/s is promising[15]. Based on our detection of 20×20 mm2 and 1.2 mm thick sample for 2 hours, the above intensity
enhancement of the four order (3.86×107 to 8×1011 n/s) enables detection of U and Pu in a few mm
fuel debris within order of ten seconds.

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Y. Kusumawati, et al./ X-band electron LINAC-based compact neutron sourcefor nuclear debris and spent fuel on-site screening

Fig. 13. Neutron energy spectrum obtained from measurement of indium and tungsten mixed platesample with 1.2 mm thickness and 20×20 mm2 detection area. The resonances formed within therespective isotopes resonance peak area determined from the simulation result can be observed.

5.

Compact and mobile X-ray source using X-band electron LINAC

In the past research, application of mobile X-band electron LINAC-based X-ray source has beendeveloped for on-site non-destructive inspection of infrastructure. As a substitute for the conventional
300 keV X-ray tube assembly NDT system that needs a long measurement time to get X-ray
radiographic image, a portable high intensity X-band LINAC-based X-ray source to inspect a PC
bridge inner wire condition was developed. The electron LINAC operating with an X-band frequency
can cause electrons to reach a high energy within a short distance owing to its high acceleration
frequency, leading to a compact system that can reach higher energies compared to conventional
X-ray sources. Figure 14 shows the photo of upgraded 950 keV X-band electron LINAC-based X-ray
source system. The upgraded side-coupled 950 keV system can provide X-ray intensity of almost
design value (0.05 Gy/min at 1 meter) at 2.5 μsec RF pulse and 330 pulse per second (pps) [10]. Thissystem has also been registered at local agency of labor supervision to comply within Regulations on
Prevention of Ionizing Hazard, allowing the external use of this X-ray source as the energy is below 1
MeV.

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Y. Kusumawati, et al./ X-band electron LINAC-based compact neutron sourcefor nuclear debris and spent fuel on-site screening

Fig. 14. Upgraded 950 keV X-band electron LINAC system.

The principle of X-ray imaging and atomic number (Z) identification is through the measurementof linear attenuation coefficient (μ) of the corresponding elements. The larger the Z of an element, thehigher the μ value will be. By using polychromatic X-ray source generated through 950 keV X-band
LINAC, imaging of the sample as well as measurement of its μ value can be performed, obtaining
necessary data for Z identification of the elements contained in the nuclear debris. Figure 15 shows
the schematic for the X-ray CT reconstruction experiment using nuclear debris dummy sample
consists of Fe and Pb, and 5 mm Pb plate as X-ray filters to eliminate the beam hardening effect. In
this experiment, Pb was used to model U and Pu as material with high number of Z. The spatial
resolution of our X-ray CT reconstruction is 1 mm according to the result from this system’s previous
application for bridge inspection [10][16].
Figure 16 shows the plot of attenuation coefficient ratio of measurement result with 950 keV
X-ray source to one with 3.95 MeV, for different thickness of Fe and Pb samples. It is clear the ratio
is rather insensitive for the sample thickness. The imaging result, which also shows a high contrast
between the materials with large gap of Z value, will be able to provide data of the corresponding
material’s size and weight inside the nuclear debris necessary for criticality calculation. As aproof-of-principle, this result proves that the 950 keV X-band electron LINAC-based polychromatic
X-ray source can be used for atomic number identification of the material inside nuclear debris,
especially in distinguishing uranium and plutonium with heavy Z from other material inside nuclear
debris with lighter Z (zirconium, SUS, etc.). If a preliminarily stack database on correlation of
attenuation constants and elements measured by both X-ray CT and NRTA were made, U and Pu canbe identified. By combining 3D X-ray CT reconstruction results with the resolutions of 1 mm and~100 mg, the density of U and Pu can be estimated for criticality safety evaluation.

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Y. Kusumawati, et al./ X-band electron LINAC-based compact neutron sourcefor nuclear debris and spent fuel on-site screening(a) Model sample with Pb and Fe, where Pb simulates U/Pu elements (heavy Z)(b) Schematic of CT imaging in full cycle while rotating the sample twice(c) Cross-section of the image reconstruction of the model sample
Fig. 15. Images related to the X-ray CT reconstruction experiment using nuclear debris dummy sample.
(a) Model sample with Pb and Fe, where Pb simulates U/Pu elements (heavy Z), (b) Schematic of CT
imaging in full cycle while rotating the sample twice, and (c) Cross-section of the image reconstruction of
the model sample.

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Y. Kusumawati, et al./ X-band electron LINAC-based compact neutron sourcefor nuclear debris and spent fuel on-site screening

Fig. 16. Plots of attenuation coefficient ratio, calculated from measurement result with 950 keV X-ray
source to one with 3.95 MeV for Fe and Pb sample with different thickness.

6. Proposal of practical system for fuel debris mass extraction and criticality
safetyThe main objective of this research is to make a meaningful contribution to criticality
calculations for nuclear debris containers, via the determination of uranium and plutonium material
density. Short-distance NRTA will complement X-ray CT system used to study nuclear debris,whereby this measurement method combination can be used to estimate the size and weight of theuranium and plutonium content in the debris, which is necessary for material density calculation. Dueto its limited sample shape for measurement and long measuring time, for the consideration ofpractical use of the screening system, this short-distance NRTA will be used at the test extraction stepfor the purpose of system calibration, as shown by the scheme in Fig. 17. When mass extraction of
nuclear debris happens, the actual screening process will be conducted by a fast line detector system
by polychromatic X-ray CT of partial angle and parallel motion that is able to do the measurement
faster for several nuclear debris samples.
Concerning the gamma rays emitted by the nuclear debris, since its energy would be very high(several hundred keV ~ several MeV) that it almost matches X-ray sources’ energy, methods to reduce
it will need further evaluation. Using higher energy X-ray source can cause beam hardening andmakes material estimation difficult. On the other hand, shielding around the sample cannot be too
heavy that it would stop X-rays from the source itself and worsen the S/N ratio. Therefore, one of theplausible solutions is to optimize the use of small debris sample and intense X-ray source.
Figure 18 shows the schematic of the line detector system and how it contributes in the criticality
estimation before proceeding to nuclear debris storage. In the current stage of development, artificial
elongation of reconstructed image happens due to the partial angle CT. This error has been evaluatedfor the X-ray bridge inspection and the diameter of reinforced iron wires and rods can be estimatedwith the spatial resolution of 1 mm [10][16]. The same treatment is planned to be introduced for the
next step of development for the practical design.

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Y. Kusumawati, et al./ X-band electron LINAC-based compact neutron sourcefor nuclear debris and spent fuel on-site screening

Fig. 17. Schematic of the practical combined use between X-ray and neutron method in the preparation of
nuclear debris screening system.

Fig. 18. Schematic of the rapid line detector system for actual nuclear debris screening system andnuclear debris storage.

7.

Conclusion

We have succeeded in making the proof of principle of short-distance NRTA system using
compact pulsed neutron source for the application of on-site nuclear debris screening, supported by
the experiment results so far. A compact pulsed neutron source with high mobility can be constructed62

Y. Kusumawati, et al./ X-band electron LINAC-based compact neutron sourcefor nuclear debris and spent fuel on-site screening

from an X-band electron LINAC coupled with photon and neutron converters. From the experimental
result using a 3.95 MeV X-band electron LINAC with tungsten as the photon converter and beryllium
as neutron converter, a neutron TOF energy spectrum up to 70 eV can be measured within 2.5-m TOF
path, with energy resolution of 4%. This system has also successfully been used to perform NRTA tosimultaneously detect neutron resonance peaks in the energy range up to 30 eV for multiple elements
within a sample. Based on the experiment result, it can be concluded that in principle, theshort-distance NRTA system using a 3.95 MeV X-band electron LINAC-based compact neutron
source would be able to confirm the existence of uranium and plutonium isotopes in a nuclear debris
sample. With its compact size, mobility, and adequate level of accuracy for screening purposes,
X-band electron LINAC-based compact neutron source can be implemented in an on-site screening
system. It will be able to obtain nuclear debris composition data necessary for nuclear debris activity
mapping of The 1F reactor core area, as a complementary in the calibration system for practicalapplication of portable line detector X-ray CT. We are going to propose more concrete and practicalsystem for the purpose.
For the improvement of neutron source’s intensity, further optimization of the 3.95 MeV system
is needed and even future development of 35 MeV system can be discussed. Aside from the on-site
nuclear debris screening, this compact and mobile uranium and plutonium detection system also has
potential for non-destructive analysis use in other field, such as nuclear security to prevent terrorism
act.

Acknowledgement
This work was supported by the Ministry of Education, Culture, Sports, Science and Technologyof Japan (MEXT) and Japan-UK collaboration with Japan Atomic Energy Agency (JAEA), The
University of Sheffield and The University of Bristol.

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