DIII-D radiation shielding procedures and experiences

The DIII-D tokamak operates with a neutron radiation shield to allow enhanced plasma operations with increased neutron production while minimizing the site boundary dose level. Neutron rates as high as 4×10¹⁵ neutron/s and total neutron production as high as 4×10¹⁵ neutrons per shot are obtained while maintaining the site dose below the DOE (Department of Energy) administrative level of 20 mrem per year. The radiation shielding has increased by a factor of 300 over the preshield value and is in agreement with the design calculation. The maximum site neutron dose since installation of the shield has been less than 0.03 mrem for a shot and less than 0.4 mrem for a day. The site neutron and gamma dose are monitored continuously during operations by a PC-based computer system that provides the means of measuring the low dose levels that occur during a shot by including postshot background subtraction. The neutron and gamma dose are measured and archived by shot, hour, and day in a database. Activation of the machine after a run day and during vessel entries is monitored, and the activated nuclides have been determined