Telescopic control rod for significant reduction in HTR height and cost

Abstract

High temperature gas-cooled reactors (HTRs) are intentionally tall and thin to enable passive decay heat removal through conduction, with a core height of 8 m being typical. Units are typically embedded in the ground, which has numerous safety advantages but requires excavating a deep silo, the cost of which grows super-linearly with depth. HTRs contain rigid control rods that add additional space above the core and make the silo deeper. A concept for a telescopic control rod containing ∼ 5 concentric annuli is proposed. Depending on the design details of the HTR, this has the potential to significantly reduce the silo depth and hence provide substantial reactor capital cost savings. Neutronic analysis demonstrates that a telescopic control rod consisting of 5 concentric annuli can achieve greater than 90% of the control rod worth of a typical articulated HTR control rod without further compensatory measures. The addition of a more control rods or a minor redesign of the rod can recover the deficit in worth if necessary to ensure shutdown margin requirements are met. The telescopic control rod will deplete faster than the reference control rod, which may necessitate more frequent replacement, but this is not anticipated to be a serious drawback as HTR control rods are not inserted extensively at power (especially in pebble bed reactors). The telescopic control rod can be constructed using materials already qualified for HTR environments. Further work is required to demonstrate comprehensively mechanical design, reactor physics and materials feasibility, and the necessary steps are identified in the paper. For mechanical design, a failure modes and effects analysis is required to identify possible failure mechanisms and their consequences, so that they can be designed out or mitigated. The friction/wear resistance of mechancial components must also be evalauted through experimentation on a prototype control rod in a high temperature helium environment. The arrangement of the upper head must also be considered. For reactor physics, the differential control rod worth must be investigated. Finally, a cost benefit analysis should be performed.