At the beginning of the development of the ISIS concept (about seven years ago) it seemed reasonable to foreseen a moderate increase of the cost of the fossil fuels in the near future which would have improved the economic competitiveness of nuclear energy. Today, instead, two facts worsen this competitiveness:

— the fossil fuels price has remained low and stable,

— the efficiency of the modern electric energy generating conventional power plants is continuously increasing.

The importance of the second fact is such that it will drastically affect the energy market, in particular it will impact the market of nuclear energy.

In the past, the efficiency of electricity production of the nuclear power plants was similar to that of the conventional power plants. Under that condition it was profitable to generate electricity by the large-size nudear power plants that dominate the nudear panorama.

Today, the effidency of the modern Combined Cyde Turbo-Gas (OCTG) Power Plants has exceeded 50% and in the near future (before the year 2000) will reach and perhaps trespass 60%, while the one of the nudear water reactors stagnates at about 33%.

This new fact will have two main consequences:

— if the cost of fossil fuels remains stable, the cost of heat will remain substantially stable, but the cost of electricity will be reduced;

— the ratio electridty/heat production will increase up to the optimum dictated by the modern fossil-fired со-generative power plants.

Qualitatively it can be affirmed that today an effiaent use of energy favours the fossil fuels for electricity and of the nudear fuel for heat production, because of the lower electric effidency of the nudear power plants.

Quantitatively, a preliminary economic evaluation carried out comparing 60% effiaent CCTG, со-generative CCTG, conventional boilers and nudear power plants, has shown that nudear power plants could recover part of their lost economic attractiveness only if exploited as

со-generating or as thermal power plants.

The со-generative use appears attractive from 3000 hours/уг. and the thermal use from 5000 hours/yr. upwards (increase of the value of the plant in the order of more then 50 %)

The increase of the value of the plant is more important for со — generative reactors and can largely exceed 100% for specific site conditions where heat can be used during the most part of the year.

A trivial condition for any interest of a prospective utility in a со-generative nuclear plant is that an adequate со-generating reactor exists. For this the reactor designers, besides providing the reactor with convincing characteristics of radiological safety, have to overcome the unfavourable scale-effect of downsizing, because the thermal power needed is mostly in the order of hundreds of megawatts against the thousands of megawatts available from the today large nuclear reactor conceived for electricity generation only.

In the view of a reactor designer, the smaller reactor can be competitive, in spite of downsizing, provided that:

— the number of systems of the larger plants is strongly reduced,

— the specific mass of steel of the NSS it is not significantly increased (scale effect neutralised),

— the specific operation & maintenance costs become not excessive.

The со-generating version of the ISIS reactor is being designed to cope with these requirements.

The technical features illustrated in this article and the results of preliminary analyses for an use of ISIS as со-generating reactor can be summarised as follows:

— no active safety system is necessary to assure safety. All active safety systems can be eliminated.

— no adverse scale effect on the specific mass of steel of the ISIS NSSS with respect to the larger modern PWRs. This is possible also thanks to the milder operating conditions of a reactor designed for co-generation (e. g, lower operating pressure ).

Ongoing studies explore furthermore the possibility of reducing operating & maintenance costs, taking profit of the predicted simple operation of ISIS, of the reduced number of systems, of the modular approach that makes possible to share facilities, such as fuel handling and component handling equipment, between the reactor modules installed in the same reactor building.

3. CONCLUSION

The ISIS is an innovative Nuclear Power Plant under development in ANSALDO. It is based on original ideas derived by ANSALDO experience on proven LWR and LMR technologies.

The main features of ISIS are as follows:

— Outstanding passive safe behaviour of the Reactor, which means core shut down and cooling functions ensured in all accident conditions and no release of primary coolant outride the Reactor Building.

— Compact reactor layout, associated with modular fabrication and erection, made possible by the integrated design of the primary circuit.

— Reactor concept flexible for combined generation of heat and electricity made possible by the modular solution and the low sensitivity to downsizing.

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