Generation III reactors will have more efficient use of fuel and better safety features but no radically new designs. Advanced Boiling Water Reactors, Advanced CANDU heavy-water Reactors, and EPRs will be added to the list of acronyms. Generation IV reactors will be of two main types: breeder reactors, either liquid-metal or gas cooled (discussed above); and very high temperature reactors (VHTRs) [44]. Of these, the most interesting is the pebble-bed modular reactor (PBMR), shown in Fig. 3.60.

The “pebbles” are tennis-ball size spheres containing both the fuel and the moderator. The small grain of fuel can be any fissionable material such as enriched uranium, plutonium, or MOX, the mixed oxides of both. The fuel is surrounded by a layer of porous graphite to absorb gaseous products of the reaction. This is covered by a thin layer of silicon carbide, which is an impenetrable barrier that can take high temperatures. The outer layer of the fuel grain is pyrolytic carbon, which

is dense and can take extremely high temperatures. These tiny fuel grains are dispersed in the graphite moderator, which forms the bulk of the pebble. The reactor core contains some 360,000 pebbles, enough to make a critical mass with the spacing fixed by the spherical pebbles. Helium is circulated through the spaces between the spheres for cooling, and the helium then carries the reaction energy to a heat exchanger.

The design has built-in safety features. The reaction products are contained within the fuel grains and the pebbles. In fact, depleted pebbles can be their own waste containers. The helium is not radioactive even if it leaks out. The reactor can operate at 1,000°C to raise the thermal efficiency to 50%. If the coolant fails, the reactor cannot go critical because the U238 part of the fuel absorbs more neutrons at higher temperature, thus slowing down the reaction if it gets hot. The pebbles might reach a temperature of 1,600°C, but the pebbles are still stable at that temperature, and the reactor core will just stay that way until cooling is restored. The pebbles can be dropped in at the top and removed from the bottom of the reactor core. This allows the pebbles to be periodically examined and removed to storage if they have been used up.

Critics of PBMRs cite the possibility that the graphite would catch fire if it contacts air or water at these extreme temperatures. PBMRs are being developed in Germany, the USA, the Netherlands, and China. The automatic safety mechanisms have been tested on a small scale.