| Electron linear accelerator is one of charged particle accelerators, in which the electron beam is charged in the longitudinal electric field by using high microwave power in traveling wave or vertical wave accelerate structure. The application areas for electron linear accelerator are becoming wider and wider, so the induced radioactivity issue brought by its decommissioning is significant.The Synchrotron Radiation Facility of National Synchrotron Radiation Laboratory (NRSL), located in Hefei, China, is one of the earliest synchrotron light sources in China. NRSL, constructed in 1989, is the home to a 200MeV electron linear accelerator and an 800MeV electronic storage ring. The 200MeV electron linear accelerator as the injector provides electron beam for nuclear physics and other research. The linear accelerator is a traveling wave linear accelerator, consisting of pre-injection (including electron gun, pre-buncher, buncher and a 3-m acceleration segment), eight 3-m acceleration segment, five beam sensing segment, and focusing inserts. The accelerator’s total length is 35.128 m. The electrons with low energy emitted from the electron gun are accelerated in the acceleration tube to 32 MeV,74 MeV,116 MeV,158 MeV, respectively, and the electronic energy reaches 200 MeV at the end of the acceleration trip. And then the electrons are ramping to 800 MeV in the storage ring. Considering the influence of ionizing radiation on environment and staffs, the linear accelerator was built in the underground tunnels, with the soil shield of 3.5 m, the cross-section of 3.5 m×3 m, and the length exceeding 140 m. The layout provides a good solution to the radiation safety problems on the surrounding environment during the operation of NSRL Linac.The NSRL Linac was retired in May 2012 due to a plan to upgrade the facility. Its decommissioning produced many real challenges:the methods to handle the device and the time required to store structural materials until the radioactivity is reduced in the waste repository. These problems should be solved as soon as possible in order for the upgrade to proceed smoothly. At present, no prior decommissioning experience existed in China on such a high-energy electron accelerator. Therefore, the decommission project provided an chance to study and evaluate the challenges, in particular, involving the induced radioactivity for the purposes of radiation protection.The electron accelerator is differ from proton and ion accelerator, because the cross section of nuclear reaction is very small for the electrons with any energy. That is to say the induced radioactivity on the electron accelerator is not due to nuclear reaction between electrons and media. Its mechanism is:the energy of electrons disappear due to bremsstrahlung, high-energy photons (higher than 10 MeV) interact with medium, subsequent neutrons and pions induce nuclear reactions. For a 200 MeV electron linear accelerator, the induced radioactivity is mainly caused photonuclear reactions.Based on the theory, this study carried out the following work:The paper carries out relevant research about the induced radioactivity of electron accelerator structural materials. And based on the elements contained in the material, radionuclides generated in the 200 MeV electron linear accelerator are predicted. Meanwhile, beam losses at different energy segments are measured using TLD. After the accelerator was shutdown, the radiation dose rate of induced radioactivity for each point in the tunnel were measured. And the radiation status for laboratory and surrounding are monitored, comprising:γ dose rate in the linear accelerator tunnel and surrounding; ambient dose rate of γ for warehouse storing removals; total α, β of soil, surface water and aerosols, total β; neutron cumulative dose. In addition, radionuclides generated on NSRL Linac are measured using HPGe gamma spectrometer at different time during shutdown.A series of simulations about the NSRL Linac are carried out using Monte Carlo procedure FLUKA. The simulations mainly focus on such problems:the kind of radionuclide problem generated at each acceleration tube with different energy. Simulated results coincide very well with the measurements, indicating the Monte Carlo simulation method can be applied to predict induced radionuclides. The distributions of electron, γ, and neutron in the scrapers of different energy are simulated. To illustrate the protection of the acceleration tube provided by the scraper, a detailed analysis of the radiation field of the scraper was performed; the distribution of photons indicated that the most highly concentrated region of photonuclear interaction in the scraper. In the paper, we presented simulations of the function of the NSRL scrapers in blocking electrons and protecting the acceleration cavity, and the distribution of induced radionuclides in the scraper. The scraper design should be carefully considered in the design of electron accelerators and radiation protection.The remaining nuclide in the scraper mainly is 60Co as time goes on due to its long half-time. The reaction is 63Cu(γ,n2p)60Co, the threshold of which is 18.86～28 MeV. To determine the high-radioactivity area and facilitate the decommissioning process, the slicing method was applied to investigate the 60Co distribution along the axial direction of the scraper. And the content of 60Co in each copper slice is measured using HPGe gamma spectrometer. The photons with threshold and the yield of 60Co are simulated by Monte Carlo program FLUKA. The simulation results are consistent with the measurements, indicating that Monte Carlo simulation method can be applied to induced radioactivity research. Meanwhile, the relationship between photon number within threshold and depth can explain the relationship between 60Co and depth fundamentally. Furthermore, the clearance time of each copper slice is given out based on attenuation calculation. This method could be extended to the decommissioning of other scrapers corresponding to different electron energies.The radionuclides were classified according to their half-lives. Such a classification provides a reliable basis for the formulation of radiation protection and facility decommissioning. Finally, the research about induced radioactivity of decommissioning NSRL Linac is given quantitative in the paper. In the future, our investigation will provide a significant reference for decommissioning of similar electron accelerator. |
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