2.1 Introduction
Quantifying nuclear materials (NM) in the debris of melted fuel (MF) formed in a
severe accident is considered to be difficult because of their variety of size, shape,
unknown composition, and strong radioactivity. Although techniques of nondestructive assay (NDA) are indispensable for the evaluation of NM in debris,
quantification methods have not been established so far [1]. In the cases of TMI-2
or Chernobyl-4, accounting for the NM was based on some estimations.
We have proposed a technique called neutron resonance densitometry (NRD)
[2, 3] to quantify NM in particle-like debris that is assumed to be produced in the
rapid cooling processes of a severe accident [4]. Small pieces are also produced
when MF are cut or broken down to be taken out of the damaged reactors [1].
To examine the NRD method, studies have begun. Some experiments were
carried out at the time-of-flight (TOF) facility GELINA [5] of EC-JRC-IRMM
under the agreement between JAEA and EURATOM in the field of nuclear
materials safeguards research and development.
In this chapter, we briefly describe the concept of NRD, give an overview of the
development of NRD, and explain some parts of the recent progress.
2.2 Neutron Resonance Densitometry
2.2.1 The Concept of NRD
Neutron resonance densitometry is a method of a combination of neutron resonance
transmission analysis (NRTA) and neutron resonance capture analysis (NRCA) or
prompt gamma-ray analysis (PGA). The fundamental principles of NRTA and
NRCA are described by Postma and Schillebeeckx [6].
In NRTA, neutron transmission is measured as a function of neutron energy with a
TOF technique. Characteristic neutron transmission dips of Pu and U isotopes
are observed in the neutron energy in the range of 1–50 eV [7, 8]. Measurements of
these transmission spectra can be carried out with a short-flight path TOF system [9, 10].
Although strong γ-ray radiation from MF samples does not interfere with NRTA
measurements, reduction of neutron flux caused by nuclei with large total
cross section (such as H, B, Cl, Fe) makes accurate NM quantification difficult.
Nevertheless, the quantities of these contained nuclei could not be determined by
NRTA only, because these nuclei do not resonantly interact with neutrons in this
energy range. To identify and to quantity the composing isotopes, the NRCA/PGA
method is required. Characteristic prompt γ rays ware utilized. Table 2.1 shows
prompt γ-rays emitted from nuclei after neutron capture reaction. Most of these
discrete prompt γ-rays have significant intensities. The information obtained by
NRCA/PGA enables us to determine the appropriate sample thickness and measurement time. This information also supports NRTA analysis.
14
M. Koizumi et al.
Quantifying nuclear materials (NM) in the debris of melted fuel (MF) formed in a
severe accident is considered to be difficult because of their variety of size, shape,
unknown composition, and strong radioactivity. Although techniques of nondestructive assay (NDA) are indispensable for the evaluation of NM in debris,
quantification methods have not been established so far [1]. In the cases of TMI-2
or Chernobyl-4, accounting for the NM was based on some estimations.
We have proposed a technique called neutron resonance densitometry (NRD)
[2, 3] to quantify NM in particle-like debris that is assumed to be produced in the
rapid cooling processes of a severe accident [4]. Small pieces are also produced
when MF are cut or broken down to be taken out of the damaged reactors [1].
To examine the NRD method, studies have begun. Some experiments were
carried out at the time-of-flight (TOF) facility GELINA [5] of EC-JRC-IRMM
under the agreement between JAEA and EURATOM in the field of nuclear
materials safeguards research and development.
In this chapter, we briefly describe the concept of NRD, give an overview of the
development of NRD, and explain some parts of the recent progress.
2.2 Neutron Resonance Densitometry
2.2.1 The Concept of NRD
Neutron resonance densitometry is a method of a combination of neutron resonance
transmission analysis (NRTA) and neutron resonance capture analysis (NRCA) or
prompt gamma-ray analysis (PGA). The fundamental principles of NRTA and
NRCA are described by Postma and Schillebeeckx [6].
In NRTA, neutron transmission is measured as a function of neutron energy with a
TOF technique. Characteristic neutron transmission dips of Pu and U isotopes
are observed in the neutron energy in the range of 1–50 eV [7, 8]. Measurements of
these transmission spectra can be carried out with a short-flight path TOF system [9, 10].
Although strong γ-ray radiation from MF samples does not interfere with NRTA
measurements, reduction of neutron flux caused by nuclei with large total
cross section (such as H, B, Cl, Fe) makes accurate NM quantification difficult.
Nevertheless, the quantities of these contained nuclei could not be determined by
NRTA only, because these nuclei do not resonantly interact with neutrons in this
energy range. To identify and to quantity the composing isotopes, the NRCA/PGA
method is required. Characteristic prompt γ rays ware utilized. Table 2.1 shows
prompt γ-rays emitted from nuclei after neutron capture reaction. Most of these
discrete prompt γ-rays have significant intensities. The information obtained by
NRCA/PGA enables us to determine the appropriate sample thickness and measurement time. This information also supports NRTA analysis.
14
M. Koizumi et al.