Change of use shielding calculations for radiation facilities

NPL recently completed a series of shielding calculations using Monte Carlo techniques to determine dose-equivalent rates arising from neutrons and high-energy photons at critical locations around a clinical linear accelerator facility to be installed in an existing bunker. These calculations are vital to ensure that adequate shielding is in place, in line with statutory requirements and the ALARP principle, especially since the energy and orientation of the beams from the proposed machine are completely different to those of the machine being replaced.

We used the FLUKA (FLUktuierende KAskade) radiation transport code to construct a detailed model of the radiation bunker (including the entrance maze, essential ventilation & cable ducts etc.) together with a simplified accelerator source. Using this model we established a set of worst-case scenarios for both electron and photon radiation beams from which we determined dose-equivalent rates and neutron/photon energy spectra at key positions in the adjacent control room and access maze. Once radiation ‘hot-spots’ had been identified we were able to simulate a series of scenarios with different shielding options in place to confirm that the desired requirements had been met. 

As an independent check, we also estimated the dose-equivalent rates and shielding requirements using analytical methods described in NCRP Report No. 151 and IPEM Report No. 75. These results agreed favourably with the results from the Monte Carlo calculations.

A radiation bunker design simulated using FLUKA showing the distribution of dose-equivalent rate in the room, maze and walls for a high energy x-ray beam striking a tungsten slab.

Neutron (left) and photon (right) fluence distributions at the entrance to the radiation bunker.

For more information, please contact David Shipley or Sebastian Galer