Helium measuring system for on-line fuel and control rod integrity surveillance in BWRs

Abstract: Accurate supervision of fuel and control rod integrity in a nuclear reactor core is very important for maintaining a safe and reliable operation of the reactor. Fuel rod failures can cause increased doses to personnel in the power plant, increased backend costs, limitations of the reactor operation and unplanned shut downs, which can lead to large economic losses. Control rod failures are normally not as severe as fuel rod failures, but can lower the reactor efficiency, cause operational restrictions and increase a need for control rod inspections during outage. Additional inspections can cause a prolonged outage, leading to economic losses. A project with the main objective to improve the detection of fuel and control rod failures in boiling water reactors (BWRs) by implementing continuous helium measurements in the off-gas system was initiated in 2007. The work was performed in collaboration between Chalmers University of Technology in Sweden, Forsmarks Kraftgrupp AB in Sweden and Kernkraftwerk Leibstadt (KKL) in Switzerland. During the project, helium detector systems have been installed in the reactor’s off-gas systems at both of the nuclear power plants (NPPs). The helium measuring system is a part of a combined on-line system which also includes nuclide specific measurements of gamma emitting noble gases. This combined system provides continuous information of both the fuel and control rod integrity. The work described in this thesis can be divided into two parts. One part includes evaluation of helium measurements for detection of control rod failures at KKL, which was the main purpose for the installation of the system at KKL in 2008. The helium measuring system at KKL has shown to be able to identify and follow control rod failures on-line. Since 2012 the helium measurement system is part of the process control and is included in the KKL core supervision system, which monitors the thermal operational parameter of the reactor core and all the relevant chemical parameters of the reactor water. The second part of the work was to investigate the ability of the helium measuring system to detect the small amount of helium that is expected to leak out in case of a fuel failure. This was also the main objective for the installation of the system at Forsmark 3 (F3) in 2010. Some helium releases from fuel failures at Forsmark 3 have been detected with the current system, however, further measurements might be needed to optimize the technique. Several factors, including operational parameters, which are influencing the helium measurements, have been identified and implemented in the developed equation for calculation of the helium background level in the off-gases. When being programmed in the core supervision system, such an automatic calculation of the helium background level, will give an immediate estimation of the released amount of helium if the helium concentration increase relative to the background level was detected.