Neutron activation characterization of the chemical elements in preparation for nuclear reactor decommissioning
At nuclear reactor facilities, intense neutron radiation fields are encountered inside and around reactor vessels. Engineering materials exposed to the neutron field absorb neutrons in nuclear reactions, and radioisotopes are produced in this way. This process is termed neutron activation. Neutron activation produces radionuclides in irradiated materials, i.e. the irradiated materials become radioactive. Some neutron activation reactions are of commercial interest, e.g. the production of the radionuclide Ir-192 from irradiated Ir-191. Most radionuclides produced by neutron activation, are undesired, long-lived radioisotopes and will place a radioactive waste burden on the licensed facility, adding to total operational costs and inflating future liabilities. After irradiation by the neutron field ends, long-lived radionuclides will remain present in irradiated materials and will present radiological and radioactive waste-disposal problems such as e.g. (1) a radiation field will be present around the activated material and will expose workers to doses of ionising radiation, and (2) some activated material may not pass clearance level criteria set by e.g. the IAEA and will therefore have to be disposed of as radioactive waste, at a significant cost. An inquiry into the systematics of neutron activation, using radiation transport and materials activation codes, was designed and successfully concluded. The systematic study showed that neutron activation will, specifically under high neutron fluence-rate conditions, depend in a profoundly non-linear way on the fluence-rate. For this reason, it is incorrect to attempt to perform a neutron activation calculation at a chosen reference integral fluence-rate 𝜙𝑟𝑒𝑓 and then attempt to scale activities and dose-rates linearly for other fluence-rates. There are no “shortcuts” i.e. every neutron activation problem is unique and must, therefore, be modelled individually. Using a representative neutron spectrum calculated for a typical light water reactor (LWR), a total of 81 chemical elements were irradiated and cooled down under specific scenarios that represent important decommissioning and operational scenarios. For selected important scenarios, the elements were ranked in terms of the unshielded dose-rate at 1 m from a point-source with a reference mass of 1 g of each irradiated chemical element. These tables clearly show which elements are high-activators, intermediate activators and low-activators, for different irradiation scenarios. This information may be used to select low-activation materials for new reactor facilities, and for components that must be replaced in existing nuclear facilities. The tabulated results can serve to guide decommissioning engineers, project managers, radiation protection specialists and neutron radiography teams. Neutron radiography teams can use the information to e.g. pre-empt which radiographed components will be highly radioactive, and which will remain radioactive for a long time, after exposure to neutrons, based on known material compositions. Decommissioning engineers can use the information to e.g. pre-empt that steels with the same neutron irradiation history as aluminium-alloys, will have a significantly higher burden of long-lived radionuclides. A comprehensive literature study on the nature and importance of the formation of long-lived radionuclides by neutron activation, for nuclear reactor decommissioning, was undertaken and presented. From the literature-study emerged a list of high-activator elements as well as problematic, long-lived radioisotopes formed by neutron activation. A total of approximately 1700 calculations with the activation code FISPACT-II 3.00 were performed, in order to describe the systematics of neutron activation in realistic irradiation-and-cooldown scenarios, focusing on reactor decommissioning. The full set of systematic FISPACT-II calculations served to verify and validate the list of high-activation materials and problematic long-lived radionuclides gathered from the literature survey. A comprehensive set of graphs are presented to show how induced activities and photon dose-rate fields will evolve over the first 50 years after the end of irradiation, for chemical elements used in important engineering materials such as low-alloys steels, stainless steel-alloys, nickel-alloys, ordinary concrete, magnetite concrete and hematite concrete. The durations of these irradiations range from 1 hour to 60 years. A notable result was that, for a decommissioning scenario, titanium-alloys are significantly more benign neutron activators compared to steel-alloys. Problematic elements that are high-activators in practically all irradiation scenarios are europium (Eu), cobalt (Co), caesium (Cs), silver (Ag) and niobium (Nb). The testing of raw materials used in concrete close to a nuclear reactor must be designed to minimise the amount of the above high-activators in the concrete. Al-alloys and steel-alloys used in intense neutron fields must also be tested to minimise the high-activators. Benign elements that are low-activators in practically all irradiation scenarios are aluminium (Al), Silicon (Si), magnesium (Mg) and titanium (Ti). Where practical and possible, aluminium-alloys and titanium-alloys must, therefore, be preferred in areas where significant neutron fluence-rates are expected.