What I found noteworthy:
Thermal-Hydraulic and Thermo-Mechanical Assessment of Dual-Cooled Annular Fuel for PWR Application
This paper discusses the work in South Korea to enable MIT annular fuel design to be adapted to Korean nuclear reactors to enable existing reactors to have power increased by 20% with minimal modification. More extensive modification would allow an increase in power of 50% for existing nuclear reactors.
A dual-cooled annular fuel for a pressurized water reactor (PWR) has been introduced for a significant amount of reactor power uprate. A previous study proposed a 13×13 annular fuel array replacing the 17×17 solid fuel in the Westinghouse PWR plant, which could increase the core power up to 50% with the considerable changes in the major reactor components. The Korea Atomic Energy Research Institute (KAERI) is conducting a research to develop a dual-cooled annular fuel for its employment in an optimized PWR in Korea, OPR1000. The dual-cooled fuel for the OPR1000 is targeted to increase the reactor power by 20% as well as reduce the fuel-pellet temperature by more than 30% without a change to the reactor components other than the fuel. Several technical issues exist for the application of the dual cooled annular fuel to the power uprate in the OPR1000. One of the important issues is the balance of heat split between the inner and outer channels since the coolant flows through the circular inner channel of annular fuel as well as the outer subchannels formed between the fuel rods. The dual-cooled fuel should be designed to maintain the heat split in reactor operation such that it does not exceed a specified acceptable fuel design limit, e.g., DNBR limit. It has been known that the heat split is largely dependent on the heat transfer in the inner and outer fuel gaps. This is because the thermal resistance in the fuel gap is very large due to a low thermal conductivity. The fuel gap is filled initially with helium gas and the heat is transferred by conduction and radiation. The heat transfer in the gap is governed by thermal gap conductance which decreases exponentially as the gap width increases. This study was performed to determine the acceptable range of gap conductance, i.e., gap width based on the DNBR analysis for the OPR1000 core with the dual-cooled fuel. The minimum DNBR (MDNBR) should be higher than the DNBR limit during anticipated operational occurrences (AOOs) as well as normal reactor operation. The DNBR calculations were made using the subchannel analysis method for the OPR1000 core with power uprate of 20%. For the wide range of the inner and outer gap conductance, the MDNBR values were predicted and compared with the DNBR limit to determine the acceptable range of the gap width. The MDNBR was found to occur in either inner channel or outer channel depending on the inner and outer gap conductance. This study also predicted the variation of the gap width using the thermo-mechanical analysis for the dual-cooled fuel in its life cycle. The outer gap width was found to reduce due to the thermal expansion while the inner one increased. The thermo-mechanical analysis included the effects of fuel swelling and creep as well. The inner and outer gap widths were predicted to vary within its acceptable region which is bounded by the DNBR limit.
Dual cooled fuel is under developing for power uprating of an existing PWR (especially for OPR-1000 in our work). The most important feature of it is an additional coolant flow passage formed inside a fuel rod to increase the surface area of heat transfer. This makes the outer diameter of a fuel rod considerably increase compared with that of a conventional fuel. It inevitably affects the design of the structural components such as the spacer grids, top and bottom end pieces and guide tubes. Primary goal of the present work is to show the possibility of composing an actual fuel assembly through changing the component design without violating the design criteria and functional requirements of the components. Main ideas for them are: moving the support location of a fuel rod in the spacer grid due to narrow gap between the fuel rods; rearranging the flow hole pattern of the top and bottom end pieces; applying another bigger diameter tube outside the conventional guide tubes. First part of the present paper will show the design results. After the possibility is shown, mechanical integrity is concerned. So another part of the paper deals with the mechanical behavior and fretting wear performance of the dual cooled fuel. Flow-induced vibration (FIV) tests were conducted with the 4X4 partial assembly to investigate the vibration behavior. Two different types of the spacer grids were applied, which had different rod supporting force. The FIV characteristics were analyzed. By using the equation suggested by Yetisir el al., which correlates the workrate for wearing and the FIV parameters, the wear rate is to be predicted. Together with the fretting wear experiments, the fretting wear margin of a dual cooled fuel is to be discussed.
M5® is the reference alloy used as cladding tube and structure material for all AREVA PWR designs. As of July 2009, over 2.8 million M5® clad fuel rods loaded in 5800 fuel assemblies in designs from 14×14 to 18×18 have been irradiated in 73 commercial PWRs in 12 countries to fuel rod burn-up of 80GWd/tU. To demonstrate superiority of the M5® alloy at burn-ups beyond current licensing limits, M5® has been operated in PWR at fuel rod burn-ups exceeding 71 GWd/tU in the United States and 78 GWd/tU in Europe.