L. CINOTTI Nuclear Division,

NSALDO,

Genoa, Italy

Abstract

ANSALDO has conceived a reactor called ISIS (Inherently Safe Immersed System), an innovative light water reactor with easily understandable safety characteristics.

The main targets are: passively safe

behaviour, no pressurization of the Reactor Containment under any accident condition, control of plant capital cost and construction schedule by virtue of the modular concept and the compact layout.

The ISIS concept, described in general terms in the paper, builds up on the Density Lock concept originally proposed by ABB ATOM for the PIUS plant (ref. /If), featuring innovative ideas derived from ANSALDO experience and based on proven technology from both LWR and LMR.

1. MAIN TARGETS OF THE ISIS CONCEIT

1.1 Safety targets

Significant progress has been made in the last years towards nuclear reactors that rely to the smallest possible extent on safety-related active systems, which, even using up-to-date technology, are felt by the public as prone-to-fail, no matter how low the frequency target for their loss is set.

The ISIS concept, under development in ANSALDO, largely embodies this progress. The main safety targets may be summarized as follows:

— No core melt-down and negligible release of radioactivity in any accident condition, by virtue of the reactor concept itself.

— Prompt reactor shut down occurring naturally after any abnormal condition.

— Reactor cooling in natural circulation for unlimited time.

— Self-depressurization of the reactor after a postulated failure of the pressure boundary.

1.2 Economic targets

The economic target aims at a viable industrial power plant based on specific overnight-capital cost and construction time competitive with those of the Light-Water Reactors under development This is achievable by means of following features:

— Modular reactor.

— Integrated components (Compact layout).

— No pressurization to be taken into account in

the design of the reactor containment

— Primary system installation after reactor

building completion.