tokamak
Nuclear physics

design of shielding for active irradiation.

The production of medical isotopes Mo-99 and Tc-99m is conventionally achieved in nuclear reactors, where a nuclear reaction with highly enriched uranium takes place. Alternatively, high energy electrons impinging on a Mo-100 target can be used to produce Mo-99 through a (γ,n) reaction. This method of production is currently of great interest, and many experiments heve been performed. See e.g. [1] and [2].

In such a process, radiation is released that endangers people and could potentially cause failure of equipment. To guarantee safety, shielding is required. Below we present a comparable scenario and demonstrate how shielding ensures human safety, both during the experiment and after. The method we use is through Monte-Carlo analysis in FLUKA.

shielding strategy.

To correctly represent the radiation environment, the profile of the electron beam is modelled in detail. Next, the reactor setup is modelled in sufficient detail to account for potential shielding weaknesses, and with suitable materials. Figure 1 shows a shielded vacuum chamber. Inside is the target and beam dump, electron beam coming form the left. 

Radiation leaving the target is made up of high-energy photons and neutrons. Knowing the radiation spectra is of important for devising a successful shielding strategy.

In this case the following shielding strategy was chosen: A first layer of lead is placed (efficient photon shielding). A high-density material like lead is well-suited at these energies due to the generation of electron-positron pairs through a process called pair-production. Next, a layer of borated polyethylene is place, to shield and moderate neutrons. As a final shielding layer, a 1.5 m thick borated concrete wall is present. Adding boron increases neutron moderation and absorption.

Figure 1: Geometry of the FLUKA model, with the material colors indicated. The beam is incident from the left.
Figure 1: Geometry of the FLUKA model, with the material colors indicated. The beam is incident from the left.

results.

A general challenge in Monte Carlo simulations is ensuring sufficient statistics in regions of interest. With advanced biasing techniques, we can achieve good results on the (low) dose rate behind the concrete wall. 

Figure 2 shows the dose equivalent rate during operation. The obtained results for the dose equivalent rate are then compared to human safety standards, for example those of the regulatory commission ICRP. Using these guides, we can identify regions that are safe, or potentially hazardous.

During primary irradiation, radioactive isotopes are produced in the setup and its structural materials. This information is critical when considering e.g. waste treatment in facilities on the long term, as well as a safety issue.
 

Figure 2. The prompt dose equivalent rate in µSv/h.
Figure 2. The prompt dose equivalent rate in µSv/h.

We mapped the activated materials and their emitted residual radiation. Figure 3  shows the activity density map and the resultant residual radiation after a certain irradiation time and cooldown period. We can see the beamline penetration of the shielding layers results in large residual dose rates along the beamline opening. This is a potentially hazardous area for humans. 

Bibliography
[1] International Atomic Energy Agency, Non-Heu Production Technologies For Molybdenum-99 And Technetium-99M, Bernan Distribution / International Atomic Energy Agency, 2012.
[2] “Four US companies chosen for Mo-99 production funding,” [Online]. Available: https://world-nuclear-news.org/Articles/ Four-US-companies-chosen-for-Mo-99-production-fund.

Figure 3: The residual dose equivalent rate in µSv/h (top) and a map of the volumetric activity (bottom).
Figure 3: The residual dose equivalent rate in µSv/h (top) and a map of the volumetric activity (bottom).

"making radiation visible"

As multiphysics engineer I use Monte Carlo methods to simulate radiation environments. Based on these results, both prompt and residual dosimetric quantities can be evaluated. Moreover, designing shielding systems in specific environments require a multiphysics approach, e.g. with regards to heat loads and material damage caused by the ionising radiation. All these factors contribute to the challenge of designing shielding systems to ensure human safety.

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