Full error study of ESS linac - ESS Linac Technical Note

The maximum dose that a European 'highly exposed' worker can absorb in one year is 50 mSv (01/2002). Euratom is recommending 30 mSv, a level that should be adopted in a near future. The ESS accelerator will operate about 6000 hours a year for the user program. This leaves 2800 hours per year for maintenance. Assuming a worker participates in the full maintenance period, the maximum average dose rate is: 50 mSv / 2800 hrs = 18 μSv/h. Because the instantaneous dose rate decreases with time while the workers are working on the shutdown machine, their average dose rate will be lower than the entering dose rate defined as the dose rate when they are allowed to enter the linac tunnel. The ratio between the entering dose rate and average dose rate depends on the decay time of the dose rate, the number of maintenance interventions and the time each intervention takes. The allowed entering dose rate would then be between 200 μSv/h and 20 μSv/h. The higher the energy, the lower should be the gas pressure (the gas pressure should be 10 times lower at 1334 MeV than at 185 MeV). To allow an easy maintenance, the gas pressure should be of the order of 10^-8-10^-9 at high energy. Residual gas stripping losses can be reduced by a factor 2 by using protons for the long pulse. These protons would allow to work below 1 W/m (200 μSv) in the all linac of the sc-option, but not in the high energy part of the nc-option. A strong effort needs to be investigated in the vacuum system