Measurement of Residual Radioactivity of Machine Elements and Concrete on the Cyclotron Decommissioning

Pre-discard estimation of radioactivated materials in positron emission tomography cyclotron systems and concrete walls of a cyclotron vault

PURPOSE

The concrete vault, cyclotron body, and peripheral equipment in a cyclotron room become radioactivated by neutrons generated by operating an unshielded cyclotron. Radionuclides and the amounts of radioactivated materials must be identified before discarding a cyclotron system. The present study aimed to reduce the amounts of concrete from cyclotron vaults, as well as cyclotron components and peripheral equipment, that will be disposed of as radioactivated waste by clarifying the nature and quantity of radioactivated materials remaining in facilities after cyclotron operations have ceased.

METHODS

Cylindrical concrete cores were bored into all four walls, ceiling, and floor of a room where a Cypris 370 cyclotron had been operated for 22.8 yr and then cooled for 40 months. The accelerated particles comprised protons and deuterons with constant energy of 18 and 10 MeV, respectively. The types and amounts of radionuclides in these cores, in 38 components of the cyclotron including the yoke, and in 13 pieces of equipment in the room, were determined by γ-ray spectrometry. Concentrations of radioactivity were also calculated using an updated version of Particle and Heavy Ion Transport System and DCHAIN-SP. Amounts of materials with both measured and calculated total radioactivity concentration (ΣD) of <0.1 Bq/g were identified as being nonradioactivated.

RESULTS

The major radionuclides in the concrete were ⁶⁰ Co and ¹⁵² Eu. The radioactivated concrete was distributed to a depth of <38 cm. Most cyclotron components and equipment were radioactivated by neutrons. The major radionuclides in cyclotron components and equipment were ⁵⁴ Mn, ⁶⁰ Co, and ⁶⁵ Zn. A 33% volume of the yoke was regarded as nonradioactivated.

CONCLUSIONS

The estimated amount of radioactivated waste in the concrete was about 70,000 kg (12.5% of the total concrete). Most components of the cyclotron except for the 33% volume of the yoke (20% of the cyclotron body), as well as most peripheral equipment in the room, were radioactivated. Part-by-part assessments of radioactive materials using measurements and calculations could distinguish nonradioactive from radioactive materials before they are discarded.