An Autonomous Reactivity Control system for improved fast reactor safety

• ARC, a new system for improved fast reactor inherent safety is presented.
• An overview and comparison to existing systems and design approaches is given.
• We analyze the impact of installing the system in a large fast reactor core.
• With the ARC system, fast reactor safety can be greatly improved.

The Autonomous Reactivity Control (ARC) system is a new safety device that can passively provide negative reactivity feedback in fast reactors that is sufficient to compensate for the positive coolant density reactivity feedback even in large low-leakage cores. The ARC system is actuated by the inherent physical property of thermal expansion, and has a very small effect on core neutronics at standard operating conditions. Additionally, the ARC system does not have an identified failure mode that can introduce positive reactivity in to the core. An ARC system can be installed in conventional fuel assemblies by replacing a limited number of fuel rods with rods that fill a safety function, providing negative reactivity to the core in the event of coolant temperature rise above nominal. These rods are of the same outer dimensions as the fuel rods, but contain smaller-diameter inner rods that are connected to liquid-filled reservoirs at the top and bottom of the assemblies. The reservoirs are filled with two separate liquids that stay liquid and immiscible throughout the applicable temperature range of fast reactor operation. The lower reservoir contains a “neutron poison” liquid with a high neutron absorption cross-section. The upper reservoir is filled with a separate liquid with a small neutron absorption cross-section. As the temperature in the assembly increases, the liquids in the reservoirs thermally expand, effectively pushing the absorbing liquid up toward the active core region while compressing the inert gas that fills the volume above the liquid between the inner and outer tubes of the ARC rods. The ARC system can be installed, or retrofitted in to existing systems, in every fuel assembly in the core. Since ARC installations in individual fuel assemblies operate independently, the system has a high level of redundancy. ARC-systems respond to local transients as well as core-wide accident scenarios. After actuation, the system automatically returns to its initial state as temperatures decrease, without the need for intervention by reactor operators. The ARC system concept and design considerations are described and illustrated.