Dose Rate Distribution in the Vicinity of Nuclear Facilities

These projects cover extensive dose rate calculations for several interim storage facilities, waste treatment buildings, and an accelerator facility. The aim was to plan shielding measures and to prove compliance with the existing dose rate limits. The simulation models cover a very large area, so that sophisticated variance reduction techniques are used.

General procedure

For this type of calculations, the internationally recognized code MCNP with current data libraries is used.

  1. Development of a detailed geometry model including all relevant absorbing structures as well as all openings to the outside
  2. Implementation of all relevant radiation sources according to their geometrical structure, material composition, and nuclide-specific activity inventory (usually gamma emitters and/or neutron emitters)
  3. Application of deterministic variance reduction techniques to speed up probabilistic MCNP calculations; especially important for highly absorbing structures or large simulation models
  4. Parameter studies to optimize radiation doors, wall thicknesses, or mobile shielding

The main simulations with MCNP yield high-resolution spatial distributions of the dose rate (see example figure). These are analyzed and visualized in an appealing way. A detailed final report is provided for direct use by the customer. A more concise report is prepared for the approval procedure.

Special aspects of accelerator facilities

In this project, the central task is the design of the underground radiation protection bunker for an electron accelerator and its beam dump.

  • Challenge: high photon energies when stopping the electrons in the beam dump
  • Thus, high-energy neutrons due to photonuclear reactions (γ,n) are created.
  • Design of additional shielding for neutrons is necessary (in addition to optimizing wall thicknesses and labyrinth entrances)

Since simulations with both electrons and photonuclear processes are very slow, no reliable results can be obtained with acceptable computational effort. Therefore, a self-developed method is used, in which realistic surrogate sources are calculated and modeled for the primary photons and neutrons. Therefore, neither the electrons nor the photonuclear (γ,n) reactions need to be simulated in the final calculations.