Preliminary neutronic analyses of ITER high resolution neutron spectrometer collimator

The High Resolution Neutron Spectrometer (HRNS) will be one of the largest and most complex neutronic diagnostics in ITER. It will be located in Equatorial Port Plug number 1, in which many other diagnostic devices are placed, increasing the interface problem. HRNS will be able to perform a wide range of measurements of plasma parameters in both DD and DT operations. Since detectors have to be placed in the port cell far away from plasma surface, an efficient collimating system has to be provided. The collimator has to allow an output neutron flux as intense as possible taking into account the ITER requirements in terms of nuclear safety and radioprotection. The collimator has also to control the quality of the spectral response arriving to the detectors, ensuring an acceptable Signal-to-Noise ratio. This study presents a set of parametric analyses performed with MCNP5 using a simple model. With this simple model the effect of geometric key parameters on the response of the collimator can be isolated, without the complexion of the geometry. Parameters such as the aperture of the cone of vision or the materials composing the collimator walls and the Port Plug have a significant effect on neutron flux at collimator output and interspace volume. Finally the results of all those geometric and material variations have been resumed to give a set of recommendations useful for a preliminary design of the collimator.