The fuel for current water reactors is in the form of pellets of uranium dioxide or a mixture of uranium and plutonium dioxide (MOX fuel). The uranium enrichment (content of uranium-235) is typically 3-5% in light water reactors. The pellets are very stable ceramic cylinders about 1 cm in diameter and 1 cm high. The pellets are placed in sealed thin metal tubes (e. g. of stainless steel or zirconium alloy), which are kept together as bundles to form a fuel element. The fuel element, which typically contains between 60 and 300 fuel pins, can be handled as an entity (Fig. 14.2). Fresh nuclear fuel elements need to be handled with care to avoid contamination and mechanical failures, but do not require radiation shielding. After the fuel has been used in the reactor it can still be removed and handled as an intact

fuel element. It is, however, highly radioactive due to the formation of fission products and transuranic elements in the fuel and activation prod­ucts in the fuel element structure. The typical composition of spent fuel (excluding the fuel element structure) is:

• 95% uranium (remaining enrichment about 0.8%)

• 1% plutonium

• 4% fission products and transuranic elements other than plutonium.

A more detailed composition of a typical LWR fuel element is given in Table 14.1. Some of the fission products are very short-lived with half-lives of a year or less, while others have half-lives ranging from 30 years to mil­lions of years.

The spent fuel element has a high concentration of different radionu­clides that decay by emitting a-, P — or y-radiation or undergo spontaneous fission that emits neutrons. The a — and P-radiation is mainly absorbed in the fuel itself and is the energy dissipated as heat (decay heat), while the y — and neutron radiation is more penetrating so that the spent fuel will require shielding. Some neutrons also generate additional fission in the fuel, which will require control of the spent fuel configuration to avoid criticality. The spent fuel thus needs shielding and cooling during the subsequent handling. After removal from the reactor the fuel is stored under water for several years to allow cooling. During the first year the decay heat goes down rapidly as the short-lived fission products decay. After about five years the decay heat is dominated by cesium-137 and strontium-90, which both have a half-life of about 30 years.

Spent fuel remains radioactive for very long times, hundreds of thousands of years, and will eventually need final geological disposal to ensure long­term isolation from humans and the environment. In Fig. 14.3 the radioac­tive decay for spent fuel is shown. The curve shows the toxicity index, which takes into account not only the activity but also the harm the radioactive substance would give if incorporated into the body (essentially eaten). After the first few years the toxicity is dominated by cesium and strontium. After a few hundred years the toxicity will be dominated by the transuranic ele­ments, such as plutonium and americium. By removing plutonium and pos­sibly also some other transuranic elements the long-term toxicity and heat release can be reduced, but it is generally considered that long-term geo­logical isolation will still be needed.