Atucha II Nuclear Reactor: Design Safety and Licensing

Atucha II is the third reactor built and operated in Argentina, following Atucha I and Embalse. This is the First-Of-A-Kind (FOAK) in the world notwithstanding Atucha I. The following main facts relate to Atucha II: \n This is a natural uranium nuclear reactor (like a CANDU, i.e. Canadian Deuterium Uranium), vessel equipped (like a PWR, i.e. Pressurized Water Reactor) and with established safety features (namely, basic philosophy and protection and control systems) of latest German PWR. \n It has a positive coolant void reactivity coefficient (fission power tends to increase if coolant is removed from the core): excluding one (standard) CANDU reactor built in China and CANDU-type reactors in India, this is the only reactor entered in operation with a such a feature after the Chernobyl accident in 1986. \n A relationship establishes among Break Opening Time (BOT), fast Boron Injection Time (BIT), Peak Clad Temperature (PCT) and Fission Power Peak (FPP) following a loss of coolant accident. This puts a challenge to the safety of the Atucha II reactor. The availability of computational tools, including uncertainty evaluation, allowed the characterization of the relationship BOT-BIT-PCT-FPP: not necessarily, the highest PCT coincides with the highest FPP. \n The construction of the reactor, started at the end of 1980 has stopped and finally restarted in 2006: all main components were stored at the site, but no industry (noticeably, not even the initial designer who supplied the components) was available for planning or supervising the construction. \n The owner and operator of the Plant, Nucleoléctrica Argentina SA (NA-SA), created a single-purpose multidisciplinary team, headed by José Luis Antúnez (JLA) to undertake the completion of the construction and the start-up of Atucha II. The team completed the commissioning in due time. \nA system encompassing the complexity of CANDU and PWR had to be built (in 2006) in a Country having a long-lasting history within nuclear technology. Furthermore, a competent regulator was ready to act based on the latest findings by the international community in terms of nuclear safety technology. Then, the utility recognized the need for a deep understanding of the design principles of the reactor beyond and over the capability to install the various components. \nIn order to accomplish the construction of Atucha II, international groups of experts formed to support the utility and the regulators, respectively. Those groups, under the guidance of regulator and utility expert-staff, confronted each other in relation to the critical issues of the system. \nAn unprecedented and unrepeatable endeavour took place: the utility leaded group accessed and reviewed the documentation of Atucha II (i.e. a few million pages) and confirmed, independently of the original designers, the validity of the design choices and the safety margins. The blue prints, the measurements taken on the site and the material properties were the only basis of the analyses performed by the latest computational tools \nAtucha II Nuclear Reactor: Design, Safety and Licensing @2021 \nF. D’Auria, O. Mazzantini, G. M. Galassi \n16 \navailable from the scientific community. All of this allowed the reconstruction of the steps of the reactor design without adopting any assumption or coefficient proposed by original designers: a pioneering application was completed of the Best Estimate Plus Uncertainty (BEPU) approach. The regulators timely assessed and, following deep discussions, endorsed the approach and the results. \nThe BEPU application, first of a kind to the analysis of all transients, part of FSAR Chapter 15, is the result of a technology grow-up in the area of accident investigations that started in the middle of 1970, i.e. at OECD/NEA/CSNI. Planning and analysis of complex experiments, code, code-user and nodalization qualification, accuracy quantification, scaling, uncertainty evaluation, code coupling, probabilistic safety assessment and licensing connection of thermal hydraulics are among the concerned topics. \nTherefore, we deemed the description of that endeavour worthy for the present book. The current authors directed a few dozen scientists (see acknowledgments where not all of them are listed) contributing to the efforts during less than ten years to accomplish the mission proposed by JLA. Four parts constitute the book. \n Part 1, Chapters 1 and 2 – Atucha II reactor description. \n Part 2, Chapters 3 to 8 – The BEPU approach. \n Part 3, Chapters 9 to 13 – The Large Break Loss of Coolant (LBLOCA) issue. \n Part 4, Chapters 14 to 24 – Insights from Accident Analysis. \nThe Part 1 deals with a short history of the Atucha II project and provides key reactor features. The heavy water moderator and coolant fluids enter in contact into the vessel through proper bypass flow paths and ensure cooling of the fuel rods and moderation of neutrons at high and reduced average temperatures, respectively. \nThe Part 2 discusses the BEPU approach, i.e. a prerequisite for understanding the reactor features. The BEPU approach, established in nuclear thermal hydraulics, includes the application of qualified numerical codes and uncertainty evaluation procedures. A pioneering effort brought to adopting the approach for the entire Accident Analysis (AA), part of ‘Chapter 15’ part of the standard Final Safety Analysis Report (FSAR). This required a previous acceptance of the approach by regulators. \nThe Part 3 deals with the LBLOCA. Following the initiating event or the double-ended guillotine break, the hot fluid in the core, which behaves as a neutron absorber in nominal conditions, vaporizes starting at a few milliseconds. The moderator remains liquid for a few seconds and induces a positive reactivity input. A power excursion follows in a situation of degraded cooling (FPP). \nThe BEPU application to the envelope of transients addressed in AA, Part 4, implied the coupling of a wide variety of numerical codes in the areas of neutron physics, nuclear fuel performance, structural mechanics, fission products source term in the core and radiological impact on the environment, in addition to thermal hydraulics. \nAtucha II Nuclear Reactor: Design, Safety and Licensing @2021 \nF. D’Auria, O. Mazzantini, G. M. Galassi \n17 \nA dozen papers dealing with the above topics have been published, e.g. Adorni et al., 2011; Araneo et al., 2011; D’Auria et al., 2012; D’Auria et al., 2012a; Pecchia et al., 2015; Petruzzi et al., 2016; Moretti et al., 2018; Mazzantini et al. 2019; D’Auria, 2019; D’Auria et al., 2019. \nThe papers by Adorni et al., 2011; Araneo et al. 2011; D’Auria et al., 2012; and Pecchia et al., 2015, deal with nuclear fuel, Pressurized Thermal Shock (PTS), Instrumentation and Control (I & C) and neutron physics modelling, respectively: related matter is part of the BEPU approach of Atucha II. \nThe paper by Moretti et al., 2018, deals with a scale-1 experiment performed to confirm the quality of the fast boron injection system of Atucha II: this is essential to mitigate the fission power excursion that is consequent to the LBLOCA. \nThe paper by Mazzantini et al., 2019, discusses various issues connected with the Large Break LOCA analysis. \nThe papers by D’Auria et al., 2012a; Petruzzi et al., 2016, and D’Auria, 2019 provide a summary of the BEPU use in licensing, including achievements and perspectives in the area. Finally, BEPU constituted the key element for a proposal aimed at creating a connection between safety of existing rectors and advancements in nuclear science and technology, paper by D’Auria et al., 2019. \nWe issued several hundred documents within the framework of the cooperation between NA-SA and University of Pisa. Three summary documents are GRNSPG, 2008; GRNSPG 2008a, and GRNSPG, 2010. Industry property information is involved. \nAll those papers and documents guided the planning of the present book where we avoided infringing the property rights. Thus, the idea here is not to replicate the contents of those papers and documents, with one noticeable exception that is the paper by Mazzantini et al., 2019: this constitutes the basis for issuing the Part 3 of the book.

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