Massimo Forni

SEISMIC ISOLATION OF THE IRIS NUCLEAR PLANT

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.Copyright

Recent Development and Application of Seismic Isolation and Energy Dissipation and Conditions for Their Correct Use

More than 23,000 structures, located in over 30 countries, have been so far protected by passive anti-seismic (AS) systems, mainly by the seismic isolation (SI) and energy dissipation (ED) ones. The use of such systems is going on increasing everywhere, although its extent is strongly influenced by earthquake lessons and the features of the design rules used. As to the latter, SI is considered as an additional safety measure (with consequent significant additional construction costs) in some countries (Japan, USA, etc.), while, in others (including Italy), the codes allow to partly take into account the reduction of the seismic forces acting on the superstructure that is induced by SI. Applications of the AS systems have been made to both new and existing civil and industrial structures of all kinds. The latter include some high risk (HR) plants (nuclear reactors and chemical installations). The applications in a civil context already include not only strategic and public structures, but also residential buildings and even many small private houses. In Italy, the use of the AS systems has become more and more popular especially after the 2009 Abruzzo earthquake (nowadays more than 400 Italian buildings are seismically isolated). Based on the information provided by the authors at the ASSISi 13th World Conference, held in Sendai (Japan) in September 2013, and on more recent data, the paper summarizes the state-of-the-art of the development and application of the AS systems and devices at worldwide level, by devoting particular attention to SI of buildings in Italy, in the context of recent seismic events. Moreover, it outlines the benefits of the aforesaid systems for ensuring the indispensable absolute integrity of strategic and public structures, as, primarily, schools, hospitals and HR plants, but also (for an adequate protection of cultural heritage) museums. Finally, based on Italian experience, it provides some remarks on costs of SI, stresses the conditions for the correct use of this technique and mentions some recent initiatives of the Italian Parliament to ensure such a correct use and to widely extend such an use to the HR chemical plants too (for which only very few applications already exist in Italy).

Methods for the seismic verification of a fast reactor core

Abstract This paper presents the main features of the methods applied by the Fast Reactor Department of the Italian Energy Authority (ENEA) to the seismic verification of the core of the Italian PEC fast reactor test facility, which is in advanced construction at the Brasimone site. It also points out the problems which in general remain open in the fast reactor core seismic analysis. The aim is to contribute towards a generally acceptable philosophy on core seismic verification techniques and suggest items of possible co-operation for future developments.

SEISMIC ISOLATION OF LEAD-COOLED REACTORS: THE EUROPEAN PROJECT SILER

SILER (Seismic-Initiated event risk mitigation in LEad-cooled Reactors) is a Collaborative Project, partially funded by the European Commission in the 7th Framework Programme, aimed at studying the risk associated to seismic-initiated events in Generation IV Heavy Liquid Metal reactors, and developing adequate protection measures. The project started in October 2011, and will run for a duration of three years. The attention of SILER is focused on the evaluation of the effects of earthquakes, with particular regards to beyond-design seismic events, and to the identification of mitigation strategies, acting both on structures and components design. Special efforts are devoted to the development of seismic isolation devices and related interface components. Two reference designs, at the state of development available at the beginning of the project and coming from the 6th Framework Programme, have been considered: ELSY (European Lead Fast Reactor) for the Lead Fast Reactors (LFR), and MYRRHA (Multi-purpose hYbrid Research Reactor for High-tech Applications) for the Accelerator-Driven Systems (ADS). This paper describes the main activities and results obtained so far, paying particular attention to the development of seismic isolators, and the interface components which must be installed between the isolated reactor building and the nonisolated parts of the plant, such as the pipe expansion joints and the joint-cover of the seismic gap.

Shaking Table Tests on a Spherical Tank Mock-Up Provided With Seismic Isolation and Flexible Piping Connections

The Project INDEPTH (Development of INnovative DEvices for Seismic Protection of PeTrocHemical Facilities), supported by the European Commission, has the objective of developing and applying innovative seismic isolation and/or dissipation devices for critical structures at petrochemical facilities. In the framework of INDEPTH, integrated seismic protection systems have been conceived, developed and tested. They are aimed at protecting liquid-filled structures (product storage, spherical and LNG tanks), with new devices (fiber-reinforced isolators, buckling reinforced braces) specific for each application and new flexible piping couplings, to compensate the displacements resulting from the use of isolation systems. The research program has been focused on the selection of critical structures, the design and manufacturing of the devices, the numerical assessment and the experimental validation through shaking table tests. A quantification of technical/economical/safety benefits with respect to the conventional state-of-the-art measures presently adopted and potential application to retrofitting has been performed. Validation through shaking table tests of the effectiveness of the isolation systems on the spherical mock-up (Figure 1), and the related piping system equipped with flexible joints (Figure 2), had been performed. Two types of seismic input have been applied, both synthesized from the 5% damping spectra of EC8 (medium and soft soils); the target peak acceleration value of the time histories was 0.4 g. Different configurations of the mock-up have been tested, such as: fixed base, isolated base with High Damping Rubber Bearings, Fiber Reinforced Rubber Bearings and Lead Rubber Bearings. Furthermore, each configuration has been tested for both time histories and at three different level of filling to verify the sloshing behavior in the sphere and the effectiveness of the isolation systems at levels of filling different from the design one (full sphere). Comparison among all the above mentioned conditions could be done. The presentation will show the main results of the shaking table campaign.Copyright

Research and development studies on plant and core seismic behaviour for a fast reactor

Abstract This paper presents the main features and results of the numerical and experimental studies that were carried out by ENEA in co-operation with ANSALDO and ISMES for the seismic verification of the Italian PEC fast reactor test facility. More precisely, the paper focuses on the wide-ranging research and development programme that has been performed (and recently completed) on the reactor building, the reactor-block, the main vessel, the core and the shutdown system. The needs of these detailed studies are stressed and the feed-backs on the design, necessary to satisfy the seismic safety requirements, are recalled. The general validity of the analyses in the framework of the research and development activities for nuclear reactors is also pointed out.

SILER Project: Design of the Seismic Isolators

This paper describes the SILER (Seismic-Initiated event risk mitigation in LEad-cooled Reactors) Project results obtained so far in the design of the seismic isolation system of two nuclear power plants: the ELSY configuration for the LFR (Lead-Cooled Fast Reactor) design and the MYRRHA configuration for the accelerator-driven systems (ADS).The seismic protection of the nuclear buildings by means of seismic isolation has been chosen in order to minimize changes to the standard design of the civil works and internal components of the Nuclear Power Plant. The work led to the identification of the optimal design solution, in terms of type and location of seismic devices, to achieve compliance to the floor response acceleration spectra in horizontal and vertical direction, with levels of horizontal displacements not exceeding the maximum acceptable values for structural and non-structural elements.The isolators studied in the project are of the type elastomeric, both High Damping Rubber Bearings and Lead Rubber Bearings; moreover the adoption of a fail-safe system to limit the horizontal isolator deformation in case of beyond design earthquakes is studied.© 2014 ASME

Seismic Safety Margin of an Isolated SMR Reactor Under Severe Earthquake

The dramatic consequence of the magnitude 9.0 earthquake in Fukushima Daiichi nuclear power plant, reactors 1, 2, 3 and 4, highlighted and confirmed that the existing and the future nuclear installations should be designed to be highly secure and capable to withstand a wide range of internal and external extreme loads, such as pressure, aircraft crash and, of course, earthquakes. The aim of this paper is the evaluation the seismic behavior of an innovative SMR hit by an exceptional seismic event, characterized by a magnitude well beyond the design basis value (e.g. also 2007 Niigataken Chuetsu-Oki or 2010 Chile earthquakes), in order to understand the true state of the SSCs in terms of their required safety functions and capacity, and, as a result, to be able to assess correctly the seismic safety margin of the considered installation. In this context, it has been also considered the adoption of the highly attractive strategy of the seismic isolation to increase the reliability or safety margin of the nuclear safety relevant structures, during and after the seismic event, with the aim of avoiding or mitigating the related structural damaging effects. To the purpose a rather refined numerical methodology was employed and several three-dimensional models (FEM approach) of the SMR reactor containment and its safety relevant structures were set up and used in the performed analyses, taking also into account a suitable materials behaviour and constitutive laws for both the reactor materials and the isolators. In addition the real behaviour and characteristics of isolators, experimentally determined, have been used as input in the carried out simulations. The obtained results were used to appropriately check mainly the NPP containment strength reserve and the isolators safety factor.Copyright

Shaking Table Tests on Innovative Anti-Seismic Systems Developed in the Framework of the LESSLOSS European Integrated Project

The Integrated Project LESSLOSS (Risk Mitigation for Earthquakes and Landslides), partially funded by the European Commission within the 6th Framework Programme, started on September 2004 and was concluded in August 2007. Activities were carried out by 46 European partners, with a total budget of 9.4 ME. The Project was divided into 11 Sub-Projects. This paper describes the activities performed in the framework of Sub-Project 6 (Development and Manufacturing of Energy Dissipation Devices and Seismic Isolators), which was coordinated by ENEA and whose partnership included two manufacturers of antiseismic devices (ALGA, Milan, Italy and MAURER SHONE, Munich, Germany) and two consulting and construction companies (STAP, Lisbon, Portugal and VINCI, Paris-Rueil, France). Aim of Sub-Project 6 was the development and validation of two innovative antiseismic devices (a low stiffness isolator and an electroinductive damper), the improvement of the performances of a slider with curved surface and the evaluation of benefits and limits of isolation systems based on steel hysteretic dissipaters coupled with flat sliders. The Low Stiffness Isolator (LSI) was developed by ALGA; it is a natural rubber seismic isolator particularly addressed to light structures like family houses. The Electroinductive Damper (DECS), developed by ALGA, is an energy dissipater based on the interaction of a diamagnetic material, like aluminium, with an electric field generated by permanent magnets. The Sliding Isolation Pendulum (SIP) developed by MAURER is an improved curved surface slider, capable of withstanding high weights for long periods without creep effects and high velocity deformations without damages due to friction. Finally, several types of Steel Hysteretic (SH) elements of different geometries and materials have been analyzed and tested in order of evaluating the benefits and the limits of such devices, with particular regard to the re-centering capabilities. All the abovementioned devices have been tested on the ENEA shaking table of the Casaccia labs (near Rome), with a suitable mock-up capable of providing a 300 kN force on the devices in the acceleration and frequency ranges of interest, using several natural and artificial acceleration time histories purposely developed by ENEA. The paper describes the main features of the devices, the numerical activity aimed at the optimization of their behavior and the results of the four shaking table campaigns carried out on each device.Copyright

State of the Art of the Development and Application of Anti‐Seismic Systems in Europe and Other Countries

There are already approximately 5,000 structures in the world, located in over 30 countries, that have been protected by seismic isolation (SI), energy dissipation (ED) and other modern seismic vibration passive control (SVPC) systems. The number of such applications is increasing everywhere more and more. It has been confirmed that, in each country, the extension of the use of the SVPC systems is conclusively influenced by earthquake experience, the availability of specific design rules and the features of those adopted. With regard to application, Japan has consolidated its worldwide leadership, with over 3,000 seismically isolated buildings, many others protected by ED systems and several isolated bridges & viaducts. Second is now the P. R. China, with 610 isolated buildings and 45 with dampers, in addition to numerous isolated bridges & viaducts. The Russian Federation is third for the number of isolated buildings, which are approximately 600. In the USA, due to the very penalizing design code in forc...