Decommissioning of medical cyclotrons: In-depth analysis of radiological inventory with regard to systems, structures and components evacuation routes acceptance (Case Study: 40 MeV CGR Cyclotron VUB (Brussels))

SCK•CEN Mentor

Çelik Yurdunaz, ycelik@sckcen.be, +32 (0)14 33 21 96

Expert group

Nuclear Systems Physics

SCK•CEN Co-mentor

Van den Dungen Kurt , kvddunge@sckcen.be , +32 (0)14 33 26 83

Introduction

The intensive exploitation of multi-particle accelerators – more specifically medical cyclotrons – leads to activation of systems, structures and components (SSC) involving considerable future decommissioning costs for both industrial companies and universities/hospitals. Material evacuation routes are comparable to those for industrial nuclear installations such as nuclear power reactors, e.g. reuse, conditional and unconditional release in some cases after melting and disposal as radioactive waste. The elimination of an SSC item according to a specific route requires the knowledge of the radionuclide inventory to prove accordance with radiological acceptance criteria, and therefore of the induced activation source term. However, the inventory and the specific activities of the radionuclides found in the irradiated materials of accelerators differ considerably from those found in nuclear reactors because the activation mechanisms and activating particles are generally different[1].

Today, preliminary assessment of the radiological inventory is mainly supported by sampling and analysis, a costly and labour-intensive approach subjected to several uncertainties among which the heterogenous activation of SCC, the potential presence of difficult to measure pure β- emitters and the extrapolation to larger volume/mass. Moreover, in the framework of making provisions for future decommissioning of new installations, this partly destructive approach is useless for newly installed cyclotrons for which the activation level is limited at the start of the exploitation.

This means that relying solely on a sampling approach can lead to unexpected delays and costs. Past years some initiatives [2,3] were taken to use Monte Carlo tools to characterise particle transport and neutron fluxes in the framework of shielding design, but long-term activation of SSC materials was poorly addressed. For this reason, the need for a reliable methodology based on computational tools and validated by nuclide-specific measurements should be studied.

References

[1] IAEA Safety Report Series No.95, Methodologies for Assessing the Induced Activation Source Term for Use in Decommissioning Applications, 2019.

[2] Brad D.Jeffries et al. Characterization of the neutron flux during production of 18F at a medical cyclotron and evaluation of the incidental neutron spectrum for neutron damage studies. Applied Radiation and Isotopes (154), 2019.

[3] D.Alloni et al. Characterisation of the secondary neutron field generated by a compact PET cyclotron with MCNP6 and experimental measurements. Applied Radiation and Isotopes (128), 2017.

Objective

  • Perform a literature study on cyclotron decommissioning, with the focus on radionuclide inventory and approach towards material evacuation.
  • Assemble and review the available information for the 40 MeV CGR Cyclotron VUB
  •  Material composition,
  • Geometry of the components (magnets, beam line, Cyclotron bunker, etc.),
  • Beam source distribution (beam current, energy, and spatial distribution),
  • Irradiation history of the facility.
  • Prepare an MCNP input with the information above.
  • Source term calculations with ALEPH code in order demonstrate capability and feasibility of decommissioning of the facility.
  • Residual dose calculations associated with the decommission process.

The minimum diploma level of the candidate needs to be

Academic bachelor

The candidate needs to have a background in

Physics