의료용 사이클로트론 방사화폐기물 내 방사능 장기운전 포화 특성 및 감쇠 평가
Saturation Behavior and Decay of Radioactivity in Medical Cyclotron Activation Waste during Prolonged Operation
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The use of cyclotrons in medical institutions worldwide has continued to increase, and this trend hasbecome more pronounced with the expanded deployment of proton therapy systems in radiation oncology. Due toinherent operational characteristics, cyclotrons inevitably experience beam losses during acceleration and extraction,which generate secondary radiation fields and induce activation of internal components. As a result, cyclotroncomponents may be classified as radioactive waste during facility decommissioning, making a quantitative assessmentof radionuclides within components essential. The purpose of this study is to systematically evaluate the typesand inventories of radionuclides accumulated in a cyclotron under long-term operating conditions and to providebaseline data applicable to future waste management and disposal planning. To this end, the geometry of a 250 MeVsuperconducting cyclotron was modeled using the PHITS code. The PHITS results were coupled with the DCHAINcode to assess the dependence of activation on beam current and to establish the relationship between beam current,radionuclide production, and total activity. Beam currents of 800 nA and 30.8 nA were considered to examine currentdependence. Subsequently, under long-term operation up to 20 years, the radionuclide composition and inventoriesaccumulated in major internal components were estimated at selected operating times, and the activity decay behavioras a function of cooling time after beam shutdown was analyzed. The results showed that changing the beam currentdid not alter the types of radionuclides produced in the components, whereas the total activity increased linearly withbeam current. Under long-term operation, short-lived radionuclides produced in Cu-based components, such as Cu62, Cu-64, and Cu-66, reached saturation within 10 days of operation. In contrast, relatively long-lived radionuclides,including Mn-54, Fe-55, and Co-60, accumulated gradually as the operating period increased, and Mn-54 and Co60 were identified as the dominant radionuclides for disposal considerations. In addition, assuming a 10-year coolingperiod after 20 years of operation, Co-60 remained above the self-disposal screening criterion in all componentsexcept the Iron Yoke. These findings are expected to serve as reference data for decommissioning and componentdisposal planning for proton therapy cyclotron facilities.
Abstract
The use of cyclotrons in medical institutions worldwide has continued to increase, and this trend hasbecome more pronounced with the expanded deployment of proton therapy systems in radiation oncology. Due toinherent operational characteristics, cyclotrons inevitably experience beam losses during acceleration and extraction,which generate secondary radiation fields and induce activation of internal components. As a result, cyclotroncomponents may be classified as radioactive waste during facility decommissioning, making a quantitative assessmentof radionuclides within components essential. The purpose of this study is to systematically evaluate the typesand inventories of radionuclides accumulated in a cyclotron under long-term operating conditions and to providebaseline data applicable to future waste management and disposal planning. To this end, the geometry of a 250 MeVsuperconducting cyclotron was modeled using the PHITS code. The PHITS results were coupled with the DCHAINcode to assess the dependence of activation on beam current and to establish the relationship between beam current,radionuclide production, and total activity. Beam currents of 800 nA and 30.8 nA were considered to examine currentdependence. Subsequently, under long-term operation up to 20 years, the radionuclide composition and inventoriesaccumulated in major internal components were estimated at selected operating times, and the activity decay behavioras a function of cooling time after beam shutdown was analyzed. The results showed that changing the beam currentdid not alter the types of radionuclides produced in the components, whereas the total activity increased linearly withbeam current. Under long-term operation, short-lived radionuclides produced in Cu-based components, such as Cu62, Cu-64, and Cu-66, reached saturation within 10 days of operation. In contrast, relatively long-lived radionuclides,including Mn-54, Fe-55, and Co-60, accumulated gradually as the operating period increased, and Mn-54 and Co60 were identified as the dominant radionuclides for disposal considerations. In addition, assuming a 10-year coolingperiod after 20 years of operation, Co-60 remained above the self-disposal screening criterion in all componentsexcept the Iron Yoke. These findings are expected to serve as reference data for decommissioning and componentdisposal planning for proton therapy cyclotron facilities.