Assessment of Variable Reflector Reactivity Envelope in Multi-Module HTGR Special Purpose Reactor

Abstract

In this report we examine the reactivity that is available in the reflectors (i.e. the reactivity envelope) through several mechanisms that may be understood notionally as a “variable reflector”. Specifically we assess in detail the reactivity of: • Subcritical Power Module (SPM) • Control Drum Rotation Patterns • Moderating power of the outer radial reflector • Moderating power of the central cross reflector for the Holos-Quad microreactor design. All mechanisms demonstrate quantitatively a sufficient range of reactivity that control is possible for load follow operation. The reactivity due to the SPM position is quite large at 220 pcm/cm in position. As a result this mechanism is not ideal for control for Flexible Power Operation (FPO). Moreover, the Holos-Quad design as undergone further development, and positioning of the SPM is no longer a feature of the design. The reactivity of the control drums is quite flexible by varying the pattern in which the drums are rotated. Depending on the number of drums rotated the reactivity worth can be anywhere from 1 up to 400 pcm per degree rotation. This is determined to be a suitable control mechanism for FPO. Furthermore, analytic models for the integral drum reactivity worth and differential drum reactivity are derived based on first order perturbation theory and shown to suitable for representing the reactivity worth curves determined from detailed Monte Carlo calculations. The development of the analytical models will be an important tool for future work on the design and assessment of control algorithms for FPO. The moderating power of the reflectors, either central or radial, is examined for 10% to 200% nominal density–while there is not a physical mechanism or ability to create these materials it neverthless provides important information as to the reactivity as parameterized by the reflector effective total cross section and scattering ratio. From this assessment we observe that the outer reflector worth is about 44pcm per V %V and 22pcm per V %V . These values also suggest this as a mechanism suitable for reactivity control enabling FPO. However, we have not yet determined a passive mechanism through which these changes might be achieved. One option would be to develop a stratified radial reflector with plates of reflector material where the spacing in between can be adjusted using changes in temperature of gas or thermal expansion of another material. Alternatively, mechanisms to rotate the drums that operate passively may also be feasible.