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3.05: Fast Neutron Reactors– Transient and Safety Analysis
11:30am - 1:00pm
Session Chair: Didier De Bruyn, SCK•CEN, Belgium
A design criterion for SFR behavior during loss of primary flow transients
Alice Jardel, Timothee Kooyman, Jean-Baptiste Droin, Pierre Sciora
Various macroscopic estimators are used during the early design phase of a Sodium-cooled Fast Reactor (SFR) to guide its conception. Among them, the integral sodium void worth is often used as a preliminary indicator of the core transient behavior. This estimator corresponds to the reactivity effect due to the complete removal of the coolant in the active part of the core and its upper structures. It is generally positive due to the competition between a positive spectrum hardening effect caused by the sodium voiding and an increase in the neutron leakage. It can be shown that the behavior of a homogeneous SFR core during a loss-of-flow transient is mainly dependent on the magnitude of the sodium void worth.
Considering this, CEA has designed a low-void core, the so-called CFV core, which features axial heterogeneities to maximize the leakage component of the sodium void worth. One specific design feature of this core is the presence of a sodium plenum above the core, which acts as a reflector during nominal operation and is voided in the case of loss of primary flow. This design creates a significant spatial heterogeneity in the void feedback coefficients, with a strong positive zone in the center of the core and a strongly negative one above it. It is shown here that due to this uneven spatial repartition, the integral sodium void worth is not an adequate estimator of a heterogeneous core behavior during a loss-of-flow transient. A new criterion, based on the ratio of the void feedback spatial component is thus discussed and analyzed.
Sodium coolant: interaction with its environment and coolant processing
Due to its very attractive properties, Na is used for nuclear and solar applications, in similar operating ranges. It is also used to study the dynamo effect, to get a better understanding of the magnetic field of the earth, to purify metals such as tantalum or silicon for photovoltaic cells... In order to operate in reliable and safe conditions a SFR, it is necessary to master the coolant’s quality. Na chemical control is performed versus the different major chemical compounds: oxygen (corrosion control), hydrogen (detection of the sodium-water reaction), and to a less extent carbon (carburization, decarburization phenomena). Oxygen contributes to the corrosion of the cladding steel and activated corrosion products are transported from the core towards the components and mainly to the Intermediate Heat Exchangers, leading to their contamination. Moreover, other detrimental effects should be avoided: plugging of narrow sections, loss of heat transfer efficiency in heat exchangers…Oxygen and moisture are introduced mainly during handling operations; hydrogen is due to the aqueous corrosion of the Steam
Generator Units and thermal decomposition of hydrazine, used to control the oxygen content in the water. The purification of oxygen and hydrogen is performed satisfactorily thanks to cold traps. Several innovative options have been developed in order to satisfy new requirements: high purification rate, optimized capacity and reduced heat losses. It is also possible to get a very high purity thanks to getters.
Before repair operations, after sodium draining, it is necessary to clean-up the internal structures, wetted by residual Na: efficient processes have been developed thanks to the reactivity of Na with steam.
Some stress corrosion cracking phenomena can occur in presence of aqueous soda; therefore it is necessary to avoid any presence of residual aqueous soda on components, gaps…by appropriate rinsing and drying processes.
A Design Study on a Metal Fuel Fast Reactor Core for High Efficiency MA Transmutation by Loading SiC/SiC Composite Material
1Japan Atomic Energy Agency; 2Central Research Institute of Electric Power Industry
A metal fuel fast reactor core for high efficiency Minor Actinide (MA) transmutation was designed. In this study, SiC/SiC composite material was loaded into the core to improve sodium fast reactor (SFR) core safety characteristics such as sodium void reactivity worth and Doppler coefficient due to neutron moderation as confirmed in the past study. Based on a 750 MWe metal fueled SFR core concept designed in the past study, the core loading fuel subassemblies with SiC/SiC wrapper tubes and moderator subassemblies was studied. Firstly, an impact on reactivity swing by replacing fuel subassemblies with SiC/SiC moderator subassemblies was evaluated. The reactivity swing of the core with highly concentrated MA-containing fuel (Pu:MA=1:1) was remarkably improved due to the excellent characteristics of MA as a fertile and superior neutron economy of the metal fuel core. Thus, about 30% of fuel subassemblies was possible to be replaced with SiC/SiC moderator subassemblies without deterioration of the reactivity swing. Secondly, neutron spectrum of fuel subassemblies next to a SiC/SiC moderator subassembly was evaluated. The result indicated that the neutron moderation was limited within one layer of fuel subassemblies next to the moderator. To maximize the improvement of the core safety characteristics based on the findings, three layers of SiC/SiC moderator subassemblies were loaded in the core by replacing 108 out of 393 fuel subassemblies with the moderators. A layer of SiC/SiC moderator subassemblies were loaded in every two fuel subassembly layers. The core with about 20wt% MA-containing metal fuel satisfied all provisional safety design criteria and achieved five times larger MA transmutation amount than the reference metal fuel core.