The strategy in most decommissioning activities is to reduce the radiological hazard in a systematic way, until the delicensing condition for the site is reached. As noted earlier, after operations have ceased in a reactor, the removal of fuel reduces the hazard by a significant degree. After that the POCO will result in a further reduction in level.

In general, radioactive decay will result in a reduction of radioactivity and deferral of operations may be of benefit. However, there may be an issue if radioactive daughter chains exist producing isotopes that present greater problems than with the parent isotope. For example (Twidale, 1999), the 241Pu isotope primarily emits Beta radiation but it has 241Am as a gamma emitter. Furthermore, the parent isotope has a half-life of 12 years but the daughter has a half-life of 432 years.

The activation of the construction materials of the reactor and the presence of gamma-emitting isotopes are a significant problem in decommissioning. The areas concerned are the core internals, the biological shielding and the pressure vessel. The most problematic isotopes are 60Co, 108Ag and 94Nb, which have half-lives of 5.27, 418 and 20,000 years, respectively. It is therefore possible to achieve reductions in activity from, e. g. 60Co after a timescale of several decades, but the other problematical radioisotopes will remain.

The quantity of activated components varies considerably with the type of reactor and the size of the vessel. A large PWR has a reactor vessel of diameter 4.5 m and total weight 600 Te compared with a Magnox reactor that has a vessel of about 20 m of weight 5000 Te. The PWR vessel can be moved as a single item but this would not be possible for a Magnox reactor (Twidale, 1999).