Japan demonstrates floating seismic isolation system for SMRs

The series of tests - which ran from 29 January to 9 February - used a 1/15-scale mockup of the structure of a small modular reactor (SMR) plant design (measuring 9 metres in length, 3 metres wide and 5 metres high) floating in a water tank mounted on the world's largest shaking table, operated by the National Research Institute for Earth Science and Disaster Resilience (NIED) in Miki City, Hyogo Prefecture, Japan.

NIED and IHI Corporation, a major nuclear equipment vendor, cooperated with Japan Atomic Energy Agency (JAEA) in the testing. The seismic waves input to the shaking table are based on some of the largest earthquakes observed, including the March 2011 Great East Japan Earthquake. Multiple accelerometers installed on the floating body and in the surrounding water pool measure the characteristic response of the floating seismic isolation structure and demonstrate its seismic isolation performance.

Japan's proprietary floating seismic isolation system (FSIS) technology is focused on the use of a series of air cavities for seismic response damping and associated orifices for energy dissipation of seismic motion as seismic isolators in a structural platform paired to the base of an SMR plant that floats in a water pool. JAEA says it is a passive system with design adaptable to a broad range of site seismic conditions and capable of reducing the seismic response of the paired SMR in the horizontal and vertical directions.

A schematic showing the pairing of the FSIS technology with an SMR (Image: JAEA)

JAEA claims the system provides such effective and omnidirectional seismic isolation that it is expected to yield increases in design margin and enhanced design standardisation for an SMR because the seismic damping will permit a single reactor/seismic isolation pairing design to be safe against seismic hazards even for sites with challenging seismic design conditions.

"The FSIS paired reactor plant is supported by buoyancy while the isolators of the cavities and orifices resist seismic load," JAEA said. "The effective and relatively simple concept is intended to greatly enhance both safety and reliability for the paired SMR."

The tests are part of a national project funded by Japan's Ministry of Economy, Trade and Industry through its nuclear energy innovation programme (NEXIP). In parallel, the project is evaluating specific safety and regulatory approaches to pairing of the FSIS and SMR plant concepts in cooperation with industry partners. The data acquired in the current tests will be used for validation of the seismic design and safety analysis methods for the FSIS-paired SMR concepts.

JAEA and its industry partners have begun pre-application engagement with the US Nuclear Regulatory Commission (NRC) with the expectation that JAEA's industry partners will eventually submit a Standard Design Approval application for an SMR design paired with the FSIS. The current test will provide supporting data for the regulatory engagement. The NRC observed the tests in progress and audited quality assurance program for the FSIS testing.

The FSIS is potentially deployable with reactors of any type and with a plant containing single or multiple reactor units, JAEA said. The FSIS paired SMR design concept as described by the JAEA will withstand the terrestrial ground accelerations associated with high seismic risk zones that may not be suitable for conventional nuclear plant construction.

"Nuclear energy is expected to play a key role of decarbonisation in many countries," JAEA said. "Offering the FSIS will enable potential deployment of SMRs in Japan and elsewhere, where challenging seismic conditions would otherwise limit siting availability for new builds."

In February last year, Japan's Cabinet approved a policy that calls to develop and construct "next-generation innovative reactors." Under the new policy, Japan will also collaborate with overseas projects by maintaining and strengthening domestic supply chains and contributing Japan`s technology and expertise.