KNFC Crud Sampling and Fuel Cleaning Technology Development

Transcription

1 KNFC Crud Sampling and Fuel Cleaning Technology Development Park, J.Y. 1, Kwon, Y.B. 1, Shin, J.C. 1, Park, J.Y. 1, Choi, J.S. 1, Kim, Y.K. 1, Choi, K.S. 1, Choi, I.K. 2 1 Korea Nuclear Fuel Co., Ltd., Daejeon, South Korea 2 Korea Atomic Energy Research Institute, Daejeon, South Korea 1. Introduction Crud deposited onto the fuel elements in nuclear power plants was not a serious problem until an upper core flux depression was found at Callaway, USA in 1989 [1]. The unexpected flux depression was named as an Axial Offset Anomaly(AOA). Though the mechanism of an AOA is not completely understood, crud is believed to be a key component of it [2,3]. After the corrosion products in the reactor cooling system are deposited onto the fuel clad by a sub-cooled nucleate boiling at the clad surface, boron is adsorbed in the crud [4]. Then a measurable reduction in the neutron flux occurs to cause an AOA problem. KNFC has developed ultrasonic cleaning system under license agreement with EPRI and it is scheduled to perform a crud removal campaign at the end of this year. In addition, the elemental compositions and particle size distribution of the crud were measured for applying to adequate filtration system design. All the experiments were performed in the post irradiation examination facility at KAERI and crud sampling equipment is being developed for sampling in the plant for more exact composition of crud. 2. Crud sampling equipment and analysis of crud 2.1 Crud sampling equipment At present, the crud sampling is carried out in the post irradiation examination facility and it is possible to be contaminated or lost some elemental compositions because of different condition with spent pool in nuclear power plant. For more exact analysis, crud sampling equipment is being developed and will be applied as shown in Fig Analysis of crud Figure 1 Crud sampling equipment Crud samples were taken from two spent fuel rods of Yonggwang Unit-1 cycle 14 in the post irradiation examination facility. Fuel rod average burnups were 40,531 GWD/MTU and 50,111 GWD/MTU for the P09-B17 and O59-L14 fuel rod, respectively. Sticky tape and steel knife were used to collect the cruds from the fuel rods. For the P09-B17 fuel rod, samples were taken with sticky tapes at 3,000mm and 400mm from the bottom of the fuel rod. Cruds at 3,000mm produced well crystallized particles while 1

2 those taken at 400mm produced non-crystalline particles. It indicates that the upper part of the fuel assembly provides a better condition to form a crud deposit due to its higher temperature Particle size Particle size information is necessary to select an adequate pore size of the filter to collect the cruds released from the fuel assembly by an ultrasonic operation. Thus, a size distribution measurement was carried out with well crystallized samples. Figure 2 shows the size distribution of the crud particles taken at 3,000mm of the P09-B17 fuel rod. Even though fine particles smaller than 0.5µm were abundant, the total weight was less than 5%. It indicates that more than 95wt% crud particles can be collected by using a 0.5µm filter system[5]. Figure 2 Particle size distribution of crud sample at 3,000mm of P09-B Composition of crud Composition analyses were carried out by using Scanning Electron Microscope-Energy Disperse X-ray Analysis, Electron Probe for Micro Analyser, and ICP-AES. Fe, Ni and Cr were found as the main elements in the crud but boron was not found in any of the crud samples. The composition of Fe and Ni in the crud particles varied from particle to particle. In most particles, Ni was found more than Fe. Figure 3 shows a sub-micron particles and analysis table. Element point Fe Ni Cr Zr Figure 3 Sub-micron particles in crud sample and analysis table 2

3 3. Fuel cleaning system This system is designed for crud removal and consists of two main parts : a cleaning fixture and a filtration system. Fuel assemblies are lowered into the cleaning chamber of the cleaning fixture and then crud deposits are removed by ultrasonic energy from transducer and then sweep crud from the cleaning fixture by a suction pump on the filtration system. Finally, the removed crud is captured in a bank of filters. Figure 4 illustrates the installation of cleaning system in power plant. 3.1 Cleaning fixture Figure 4 Crud cleaning system Normal Coolant Flow Natural Circulation (in Pump Stop) Figure 5 Cleaning fixture Figure 5 shows the general arrangement of the fuel cleaning fixture. The fuel assembly to be cleaned is lowered into a box tube which runs the full length of the fuel assembly and has the same internal dimensions as a spent fuel storage rack cell. The fuel assembly remains suspended from the spent fuel handling bridge hoist throughout the cleaning operation and there are no openings in the box tube which could cause the fuel assembly to become stuck during insertion or removal. 3

4 Four advanced push-pull ultrasonic transducers are mounted on each side of the box tube (sixteen(16) total). The ultrasonic transducers are mounted along the top half of the box tube. When a fuel assembly is fully inserted, the ultrasonic energy is concentrated in the upper half of the fuel assembly, where the crud deposits causing AOA are reported to be concentrated. This transducer arrangement permits the lower half of the fuel assembly to be cleaned as well by positioning the fuel assembly higher in the fixture. The transducers have been demonstrated in the laboratory to be capable of removing simulated nickel ferrite deposits throughout the full cross section of the fuel bundle in three to ten(10) minutes time depending upon the power level applied and the deposit tenacity. Figure 6 Set up ultrasonic transducers A small hole is provided at the bottom of the piping on the cleaning fixture to aid in filling and draining, and to provide natural convection for decay heat removal in the unlikely event that power is lost to the filter assembly pumps and that there is some delay in removing the fuel assembly from the cleaning fixture using the fuel handling hoist. 3.2 Filtration system Figure 7 shows the independent underwater filtration system used to collect the deposits that are loosened in the cleaning fixture. This system is assembled on a system mounted to the wall of the cask storage pit and supported by the adjacent floor. The system comprises: 1) fully redundant process pumps, 2) parallel flow filter sets [four(4) filters in each set], 3) flow control valves 4) flow, pressure and temperature elements, 5) radiation monitor devices on both the filter inlet stream and at the filter cartridges, and 6) sampling capabilities for the filter inlet. The system takes suction from the fuel cleaning fixture via a hose connection. Figure 7 Filtration system 4

5 The cleaning process is monitored by a gamma detector at the inlet to the pumps, tuned to Co 58 and Co 60. Gamma detectors are provided on each filter canister in each filter bank to monitor the activity of crud captured by the filters. Water flow rate, water temperature, and differential pressure across the filters are also monitored during cleaning. Cleaning performance is visually confirmed by TV cameras aimed at the fuel assembly as it is inserted into and removed from the cleaning fixture. 4. Conclusion Particle size of crud was measured about 0.5 and it means that more than 95wt% crud particles can be collected by a 0.5 filtration system. It may affect both the collecting efficiency of the crud and the filtering time. Fe, Ni and Cr were found as the main elements in the crud but boron was not found in any of the crud samples. For more exact analysis, KNFC is developing the crud removal equipment which can sample crud in the irradiated fuel storage pit. KNFC will construct and qualify the complete fuel cleaning system until September The system will be demonstrated on several spent fuel assemblies at Kori plant in December References [1] W.A. Byers, and J. Deshon, Structure and Chemistry of PWR Crud, International Conference Water Chemistry in Nuclear Reactors Systems Conference, San Francisco-USA, October [2] A. Tigeras, J-L. Bretelle, E. Decossin, EDF AOA Experience: Chemical and Thermal Hydraulic Analysis, International Conference Water Chemistry in Nuclear Reactors Systems Conference, San Francisco-USA, October [3] Fuel Safety Criteria Technical review, NEA/CSNI/R(99)25. [4] Effects of Crud Buildup and Boron Deposition on Power Distribution and Shutdown Margin, NRC Information Notice 97-85, [5] In Kyu Choi, et al., Characteristics of Cruds Deposited on the Spent Fuel Rods of Yonggwang-Unit 1, Korea Nuclear Society, May