The uranium fuel in the pebble bed reactor can not melt down. The nuclear chain reaction slows down as the temperature rises.
Naturally occurring uranium contains two isotopes, 0.7% U235 and 99.3% U238. The fissionable U235 in the fuel spheres is enriched to exceed 3% so that a chain reaction can be sustained. As a U235 atom nucleus decays it releases neutrons and creates heat. The neutrons bounce off moderating carbon atoms and slow down enough to be captured by other U235 atoms, continuing the chain reaction. If unchecked this process could overheat a conventional reactor, so control rods and other moderators are introduced to prevent a melt-down such as happened at Three Mile Island.
In the pebble bed reactor high temperature U238 atoms absorb neutrons to prevent run away overheating. At normal operating temperatures the U238 atoms have a low probability of absorbing a neutron, because the relative velocities of the neutron and U238 must be within a narrow range. The process is said to have a low cross section for absorbtion of the neutron by U238. As the reactor heats the U238 atoms vibrate more rapidly, increasing the chance the neutron and U238 nucleus will have the right relative velocity to absorb a neutron. This is called Doppler broadening, much like the Doppler effect that raises the pitch of oncoming train whistles. The captured neutrons can not further the U235 chain reaction, so as the reactor heats up the chain reaction is checked and the temperature approaches a steady state.
The graph above results from a computer simulation of the temperature of a pebble bed reactor in a worst case scenario. The temperature rises to a maximum of 1500 degrees Celsius. The pyrolytic carbon and silicon carbide structures of the fuel pebbles maintain physical integrity to about 2000 degrees Celsius. The hot pebble bed reactor will remain stable and safe until corrective measures are undertaken. This intrinsic, passive safety has been verified by experiments with the 1980s German AVR reactor and the 2006 Chinese pebble bed reactor. Continuing reasearch and experimentation within the US will be key to calming public concerns about nuclear reactor meltdowns.
Operator errors are reduced.
Operator errors were causes of the meltdown at Three Mile Island and the fire at Chernobyl. The PBR design is much safer. Suppose that the operators were suicidal terrorists and they undertook to create havoc and they
- Shut off all cooling.
- Withdrew all control rods.
- Undertook no emergency actions.
PBR design has important safety factors.
- High heat capacity of the graphite core.
- High temperature capacity of the core components.
- Chemical stability and inertness of the fuel, coolant, and moderator.
- High retention of fission products within fuel coatings.
- Single phase characteristics of the helium coolant.
- Low energy density of the fuel core.