Passive control features of a small pebble-bed HTR for a gas turbine cycle

A recent study outlines possible variants of the pebble-bed high-temperature reactor characterized by simplifications in design and operation. Common to them all is the passive response of the reactor to a loss-of-coolant accident (LOCA) in which the decay power is transmitted to the environment by thermal conduction and radiation without the danger of overheating the fuel elements. The simplest way of fueling a pebble-bed reactor is the Peu a Peu modus: Reactor operation starts with the core cavity partially filled with fuel elements, and little by little, new elements are loaded to compensate for burnup. At the end, they are unloaded in one step. This fueling modus avoids the handling of irradiated elements over the whole loading period, and devices for the onload unloading are superflouous. A small 20-MW(thermal) Peu a Peu-fueled reactor operating with a gas turbine cycle is introduced in this paper. Beyond the properties mentioned, it is characterized by additional simplifying features: (1) A single loading period is extended over the whole lifetime, i.e., ∼20 yr of full-power operation. (2) Passive response to transients is extended to the control of a regular load follow