MARVEL Microreactor’s Control Drum and Control Rod System Design, Analysis, and Testing

MARVEL is an 85-kW thermal microreactor with a 36-pin three-layer hexagonal patterned core being developed and deployed at the Idaho National Laboratory. Its reactivity control system (RCS) consists of two separate systems of different design, functionality, and technology. The first is the primary means of control and safety-related shutdown, called the control drum (CD) system, which employs four rotating CDs that are evenly distributed around the peripheral of the reactor core. The second is the defense-in-depth central insurance absorber (CIA), which consists of a translating central control rod operating in a port located in the core’s center. As is conventional, the systems adjust the position of their reflecting and absorbing materials relative to the core to control reactivity. MARVEL’s RCS system is designed around specific operating requirements, which were established by thorough hazard evaluation and accident analysis. This drives the requirements for relative positioning and the consequential operating limits, such as maximum speed and sufficient position resolution, as well as scram demands such as maximum duration compliance and minimized impact. MARVEL’s drum and rod subsystem components and characteristics have been designed to satisfy the enveloping form of these demands as verified through finite element analysis and the mathematical computation of each system’s driving nonlinear second-order differential equation form. Iterative prototyping and testing were also performed to give intermediate assurance that the designs and their supporting analyses matched physical behaviors. A regimented qualification plan under continual maturation during the prototyping process pitted the evolving prototypes’ performance against the requirements to drive the design to an acceptable state. The resulting final designs’ critical characteristics are propagated into rigorous procurement and assembly requirements, satisfied via the execution of vendor specifications, in-house commercial-grade dedication plans, and detailed supplemental work instructions. These systems will be employed as a tool during assembly to intermittently verify that the RCS has retained its functionality after each subsystem’s inclusion, thus providing additional assurance that the new subsystem has been sufficiently incorporated. These efforts have culminated in system designs that are robust and have a unique rapid tunability to meet the system demands. Tunability is a critical capability, as the final system’s physical demands are nonconfirmed during initial startup and can shift during operation and throughout the system’s lifetime. Though developed for the MARVEL reactor, the presented CD and CIA system designs and methods can be leveraged in whole or in part for the benefit of the reactor development community.