Graphite fuel elements and reactor internals which make up the reactor
core have a high heat capacity and maintain their strength at temperatures
beyond 2760 C (5000 F). As a result, temperature changes in the core occur
very slowly and without damage to the core structure in the event of design
basis transients and accidents.
The annular core geometry, core power density, and the total module
power of the MHTGR have been chosen such that the decay heat generated
within the core can be passively removed by means of conduction, radiation,
and natural convection without the fuel reaching a temperature at which
fission products contained within the fuel kernels are released during an
accident. In addition, a negative power coefficient is maintained for the
reactor module over all times in the fuel cycle for initial or equilibrium cores
over a temperature range which includes accident temperatures, thus
ensuring inherent core feedback characteristics to control heat generation.
The main loop cooling system and the maintenance-related shutdown
cooling system are available to remove core decay heat under all operating
and transient conditions. The resulting core temperatures do not exceed
normal levels. Even if both active cooling systems are unavailable, decay
heat is dissipated by conduction and radiation to the RCCS in the reactor
enclosure under primary pressurized and depressurized conditions. The
maximum fuel temperature remains well below the temperature, 2000 C
(3630 F), at which silicon carbide degradation begins. No operator action is
required to meet the top-level regulatory requirements during any licensing
basis event.
No public evacuation or sheltering is required for licensing basis events or
for severe, low probability accidents, because the consequences are
accommodated by the inherent and passive features of the MHTGR.