Advanced Reactors

"Typical design objectives imply that safety actions by the operators shall not be needed within at least 30 minutes, leaving them time to assess the situation carefully before acting. Increased thermal inertia in the reactor system and reduced core power densities are two such design related factors." (IAEA website)

Often called Generation III or Generation IV reactors, advanced reactors are modern nuclear power reactor designs that are very new (built in the last 5 or so years), currently being built, or are in the design/planning stage. They fall into three general categories (with a few novel exceptions):

1. Water Cooled Advanced Reactors
2. Advanced Gas-Cooled Reactors
3. Advanced Fast Reactors

Water Cooled Advanced Reactors

These fall into two general categories; light-water and heavy-water reactors. These all also fall into one of two subcategories - pressurised water reactors and boiling water reactors. They are in principle the same as the older gen I and gen II designs, except that they employ more advanced technology and engineering expertise. Currently most power reactors worldwide are light-water reactors.

Fuel: Light-water: 3.5% enriched U-235.

Heavy Water: Ordinary Uranium ore (requires no enrichment as the moderator absorbs so few neutrons).

Moderator: Light-water: Ordinary water.

Heavy Water: Deuterium (D2O). This is water where the hydrogen atoms also have a neutron in their nucleus.

Coolant: Light-water: Ordinary water.

Heavy Water: Deuterium (D2O).

Advancements: Both heavy and light-water advanced reactors contain evolutionary changes, such as the ability to use mixed oxide fuels (MOX) to recycle plutonium (a waste product of most reactors), employ passive safety features (which depend upon the unchanging physical properties of materials instead of man-made fail-safes which may malfunction), have increased plant lifetime (to ~100 years), and cost effectiveness.

Example:
Supercritical Water Cooled Reactor (SCWR)

• SCWR follows BWR design, but heats water to it's critical point rather than boiling it.
• Simplified design - water doesn't change phase & is ~33% more efficient than LWR's.
• Still in design phase - may be altered to run with fast neutrons.

Advanced Gas-Cooled Reactors

High temperature gas-cooled reactors (HTGCR's) have been in operation for some time. They characteristically have a high thermal efficiency but often require a high enrichment of fuel (especially for the initial load). The fuel assemblies have traditionally consisted of fuel pellets (2-6mm diameter) coated with a ceramic protective layer dispersed in graphite. The two foremost advanced gas-cooled designs are the Pebble Bed Modular Reactor (PBMR) and the Gas-Turbine Modular Helium Reactor (GTMHR).

Fuel: A mixture of 20% enriched UCO or ThO fuel in sphere form coated with multiple ceramic layers or with graphite.

Moderator: Graphite - either as fuel pellets dispersed in a solid block (GTMHR) or a shell encapsulating the fuel pellet (PBMR).

Coolant: Helium Gas. As Helium is inert, it is a suitable choice.

Advancements: Again, primarily evolutionary changes, such as improved engineering leading to much grater efficiency, passive safety features and standardised parts (to allow more cohesion in the supporting industry). The advantage of a graphite core is that it cannot melt down due to graphite's extremely high melting point.

Example:
Gas-Turbine Modular Helium Reactor (GTMHR)

• Fuel is 20% enriched and dispersed in a graphite array.
• 16% more efficient than standard LWR's, 1128Mw electricity output (the average Australian home uses 6000MwHrs per year). Reduces Uranium consumption by 35%! Still in design phase.
• Gas turbine coupled directly to the core provides the extra efficiency.

Advanced Fast Reactors

"Breeder Reactors are capable of satisfying the electrical energy needs of the world for thousands of years." (Berkeley Dept. Of Nuclear Engineering website).

Fast reactors are often called breeder reactors for their ability to produce more fuel than they consume. The revolutionary aspect to the design of fast reactors is that they have no moderator. Instead of slowing neutrons down, fast neutrons are employed. All nuclei above Ac on the periodic table are fissile. Thus fast reactors are of profound significance to any nuclear fuel cycle being developed, as the quote from Berkeley University above points out. U-235, like oil and coal is a finite resource; and many estimates put a timeframe of 50 years before all the U-235 on the planet runs out, were the whole world to turn to nuclear power as their primary power source. It is the unique ability of fast reactors to run on not only unenriched uranium, but also on the spent fuel of other reactors that makes them so handy. Fast reactors extract 60 times as much energy from fuel than other reactors.

Fuel: Varies. Can run on depleted fuel from other reactors.

Moderator: None! Neutrons are not required to slow down.

Coolant: Traditionally liquid sodium. Sodium does not appreciably show down neutrons, has excellent thermal properties, and has a high boiling point (allows reactor to be operated at lower pressures). Unfortunately, in the process of running the reactor the sodium becomes radioactive. Also sodium reacts violently with water and catches fire when in contact with air, which leads to engineering difficulties.

Advancements: Newer uranium (or plutonium) carbide fuels allow potentially for greater fuel efficiency. Pool designs emerging allow use of passive safety features, lower radiation doses for workers, and longer intervals before refuelling is necessary (reactors often have to shutdown to refuel). Liquid lead coolant overcomes engineering problems with liquid sodium (though creates new health risk for workers - heavy metal).

Example:
Lead Cooled Fast Reactor (LCFR)

• Still in design stage, envisioned to run on depleted fuel.
• Pool type - liquid lead cools by convection - passive safety feature.
• Designed for very long interval before refuelling- 15 to 20 years!

The Future?

More radical designs exist that envision things such as a molten salt (liquid) fuel that circulates through the reactor (Molten Salt Reactor), or even a reactor that requires a beam of high energy protons to be fired into it to maintain criticality - otherwise the reaction would stop (Subcritical Proton Accelerator Driven Reactor). Practically, these designs - although fascinating - have little to no chance of ever being implemented. Future use of nuclear energy is contingent on overcoming proliferation and spent fuel issues in the public's eyes. Fast reactors present us with a real opportunity of at least addressing the proliferation issue.