11.1 Introduction
The accelerator-driven system (ADS) has been developed as the next-generation
nuclear energy system and is expected to be used as a nuclear transmutation process
[1]. ADS is a hybrid system that consists of a high-intensity proton accelerator,
a nuclear spallation target, and a subcritical core. Lead–bismuth eutectic (LBE)
is considered as an option of the spallation target and can also be used as the
coolant of the reactor. Neutrons are produced by a nuclear spallation reaction
between the protons supplied from the accelerator and the LBE target, and a
chain reaction of nuclear fission can then be maintained by the contribution of
spallation neutrons. The chain reaction in the core will stop when the supply of
protons stops. Therefore, the ADS has a higher safety margin, in principle, than
other nuclear energy systems.
As research toward the development of the ADS, a subcritical reactor physics
study, a reactor thermal-hydraulics study, and studies on the material, accelerator,
and fuel for the ADS have been carried out. However, safety assessment is very
important in preparation for a possible severe accident. A pipe rupture in a steam
generator is one of the severe accidents of a LBE-cooled ADS. In this case, the
direct contact between the LBE and the water ejected from the ruptured pipe of the
steam generator might lead to LBE–steam two-phase flow in the reactor pool. If the
gas bubble comes into the fuel region, the core reactivity might be affected. Thus,
the gas–liquid two-phase flow appearing in the ADS core should be understood in
taking measures for such an accident. The gas–liquid two-phase flow in an ADS has
density ratio that is an order larger than that of air–water two-phase flow. Although
flow models of gas–liquid two-phase flow with a large liquid-to-gas density ratio
are required for severe accident analysis, there are fewer studies on two-phase flow
in the physical property range of large density ratio mixture. Thus, an experimental
database on two-phase flow properties in two-phase flow with a large density ratio
should be built and the two-phase flow model should be developed based on the
database. In this study, an LBE two-phase flow was measured by using a foursensor electrical conductivity probe and a miniature electromagnetic probe, and
knowledge of the flow structure and the turbulent characteristics in two-phase flow
with a large density ratio was obtained.
11.2 Measurement Techniques
11.2.1 Four-Sensor Probe
The four-sensor probe [2] used in this study consists of a central front sensor and
three peripheral rear sensors (Fig. 11.1a). Tungsten acupuncture needles with a
maximum diameter of 0.1 mm were coated with epoxy resin varnish except the tip;
the diameter of the tip is less than 1 μm. The insulated needles were inserted into a
108
G. Ariyoshi et al.
The accelerator-driven system (ADS) has been developed as the next-generation
nuclear energy system and is expected to be used as a nuclear transmutation process
[1]. ADS is a hybrid system that consists of a high-intensity proton accelerator,
a nuclear spallation target, and a subcritical core. Lead–bismuth eutectic (LBE)
is considered as an option of the spallation target and can also be used as the
coolant of the reactor. Neutrons are produced by a nuclear spallation reaction
between the protons supplied from the accelerator and the LBE target, and a
chain reaction of nuclear fission can then be maintained by the contribution of
spallation neutrons. The chain reaction in the core will stop when the supply of
protons stops. Therefore, the ADS has a higher safety margin, in principle, than
other nuclear energy systems.
As research toward the development of the ADS, a subcritical reactor physics
study, a reactor thermal-hydraulics study, and studies on the material, accelerator,
and fuel for the ADS have been carried out. However, safety assessment is very
important in preparation for a possible severe accident. A pipe rupture in a steam
generator is one of the severe accidents of a LBE-cooled ADS. In this case, the
direct contact between the LBE and the water ejected from the ruptured pipe of the
steam generator might lead to LBE–steam two-phase flow in the reactor pool. If the
gas bubble comes into the fuel region, the core reactivity might be affected. Thus,
the gas–liquid two-phase flow appearing in the ADS core should be understood in
taking measures for such an accident. The gas–liquid two-phase flow in an ADS has
density ratio that is an order larger than that of air–water two-phase flow. Although
flow models of gas–liquid two-phase flow with a large liquid-to-gas density ratio
are required for severe accident analysis, there are fewer studies on two-phase flow
in the physical property range of large density ratio mixture. Thus, an experimental
database on two-phase flow properties in two-phase flow with a large density ratio
should be built and the two-phase flow model should be developed based on the
database. In this study, an LBE two-phase flow was measured by using a foursensor electrical conductivity probe and a miniature electromagnetic probe, and
knowledge of the flow structure and the turbulent characteristics in two-phase flow
with a large density ratio was obtained.
11.2 Measurement Techniques
11.2.1 Four-Sensor Probe
The four-sensor probe [2] used in this study consists of a central front sensor and
three peripheral rear sensors (Fig. 11.1a). Tungsten acupuncture needles with a
maximum diameter of 0.1 mm were coated with epoxy resin varnish except the tip;
the diameter of the tip is less than 1 μm. The insulated needles were inserted into a
108
G. Ariyoshi et al.