Abstract :
Secure safety, resistance to weapons material proliferation and problems of long-lived wastes remain the most important “painful points” of nuclear power.
Many innovative reactor concepts have been developed aimed at a radical enhancement of safety. The promising potential of innovative nuclear reactors allows for shifting accents in current reactor safety “strategy” to reveal this worth. Such strategy is elaborated focusing on the priority for intrinsically secure safety features as well as on sure protection being provided by the first barrier of defence.
Concerning the potential of fast reactors (i.e. sodium cooled, lead-cooled, etc.), there are no doubts that they are able to possess many favourable intrinsically secure safety features and to lay the proper foundation for a new reactor generation. However, some of their neutronic characteristics have to be radically improved.
Among intrinsically secure safety properties, the following core parameters are significantly important: reactivity margin values, reactivity feed-back and coolant void effects. Ways of designing intrinsically secure safety features in fast reactors (titled hereafter as Intrinsically Secure Fast Reactors – ISFR) can be found in the frame of current reactor technologies by radical enhancement of core neutron economy and by optimization of core compositions.
Simultaneously, respecting resistance to proliferation, by using non-enriched fuel feed as well as a core breeding gain close to zero, are considered as the important features (long-lived waste problems will be considered in a separate paper).
This implies using the following reactor design options as well as closed fuel cycles with natural U as the reactor feed:
• Ultra-plate “dense cores” of the ordinary (monolithic) type with negative total coolant void effects.
• Modular type cores. Multiple dense modules can be embedded in the common reflector for achieving the desired NPP total power. The modules can be used also independently (as a small power NPP) then giving an attractive flexibility to the prospective NP.
For both above-mentioned ISFR options, which possess the stabilized reactivity at equilibrium, void effects in all reactor types have been favourably corrected: positive void effects in sodium cooled reactors have been radically reduced from 5 to 8$$ (the range of values for the best traditional and some innovative projects) down to $/3. As for the modular core options, all void effects in sodium cooled reactors became negligible. Besides, all void effects in lead cooled ISFR can be “designed” in a “harmonious” way: they became modest and favourably negative thus significantly increasing their natural self-protection against severe accidents.
These concepts imply using hard neutron spectra in the “dense” reactor cores identifying fast reactors with: dense fuel (mono-nitride, carbides or metallic alloys similar to known projects like BREST, IFR), elevated fuel in-core fractions (where parasitic neutron capture is significantly depressed), and optimal core-blanket configurations.
