The explosion at Chernobyl in 1986, caused by a runaway power surge in a grossly maloperated and uniquely unstable type of reactor, raised concerns about the safety of other types especially if they were used to transmute substantial amounts of long-lived waste elements with a destabilising potential. Neutrons for this purpose could alternatively be supplied by the impact of a highly energetic proton beam on a heavy metal target, but with the currents likely to be attainable in the foreseeable future, the rate of transmutation would be inadequate. However, if the beam were used to drive a sub-critical array of fissile and fertile material (like a reactor core but with a lower fissile content), the neutron flux could safely be amplified by a factor of ten or twenty. If a problem should arise, the reaction could be stopped almost instantaneously by switching off the accelerator that provided the proton beam.
In 1995 Carlo Rubbia (formerly Director-General of the CERN high-energy physics laboratory) proposed with various associates a system of thorium-fuelled "energy amplifiers" on this principle, and eventually a series of them to be built near Zaragoza in northern Spain. Such stations would require a structure differing little in essentials from a conventional power reactor besides the accelerator and its associated radio-frequency power supply, and so would be expensive. However, the scheme was eventually rejected by the local community, maybe for other reasons. Nevertheless, the principle is still being considered elsewhere for purposes of transmutation.