Tae Hyun Chun

HIGH BURNUP FUEL TECHNOLOGY IN KOREA

High burn-up fuel technology has been developed through a national R&D program, which covers key technology areas such as claddings, UO2 pellets, spacer grids, performance code, and fuel assembly tests. New cladding alloys were developed through alloy designs, tube fabrication, out-of-pile test and in-reactor test. The new Zr-Nb tubes are found to be much better in their corrosion resistance and creep strength than the Zircaloy-4 tube, owing to an optimized composition and heat treatment of the new Zr-Nb alloys. A new fabrication technology for large grain UO2 pellets was developed using various uranium oxide seeds and a micro-doping of Al. The uranium oxide seeds, which were added to UO2 powder, were prepared by oxidizing and heat-treating scrap UO2 pellets. A UO2 pellet containing tungsten channels was fabricated for a thermal conductivity enhancement. For the fuel performance analysis, new high burnup models were developed and implemented in a code. This code was verified by an international database and our own database. The developed spacer grid has two features of contoured contact spring and hybrid mixing vanes. Mechanical and hydraulic tests showed that the spacer grid is superior in its rod- supporting, wear resistance and CHF performance. Finally, fuel assembly test technology was also developed. Facilities for mechanical and thermal hydraulic tests were constructed and are now in operation. Several achievements are to be utilized soon by the Korea Nuclear Fuel and thereby contribute to the economy and safety of PWR fuel in Korea

Evaluation of a Dual-Cooled Annular Fuel Heat Split and Temperature Distribution

The heat flux and fuel temperature of an annular fuel rod was analyzed using a newly developed program, DUO_THERM. The dual-cooled annular fuel rod has both inner and outer coolant channels. The heat fluxes to inner and outer claddings can change during irradiation because of the variation in the gap conductances of the inner and outer sides. The heat flux is a very important design factor affecting fuel integrity and safety. However, in an annular fuel design, the prediction of heat flux is very complicated because it is closely connected with size changes of two gaps, which are different from each other during irradiation. A burnup increase and power change can cause pellet and cladding radial deformation and eventually change the gap width. Because an annular fuel has two gaps at both sides and their heat resistances are very large, an imbalance of the gap width can lead to heat flux asymmetry. Therefore, a pellet and cladding dimensional change including densification, swelling, creep, and thermal expansion must be considered in the calculation of heat flux and temperature. By using the DUO_THERM program, the changes in heat flux and temperature during an operation were evaluated with respect to a reference design of an annular fuel. The results of the evaluation showed that the heat flux of an annular fuel was greatly affected by the inner and outer gap conductance changes. The heat flux of the inner cladding was maximum at zero burnup, but that of the outer cladding was maximum at the burnup of the outer gap closing. In addition, the maximum fuel temperature location was changed by gap conductance asymmetry.

Development of a Bi-Axial Acceleration-Detecting Device for a Tube

A new device for measuring an acceleration in a fuel rod has been developed. The primary purpose is to apply it to the experiments for a nuclear fuel fretting, which is caused by a fuel rod and grid interaction due to a flow-induced vibration of the rods. A bi-axial accelerationdetecting device of a cylindrical shape for an insertion into a cladding tube is designed. Two unimorph piezoelectric accelerometers of small size and for special use in a high temperature condition were attached to the housing’s inner wall of the mounting device, which were oriented perpendicularly with each other to accommodate the acceleration signal during a fretting. Verification is carried out by a displacement and a frequency response analysis. The verification results of the bi-axial acceleration-detecting device showed a good response for the amplitude and bi-directional trace. This paper mainly presents the detailed design of the bi-axial accelerationdetecting device which is developed by using the design optimization of a cylindrical type and applying it to the lower and upper plugs to sustain the device in the tube wall. The verification is carried out by a displacement analysis and a frequency response analysis. An elaboration of the development procedure, calibration method and results are also given in this paper.