The aim of this project is to couple a proton accelerator with a target and a subcritical system of sufficient size to produce a sizable power. This experi­ment could be carried out in the TRIGA reactor at the ENEA Casaccia Centre (Italy) operating as a subcritical assembly. TRIGA (Training Research Isotope General Atomic immersed test reactor) is an existing 1 MW thermal power swimming pool reactor cooled by natural convection
of water in the reactor pool. The fuel elements are cylinders of uranium (enriched to 20% in 235U) with a cylinder of metallic zirconium inside.

At the present stage of the feasibility study, the TRIGA project is based on the coupling of an upgraded commercial proton cyclotron with a tungsten solid target surrounded by the TRIGA reactor scrammed to undercriticality. The flexibility offered by the swimming pool reactor is well suited for conversion to subcritical configuration, which is achieved through:

• the replacement of the outermost fuel ring with a graphite reflector,

• the removal of the innermost ring of fuel core.

The target should be hosted in the central thimble, at present used for high-neutron-flux irradiation. A beam power of a few tens of kW appears adequate to run a subcritical system with an appropriate multiplication coefficient. With ks of about 0.97, the system may produce several hundred kWth power in the reactor and a few tens of kWth in the target.

The reference design for the accelerator is a 220MeV-H2+ Super­conducting Cyclotron based on the concepts developed in the Cyclotron Laboratory of CAL (Nice) for compact superconducting fixed-frequency cyclotrons for hadron therapy. Protons are produced at 110 MeV by strip­ping; the requested beam current is in the range 1-2 mA.

The experiments of relevance to ADSR development to be carried out in TRIGA are:

• subcriticality operation at significant power,

• the possibility of operating at some hundred kWth of power and at different subcriticality levels (0.95-0.99) will allow the designers to validate experimentally the dynamic system behaviour versus the neutron importance of the external source and to obtain important information on the optimal subcriticality level,

• correlation between reactor power and proton current,

• reactivity control by means of neutron source importance variation,

• start-up and shut-down procedures.