• Energy Research

The heat transfer enhancement through turbulence augmentation is recognized as a key factor for improving the safety and economic conditions in the development of both critical and subcritical innovative advanced gas cooled fast reactors (GFR) and transmutation systems. In the present work, a new experimental facility named L-STAR has been designed and erected at the Karlsruhe Institute of Technology (KIT) to study turbulence flow behavior and its heat transfer enhancement characteristics in gas cooled annular channels under a wide range of conditions. The main objective of the experimental study is to investigate and improve the understanding of complex turbulent convective enhancement mechanisms as well as the friction loss penalties of roughened fuel rod elements compared to smooth ones and to generate an accurate database for further development of physical models. Tests are being conducted in a closed gas loop at various Reynolds numbers with nearly uniform heat release conditions. The test section consists of an annular hexagonal cross-section channel with an inner electrical heater rod element (smooth and roughened), placed concentrically within the test section, to simulate the flow area of a fuel rod element in a fast gas cooled reactor. In the first step, experimental results of the fluid flow with a smooth heater rod are presented. The pressure drops, as well as axial temperature profiles within the heater rod surface have been measured at Reynolds numbers in the range from 3·103 to 7·104. Experimental program is continued with higher temperatures and the implementation of various artificial surface structures.

The heat transfer enhancement through turbulence augmentation is recognized as a key factor for improving the safety and economic conditions in the development of both critical and subcritical innovative advanced gas cooled fast reactors (GFR) and transmutation systems. In the present work, a new experimental facility named L-STAR has been designed and erected at the Karlsruhe Institute of Technology (KIT) to study turbulence flow behavior and its heat transfer enhancement characteristics in gas cooled annular channels under a wide range of conditions. The main objective of the experimental study is to investigate and improve the understanding of complex turbulent convective enhancement mechanisms as well as the friction loss penalties of roughened fuel rod elements compared to smooth ones and to generate an accurate database for further development of physical models. Tests are being conducted in a closed gas loop at various Reynolds numbers with nearly uniform heat release conditions. The test section consists of an annular hexagonal cross-section channel with an inner electrical heater rod element (smooth and roughened), placed concentrically within the test section, to simulate the flow area of a fuel rod element in a fast gas cooled reactor. In the first step, experimental results of the fluid flow with a smooth heater rod are presented. The pressure drops, as well as axial temperature profiles within the heater rod surface have been measured at Reynolds numbers in the range from 3·103 to 7·104. Experimental program is continued with higher temperatures and the implementation of various artificial surface structures.

During the Engineering Validation and Engineering Design Activities (EVEDA) phase (2007-2014) of the International Fusion Materials Irradiation Facility (IFMIF), an advanced engineering design of the High Flux Test Module (HFTM) has been developed with the objective to facilitate the controlled irradiation of steel samples in the high flux area directly behind the IFMIF neutron source. The development process addressed included manufacturing techniques, CAD, neutronic, thermal-hydraulic and mechanical anal- yses complemented by a series of validation activities. Validation included manufacturing of 1:1 parts and mockups, test of prototypes in the FLEX and HELOKA-LP helium loops of KIT for verification of the thermal and mechanical properties, and irradiation of specimen filled capsule prototypes in the BR2 test reactor. The prototyping activities were backed by several R&D studies addressing focused issues like han- dling of liquid NaK (as filling medium) and insertion of Small Specimen Test Technique (SSTT) specimens into the irradiation capsules. This paper provides an up-todate design description of the HFTM irradiation device, and reports on the achieved performance criteria related to the requirements. Results of the vali- dation activities are accounted for and the most important issues for further development are identified.

During the Engineering Validation and Engineering Design Activities (EVEDA) phase (2007-2014) of the International Fusion Materials Irradiation Facility (IFMIF), an advanced engineering design of the High Flux Test Module (HFTM) has been developed with the objective to facilitate the controlled irradiation of steel samples in the high flux area directly behind the IFMIF neutron source. The development process addressed included manufacturing techniques, CAD, neutronic, thermal-hydraulic and mechanical anal- yses complemented by a series of validation activities. Validation included manufacturing of 1:1 parts and mockups, test of prototypes in the FLEX and HELOKA-LP helium loops of KIT for verification of the thermal and mechanical properties, and irradiation of specimen filled capsule prototypes in the BR2 test reactor. The prototyping activities were backed by several R&D studies addressing focused issues like han- dling of liquid NaK (as filling medium) and insertion of Small Specimen Test Technique (SSTT) specimens into the irradiation capsules. This paper provides an up-todate design description of the HFTM irradiation device, and reports on the achieved performance criteria related to the requirements. Results of the vali- dation activities are accounted for and the most important issues for further development are identified.

AbstractSeveral validation activities were dedicated to the High Flux Test Module (HFTM) of the International Fusion Materials Irradiation Facility (IFMIF) at the Karlsruhe Institute of Technology (KIT) in Germany. The HFTM contains 24 capsules packed densely with Eurofer specimens to facilitate their irradiation in the high flux zone directly behind the IFMIF neutron source. The small gaps among the Eurofer specimens are filled by the sodium potassium eutectic alloy NaK-78 to improve the thermal conduction among the specimens and achieve uniform and predictable temperature distribution. As a result of first trials, the filling process of NaK-78 into the specimens’ capsule had been identified as an issue worth further investigations. Therefore, the wettability of the steels Eurofer and SS 316 by NaK-78 is experimentally investigated to evaluate the applicability of this concept and identify the favorable conditions. In the experiment, the capillary rise of NaK-78 in a two-parallel-plates channel (gap) is investigated versus the following: (i) temperature of both NaK-78 and the parallel plates from 50°C to 350°C, (ii) machining techniques used for the parallel plates, (iii) thickness of the gap between the plates, and (iv) material of the parallel plates including Eurofer and SS 316. The present experimental results will help in defining the working conditions required to achieve an optimal filling of the IFMIF HFTM capsules with NaK-78 and a complete wetting of the capsules’ specimens.

AbstractSeveral validation activities were dedicated to the High Flux Test Module (HFTM) of the International Fusion Materials Irradiation Facility (IFMIF) at the Karlsruhe Institute of Technology (KIT) in Germany. The HFTM contains 24 capsules packed densely with Eurofer specimens to facilitate their irradiation in the high flux zone directly behind the IFMIF neutron source. The small gaps among the Eurofer specimens are filled by the sodium potassium eutectic alloy NaK-78 to improve the thermal conduction among the specimens and achieve uniform and predictable temperature distribution. As a result of first trials, the filling process of NaK-78 into the specimens’ capsule had been identified as an issue worth further investigations. Therefore, the wettability of the steels Eurofer and SS 316 by NaK-78 is experimentally investigated to evaluate the applicability of this concept and identify the favorable conditions. In the experiment, the capillary rise of NaK-78 in a two-parallel-plates channel (gap) is investigated versus the following: (i) temperature of both NaK-78 and the parallel plates from 50°C to 350°C, (ii) machining techniques used for the parallel plates, (iii) thickness of the gap between the plates, and (iv) material of the parallel plates including Eurofer and SS 316. The present experimental results will help in defining the working conditions required to achieve an optimal filling of the IFMIF HFTM capsules with NaK-78 and a complete wetting of the capsules’ specimens.