PROTEUS is a driven system that can rather straightforwardly be adapted to model local core environments for a large number of different reactor concepts. Since February 1968, its versatility has enabled experimental studies of a rich diversity of different reactor concepts, such as the Gas-Cooled Fast Reactor (GCFR), the tight pitch High-Converting Light Water Reactor (HCLWR) and the modular High Temperature Reactor (HTR).

The earlier investigated advanced reactor types were of interest because they offered the possibility to use alternative fissile material and/or to breed new fissile material (thus increasing the fuel economy). Although the HTR offers a more efficient fuel usage too (due to a higher power conversion efficiency), the interest in this reactor concept mainly originated from the growing demand for an enhancement of the safety standards of nuclear power plants, which can be fulfilled by the inherent safety characteristics of the HTR. Inherent safety means that the core is designed in such a way that the guarantee for its integrity under any conceivable disturbed operational condition is based on the validity of physical laws rather than on the very low probability that multiple engineered safety mechanisms will fail.

Starting in mid 1996, work was underway to realise a new and exciting experimental programme addressing the second general research philosophy mentioned above. This first of its kind new programme was known as LWR-PROTEUS and marked a departure from the more exotic reactor concepts of the past to concentrate more upon the near- and mid-term needs of the LWR-community. The LWR-PROTEUS programme represented a conscious move from traditional critical experiments, in which specially prepared mock-up lattices are investigated, to a concept in which authentic power reactor fuel was first investigated in PROTEUS before use as normal reactor fuel in a power station. It was aimed at realizing an enhanced expertise and capability for predicting LWR core characteristics more and more accurately by setting up an experimental validation database. In this way, the role played by uncertainties in conservative constraints in reactor core design can be minimized, thus leading to the possibility of an increased fuel usage under the same set of operational and safety limits. Because of the commercial implications of being able to eliminate over-conservatism in operational constraints and thus to increase the fuel usage, the LWR-PROTEUS project received financial support from swissnuclear on top of the funding provided by the Paul Scherrer Institute itself. swissnuclear comprises representatives of the Swiss electricity companies Atel, BKW, CKW, EGL, EOS and NOK which operate the Swiss nuclear power plants Beznau, Gösgen, Leibstadt and Mühleberg. Further, indirect contributions are received from nuclear fuel vendors such as Westinghouse Atom and AREVA.

On the completion of the LWR-PROTEUS experimental programme at the end of 2005, a new programme called LIFE@PROTEUS (Large-scale Irradiated Fuel Experiments) was started. This programme aims at answering questions arising from the interfaces between fresh fuel and more and more highly burnt fuel, typical at the reloading of modern nuclear power plants. Analogous support is provided by swissnuclear, as described for LWR-PROTEUS.