Meltdown Proof Core
Why is the pebble bed reactor meltdown proof?
One of the main reasons that a pebble bed reactor can be designed to be meltdown proof is the fact that the core power density is approximately 30 times lower than in most water cooled reactors. The power density is the amount of heat (from nuclear fission) typically generated in one cubic meter in the reactor core. The figure below illustrates the size and core volume of a pebble bed reactor producing 100MWt compared to a typical water cooled reactor which produces 3000MWt. It can be seen that the reactor pressure vessels are of similar size (height and diameter) and that the cores (i.e. the volume where the nuclear fuel is placed to produce heat from nuclear fission) are of similar physical size. In both cases a coolant is used to cool the core during normal operation; however the pebble bed reactor has a number of inherent safety features which ensure that the core cannot melt down when the coolant flow stops due to an accident or some unforeseen event:
The strong negative temperature coefficient together with the low power density of a pebble bed reactor means that if the active coolant flow ceases, the reactor will automatically become sub-critical (i.e. shut itself down). LWRs also have a negative temperature coefficient, however they have a high power density and require active cooling to keep the core cooled.
Comparison between Pebble and LWR reactors