Mikio Izumi

Cosmic-ray muon radiography of UO2 fuel assembly

A technical demonstration of cosmic-ray muon radiography of a UO2 fuel assembly was performed at Toshiba Nuclear Critical Assembly (NCA). The fuel assembly in the NCA was imaged through obstacles such as steel and concrete. The result suggested that the method can be applicable to assess the damage to the reactors at the Fukushima Daiichi nuclear power plant. Here, both scattering and displacement methods are presented, and the results are shown to agree with Monte Carlo simulations. In addition, detailed Monte Carlo simulations of the Fukushima Daiichi reactor were performed, which showed capability of muon radiography to locate the fuel in the damaged reactors.

Analysis of muon radiography of the Toshiba nuclear critical assembly reactor

A 1.2 × 1.2 m2 muon tracker was moved from Los Alamos to the Toshiba facility at Kawasaki, Japan, where it was used to take ∼4 weeks of data radiographing the Toshiba Critical Assembly Reactor with cosmic ray muons. In this paper, we describe the analysis procedure, show results of this experiment, and compare the results to Monte Carlo predictions. The results validate the concept of using cosmic rays to image the damaged cores of the Fukushima Daiichi reactors.

Remote detector of alpha-ray using ultraviolet ray emitted by nitrogen in air

Alpha Camera was developed that can identify 1.5-kBq alpha emitter in 30-sec from 1-m away. The alpha camera equips mirror optics so that the alpha-induced ultraviolet lights are transferred while gamma-ray backgrounds are removed.

Radon concentration dependence of alpha radioactivity measurement using ionized air transportation

We are developing an alpha radioactivity measurement apparatus for uranium contaminated waste with large and complex surfaces (alpha-clearance monitor). This monitor uses the alpha radioactivity measurement method using ionized air transportation. Our goal of the monitor is to measure 4 Bq of alpha radioactivity for a 10 minutes measurement (5.7 Bq at present). A problem in achieving the goal is fluctuation in the background (BG) ion current. This fluctuation is predicted to be mainly due to radon inside the monitor. Therefore we developed a method to measure the radon concentration inside the monitor without an additional radon detector which disturbs airflow. And as a preliminary study, we determined the radon contribution to the BG current using this method. In addition, ion mobility and ion recombination coefficient, which are the other factors influencing the detection limit, were measured and evaluated under various environmental conditions (temperature, humidity and aerosol density).

Muon trackers for imaging a nuclear reactor

A detector system for assessing damage to the cores of the Fukushima Daiichi nuclear reactors by using cosmic-ray muon tomography was developed. The system consists of a pair of drift-tube tracking detectors of 7.2× 7.2-m2 area. Each muon tracker consists of 6 x-layer and 6 y-layer drift-tube detectors. Each tracker is capable of measuring muon tracks with 12 mrad angular resolutions, and is capable of operating under 50-μ Sv/h radiation environment by removing gamma induced background with a novel time-coincidence logic. An estimated resolution to observe nuclear fuel debris at Fukushima Daiichi is 0.3 m when the core is imaged from outside the reactor building.

Development of FPGA-Based Safety-Related I&C Systems

Toshiba has developed FPGA-based systems which perform signal processing by field programmable gate arrays (FPGA) for safety-related I&C systems. FPGA is a device which consists only of defined digital circuit: hardware, which performs defined processing. FPGA-based system solves issues existing both in the conventional systems operated by analog circuits (analog-based system) and the systems operated by central processing units (CPU-based system). The advantages of applying FPGA are to keep the long-life supply of products, improving testability (verification), and to reduce the drift which may occur in analog-based system. Considering application to safety-related systems, nonvolatile and non rewritable FPGA which is impossible to be changed after once manufactured has been adopted in Toshiba FPGA-based system. The systems which Toshiba developed this time are Power range Monitor (PRM) and Trip Module (TM). These systems are compatible with the conventional analog-based systems and the CPU-based systems. Therefore, requested cost for upgrading will be minimized. Toshiba is planning to expand application of FPGA-based technology by adopting this development method to the other safety-related systems from now on.Copyright

Ion Current Prediction Model Considering Columnar Recombination in Alpha Radioactivity Measurement Using Ionized Air Transportation

We present a reinforced ion current prediction model in alpha radioactivity measurement using ionized air transportation. Although our previous model explained the qualitative trend of the measured ion current values, the absolute values of the theoretical curves were about two times as large as the measured values. In order to accurately predict the measured values, we reinforced our model by considering columnar recombination and turbulent diffusion, which affects columnar recombination. Our new model explained the considerable ion loss in the early stage of ion diffusion and narrowed the gap between the theoretical and measured values. The model also predicted suppression of ion loss due to columnar recombination by spraying a high-speed air flow near a contaminated surface. This suppression was experimentally investigated and confirmed. In conclusion, we quantitatively clarified the theoretical relation between alpha radioactivity and ion current in laminar flow and turbulent pipe flow.

Particle modeling of transport of α-ray generated ion clusters in air

A particle model is developed using the test-particle Monte Carlo method to study the transport properties of α-ray generated ion clusters in a flow of air. An efficient ion–molecule collision model is proposed to simulate the collisions between ion and air molecule. The simulations are performed for a steady state of ion transport in a circular pipe. In the steady state, generation of ions is balanced with such losses of ions as absorption on the measuring sensor or pipe wall and disappearance by positive–negative ion recombination. The calculated ion current to the measuring sensor agrees well with the previous measured data.

Imaging of a reactor with muons

A technical demonstration to image a research reactor, Toshiba Nuclear Critical Assembly, with cosmic-ray muons is presented. The demonstration was performed as a precursor to Fukushima Daiichi muon imaging. We have obtained resolution of 3 cm during 1 month of exposure time. This result is in agreement with previous simulation results conducted on Fukushima Daiichi reactors 1 and 2.

Discrimination Monitors for Various Kinds of Waste to Be Down Graded

During operation and maintenance, or decommissioning of nuclear power plant, various kind of waste should be treated, and exposure control is also required. These wastes have a wide range of contamination, different composition of nuclides, and a different shape, so each measurement instrument would be optimized for its use especially for very low level radioactivity measurement. TOSHIBA provides appropriate equipment for any needs to discriminate the very low and non radioactive waste to save cost of waste disposal, based on our original and innovative technology. For alpha emitting nuclides, we are ready to supply instruments based on ionized particle measurement technology. For beta, gamma-emitting nuclides, we are ready to customize a shape of detector based on our original plastic scintillation material. Some examples are introduced.Copyright