Research on High-performance Steam Injectors for Passive Core Cooling Systems

This is a simple static steam-driven jet pump that operates without an electric power source. Its steam injectors can supply water at higher pressures than the supply pumps, using the operating principle of direct contact condensation of steam as their energy source. A complex gas-liquid two-phase flow occurs in these steam injectors, caused by contact between the supersonic steam flow and the water jet. This laboratory is gathering data on the stable behavior of steam injectors and the behavior of uncondensed gas, in order to learn about thermal-hydraulic issues; concurrently, we’re investigating how to optimize injectors, using thermal-hydraulic code for analysis. Our focus is on explaining the shapes of components and the thermal-hydraulic operating conditions in a system capable of operating stably across a wide range of pressures, as an emergency static isolation cooling system for when all electric power has been lost; we also look forward to developing a heat exchanger for an emergency condensing system.


Schematic of the water-jet centered Steam Injector

Development of Measurement Methods for Microstructure of Gas-Liquid Two-phase Flows, Research on Broad Enhancement of Critical Heat Flux

It is necessary to learn more about the gas-liquid structure in the vicinity of the boiling surface in order to better understand the heat conduction mechanisms in gas-liquid 2-phase flow, which are key parameters in the critical heat flux, a critical factor in the safety of nuclear power plants. This research is aimed at developing microsensors such as conductance micro-probes, high-precision electrostatic capacity micro-probes, and micro-thermocouples, and conducting measurements with these. The lower figure provides a 3-dimensional distribution of the liquid films remaining between the boiling surface and the bubbles, as indicated by the conductance probe. Thus, we are taking extremely precise measurements of the 10-μm liquid film behavior and of the temperature distributions and other parameters. Also, the students who have participated in this research have received several awards, including the Young Scientists’ Prize from the Minister of Education, Culture, Sports, Science and Technology (MEXT), Young Researcher Award from the Heat Transfer Society of Japan, the AESJ Thermal Hydraulics Division Award from the Atomic Energy Society of Japan, the JSME Fellowship Outstanding Presentation Award for Young Engineers from the Japan Society of Mechanical Engineers.

Ebullient cooling is the most desirable way to stop high heat flux, but during boiling, there exists a limit to the rate of heat removal called the “limit heat flux”. We are carrying out research to identify an effective method for dramatically increasing the limit heat flux. So far, it has been found that adding trace amounts of alcohol and nanoparticles to water causes large changes in the microstructure of gas-liquid 2-phase flow and vastly increases the limit heat flux. In the future, the microsensors mentioned above and visualization technologies will be used to explain the mechanisms of enhancing the limit heat flux and to search for ways to obtain larger enhancements.

Conductance micro-probe (tip diameter 5 μm)

Conductance micro-probe (tip diameter 5 μm)

Distribution of liquid film thicknesses at bottom of air bubbles, measured with microsenso

Distribution of liquid film thicknesses at bottom of air bubbles, measured with microsenso

Super Safe Reactor Design & Safety Analysis

  • Risk assessment of fail-safe reactors, analysis and evaluation of safety
  • Re-evaluation of concept of safety of nuclear power based on system/device safety theory
  • Changes in fluid mechanics properties accompanying solid-liquid state changes in pipes containing fluoride-salt
  • System design and evaluation of risks/establishment of fail-safe in case of electric power failure
  • Analysis and evaluation of safety of passive cooling equipment (IC, etc.) and design of equipment for next-generation reactors
  • Evaluation of applications of safety theory to next-generation reactors


Transient Analysis & Safety System Design

  • Evaluations of safety margins using measurements with improved accuracy in analysis of re-enactment of emergency/accident phenomena in LWR, application of results in design of plant upgrades
  • Optimization of safety maintenance during operation of LWR based on quantitative operating data
  • Analysis of behavior of nuclear power plant in actual operation using RETRAN-3D.MOD4 code
  • Simulation of core during BOP accident and analysis/evaluation of parameters


Severe Accident / PRA Analysis

The East Japan earthquake and tsunami and the resulting accident at Fukushima Power Plant #1 pointed up the deficiencies in the safety countermeasures in Japanese nuclear power systems in case of natural disasters. In order to guarantee the safety of a nuclear power plant, the risk of damage to the core by a tsunami must be identified. Assuming the risk of a complete failure of safety systems, we will investigate and evaluate whether there are holes in the “defense in depth” and redundancy in a probability risk assessment (PRA) and use those results to reduce the vulnerability of plants to tsunamis.

FP Cesium Diffusion & Migration

Released radioactive substances can settle to the earth’s surface in 2 processes, dry deposition and wet deposition. After the Fukushima Power Plant #1 accident, most of the areas where the highest contamination was found had been contaminated by wet deposition. However, the existing wet deposition model had been developed on the assumption that rain would be the main carrier; currently, it still has never been optimized for the snow and sleet typical in northern regions. The aim of this research is to provide a dependable prediction of the diffusion of radioactives in northern regions with a model optimized for all the mechanisms of wet deposition.


Totals for ground deposits of cesium 134 and 137 throughout east Japan, reflecting the measurements in the 4th aerial monitoring sweep.

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