CONSIDERATIONS, MEASUREMENTS, and LOGISTICS ASSOCIATED with LOW-ENERGY CYCLOTRON DECOMMISSIONING Sunderland J.J., Erdahl C.E., Bender B.R.,Sensoy L., Watkins G.L
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CONSIDERATIONS, MEASUREMENTS, AND LOGISTICS ASSOCIATED WITH LOW-ENERGY CYCLOTRON DECOMMISSIONING Sunderland J.J., Erdahl C.E., Bender B.R.,Sensoy L., Watkins G.L. Positron Emission Tomography Imaging Center, University of Iowa, Iowa City, IA, USA Corresponding author email address: [email protected] pCi/gm ± Half-Life Likely Production Channel Introduction: There are currently approximately 150 active PET cyclotrons in the US. Ce-141 0.043 0.019 32 days (n,γ) on natural cerium in concrete There are likely at least 3X that number, total, in the world. Despite the fact that Co-60 1.835 0.042 5.27 years (n,γ) on natural cobalt in concrete the first wave of PET cyclotrons is in the decommissioning stage, there is still Cs-134 0.128 0.023 2.06 years (n, γ) on natural cesium in concrete little information in the literature on the science, health physics, and regulatory Eu-152 2.23 0.086 13.3 years (n, γ) on natural europium in concrete framework surrounding the process. Here we present quantitative radioactive Eu-154 0.264 0.047 8.8 years (n, ) on natural europium in concrete waste measurements associated with the University of Iowa’s cyclotron γ decommissioning. K-40 8.16 0.386 1.3e9 years Naturally Occurring La-140 0.528 0.269 40.3 hours (n, γ) on natural lanthanum in concrete Mn-54 0.436 0.036 312 days (n,p) on natural iron in concrete Material and methods: Na-24 211.4 2.08 15 hours (n, γ) on natural sodium in concrete In the summer of 2011, the University of Iowa’s 20-year-old 17 MeV And (n, α) on natural aluminum Scandatronix cyclotron underwent decommissioning. In an effort to be both environmentally and economically responsible, we investigated the possibility of Table 1. Measured concrete concentrations. All were below exempt concentrations, and so transferring ownership of our otherwise intact and functioning cyclotron. no special disposal of concrete was required. Although two potential recipients were identified, logistics precluded this most- optimal solution. Since disposal of the cyclotron was, in our case, required, we were forced to run the regulatory gauntlet associated with the transport and disposal of more Nuclide MBq Half-Life Likely Production Channel than 40 tons of low-level cyclotron related radioactive waste. Waste was Be-7 1654 53 days 10B(p,α)7Be from Boron in Havar foils classified into two categories: That associated with the cyclotron and targets, Na-22 2.0 2.6 years 25Mg(p,α)22Na from trace Mg in Havar? and that associated with the concrete vault wall that needed to be removed to remove the old cyclotron and bring in the new one. Multiple samples were Na-24 5.3 15 hours 27Al(n,α)24Na from Aluminum Tank and targets taken from the concrete wall (via hammer drill with concrete dust collection) and Sc-48 2.9 44 hours 48Ti(n,p)48Sc on Ti targets and foils analyzed with two independent High Purity Germanium detectors to assess Cr-51 247.5 27 days 54Fe(n, α) 51Cr, 50Cr(n,γ) 51Cr, 52Cr(n,2n)51Cr concentration of neutron activation products. The first measurement was 52 52 performed in-house, and the second was performed by an independent Mn-52 90.9 5.6 days Cr(p,n) Mn from Chromium in Havar foils laboratory at the request of state safety officers. Results are in Table 1. Mn-54 150 312 days 54Cr(p,n)54Mn from Chromium in Havar foils The cyclotron itself was stripped of useful spare parts, which were sent to Co-56 273 77 days 56Fe(p,n) 56Co from Iron in Havar Foils sites with still-operating 17 MeV Scandatronix cyclotrons. Accumulated Co-57 120 272 days 57Fe(p,n) 57Co from Iron in Havar Foils – other radioactive waste from 20 years of operation was bagged and placed in the channels available cyclotron vacuum chamber. This included target foils which were first placed in Co-58 877 71 days 58Fe(p,n)58Co 58Ni(n,p)58Co from Iron and nickel lead containers to minimize gamma-shine from the cyclotron. The tank was then in Havar Foils – other channels available resealed for the last time. Assessment of the identity of the quantity of Co-60 176 5.3 years 63Cu(n, α)60Co neutron activation of Copper Coils radionuclides associated with the cyclotron was required before cyclotron shipment to the waste disposal site in Clive, UT (Energy Solutions). This Ni-57 7.6 36 hours 58Ni(p,pn)57Ni from Nickel in Havar foils assessment was performed with the ISOCS (Canberra) nuclear spectroscopy Fe-59 11.3 44 days 59Co(n,p)59Fe, 58Fe (n,p)59Fe neutron activation geometric modelling package. A simplified CAD drawing of the cyclotron, with of magnet steel its associated materials, was input into the modelling system, then collimated Zn-65 1454 244 days 65Cu(p,n)65Zn stray protons in tank hitting Cu spectroscopic gamma measurements of the cyclotron were performed from Dees, etc… several angles. Calculations, taking into account attenuation and geometry resulted in the data shown in Table 2. Table 2. Measured cyclotron activities at disposal - approximately two weeks after final run. Concrete Results: Residual activity in concrete is largely the result of neutron activation of trace elements of concrete, whose primarily components are cement, aggregate, sand, and water. It is critical to understand that concentrations of trace elements are geographically highly variable. Radionuclides detected in our samples will likely vary considerably in relative quantities relative to other decommissionings for this reason, and radionuclides not found in our samples could easily be found in others. A good physicist/ geologist detective could likely trace the concrete back to its quarries of origin based upon its neutron activation signature. Concrete activation results are shown in Table 1 below. Note that Na-24 and La-140 have highest concentrations, but their 15 hour and 40 hour half-lives suggest that a two week delay before demolition would obviate radiation safety consideration. All other levels are well below naturally occurring K-40 concentrations. The wall samples were taken from was 3 meters directly in front of the target area. Both fast and slow neutrons would impact the wall. Previous work (not presented) demonstrated that radionuclides resulting from interaction with isotopes with a high fast neutron cross-section appear in higher abundance in the first few cm depth, while as expected, slow neutron reactions tend to occur deeper into the concrete. Figure 1. Radioactive waste quantities as a function of time during storage. After 15 years, all activities will fall below the 1 microCurie benchmark for all but Co-60. Cyclotron Results: Residual radioactivity from the cyclotron and cyclotron parts comes from direct proton and deuteron bombardment of materials (foils, target bodies, and internal components from stray protons and deuterons), and from neutron activation primarily of the magnet steel and copper coils. Detected products are shown in Table 2. Assuming that the quantitative measurements Conclusions: Non-commercial cyclotron operated at benign beam currents from ISOCS are remotely accurate, an assessment of the long-term activity is supporting typical PET research volumes over a 20 year period generates shown in Figure 1. According to this assessment, after 15 years, all activities only modest quantities of waste with negligible environmental impact beyond will have dropped to below 1 µCi for all radionuclides except Co-60. In reality, 15 years. the ISOCS measurement will have underestimated used-foil activities, because these were put into lead pigs to minimize their shine out of the cyclotron tank where they were stored for disposal, and this was not modeled. However, these half-lives are generally short, and do not present significant long-term hazards. .