Aphafia Seapei Maodi
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 will become radioactive. Most radionuclides produced by neutron activation are undesirable 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 (1) a field of ionising radiation 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 showed that the dependence of neutron activation levels on the neutron fluence-rate can sometimes behave in profoundly non-linear ways. For this reason, it is futile and dangerous 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, or at least must be derived from simulations at similar neutron fluence-rates. Using a representative neutron spectrum calculated for a typical Light Water Reactor, a total of 81 chemical elements were irradiated and subsequently allowed to cool down under specific scenarios that represent important decommissioning and operational scenarios. For selected scenarios of practical importance, the elements were ranked in terms of the dose-rate at 1 m from a reference mass of 1 g of each irradiated chemical element. These tables clearly show which elements are high-activators, which elements are intermediate activators and which elements are low-activators.
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 guide decommissioning engineers, project managers, radiation protection specialists, neutron radiography teams and even reactor design teams.
A comprehensive literature study 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. Approximately 1600+ 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.
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 were presented to show how induced activities and photon dose-rate fields will evolve over e.g. the first 50 years after the end of irradiation, for elements used in important engineering materials such as carbon steel-alloys, 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 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. Where possible, aluminium-alloys and titanium-alloys must be preferred in areas where significant neutron fluence-rates are expected.