Intrinsically secure fast reactors with dense cores
Introduction
Secure safety, resistance to weapons material proliferation and problems of long-lived wastes (Salvatores et al., 1995, Salvatores et al., 1998) remain the most important “painful points” of nuclear power.
For the current generation of innovative reactor designs, the word “secure” may imply essential enhancements: in safety (sure protection against all severe accidents), in proliferation (a radical increase of proliferation resistance) and in waste problems (a significant reduction of the long-lived radiological environmental pollution source).
These goals are really ambiguous and it requires special attention when new reactor concepts are elaborated. The radical recipe on this way is using those fundamental (intrinsic) properties, both of the reactor and of the fuel cycle, which are (a priori) the most favourable for the achievement of these goals. Let us call them “intrinsically secure” features which can be applied to different areas of interest: safety, non-proliferation and waste management.
A radical increase of proliferation resistance can be achieved in NP with fast reactors due to the possibility of full exclusion of enrichment technology (natural U feed) as well as of fissile-fission/fissile-fertile separation technologies during fuel reprocessing. The last is particularly real, for example, in the case of stabilized reactivity.
A significant reduction of long-lived radioactive wastes in developing NP is also relevant and will be discussed in detail in later publications.
Intrinsically secure properties in the safety area imply a capability for significant reduction of severe accident risks corresponding to the so-called “MP + MU principle” – minimum risk probability with minimum associated uncertainties – in the case of abnormal internal (including the “human factor”) and external events for the account of “purposeful designed” fundamental reactor and fuel cycle characteristics. Because of possible catastrophic consequences of an accident in NP, it is quite reasonable to respect the well known pragmatic principle – “an ounce of prevention is better than a pound of cure”. It means that the protection provided by the first and the last barriers of defence is the priority with these referring to internal or external initiating dangerous events, respectively.
Respecting intrinsically secure features, the potential of the proper “deterministic” defence against unprotected anticipated internal events is evidently the most preferable: MP + MU ≈ 0 + 0. However, its practical realization is an arduous task. It does not exclude application of other safety means.
Focusing on intrinsically secure safety potential enhancement allows shifting accents in the current reactor safety “philosophy” to reveal new worth. This simplifies and clarifies overall reactor defence, and it simultaneously may reduce the cost of a nuclear plant.
As mentioned, providing a guaranteed overall deterministic-like protection by the first barrier of defence is an extremely difficult (if even possible) task. Generally, it is rarely achievable and even principally unachievable for current reactor types (like traditional Light Water Reactors, LWR, the SuperPhenix Fast Reactor, SPX or the Russian fast reactors, BN) because it requires a radical improvement of “self-protection” against all anticipated unprotected accidents.
Among intrinsically secure safety properties, the following core parameters are significantly important: reactivity margin values, reactivity feed-back effects and coolant void reactivity effects. All of these ones can be potentially the reasons of the menacing reactivity insertion. Then intrinsically safety can be provided if the following properties to be met: “zeroed” reactivity margins at the equilibrium closed fuel cycle (Orlov et al., 1988).
Intrinsically secure safety features are more readily provided in fast reactors for many reasons, and, firstly, due to an achievable proper stabilization of reactivity at equilibrium corresponded to non-enriched feed (natural U) where there is no more evolution of the isotopic compositions.
In this case, there is almost no source of reactivity and such cores are titled hereafter as the cores with Zeroed Stable Reactivity (ZSR). It roughly corresponds to the case of “zeroed” in-core breeding gain (IBG). Speaking more precisely, it would be more convenient if the neutron net production at equilibrium slightly exceeds zero to cover the increasing neutron consumption of accumulating fission product.
This paper presents a fresh view on “designing an innovative safety strategy” based on innovative reactor concepts targeted towards appropriate enhanced intrinsically secure safety features. A considerable part of these features can be fully met if one uses the capability of fast reactor neutronics to produce the maximum number of neutrons per fission.
Innovative concepts for fast reactors based on the traditional solid fuel technology, aiming to meet these properties, are entitled below as ISFR – Intrinsically Secure Fast Reactors.
Section snippets
Innovative safety strategy related options: safety ranking
It is quite natural that the current “philosophy” of reactor safety and of safety control was “slaved” by the confined capability of the first NPP generations to withstand severe accidents. Nuclear authorities have been obliged to accept the compromising criterion: severe accidents are admitted if their probabilities/consequences are “acceptably” limited. Such reactors/reactor cores cannot be considered as deterministically secure and, hence, the corresponding technical means for mitigation of
Criticality and fuel cycle equilibrium
One of the necessary conditions for realizing intrinsically secure resistance against weapons material proliferation (Slessarev, 2006) is the use of a fuel cycle which is based on natural U or U-238 fuel feeds. Hence, such reactors have to be capable of supporting criticality at equilibrium without feed fuel enrichment. Generally, it is not easy to comply with this constraint: a sufficiently high fuel breeding is required. As already mentioned, for minimization of reactivity margins, the
On fast reactor intrinsically secure safety potentials
Several studies of fast reactor secure safety potential show (Alekseev et al., 1991, Novikov et al., 1992) that such potential is quite sufficient and could be implemented in the frame of the current technology.
Among decisive parameters respecting the “ISR → 1 strategy”, the most important are:
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Reactivity burn-up swing (particularly dangerous in the case of an instant addition of all margins of positive reactivity, such as control rod withdrawal) for all reactor types.
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Coolant void effects:
for
Void effect reduction in fast reactors with stabilized reactivity cores
Ways to reduce coolant void reactivity effects can be based on the following considerations.
If there is coolant voiding, the neutron balance BAL in cores (expressed as a perturbation of the neutron balance) gets “broken”. It may lead to the following reactivity addition:where the neutron “source/sink” is normalized to fission numbers.
As a result of coolant voiding, there are reactivity changes due to perturbations of the neutron balance components:
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neutron leakage Y,
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net neutron
Conclusions
The “new safety accents” assesses clearly the real “worth” of those innovative fast reactor (ISFR) concepts which are possessing enhanced intrinsically secure safety features focusing on the priority of the first barrier in reactor defence. The background of this “philosophy” of reactor safety has been proposed earlier (Orlov et al., 1988, Slessarev et al., 2004, Novikov et al., 1992), however, it is not yet sufficiently acknowledged.
Intrinsic protection of the first barrier makes such defence
Acknowledgments
I express my sincere gratitude to Dr. John Rowlands for his help and fruitful discussions which allowed improving the presentation of this paper.
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