Sodium Cooled Fast Reactor: In the early days of nuclear power, it was believed that uranium reserves were limited, so systems that could maximise the energetic potential of the available uranium were afforded a high priority.

The ability of fast neutron reactors to fission all trans-uranic elements, and to convert the non-fissile 238U to fissile 239Pu, provided a far higher degree of sustainability than was
possible with thermal reactors.

The very high-energy densities necessary in a fast reactor core required either a very efficient means of heat transfer or the use of highly refractory core and coolant materials.

The difficulties in developing fuel and structural materials capable of withstanding extremes of both temperature and neutron fluence led to the selection of liquid metals as a highly efficient heat transfer fluid.

Sodium was selected because of its relatively low melting temperature (98 1C), low capture cross-section, low abundance of troublesome fission products, good flow characteristics, and good compatibility with fuel and structural materials (both lead and mercury having been rejected on one or more of these grounds).

The chemical reactivity of sodium was not a problem unless exposed to air or water, and experience showed that such leaks as occurred from time to time could be managed without undue difficulty.

The UK was among the pioneers of fast reactor development, and operated SFRs at Dounreay in Scotland from 1959 to 1994.

During this time, the science underpinning the technical feasibility of the SFR system became well established.

Although the availability of uranium resources remains a concern, the discovery of new uranium deposits has reduced the urgency for fast ‘breeder’ reactors.

However, concerns over the longevity of nuclear waste have increased, and recent attention has focused on the ability of fast reactors to ‘burn’ or transmute the trans-uranic elements that form the longest-lived components of nuclear waste.

The relative abundance of uranium has helped to maintain low fuel prices, placing considerably more emphasis on fast reactor system costs than was expected in the early days.

Therefore, the principal challenges are now to improve on the economic performance of the SFR, and to re-establish the validity of a safety approach that was developed several decades ago.

Meeting both of these challenges will require scientific advances or at least the novel application of existing knowledge.

For example, the opaque coolant hinders in-core inspection and maintenance, both of which are time-consuming and expensive.

However, recent technologies offer the prospect of performing improved under-sodium imaging, allowing the conduct of inspections that would otherwise have necessitated the removal of fuel or even the partial draining of the core. A schematic of the SFR is shown in Fig. 3.