Development of Nondestructive Assay to Fuel Debris of Fukushima Daiichi NPP (1): Experimental Validation for the Application of a Self-Indication Method
Abstract We have proposed a new concept of the “self-indication method” combined with neutron resonance densitometry (NRD) for nondestructive assaying of the distribution of nuclear materials in the fuel debris of Fukushima Daiichi NPP. To verify the method, we performed experiments using a 46 MeV electron linear accelerator at the Kyoto University Research Reactor Institute. First, we measured the area densities of gold foil 10, 20, 30, 40, and 50 μm thick by area analysis at the
4.9 eV resonance region. It was confirmed that the area densities of the target nuclide can be determined by conventional NRD and the self-indication method within 3 % accuracy, respectively. As the next step, we added a silver foil of 50 μm thickness to a gold foil of 10 μm thickness and measured the area density of the gold foil. It was shown that the contribution from the other nuclide (silver foil) can be remarkably suppressed by applying the self-indication method. Finally, we have demonstrated a nondestructive assay of nuclear material using a mixture composed of a natural uranium foil, sealed minor actinide samples of 237Np and 243Am. The results indicated that the self-indication method is useful for assaying a mixture of materials with high activity such as fuel debris.
Keywords Fuel debris • Neutron resonance absorption • Pulsed-neutron source
• Self-indication method • TOF
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)  with the time-of-flight (TOF) tech-
nique based on neutron resonance transmission analysis (NRTA)  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.