Ionizing radiation affects all life on earth. Radiation is present all around us. This can be natural, for example, cosmic rays from outer space, or artificial, for example radiation from nuclear power plants. To minimise human exposure to radiation, permissible exposure levels have been set to limit radiation doses. It is also necessary however to be able to monitor the effects of irradiation inside nuclear reactors. The exposure of reactor material to radiation was measured by retrospective dosimetry. Dosimetry is measured using fluence, which is a unit of exposure dose defined as the number of particles or amount of energy per unit area. This was investigated in reactor material, mainly from the pressure vessel. The process involved the extraction and counting of niobium, Nb, present in the reactor material. Niobium is irradiated inside the reactor, converted to an excited state, and then emits x-rays, which can be measured. This is a difficult task due to the significant quantity of cobalt present which makes counting difficult. The problem was tackled by taking some scrapings or chips from the reactor material and chemically separating the niobium to allow its activity to be measured. Various chemical separation procedures were explored, and after separation, Inductively Coupled Plasma-Mass Spectroscopy and Neutron Activation Analysis were considered as means to measure the niobium activity. Quantifying very weak emissions, counting aqueous samples of niobium and the use of liquid scintillation counting were also explored. The procedure of extraction and measuring can be used in most radiochemical laboratories and does not depend on the type of steel in use. Further research and development support is required.