[en] Summary: 1. The multi purpose modular microreactor fundamentally enhance nuclear safety, reliability and opportunity of nuclear energy for zero carbon energy. 2.“All solid state core” concept and full passive safety concept realizes sufficient safety in the operation environment for the microreactors. 3. Nuclear Security is important for microreactor operation and transport. International discussion is needed to build framework on the microreactor nuclear security.

No abstract available

[en] Mitsubishi Heavy Industries, Ltd. (MHI) has been working on the development of codes for core design (nuclear and thermal-hydraulic) and safety evaluation with the aim of advancing the operation of PWR (Pressurized Water Reactor) plants in terms of long cycle operations, power uprate, the adoption of high burnup fuel, etc. This report explains firstly model advancement in the nuclear and thermal-hydraulic design codes, and then explains how those advanced models have been incorporated into the non-LOCA (Loss of Coolant Accident) analysis technique. This report also describes the large break LOCA analysis method that statistically evaluates the effect of uncertainty in safety analysis inputs. Development of the CFD (Computational Fluid Dynamics) -based evaluation technology is also described, which deals with the core melt spreading behavior important in the field of severe accident, including a comparison between experiments and calculation results. This report introduces the advanced core and safety evaluation codes of MHI which incorporate high accuracy for evaluation, in addition to their applicability to core design and safety evaluation. (author)

[en] Nuclear energy is an effective option for decarbonization, which has significant potential to be utilized in a wide range of industrial fields―not only for the purpose of power generation, but also for heat utilization and other applications. Meanwhile, with respect to the utilization of nuclear energy, operations with both an extremely high level of safety after the Fukushima Daiichi Accident and economic efficiency sufficiently competitive with other energy sources are required. Mitsubishi Heavy Industries, Ltd. (MHI) has been working on the development of the Small Pressurized Water Reactor (Small-PWR), High Temperature Gas-cooled Reactor (HTGR) and Micro-Reactor, positioning them as “future reactors,” in addition to conventional large- and medium-scale light-water reactors and fast reactors developed in accordance with the national fuel cycle policy, in order to meet various different needs in the future. This report will introduce three future reactors and their characteristics, advantages, application technologies and other relevant information. (author)

[en] Optimizing melt spreading in the aftermath of a core disruptive accident is crucial for achieving sufficient melt cooling to maintain reactor containment integrity. Two approx. = 30 kg-scale experiments performed at the VULCANO facility explore the spreading of high-temperature molten corium-concrete mixtures over ceramic and sacrificial concrete substrates. Imaging of the melt front propagation revealed a 7% increase in spreading length and a 30% increase in maximum front velocity during spreading over sacrificial concrete, despite a reduced mass partaking in spreading due to increased holdup within the crucible. Infrared imaging of the melt indicated surface temperatures around 45℃ lower during spreading on sacrificial concrete, resulting in a roughly three-fold increase in melt viscosity. The enhanced viscosity and reduced mass during the VE-U9-concrete test imply an increased spreadability on sacrificial concrete greater than the observed 7% increase in spreading length. This enhanced spreadability on sacrificial concrete could be explained by the apparent gliding motion of the melt, consistent with reduced friction at the melt-substrate interface. Reduced friction at the melt-substrate interface is best explained by a diphasic film of molten concrete and gaseous concrete decomposition products acting as a lubricant between the melt and solid substrate. (author)