3.1 Introduction
It is surmised that melted fuel debris is present in the cores at units 1, 2, and 3 of
Fukushima Daiichi NPP. Identifying the fuel debris status in the reactors is one of
the most important issues for decommissioning. Therefore, we need to determine
how to analyze the properties of actual debris collected from those cores in advance
of removal work. As the debris contains melted fuel and cladding tube and structure
materials heterogeneously in addition to salt content, nondestructive assaying of the
distribution of nuclear materials within the debris is absolutely essential for nuclear
material accountancy and critical safety.
Neutron resonance densitometry (NRD) [1] with the time-of-flight (TOF) technique based on neutron resonance transmission analysis (NRTA) [2] and neutron
resonance capture analysis (NRCA) [3, 4] is a promising way to characterize the
debris. However, there are two difficulties in applying those methods to fuel debris.
In NRD, many resonances of other nuclides that are contained in the debris may
make it difficult to identify and quantify the target nuclide. In NRCA, it is expected
that the intense decayed gamma rays from debris result in high background and
large dead time of the gamma-ray detector. In this work, we propose a new concept
of the “self-indication method” as a complementary assay to overcome those
difficulties. In the self-indication method, we set an indicator consisting of target
nuclide with a high purity at the beam downstream from a sample. By detecting the
reaction products such as neutron capture γ-rays or fission products from the
indicator with the TOF method, the transmission neutron can be measured indirectly. The self-indicator is a transmission neutron detector that has high efficiency
around the objective neutron resonance energies of the target nuclide, enabling us to
quantify effectively the amount of resonance absorption of the target nuclide.
Moreover, it is not easily affected by the decayed γ-rays from the debris.
In this work, experimental validation for application of the self-indication
method was carried out. A part of the preliminary results is shown in this chapter.
3.2 Experiment
To verify the self-indication method, we have performed three kinds of experiments
using a 46 MeV electron linear accelerator (linac) at the Kyoto University Research
Reactor Institute. The experimental arrangement is shown in Fig. 3.1. Pulsed
neutrons were produced from a water-cooled photo-neutron target assembly,
5 cm in diameter and 6 cm long, which was composed of 12 sheets of tantalum
plates with total thickness of 29 mm [5]. This target was set at the center of an
octagonal water tank, 30 cm long and 10 cm thick, as a neutron moderator. The
linac was operated with a repetition rate of 50 Hz, a pulse width of 100 ns, a peak
current of 5 A, and an electron energy of about 30 MeV. We used a flight path in the
direction of 135
to the linac electron beam. To reduce the gamma flash generated
22
J. Hori et al.
It is surmised that melted fuel debris is present in the cores at units 1, 2, and 3 of
Fukushima Daiichi NPP. Identifying the fuel debris status in the reactors is one of
the most important issues for decommissioning. Therefore, we need to determine
how to analyze the properties of actual debris collected from those cores in advance
of removal work. As the debris contains melted fuel and cladding tube and structure
materials heterogeneously in addition to salt content, nondestructive assaying of the
distribution of nuclear materials within the debris is absolutely essential for nuclear
material accountancy and critical safety.
Neutron resonance densitometry (NRD) [1] with the time-of-flight (TOF) technique based on neutron resonance transmission analysis (NRTA) [2] and neutron
resonance capture analysis (NRCA) [3, 4] is a promising way to characterize the
debris. However, there are two difficulties in applying those methods to fuel debris.
In NRD, many resonances of other nuclides that are contained in the debris may
make it difficult to identify and quantify the target nuclide. In NRCA, it is expected
that the intense decayed gamma rays from debris result in high background and
large dead time of the gamma-ray detector. In this work, we propose a new concept
of the “self-indication method” as a complementary assay to overcome those
difficulties. In the self-indication method, we set an indicator consisting of target
nuclide with a high purity at the beam downstream from a sample. By detecting the
reaction products such as neutron capture γ-rays or fission products from the
indicator with the TOF method, the transmission neutron can be measured indirectly. The self-indicator is a transmission neutron detector that has high efficiency
around the objective neutron resonance energies of the target nuclide, enabling us to
quantify effectively the amount of resonance absorption of the target nuclide.
Moreover, it is not easily affected by the decayed γ-rays from the debris.
In this work, experimental validation for application of the self-indication
method was carried out. A part of the preliminary results is shown in this chapter.
3.2 Experiment
To verify the self-indication method, we have performed three kinds of experiments
using a 46 MeV electron linear accelerator (linac) at the Kyoto University Research
Reactor Institute. The experimental arrangement is shown in Fig. 3.1. Pulsed
neutrons were produced from a water-cooled photo-neutron target assembly,
5 cm in diameter and 6 cm long, which was composed of 12 sheets of tantalum
plates with total thickness of 29 mm [5]. This target was set at the center of an
octagonal water tank, 30 cm long and 10 cm thick, as a neutron moderator. The
linac was operated with a repetition rate of 50 Hz, a pulse width of 100 ns, a peak
current of 5 A, and an electron energy of about 30 MeV. We used a flight path in the
direction of 135
to the linac electron beam. To reduce the gamma flash generated
22
J. Hori et al.