Passive acoustic leak detection in energy conversion systems of sodium fast reactors

Abstract: Reaching the standards set forth for Generation IV nuclear power is a challenging task, demanding increased resource utilization, decreased volumes of long-lived radiotoxic waste and increased safety - all while being economically competitive. However, no less than six reactor concepts have been identified as capable of fulfilling the demands. Among these, the Sodium Fast Reactor (SFR) probably represents the most mature technology as more than 20 plants of this type have been operated to this day.One design-specific issue for the SFR is the risk of a leak inside the steam generator, resulting in a violent sodium-water reaction. Such an event is not catastrophic but neeeds to be detected and mitigated in a fast and reliable way. The standard detection method today is based on monitoring of dissolved hydrogen, which gives high sensitivity but slow response. The possibility of using acoustic instrumentation instead has been an active research topic since the 1970s and such a method would indeed be able to respond much faster. Demonstrating that it is, at the same time, robust against false alarms while detecting the fairly weak and possibly unknown acoustic signals associated with leaks has however proven to be a difficult problem.Today, the Commisariat à l’énergie atomique et aux énergies alternatives (CEA) performs research and development, notably within the scope of the ASTRID (Advanced Sodium Technological Reactor for Industrial Demonstration) project, with the aim of entirely eliminating the sodium-water reaction risk. This is achieved by a Brayton cycle design, using a nitrogen turbine and innovative compact sodium-nitrogen heat exchangers. In case of a leak in this system, the low solubility of nitrogen in sodium and the high pressure in the tertiary circuit would increase the secondary circuit pressure, locally deteriorate the exchanger performance and possibly result in harmful hydrodynamic effects. Together with the risks associated with a potential gas leak over to the reactor vessel, this motivates the use of leak detection even for a system where the chemical risk is excluded.This thesis concerns passive acoustic detection of small leaks, primarily for a SFR sodium-nitrogen heat exchanger concept, arguing that this method is indeed suitable based on new experimental results, numerical simulations and studies on existing and novel detection algorithms. The word passive in this case refers to a monitoring system that does not send any acoustic signals on its own, but rather records the noise of the plant online and aims to detect leaks through changes in this signal.The work is presented in five chapters, with the first being a general introduction and the second a review of the scientific literature on the steam generator problem. The third chapter concerns experimental acoustics on leak-simulating nitrogen injections as well as on normal operation noise from a mock-up sodium-nitrogen heat exchanger. The fourth chapter is devoted to development of new detection concepts and methods, intended to be of general interest also for other fields where fast and reliable change detection in noisy signals is needed. Conclusions, discussion and ideas for further research are given in the last and fifth chapter.