The HTMR100 core is a cylindrical cavity formed by graphite blocks (the reflector) within a Reactor Pressure Vessel (RPV). This cylindrical cavity is filled with pebble fuel, forming what it is called a "pebble bed", which is ultimately responsible for the power generation. Two independent Reactivity Control Systems are inserted in the reflector and are able to control the reactivity of the core, as well as shut it down indefinitely.
All other modular pebble bed reactors built, designed and under construction, include the ability of recirculating used pebbles back into the core. A pebble extracted from the bottom of the core is evaluated to measure its burn-up, if there is enough fissile material left, the pebble is recirculated. The objective of recirculating pebbles is to achieve a more homogeneous axial distribution of fissile material, and a narrower extraction burn-up distribution (maximizing the average burn up of the fuel). Typical pebble bed reactors are designed to recirculate pebbles between 5 and 15 times. This feature has implications in the design of the Fuel Handling System making it more complex, more difficult to maintain, more expensive, and it increases the probability of failure of the pressure boundary.
HTMR100 has been designed as a single pass fuel cycle, therefore eliminating all the complexities of recirculating pebbles. A direct consequence of the OTTO (Once Through Then Out) cycle is a higher concentration of fissile material and power in the upper half of the core. This is a beneficial feature, as high helium temperatures can be obtained using relatively low fuel temperatures, but it limits both the maximum reactor power and the pebble bed height-to-diameter ratio. The current design of HTMR100, using standard fuel pebbles, ensures passive cooling after a postulated reactivity incident (a requirement of Generation IV reactors), but thermal power can be enhanced with the addition of a Burnable Poisons (BP), and this option is currently under study with the aim to improve and maximize safety margins.