Medical cyclotrons have been used for dedicated medical of commercial applications such as positron emission tomography (PET) for the past tens of years. These cyclotron facilities have produced positron-emitting radionuclides (i.e. 11C, 13N, 15O, 18F, etc.). Among them, 18F, produced by 18O(p,n)18F reaction is the most widely used which has longer half-life (around 110 m) and lower energy of emitted positrons (around 0.63 MeV). Secondary neutrons produced during 18O(p,n)18F reaction could cause neutron activation of structures, systems, and components of cyclotron facilities. Therefore, International Atomic Energy Agency (IAEA) had addressed that during the operation of cyclotrons, concrete walls become radioactive over time and this radioactivity needs to be characterized for planning of the facility decommissioning. Moreover, several prior studies had estimated the neutron activation and levels of radioactivity of concrete wall of cyclotron facilities. Although those studies assessed the neutron activation of actual cyclotron facilities, however, the purpose of assessment was only for decommissioning each individual facility. Also, the assumptions, conditions or insights of conclusion may be limited to each individual case. For these reasons, this study focused on analysis of effects of major factors (e.g. concrete type, impurity contents of structural materials, etc.) about neutron activation of cyclotron facilities. In this study, the well-known methodology of neutron activation estimation was established and neutron activation products of concrete wall of cyclotron vault was calculated. Also, sensitivity analyses were conducted to figure out the effects of major factors of neutron activation and production of radioactive wastes during decommissioning of the facility. The methodology and results were validated by two steps: comparing with prior studies and comparing with another computer code. Concrete type did not affect that the decision of level of radioactivity waste criteria. Because of relatively longer half-lives, impurity contents of structural materials especially Co and Eu were turned out one of the most important factors for planning the facility decommissioning. It is hard to simply figure out the radioactivity levels of cyclotron facilities, however, rough predictions of minimum period for decay-in-storage as radioactive waste management can be possible with using information of thermal neutron spectra and major impurity nuclides (e.g. 59Co, 151Eu and 153Eu) for minimization of radioactive waste production and relief of charge of radioactive waste management.

Important medical radionuclides for Positron Emission Tomography (PET) are producing using cyclotrons. There are about 1,200 PET cyclotrons operated in 95 countries based upon IAEA database (2020). Besides, including PET cyclotrons, demands for particle accelerators are continuously increasing. In Korea, about 40 PET cyclotrons are in operating phases (2020). Considering design lifetime (about 30-40 years) and actual operating duration (about 20-30 years) of cyclotrons, there will be demands for decommissioning cyclotron facilities in the near future. PET cyclotron produces radionuclides by irradiating accelerated charged particles to the targets. During this phase, nuclear reactions (18O(p,n)18F etc.) produce secondary neutrons which induce neutron activation of accelerator itself as well as surrounding infrastructures (the ancillary subsystems, peripheral equipment, concrete walls etc.). Generally, experienced cyclotron personnel prefer an unshielded cyclotron because of the repair and maintenance time. In unshielded cyclotron, water cooling systems, air compressor, and other equipment and structures could be existed for operating purposes. Almost all the equipment and structures are consisted of steel, and these affect neutron distribution in vault especially thermal neutron on the concrete wall. In addition, most of them can be classified as very low level radioactive wastes by Nuclear Safety and Security notice (NSSC Notice No. 2020-6). However, few studies were estimating radioactivity concentrations (Bq/g) of surrounding structures using mathematical calculation/simulation codes, and they were not evaluating the effect of surrounding structures on neutron distribution. In this study, by using computational neutron transport code (MCNP 6.2), and source term calculation code (FISPACT- II), we evaluated effect of the interaction between surrounding structures (including surrounding equipment) and secondary neutrons. Discrepancies of activation distribution on/in concrete wall will be occur depending on thickness of structure, distance between structures and walls, and consideration of interaction between structures and neutrons. Throughout this study, we could find that the influence of those structures can affect neutron distribution in concrete walls even if, thickness of the structure was small. For estimating activation distribution in unshielded cyclotron vault more precisely, not only considering cyclotron components and geometry of target, but also, considering surrounding structures will be much more helpful.

For producing radionuclides which were mostly used in medical purposes, for instance, Positron Emission Tomography (PET), there were about 1,200 PET cyclotrons operated in 95 countries based upon IAEA database (2020). Besides, including PET cyclotrons, demands for particle accelerators are continuously increasing. In Korea, about 40 PET cyclotrons are in operating phases (2020). Considering design lifetime (about 30–40 years) of cyclotrons, there will be demands for decommissioning cyclotron facilities in the near future. PET cyclotron produces radionuclides by irradiating charged particles to the targets. During this phase, nuclear reactions (18O(p,n)18F, 14N(d,n)15O etc.) produce secondary neutrons which induce neutron activation of accelerator itself as well as surrounding infrastructures (the ancillary subsystems, peripheral equipment, concrete walls etc.). Most of the ancillary systems including peripheral equipment can be neutron activated, since, most of them were made of steels. Steels like stainless steel or carbon steel may contain some impurities, typically cobalt. Although, there were several researches evaluating activation of concrete walls and accelerator components, estimating the activation and influence on neutron interaction of the other surrounding infrastructures were insufficient. In this study, by using computational neutron transport code (MCNP 6.2), and source term calculation code (FISPACT- II), we estimated neutron distribution in cyclotron vault and activation of ancillary subsystems including some peripheral equipment. Also, using Au foil and Cd cover, we measured thermal neutron distribution at 16 points on the concrete wall, and compared it to calculated results (MCNP). Even though, the compared results matches well, there was a discrepancy of neutron distributions between presence and absence of those equipment. Additionally, in estimating activation distributions by calculating, most of the steel-based subsystems including peripheral equipment should be managed by radioactive wastes after 20 years of operation. Throughout this study, we could find that influence on neutron interaction of those equipment can affect neutron distribution in concrete walls. This results vary the activation depth as well as location of the hot contaminated spot in concrete wall. For estimating or evaluating activation distributions in cyclotron facilities, there was need to consider some equipment located in cyclotron vault.

In worldwide, tens of thousands of units of particle accelerators have been used and more than 97% of those accelerators are used for dedicated medical of commercial applications. Radionuclide production cyclotron produce several positron-emitting radionuclides such as 18F by 18O(p,n)18F reaction which generates secondary neutrons. It is of note that these neutrons cause neutron activation in structures and components of cyclotron facilities. Therefore, International Atomic Energy Agency had addressed that a well-developed estimate of the neutron activation induced radioactive inventory of accelerator facilities is needed for the proper planning and safe implementation of decommissioning using proven methods or codes that can be used to perform activation calculations. Moreover, IAEA suggested that during the operation of cyclotrons, concrete walls become radioactive over time and this radioactivity needs to be fully characterized as part of early decommissioning planning. In this study, Neutron activation in the medical cyclotron facilities was evaluated with the MCNP and FISPACT-II code to analyze the generation of decommissioning radioactive wastes during facilities dismantling. For the reference case, residual radioactivity concentration of each activation product (e.g. 60Co, 152Eu, etc.) was calculated and the sum of fractions of the activity concentration of each radionuclide divided by its clearance level was exceeded 1.0 at each calculation point which means radioactive waste generations during decommissioning of the facility. Several points show the calculated sum of fractions (SoF) at inside wall were bigger than the surface wall. The reason of these phenomena is that the slowdown of the incident neutron energy at the inside wall due to neutron attenuation and larger thermal neutron flux than surface wall. It is of note that each activation reaction cross-section was dominant at thermal neutron energy band. Sensitivity analysis was conducted to analyze the effects of design characteristics (e.g. beam energy and current, operation period, and workload). The SoF was exceeded 1.0 at the least activation condition (i.e. 9 MeV, 10 μA) if the operation period was 10 years. For the realistic condition such as 13 MeV, only 10 μA of beam current case shows the SoF was under union. On the other hand, 19 MeV, 60 μA, and 10 years operation case shows the SoF as 20.4 which means the clearance rule can be applied only after 21 years of decay-in-storage. The result of this study can be used for proper planning of decommissioning and/or new installation of cyclotron facilities include considerations of radioactive waste management.