Radiation Shielding for ECH Launchers in ITER

Radiation transport analyses were carried out for the ITER electron cyclotron heating launchers. Elevated dose rates were predicted to the south of the tokamak due to activated water in the tokamak cooling system. These are among the most challenging radiation transport calculations conducted at ITER: they required unprecedented methods to accelerate the calculations as well as code development for the handling of very large models and the visualization of the results. Long (~25m) streaming paths defined by waveguides with a radius of less than 8 cm that cross several walls of the port cells and through the south wall into the assembly hall create a highly collimated beam of photons.

There are several constraints on the shielding of waveguides that penetrate the bioshield and the outer walls of the ITER complex building. Shielding is needed for both neutrons and high-energy gamma rays; it must allow differential movement of systems, provide tritium confinement, and be fire retardant. Borated polyethylene in the penetrations and around the waveguides was replaced by Shieldwerx SWX-277Z-5 to reduce the fire load. This proved insufficient to lower the dose rate in the assembly hall. The introduction of an additional bend would eliminate the direct line of sight but lower the power delivered to the plasma, and it would be impractical to implement at this stage of the construction. A repositioning of the doglegs behind the bioshield would be effective in removing the direct streaming path, but this cannot now be implemented. Shielding on the cooling water system is the preferred solution.