Among the innovative Western nuclear reactors, the High Temperature Gas Reactor (HTR) represents one of the most interesting candidates, both in terms of safety (very high) and of high cheapness and low environmental impact, even regarding thermal pollution.

Since 1945, a HTR design had been proposed in the United States by Farrington Daniels. Later on, in the '50, a series of studies on the HTR has been begun in various countries, among which United Kingdom, USA and Germany. They have finally lead to the construction of three prototypes (DRAGON in United Kingdom, Peach Bottom in United States and AVR in Germany). The fundamental innovation in the field of the fuel technology of the HTR has been the invention of the Coated Particle (CP) with its exceptional qualities in terms of resistance and retention of the fission products. Also at the University of Pisa these reactors have been studied since long time, starting from the first researches got ahead by Prof. Poggi and Prof. Cerullo.

These reactors are characterized by a fully ceramic core and a neutronically non-active and non-corrosive cooling (helium or carbon dioxide): due to these characteristics it is possible to have high operating temperatures. Large thermal capacity and low power density of the core constitute the reasons of the slow progression of potential accidents. These characteristics are some of the reasons for the interest in the development of the HTR (or HTGR, High Temperatures Gas Cooled Reactor, as these reactors are known outside the EU).

The fundamental element of HTR safety is that, even in accident conditions, the fission products are, de facto, fully retained (in air and water absence) into the CPs (TRISO type) for temperatures lower than 1600°C. Moreover, the already mentioned low power density, typical of these reactors (some KW/l), prevents to reach this temperature limit.

The great quantity of proposed HTR designs has often hidden the evolution phases of this type of reactors. However, today, these reactors can be considered an innovative answer to the world energy demand, both for the electric energy and for the hydrogen production and for the desalinization systems. The results that could be obtained from operation of HTTR in Japan and HTR-10 in China in conjunction with the experience already available from other GCRs, constitute a strong technological basis for a present (PBMR - South Africa) and future (GT-MHR - USA) commercial development of HTRs.

In addition to previously described thermal reactors, recently Gas Cooled Fast Reactors (GCFRs) have become to be studied by the international scientific community. Due to the positive characteristics common to all the fast reactors (possibility of self-generation and therefore improved fuel exploitation, high fluence and therefore greater potentialities of burning nuclear waste) and to those common to the reactors cooled by an inert gas (no phase change and no nuclear and/or chemical reactions  between the coolant and the structural materials and/or the fuel), this kind of reactors, even if they are still in a preliminary stage of development, represents a very interesting prospect for future nuclear technology development.

Today it is quite popular to talk about the so-called hydrogen economy: many studies have demonstrated that the use of this element as fuel for the motor vehicles would help to reduce drastically (or even to avoid at all) every kind of local atmospheric pollution due to the transports. However the hydrogen, even if quite widespread in nature, can not be found in an unbounded state. Therefore it is necessary to spend energy in order to produce it: hydrogen is, in fact, just an energy carrier. Currently it is produced mainly by using fossil energy source (not renewable and subject to remarkable carbon dioxide emissions); its production by nuclear energy source (using HTRs and/or GCFRs) could constitute the key point for the reduction of urban pollution (smog). The residual heat at low temperature could be also used as domestic heat or for sea water desalinization.

Moreover, the good GCFRs' characteristics in terms of neutronics could allow the adoption of optimized fuel cycles in order to obtain the burning of nuclear waste (particularly of actinides). Some studies regarding plutonium consumption (both Weapon and Reactor Grade) have demonstrated (also by means of advanced software ad hoc developed) that the problem of the dangerousness of such chemical element (main contributor to the long term waste radiotoxicity) can meaningfully be reduced (of a factor 10, both in terms of radiotoxicity  for ingestion and in terms of mass) by adopting Th/Pu fuel cycle in the HTRs. In this way it should be possible to fertilize (and therefore to use) thorium, much more widespread (more than 2.5 times) than uranium and currently not used as nuclear fuel. Even better results could be obtained using symbiotic LWR-HTR-GCFR fuel cycles.