Seismic Isolation of the IRIS Nuclear Plant

Abstract

The safety-by-design™ approach adopted for the design of the International Reactor Innovative and Secure (IRIS) resulted in the elimination by design of some of the main accident scenarios classically applicable to Pressurized Water Reactors (PWR) and to the reduction of either consequences or frequency of the remaining classical at-power accident initiators. As a result of such strategy the Core Damage Frequency (CDF) from at-power internal initiating events was reduced to the 10−8/ry order of magnitude, thus elevating CDF from external events (seismic above all) to an even more significant contributor than what currently experienced in the existing PWR fleet. The same safety-by-design™ approach was then exported from the design of the IRIS reactor and of its safety systems to the design of the IRIS Nuclear Steam Supply System (NSSS) building, with the goal of reducing the impact of seismically induced scenarios. The small footprint of the IRIS NSSS building, which includes all Engineered Safety Features (ESF), all the emergency heat sink and all the required support systems makes the idea of seismic isolation of the entire nuclear island a relatively easy and economically competitive solution. The seismically isolated IRIS NSSS building dramatically reduces the seismic excitation perceived by the reactor vessel, the containment structure and all the main IRIS ESF components, thus virtually eliminating the seismic-induced CDF. This solution is also contributing to the standardization of the IRIS plant, with a single design compatible with a variety of sites covering a wide spectrum of seismic conditions. The conceptual IRIS seismic isolation system is herein presented, along with a selection of the preliminary seismic analyses confirming the drastic reduction of the seismic excitation to the IRIS NSSS building. Along with the adoption of the seismic isolation system, a more refined approach to the computation of the fragility analysis of the components is also being developed, in order to reduce the undue conservatism historically affecting seismic analysis. The new fragility analysis methodology will be particularly focused on the analysis of the isolators themselves, which will now be the limiting components in the evaluation of the overall seismic induced CDF.

Innovative seismic resistant structure of shield building with base isolation and tuned-mass-damping for AP1000 nuclear power plants

“…Another two nuclear power stations under construction also adopted the base isolation technology (Forni et al, 2012,Whittaker andKumar, 2014). (Forni, 2010;Ali et al, 2014), (Whittaker and Kumar, 2014), (Xia, 2013), (Xie and Zhai, 2012), (Forni et al, 2009), etc. systematically presented the advantages, challenges, and potential drawbacks of the NPPs with base isolation (BIS), and provided the roadmap of the development of such a system.…”

A European Association for the Control of Structures joint perspective. Recent studies in civil structural control across Europe

“…The improved performance in terms of acceleration response to design seismic actions of isolated buildings with respect to traditional ones makes isolation techniques a very attractive choice also for reactor buildings in future nuclear power plants. In light of this consideration, the probabilistic evaluation of the seismic fragility of the reactor buildings of a nuclear power plant including passive seismic isolation was the object of recent research activities carried out at Politecnico di Milano , at ENEA , and at other partners' facilities.…”

Shaking table test and numerical analysis of mid-story isolation eccentric structure with tower–podium

“…6 Analytical approaches for calculating the optimal parameters of a mid-story isolation system were formulated by Zhou et al 7 to optimize the design of the isolation system. Zhang et al 8 and Forni et al 9 suggested that the design of isolation layer needs to be optimized to not only minimize the acceleration response of the structure and the relative displacement of the isolation layer but also reduce the difficulty of constructing the isolation structure and designing the tubes resulting from the connection between the isolation layer and the structures above and below it. The construction of the isolation system is simpler than that of the base isolation system.…”