Abstract
Passive safety systems in a nuclear reactor allow to simplify the overall plant design, beside improving economics and reliability, which are considered to be among the salient goals of advanced Generation IV reactors. This work focuses on investigating the application of a self-actuated, gravity-driven shutdown system in a small lead-cooled fast reactor and its dynamic response to an initiating event. The reactor thermal-hydraulics and neutronics assessment were performed in advance. According to a first-order approximation approach, the passive insertion of shutdown assembly was assumed to be influenced primarily by three forces: gravitational, buoyancy and fluid drag. A system of kinematic equations were formulated a priori and a MATLAB program was developed to determine the dynamics of the assembly. Identifying the delicate nature of the balance of forces, sensitivity analysis for coolant channel velocities and assembly foot densities yielded an optimal system model that resulted in successful passive shutdown. Transient safety studies, using the multi-point dynamics code BELLA, showed that the gravity-driven system acts remarkably well, even when accounting for a brief delay in self-actuation. Ultimately the reactor is brought to a sub-critical state while respecting technological constraints.
Highlights
Safety is one of the important aspects in the design of a nuclear reactor
The underlying idea here is to make use of the self-weight of the absorber rods to drive the assembly without any external drive mechanism
This has not been the case in lead-cooled fast reactors (LFRs)
Summary
Safety is one of the important aspects in the design of a nuclear reactor It has been accorded greater attention since the accidents in Chernobyl and, more recently, in Fukushima. Gravity-driven or -assisted insertion is one of the preferred solutions to the question of passive safety in shutdown system. A carefully designed gravity-driven shutdown system is reliable in diverse accident scenarios. Such passive systems have been used in water-cooled reactors, for instance in CANDU reactors, and have been tested in sodium-cooled fast reactors. LFRs have the potential for buoyancy-driven insertion, this shutdown route is fraught with design challenges, requiring insertion from below the core and is not suited for small modular reactors (SMRs). Numerical study to demonstrate the viability of implementing this concept in a small LFR is presented in this paper
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