HTMR-100 General Design Features
The HTMR-100 nuclear energy system is a 100 MWth high temperature helium cooled pebble bed modular reactor of the Generation IV Design. The reactor is versatile enough to accommodate various types of fuelling schemes such as the uranium, uranium/thorium or plutonium/thorium based fuel cycles. With a Once-Through-Then-Out (OTTO) fuel cycle, pebble fuels slowly pass through the core and leave the core when they achieve high fuel burn-up. These contribute to the significant simplicity of the HTMR-100 design and its high neutron-economy.
The HTMR-100 produces high quality steam and is therefore useful for various applications. Steam can be used for producing power via a steam turbine (35MWe), or it can be used for process heat in petrochemical plants, oil refineries and many other process heat applications.
The HTMR-100 exhibits the following intrinsic safety characteristics:
• Inherently safe due to huge negative reactivity feedback that naturally eliminate the diverse shutdown requirements that are normally required for water-cooled reactors;
• Use of fully ceramic fuel elements, which cannot melt, even in extreme accidents which may result in the total loss of active core cooling;
• Use of helium as coolant, which is both chemically and radiologically inert and does not influence the neutron balance. It allows for very high coolant temperatures during normal operation;
• Use of fully ceramic (graphite) core internal structures, which enables operation at high temperatures;
• A reactor core with a low power density, providing a thermally robust design with a high heat capacity renders the reactor thermally stable during all operational and control procedures;
• The reactor core can tolerate a loss of forced cooling event. Passive decay heat removal is possible and fuel temperatures stay below admissible values. Therefore, the fission products remain inside the fuel particles even in extreme accidents;
HTMR Core Design
The HTMR-100 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.
HTMR-100 is designed as a single pass (OTTO) fuel cycle, therefore eliminating all the complexities of recirculating pebbles. A direct consequence of this 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.