UPTEC F11 011 Examensarbete 30 hp Februari 2011 Simulation of Reactor Transient and Design Criteria of Sodium- cooled Fast Reactors Filip Gottfridsson Abstract Simulation of Reactor Transient and Design Criteria of Sodium- cooled Fast Reactors Filip Gottfridsson Teknisk- naturvetenskaplig fakultet UTH-enheten The need for energy is growing in the world and the market of nuclear power is now once more expanding. Some issues of the current light-water reactors can be solved Besöksadress: by the next generation of nuclear power, Generation IV, where sodium-cooled Ångströmlaboratoriet Lägerhyddsvägen 1 reactors are one of the candidates. Phénix was a French prototype sodium-cooled Hus 4, Plan 0 reactor, which is seen as a success. Although it did encounter an earlier unexperienced phenomenon, A.U.R.N., in which a negative reactivity transient Postadress: followed by an oscillating behavior forced an automatic emergency shutdown of the Box 536 751 21 Uppsala reactor. This phenomenon lead to a lot of downtime of the reactor and is still unsolved. However, the most probable cause of the transients is radial movements of Telefon: the core, referred to as core-flowering. 018 – 471 30 03 Telefax: This study has investigated the available documentation of the A.U.R.N. events. A 018 – 471 30 00 simplified model of core-flowering was also created in order to simulate how radial expansion affects the reactivity of a sodium-cooled core. Serpent, which is a Hemsida: Monte-Carlo based simulation code, was chosen as calculation tool. Furthermore, a http://www.teknat.uu.se/student model of the Phénix core was successfully created and partly validated. The model of the core has a k_eff = 1.00298 and a neutron flux of (8.43+-0.02)!10^15 neutrons/cm^2 at normal state. The result obtained from the simulations shows that an expansion of the core radius decreases the reactivity. A linear approximation of the result gave the relation: change in k_eff/core extension = - 60 pcm/mm. This value corresponds remarkably well to the around - 60 pcm/mm that was obtained from the dedicated core-flowering experiments in Phénix made by the CEA. Core-flowering can recreate similar signals to those registered during the A.U.R.N. events, though the absence of trace of core movements in Phénix speaks against this. However, if core-flowering is the sought answer, it can be avoided by design. The equipment that registered the A.U.R.N. events have proved to be insensitive to noise. Though, the high amplitude of the transients and their rapidness have made some researcher believe that the events are a combination of interference in the equipment of Phénix and a mechanical phenomenon. Regardless, the origin of A.U.R.N. seems to be bound to some specific parameter of Phénix due to the fact that the transients only have occurred in this reactor. A safety analysis made by an expert committee, appointed by CEA, showed that the A.U.R.N. events are not a threat to the safety of Phénix. However, the origin of these negative transients has to be found before any construction of a commercial size sodium-cooled fast reactor can begin. Thus, further research is needed. Handledare: Hans Henriksson Ämnesgranskare: Henrik Sjöstrand Examinator: Tomas Nyberg ISSN: 1401-5757, UPTEC F11 011 Sponsor: Vattenfall AB Acknowledgements I would like to thank the following persons for their guidance and criticism Andrei Fokau Anna-Maria Wiberg Bruno Fontaine Hans Henriksson Henrik Sjöstrand Peter Wolniewicz IespeciallywouldliketothankBruno Fontaine for all the valuable information and guidance he has provided, which have been essential for this thesis. I am also grateful for the time Andrei Fokau spent in order to help me learn Monte-Carlo simulation codes. Contents 1 Introduction 1 1.1 Background . 1 1.2 Aims and Objectives . 2 1.3 Limitations . 2 1.4 Outline of this report . 3 2 Fundamentals of fast reactors 5 2.1 Overview . 5 2.2 Fission . 5 2.3 Breeding . 6 2.4 Transmutation of long-lived radio-active elements . 7 2.5 Coredesignoffastreactors . 7 2.5.1 Configuration of fast breeder reactors . 7 2.6 Effective neutron multiplication factor, keff ..................... 9 3 Sodium-cooled fast reactors 13 3.1 Sodium-cooled fast reactors in the world . 13 3.2 Sodium-cooled reactor design . 14 3.2.1 Advantages and disadvantages . 14 3.2.2 Technical overview . 14 3.3 Phénix.......................................... 16 3.3.1 A.U.R.N. 17 3.3.2 Core-flowering ................................. 21 3.3.3 Core-floweringtestsofPhénix. 22 3.4 ASTRID......................................... 22 3.4.1 Preliminary design . 23 4 Method and materials 25 4.1 Monte-Carlosimulationcode . 25 4.1.1 Difficulties using Monte-Carlo simulation code . 25 4.1.2 Choice of Monte-Carlo simulation code . 26 4.1.3 Advantages and disadvantages of Serpent . 27 4.2 Model of Phénix . 27 4.3 Model of core-flowering . 30 5 Result 33 5.1 Model of Phénix . 33 5.2 Core-flowering...................................... 33 6 Discussion 37 6.1 Simulations . 37 6.2 A.U.R.N. 38 7 Conclusions 41 7.1 Conclusions ....................................... 41 7.2 Suggestions for further work . 42 References 43 List of Figures 45 List of Tables 47 Nomenclature 48 Appendices 49 A Definitions of the units in the Four Factor Formula A-1 B Code of the Phénix Model B-1 C Output data from a test run of the Phénix model C-1 D Results of the PFBR-model D-1 "The first country to develop afastbreederreactor will have a commercial advantage for the exploitation of nuclear energy" Enrico Fermi - 1945 1 Introduction The need of energy in the world is growing and since we now are facing possible climate changes the search for alternative energy sources to fossil fuels is greater than ever. Nuclear power has for some time been a non-expanding market, though today the view has changed. It is now seen as one of the alternatives to fossil fuel due to its low emission of CO2 and low environmental impact. However, nuclear power has its disadvantages, for example the waste produced in current reactors needs to be stored for more than 300 000 years [1]. A new generation of nuclear power plants named Generation IV is under development, which can solve some of the problems related to the current nuclear power. The aim of Generation IV is to have safer, more reliable and efficient power plants with a physical protection against terrorism in a closed fuel cycle [2]. The goal is also to improve the environment, for example by introducing nuclear-produced hydrogen for transportation. 1.1 Background There are six different reactor designs of Generation IV [2], Sodium-cooled Fast Reactor (SFR), Lead-cooled Fast Reactor (LFR), Molten Salt Reactor (MSR), Very High Temperature Reac- tor (VHTR), Gas-cooled Fast Reactor (GFR) and SuperCritical-Water Reactor (SCWR). All of these designs are being developed throughout the world in order to achieve the goal of commer- cialization. It is possible to have different neutron spectra by using different coolants. For example us- ing liquid metal as coolant results in having a fast neutron spectrum1 in the reactor, more about this in Chapter 2, which in turn leads to the possibility to use up to 99.9 % of the fuel. This can be compared to the fuel usage of todays water reactors’ usage of a few percent. Recycling of the spent nuclear fuel is then possible and it can result in a reduction of storage time from 300 000 years to several 1000 years [1]. Hence, the process of storing might be easier for a country to manage. The main drawbacks of using these coolants are the increase of temperatures and 1Fast neutron spectrum: The neutron spectrum is dominated by fast/high energetic neutrons. 1 INTRODUCTION Aims and Objectives the high irradiation in the core, which makes it difficult to develop feasible materials that can sustain such severe environment. The sodium-cooled fast reactor is the candidate of Generation IV that lies furthest ahead in research and development [2]. Even though sodium-cooled fast reactors are part of a new gener- ation of nuclear power, the idea is quite old. In fact, the first reactor connected to the electrical grid, EBR-I, was cooled with a combination of sodium and potassium [3]. However, the com- mercialization of sodium-cooled fast reactors is still far away in time due to the lack of proper materials. Downtime due to sodium-leaks is one of the most common issues with the operation of SFRs, though earlier unexperienced very rapid negative reactivity transients2 have caused major prob- lems in the French reactor Phénix. The French call these transients A.U.R.N., which is short for Arrêt d’Urgence par Réactivité Négative. In English this means automatic emergency shutdown by negative reactivity. No final explanation of A.U.R.N.s has yet been established, though the most probable cause is radial movement of the core called core-flowering. This is one of the issues that needs to be solved before introducing SFRs of commercial size. 1.2 Aims and Objectives This master thesis investigates the cause of the A.U.R.N. events. The objectives are to survey published reports and use Monte-Carlo simulation code in order to simulate core-flowering and analyze how it affects the reactivity of an SFR core. Simplified models of the phenomenon have been used in order to make the simulations possible. The aims of the study are to determine a possible cause of the negative reactivity transients, give a solution on how to avoid the problem and point to further research.