In a nuclear power plant there are two sources for the production of radio­active substances, the fission by neutrons and absorption of neutrons (trans­mutation by neutron absorption) taking place in the fuel itself, and the irradiation of material in the reactor that is exposed to the neutrons from the fission process (activation). The radioactive substances produced from the first source are fission products and transuranic elements (elements heavier than uranium). The fission products are the lighter elements (e. g. cesium, strontium and iodine) that are created when the heavier atoms (e. g. uranium or plutonium) are split (fissioned) and energy is released. The transuranic elements (e. g. plutonium, americium and curium) are generated by the absorption of neutrons in uranium and the successively created transuranic elements. The amount of fission products and transuranic ele­ments is directly coupled to the energy that has been generated. The spent fuel is highly radioactive and will need shielding and cooling for the subse­quent handling.

A typical composition of spent nuclear fuel is shown in Table 14.1. The fission products and transuranic elements are kept in the fuel and contained by the fuel cladding. They will only be released to other parts of a nuclear power plant if the fuel cladding is damaged. Minor amounts could also

emanate from fuel contamination on the outside of the fuel cladding that remains after the fuel fabrication.

The second source of radioactive substances in a reactor, activation prod­ucts, is the result of irradiation of material in the reactor by neutrons from the fission process. Only material inside the reactor pressure vessel and in the concrete that immediately surrounds it will be exposed to sufficient neutron fields for activation. The highest activity will be generated in the core components holding the fuel and in other internal parts in the pressure vessel. Also material contained in the coolant or coolant-moderator water, which passes through the reactor core, could become activated.[80] This could be metal ions or particles from corrosion in the primary circuit of the reactor or other trace elements contained in the coolant or coolant-moder­ator. Radioactive substances thus created could then be transported through the primary system of the reactor and contaminate surfaces and filters, thus creating a radiation field around these components and in the end a radio­active waste.

A list of typical activation products is given in Table 14.2. To minimize the creation of activation products, one strives to keep the primary circuit water very clean through ion exchange and mechanical filtering as well as

to reduce the corrosion by adjusting the chemical environment, e. g. by adding lithium hydroxide or hydrazine to the coolant. Also gaseous radioac­tive fission and activation products are formed and transported by the coolant and coolant-moderator to a degasification system.