The Design and Testing of a Self-Actuated Shut down System for the Protection of Liquid Metal Fast Breeder Reactors (LMFBRS)

Most nuclear reactors are shut down automatically upon the receipt of an external electrical signal which actuates the shutdown device. The design and testing of a simple and reliable Self-Actuated Shutdown System (SASS) for the protection of Liquid Metal Fast Breeder Reactors (LMFBRs) is described here. Self-actuation refers to the capability of the system to be triggered by abnormal changes in reactor variables at the location of the device which can be in the reactor core itself. A ferromagnetic Curie temperature permanent magnet holding device has been selected for the design of the Self-Actuated Shutdown System in order to enhance the safety of liquid metal cooled fast reactors (LMFBRs). The self-actuated, self-contained device operates such that accident conditions, resulting in increased coolant temperature or neutron flux reduce the magnetic holding force suspending a neutron absorber above the core by raising the temperature of the trigger mechanism above the Curie point. Neutron absorber material is then inserted into the core, under gravity, terminating the accident. Two possible design variations of the selected concept are presented. The proposed designs derive the magnetic holding force from an Alnico V permanent magnet which drives the magnetic holding flux through the parting plane of a carbon steel magnetic circuit. Magnetic and thermal-hydraulic analyses have been performed which show the feasibility of the concept with respect to response time, holding force, and coolability. A series of experiments have been performed which demonstrate proof of principle of the selected Curie point magnetic hold device.