Nuclear fuel reprocessing has been carried out at Sellafield in the UK since the 1950s and, over that period, has presented engineers with a wide variety of challenging materials problems.

The process carried out at Sellafield is basically the dissolution of spent nuclear fuel in hot concentrated nitric acid and then the separation of its constituent parts by a solvent extraction process. During this process, various waste products arise - gases, liquids and solids, all of which have to be made safe to minimise discharges to the environment. There is also a need to transport nuclear materials, i.e spent nuclear fuels from the power generators in the UK and abroad and wastes generated in the reprocessing operations which are transported back to the customer.
Materials Selection Criteria

The selection of a material for any chemical process plant is a complicated decision involving many parameters such as corrosion resistance, mechanical properties, availability, cost, etc. What makes nuclear plant more demanding is the presence of radiation, particularly gamma, and of radioactive contamination of surfaces, usually alpha radiation emitting particles, which need to be decontaminated either on a routine basis or at the end of their life. Not only must the materials perform their function for extended periods of time in non-maintainable areas but this performance has to be demonstrated. Even where radiation levels are very low and do not present risk to human life, great care is still taken. The result of this is that the nuclear industry generally is more conservative than most with regard to new materials which, by definition, will have no track record of satisfactory service. The tendency is for an evolutionary approach rather than great leaps into the unknown.

In addition, materials that play a crucial role in maintaining the high safety standards necessary in any modern chemical plant must be obtained under a strict quality assurance regime. There is little point in compiling exacting technical specifications for materials and then discovering that there are doubts about their origin, processing, identity, etc.
Materials Used

The engineering materials utilised at Sellafield are those that are used in many large chemical process plants, but they will have been put under greater scrutiny for the reasons given earlier. By far the largest in tonnage terms is concrete; this is followed by steel, both carbon and stainless, which is widely used in large amounts with much smaller quantities of nickel alloys, titanium, zirconium, hafnium, polymers, glass and various coatings.
Concrete

Concrete is used extensively, as in any other large chemical plant, for building foundations, bridges, drainage channels, etc. But there are some situations at Sellafield which deserve special mention. There is a need for biological shielding in a number of operating areas and concrete offers the most cost effective way of achieving this; if necessary, additives such as lead shot can be used to enhance its shielding properties, or polymer fibres to increase its toughness. Since the concrete has to be penetrated to allow access for power and instrument cables, water, air, etc, chloride levels in the concrete must not be excessive in order to minimise the likelihood of corrosion of metallic conduits carrying these services; concrete to BS 8110 is usually specified and the use of marine sand and aggregate is forbidden. Concrete is also the main structural material for water filled ponds in which spent nuclear fuel is stored prior to reprocessing. A number of important structures containing active materials have to survive seismic events and hence some of the concrete is more heavily reinforced than would normally be the case.

An important use of concrete in the non-structural sense is particular to the industry, namely the immobilisation of certain radioactive wastes by encapsulation, figure 1. The active material is mixed with the concrete, a mix of ordinary portland cement (OPC) and blast furnace slag. It is then cast into stainless steel drums for eventual disposal in an underground repository, in which the drums are embedded within a highly alkaline backfill. The high alkalinity of the concrete matrix and backfill not only suppresses corrosion processes on the interior and exterior of the drum but it also acts as a very effective barrier to radionuclide migration. The concrete walls of the repository will also prevent the migration of radioactive species, as will the surrounding rock strata.

Steels

As in any large plant, carbon steels are used extensively for structural steelwork, cranes, lifting beams, drive shafts, reinforcing bars, bolts, etc. Their advantages are well known they are widely available, cheap, easily fabricated, heat treatable to give a wide range of properties and are readily amenable to simple, conventional and established non-destructive testing methods. Equally well known are their limitations - poor corrosion resistance and ductility at low temperatures.

The grade of steel usually employed for steel framed factory buildings is BS4360 43A (nowadays, BS EN 10025 Fe430A), which is usually clad with organically coated steel or aluminium sheets. Grade 43A steel is not supplied with any guaranteed impact properties and where these are needed, in the relevant design code, then grades 43E or 50D are used. There may also be parts of a fabrication where material is are subjected to high through thickness stresses and here, metallurgically cleaner steels are used (as specified in BS EN 10164 - formerly BS 6780). The advantages of these steels in reducing lamellar tearing are well known.
C-Mn-Ni Steels

C-Mn-Ni steels, as forgings or rolled plates, are used in the specialised area of fuel transport flasks which have to meet national and international regulations before they can be approved for use, figure 2. The impact properties of these materials are extremely important because of the potential, albeit slight, for accidents. Much work has been done, and is continuing, on characterising their dynamic fracture toughness properties and on the development, in various countries, of more cost effective materials such as ductile cast iron and cast steel.

Austenitic Stainless Steels

Austenitic stainless steels are used extensively at Sellafield as the main material of construction for process vessels and pipework. Much effort has been expended in defining the operating limits of these steels. After many thousands of hours of laboratory testing and decades of plant experience, it has been possible to define realistic and predictable corrosion rates before and after welding, cold working and hot working. An example is shown in figure 3, which depicts the corrosion rate of 18Cr‑13Ni‑1Nb; this was the primary grade of stainless steel used at Sellafield until the 1980s when it was superseded by the NAG 18/10L grade. The main advantages of austenitic stainless steels are their inherently high resistance to corrosion in oxidising media such as nitric acid and the relative ease with which they can be decontaminated. They also have excellent impact resistance down to sub-zero temperatures, they are readily available and are easy to fabricate and weld.