Because nuclear power plants do not emit greenhouse gases, nuclear energy is effective to reduce their emission. The thermal cycle efficiency of current LWRs is about 34%: two-thirds of the energy generated in the core is dissipated into the ocean as lukewarm water through steam condensers. For that reason, such plants’ natural resource utilization efficiency is inferior to that of fossil-fired plants because the LWR cycle efficiency is considerably lower than that of modern fossil-fired power plants (about 43%) and liquid natural gas-fired cogeneration plants (about 53%). Nuclear energy faces negative public opinion based on popular anxiety regarding its safety and buildup of long-half-life radioactive wastes.

     If a safe and simplified small reactor were commercialized for construction near or even under cities, its waste heat could be recovered and utilized economically. Thereby, its total energy utilization efficiency or natural resource utilization efficiency would be greatly enhanced. In fast reactors, especially in gas-cooled fast reactors (GCFRs), trans-uranium elements produced in LWRs, which are long-lived radioactive wastes and require geological disposal, can be used as fuel by taking advantage of a harder neutron spectrum. A safe and simplified small fast reactor can thereby allay anxieties.

Our group aims to establish an advanced nuclear system through developing a safe and simplified small fast reactor and a heat recovery system that affords the following three “zero-release” guarantees to the public and the surrounding environment.

a) zero emission of greenhouse gases
b) zero accumulation of trans-uranium elements
c) zero release of waste heat

     Our group also strives to establish an advanced energy system in collaboration with other groups in our institute. Such a system would foster a recycling society that consumes locally-produced garbage, waste wood and used paper, using waste heat and solar energy along with the excreta produced in farming to generate hydrogen, methane and methanol for fuel cells.

2. Recent Research Topics

1. Supercritical CO₂ Gas Turbine Reactors

      Our group has been developing supercritical CO₂ gas turbine reactors since 1999 and has obtained the following results: the proposed CO2 gas turbine cycle achieves a 4% to 11% higher cycle efficiency (see Fig. 1) than a He-gas turbine cycle through reduction of the compression work around the critical point of _CO2 (see Fig. 2), allowing a five-times smaller turbine size. Because the cycle efficiency exceeds 40% at a representative core outlet temperature of 530℃ in liquid metal cooled fast reactors (LMFRs), the CO₂ gas turbine FR offers an alternative to conventional sodium-cooled FRs, eliminating problems related to safety, plant maintenance and construction cost. In this FR core, addition of ²³⁷Np reduces the burnup reactivity loss, minimizing it to the rate of 0.17% ΔK per ten years at a content of about 4.5%. ²³⁷Np is transmuted to ²³⁹Pu after two neutron capture reactions via ²³⁸Pu: it functions mainly as a neutron absorber in the early stage and a fissile material in the later stage. The total amount of ²³⁷Np burned is equivalent to the quantity produced from about 20 LWRs, each with similar electrical output.

     A research plan of supercritical CO₂ gas turbine fast reactors was selected for the advanced reactor system development project by MEXT in 2003. The research aims to evaluate feasibility of the supercritical CO₂ gas turbine fast reactors. The project comprises the following four subjects:

1) Supercritical CO₂ gas turbine cycle mockup test
2) Corrosion test of structural materials in supercritical CO₂
3) Reaction test between Na and CO₂
4) Design study of supercritical CO₂ gas turbine fast reactors

Movie 1.  CO₂ critical point: transition from transparent to black (*.avi, 4.8 Mb).

2. Supercritical CO₂ Cycle Waste Heat Recovery System

      An innovative heat recovery system (see Fig. 3) was proposed. It recovers waste heat from steam condensers in power plants using liquid CO₂ instead of seawater as a cooling medium. Thereby,　it achieves total energy utilization efficiency of greater than 85%. Its heat supply cost is estimated to be lower than those of conventional fossil-fired boilers beyond operation of about four years. A mockup experimental facility was constructed under contract with NEDO in 2003 to confirm its heat recovery performance by adding a steam supplier and a condenser (see Photo 1) to the supercritical CO₂ thermal hydraulic test loop (see Photo 2). Through this project, Micro Channel Heat Exchangers (MCHEs) will be developed for steam condensers with heat transfer between boiling liquid CO₂ and steam condensing two-phase flows and for hot water suppliers with heat transfer between water and gaseous CO₂ single-phase flows.

3. High-Efficiency Compact Heat Exchanger

      MCHE is manufactured by virtue of the two technologies of chemical etching and diffusion bonding. Flow channels are etched chemically onto the metal plates. Etched plates are stacked to produce one block by diffusion bonding. MCHE is categorized as a plate-fin type heat exchanger, but it has the capability of achieving higher efficiency and reducing its size to half that of usual plate fin models. PCHEHEs have been developed through optimization of the flow channel configuration by 3D-CFD for single phase flow heat transfer such as in recuperators of gas turbine systems and hot-water suppliers in the heat recovery system; and by observation of the flow in the MCHE for two-phase flow heat transfer using a high-speed video camera (see Photo 3) such as in steam condensers of the heat recovery system. Chemical etching and diffusion bonding technologies have also been developed.

    An optimized flow channel configuration for the recuperator of the supercritical CO₂ cycle is shown in Fig. 4. It has discontinuous fins with an S-shape, similar to a sine curve, in contrast to the conventional continuous zigzag configuration. The new configuration reduces the pressure drop to one-fifth of the conventional zigzag configuration with equal thermal-hydraulic performance. This promising result has beenconfirmed experimentally using the test loop shown in Photo 2.
 

3. Selected Publications and Projects

Publications:

Y. Kato et al., Nucl. Eng. Design, 230, pp.195-207 (2004). Y. Kato et al., 7.
"Cellular Automaton Methods for Simulations of Complex Phenomena (in Japanese)," Morikita Shuppan, September, 1997.
 

Patents:

Nuclear reactor plant, Patent #3530939
 

National Projects:

MEXT Advanced Reactor System Development Project, FY2003-2007.
NEDO Energy Usage Rationalization Technology Strategic Project, FY2003-2005