Neutron activation of a cyclotron bunker to produce radiopharmaceuticals

During the operation of the PETtrace cyclotron for the production of 18-F used in Positron Emission Tomography (PET), activation of the cyclotron’s structures and surrounding infrastructure occurs. The activation can penetrate several centimeters into the concrete of the bunker walls of the facility. This article presents a facility that will undergo modifications due to the cessation of activity and relocation of the cyclotron. It is necessary to assess the activation state of the bunker in order to define the work procedure and the radioactive waste to be considered during the modification..

Due to the presence of the cyclotron, there is a radiological background that complicates characterization through direct non-destructive measurements. Therefore, ENUSA-ENSA AIE (EE-AIE) proposed the extraction of samples from areas of greater interest.

To optimize the number and position of the extractions, EE-AIE designed a model using the MCNP6.2 code to determine the neutron fluence in the bunker, considering both the cyclotron’s structural elements, the neutron moderator, and the source term. With the results of the model, areas of higher and lower neutron impact were determined, creating a fluence map that guided the sampling points. Ten samples were extracted, distributed across walls, floors, and doors. Gamma spectrometry was used to determine the isotopic composition and activity concentration at various depths, with the activity results of the samples aligning with the areas of highest and lowest impact as predicted by the Monte Carlo model.

By comparing the analysis of the extracted samples with the model, it was concluded that the model is valid for representing the radiological impact of the cyclotron in the bunker, and the applied methodology is transferable to other facilities with similar characteristics.