The Fixed Bed Nuclear Reactor For Decentralized Energy Needs Sümer Sahin Gazi Üniversitesi, Teknik Eğitim Fakültesi,Teknikokullar, Ankara, TURKEY E-mail: sumer@gazi.edu.tr Farhang Sefidvash Nuclear Engineering Departament, Federal University of Rio Grande do Sul Av. Osvaldo Aranha, 99 - 4° andar, 90.046-900 Porto Alegre, RS, Brazil Email: Farhang.Sefidvash@pesquisador.cnpq.br ABSTRACT In the present work, the basic features of a new reactor type, the so called Fixed Bed Nuclear Reactor (FBNR) is presented. FBNR is a small reactor (40 MWe) without the need of on-site refueling. It utilizes the PWR technology but uses the HTGR type fuel elements. It has the characteristics of being simple in design, modular, inherent safety, passive cooling, proliferation resistant, and reduced environmental impact. The study comprises reactor description, fuel element description, criticality calculations. A series of one dimensional criticality calculations are conducted with SCALE5 using SN methods. SN calculations with SCALE5 have resulted for the cold reactor (20 °C, 1 bar) kf = 1.4408 and for the hot reactor (308 °C, 160 bar), based on the average inlet-outlet temperatures, kf = 1.40003 for the spherical fuel element cell. Time calculations have been pursued for 12 years. Three dimensional criticality calculations benchmarking are conducted with MCNP5-1.4 using Monte Carlo methods and have yielded kf = 1.456 73 for the hot FBNR unit cell. 1. INTRODUCTION The Small Reactors without On-Site Refueling are defined by IAEA “As reactors which have a capability to operate without refueling and reshuffling of fuel for a reasonably long period consistent with the plant economics and energy security, with no fresh and spent fuel being stored at the site outside the reactor during its service life. They also should ensure difficult unauthorized access to fuel during the whole period of its presence at the site and during transportation, and design provisions to facilitate the implementation of safeguards. In this context, the term “refueling” is defined as the ´removal and/or replacement of either fresh or spent, single or multiple, bare or inadequately confined nuclear fuel cluster(s) or fuel element(s) contained in the core of a nuclear reactor`. This definition does not include replacement of well-contained fuel cassette(s) in a manner that prohibits clandestine diversion of nuclear fuel material.“ FBNR (FBNR site, Sefidvash 2003,2007) is designed to be such a reactor and to meet the requirements for an innovative reactor established by the IAEA-INPRO Program (IAEA 2004). 2. PAGE SIZE The Fixed Bed Nuclear Reactor (FBNR) is a small reactor (40 MWe) without the need of on-site refueling. It utilizes the PWR technology but uses the HTGR type fuel elements. It has the characteristics of being simple in design, modular, inherent safety, passive cooling, proliferation resistant, and reduced environmental impact. The fixed bed concept is a special case of the fluidized bed concept. Here, the fuel elements (pebbles) are fixed in the core through the pressure put on them by the flow (~ 10 bar). The pebbles become fluidized in the fuel chamber and leave the chamber when the velocity reaches ~1.4 m/sec and go to the core where they stay in a fixed position while the flow velocity is ~7 m/sec. The FBNR is modular in design, and each module is assumed to be fuelled in the factory. The fuelled modules in sealed form are then transported to and from the site. The FBNR has a long fuel cycle time and, therefore, there is no need for on- site refueling. The reactor makes an extensive use of PWR technology. It is an integrated primary system design. The basic modules have in its upper part the reactor core and a steam generator and in its lower part the fuel chamber, as shown in the schematic figure 1. Reactivity calculations in chapter 4.1 reveal that the FBNR would have very high excess reactivity at start up. Conventional reactors compensate this with a great number of control rods and/or burnable poisons in the core, which cause neutron flux and power distortions and affect negatively the neutron economy. Whereas in the FBNR, a core level limiter is used to add reactivity during reactor operation by introducing fresh fuel to the core and compensate for long term reactivity change caused by fuel depletion or other factors. At start up, the reactor reactivity is manipulated by the adjusting the level limiter. This reduces flux and power distortions to a great degree. On the other hand, the temperature effects and xenon effect are manipulated by the fine control rods. The coolant flows vertically up into the inner perforated tube and then, passing horizontally through the fuel elements and the outer perforated tube, enters the outer shell where it flows up vertically to the steam generator. The reserve fuel chamber is a 40-cm diameter tube made of high neutron absorbing alloy, which is directly connected underneath the core tube. The fuel chamber consists of a helical 25 cm diameter tube flanged to the reserve fuel chamber that is sealed by the international authorities. A grid is provided at the lower part of the tube to hold the fuel elements within it. A steam generator of the shell-and-tube type is integrated in the upper part of the module. The control rods slide inside the core. The reactor is provided with a pressurizer system to keep the coolant at a constant pressure. International Symposium on the Peaceful Applications of Nuclear Technology in the GCC Countries, Jeddah 2008 Nuclear Power (Session 6/No 3)