Sample thickness effect on nuclear material quantification with NRTA for
particle like debris of melted fuel
H. Tsuchiya1, H. Harada1, M. Koizumi1, F. Kitatani1, J. Takamine1,
M. Kureta1, H. Iimura1, B. Becker2, S. Kopecky2, K. Kauwenberghs2,
A. Moens2, W. Mondelaers2, P. Schillebeeckx2
1
Japan Atomic Energy Agency (JAEA)
Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan
E-mail: [email protected], [email protected]
2
EC-JRC-IRMM
Unit D.4. Standards for Nuclear Safety, Security and Safeguards
Retieseweg 111, B-2440 Geel, Belgium
E-mail: [email protected]
Abstract:
A method to quantify the amount of uranium and plutonium in melted fuel derived from a
nuclear accident such as the one occurred at the Fukushima Daiichi nuclear power plant has not
been established yet. For this reason, neutron resonance densitometry, combining neutron
resonance transmission analysis and neutron resonance capture analysis, is proposed and its
feasibility study has been defined in a collaboration between Japan Atomic Energy Agency
(JAEA) and Joint Research Center, Institute for Reference Materials and Measurements (JRCIRMM). Within this contribution, transmission experiments using three Cu metal disks with
different thickness of 0.125 mm, 0.25 mm, and 0.7 mm were made between November 2012 and
February 2013 at the Geel Electron LINear Accelerator (GELINA) to investigate sample
thickness effect on neutron resonance transmission analysis. We experimentally derived the
areal density for the individual Cu samples with the resonance shape analysis code REFIT, and
then compared them with the declared areal density. It was found that the REFIT-evaluated areal
density was consistent with declared ones for each sample.
Keywords: neutron resonance transmission analysis; neutron time-of-flight; melted fuel; Cu sample
1. Introduction
The earthquake and the subsequent gigantic tsunami, which occurred on March 11, 2011 in
Japan, resulted in a failure of electricity of the Fukushima Daiichi nuclear power plants. As a
consequence, the cooling system for the nuclear fuel installed at the reactors Units 1−3 stopped
its operation. Consequently, the nuclear fuel melted in the pressure vessels. In addition, a certain
part of melted fuel (MF) most likely penetrated through the pressure vessel. Therefore, it is
possible that melted fuel dropped on the concrete floor and solidified together with other
surrounding materials. At present, it is planned that MF will be removed from the reactors after
a cooling time of at least 10 years.
From the viewpoint of nuclear safeguards and security, special nuclear materials (SNM) of
uranium and plutonium in the MF should be accurately quantified after its removal. A possible
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technique for the measurement is neutron resonance transmission analysis (NRTA). A detailed
description of its application for various materials as well as principle of NRTA is given in Refs.
[1, 2]. With respect to its application for nuclear fuel, Bowman et al. [3] and Behrens et al. [4]
applied NRTA to quantify SNM in fresh and spent fuel pins and determined the abundance of
239,240,242
Pu and 235,236,238U in the pins with an accuracy better than 4%. Furthermore, a feasibility
study with Monte Carlo simulations was carried out to investigate how NRTA can quantify the
amount of SNM in spent fuel from commercial light water reactors [5]. This showed the great
potential of NRTA to assay intact spent fuel assemblies. However, unlike such fresh and spent
fuel assemblies, a method to assess SNM in MF caused by a severe accident like the Fukushima
case has not been established yet.
It is expected that the accuracy of NRTA for MF will be significantly affected by the
characteristics of the sample to be measured: thickness, inhomogeneity, and presence of
impurities such as 10B and concrete. It is most likely that particle-like debris of MF were formed
due to e.g. a steam explosion [6]. Moreover, debris will also be produced during the removing
process of the MF from the reactors. Therefore, such particle-like debris will have a wide variety
of size and composition, which complicates the measurement. Accordingly a new technique that
considers these difficulties is required. For this reason, neutron resonance densitometry (NRD),
which is based on NRTA combined with a kind of neutron resonance capture analysis (NRCA),
is proposed and under development. A description of NRD can be found in Ref [7,8]. In addition,
its feasibility studies considering 10B contamination in MF were made with numerical
calculations [9,10]. Also, particle size effect on NRTA was examined in Ref [11].
In this paper, we focus on a NRTA experiment conducted at GELINA. The experiment utilized
three Cu-metal disk samples with different thickness in order to investigate the sample thickness
effect on NRTA. We first present the basic of NRTA. Then, experimental transmissions for the
three Cu samples are shown. Lastly, we compare the declared areal density for the Cu samples
with the ones determined using the resonance shape analysis code REFIT [12].
2. NRTA experiment
2.1. Basic of NRTA experiment
NRTA utilizes an intense pulsed white neutron source as a diagnostic beam. The probability that
a neutron beam is transmitted through a sample is measured as a function of neutron energy. The
neutron energy is obtained by the time-of-flight (TOF) technique. That is by measuring the time
difference between a start signal and a stop signal, provided by the neutron detector and
accelerator, respectively. The transmitted spectrum has characteristic dips resulting from
resonance structures in neutron-induced reaction cross sections of nuclides in the sample.
In an actual measurement, the observed quantity is the fraction of the neutron beam that
traverses the sample without any interaction. For a parallel neutron beam that is perpendicular to
a sample material, the transmission T is represented by
T = 𝑒 ! ! !! !!"!,!
,
(1)
where 𝜎!"!,! is Doppler broadening total cross section and nk is the number of atoms per unit
area of nuclide k, which is also denoted as areal density.
2
Experimentally, the transmission Texp is computed from the ratio of the counts of a sample-in
measurement Cin and a sample-out measurement Cout, after subtraction of the background
contributions Bin and Bout, respectively,
𝑇!"# = 𝑁!
𝐶!" − 𝐵!"
,
𝐶!"# − 𝐵!"#
(2)
where NT is a normalization factor that is the ratio of the total intensities of the incident neutron
beam during the sample-out and sample-in cycles. The background (Bin and Bout) is determined
by an analytical expression applying the black resonance technique [13].
Eq. (2) indicates that the experimental transmission is independent of both the detection
efficiency and neutron flux incident to the sample. Therefore, we can consider that NRTA
provides an absolute measurement that does not require additional calibration experiments.
2.1. Experiments at GELINA
Thickness
(mm)
0.125
0.25
0.7
Diameter
(mm)
80
80
80
Mass
(g)
5.25
11.1
31.4
Areal density
(at/b)
9.80×10-4
2.09×10-3
5.92×10-3
Table 1: Characteristics of individual Cu samples. The declared areal density is derived from a measurement of the
weight and the area.
To investigate the applicability of NRD, the Japan Atomic Energy Agency and the Institute for
Reference Materials and Measurements of the Joint Research Centre (EC-JRC-IRMM) started
collaboration in 2012. Within the collaboration various experiments are scheduled at the neutron
TOF facility Geel Electron LINear Accelerator (GELINA) of the EC-JRC-IRMM. The
experiments focus on the influence of the characteristics of debris on the accuracy of NRD, in
particular, the impact of the thickness of the sample, distribution of the particle size, the
presence of neutron absorbing impurities, and the radioactivity of the sample.
The sample thickness effect on NRTA was investigated between November 2012 and February
2013 at GELINA of the EC-JRC-IRMM. Here, we give a brief description on the neutron TOF
facility of GELINA. A linear electron accelerator delivers a very short electron pulse with
energies up to 150 MeV and a maximum repetition frequency of 800 Hz. Electron bunches, with
peak currents of 12 A in a 10 ns time interval, are compressed by a compression magnet to a
width of less than 1 ns. Neutrons are produced via photonuclear and photofission reactions by
electrons impinging on a rotating target consisting of an U-Mo alloy. To produce a white
neutron spectrum from thermal energy up to a few MeV, two 4-cm thick beryllium containers
filled with water, placed beneath and above the target, were used as moderators. Produced
pulsed neutrons travel through vacuum flight paths to measurement stations in which detectors
and samples are installed. There are 10 flight paths with a length ranging from 10 m up to 400 m.
Several measurement stations are arranged at various nominal distances of 10, 30, 50, 60, 100,
200, 300, and 400 m. A detailed description of GELINA including the accelerator and its
neutron-producing target can be found in Ref. [14].
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For the purpose of assessing the performance of
NRTA for the determination of the areal density,
transmission experiments using Cu samples were
performed at a 25 m neutron flight path with the
accelerator operating 800 Hz. The Cu samples
consisted of metal disks with a different nominal
thickness of 0.125, 0.25, and 0.7 mm. Table I gives
the characteristics of the Cu samples. Each sample
was placed at 9 m from the neutron target in a multiposition sample changer. A 6.35 mm thick and 101.6
mm diameter Li-glass scintillator (NE912) enriched
to 95% in 6Li detected neutrons penetrating through
the samples. The Li-glass scintillator was equipped
with a boron-free quartz windowed photomultiplier
(RMI9823-QKB). The detector was placed at 25 m
from the face of the moderator viewing the flight path.
In all measurements, we used a 10B overlap filter and
black resonance filters of sulphur, bismuth, and cobalt.
3. Results
Figure 1 (Top panel) shows experimental
transmission [Eq. (2)] for the different Cu samples,
together with uncertainties only due to counting
statistics. For comparison, the total cross sections for
63
Cu and 65Cu taken from JENDL-4.0u [15] are
plotted in the bottom panel. It is found that several
dips are seen in the experimental transmission,
corresponding to resonance structures in the Cu cross
sections (bottom). Especially, three pronounced dips at
TOF of 39 𝜇𝑠, 72 𝜇𝑠, and 115 𝜇𝑠 exist in all the
transmission, corresponding to resonance energies of
2038 eV, 579 eV, and 230 eV, respectively. In this
study, we used the 2038-eV resonance dips to derive
the areal density of each sample. Figure 2 shows the
transmission at the 2038-eV resonance region. It
clearly shows that the dip structure in the transmission
is stronger for a thick sample.
We analyzed the transmission data (Fig. 2) with the
resonance shape analysis code REFIT [12], to derive
the areal density for the 0.125-mm, 0.25-mm, and 0.7mm thick Cu samples. Figure 3 compares the
experimental transmission with the transmission
evaluated by REFIT. The evaluated values are found to
reproduce well the measured transmission (𝜒 ! /𝑑. 𝑜. 𝑓=
1228/989). Figure 4 presents the ratio of the areal
density derived from the transmission data and the
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Figure 1: Experimental transmission (top) and
corresponding total cross sections (bottom).
Horizontal axis in both panels corresponds to
TOF of neutrons in ns. (Top) Black, red, and
green lines show transmission with Cusample thickness of 0.125, 0.25, and 0.7 mm,
respectively. (Bottom) Black and red lines
represent total cross sections of 63Cu and 65Cu,
respectively. Numbers in the panel show
resonance energies.
Figure 2: Experimental transmission close
to the 2038-eV resonance region. Black,
red, and green lines show transmission
through a Cu-sample thickness of 0.125,
0.25, and 0.7 mm, respectively.
declared ones. We may conclude that the derived
areal densities for the individual samples are in very
good agreement with the declared ones.
4. Summary
Figure 3: Comparison between the
measurement for 0.7-mm thick Cu sample
(black points) and the REFIT (red line).
Errors quoted are 1 𝜎 statistical ones.
Horizontal axis shows neutron energy in eV.
In order to quantify SNM in MF caused by a severe
accident such as the Fukushima case, Neutron
Resonance Densitometry is under development. NRD
combines neutron resonance transmission analysis
and neutron resonance capture analysis. One of
activities within the R&D programme of NRD
concerns the assessment of the sample thickness
effect on NRTA. In the present work, using three Cu
disk samples with different thickness of 0.125 mm,
0.25 mm, and 0.7 mm, we performed NRTA
experiments at the GELINA facility of the EC-JRCIRMM as part of collaboration between the JAEA and
EC-JRC-IRMM. Consequently, we found that the
areal density for the individual samples was, within
2%, consistent with the declared values.
5. Acknowledgements
Figure 4: Ratio of the derived areal density to
the declared ones, plotted as a function of
thickness of the Cu sample. Errors originate
from count statistics only.
This work was done under the agreement between
JAEA and EURATOM in the field of nuclear
materials safeguards research and development. This
work is supported by JSGO/MEXT. We are very
grateful for the technical assistance of J.C. Drohe, D.
Vendelbo, and R. Wynants during the measurements
at GELINA.
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