D’Auria, and two co-authors, reported 2019 that the authors dealt with the proposal of an additional safety barrier for the class of large (1000 MWe or more) Light Water Reactors (LWR) now in operation, in construction, or under design. Emphasis is given to the motivations or the needs for the barrier. Two main issues are discussed.
The following topics are discussed in the former part (section 2): (a) the weakness of the barrier constituted by the current design of nuclear fuel; (b) the continuously increasing complexity of the system, with main reference to the Instrumentation and Control (I&C); (c) the role that the Large Break Loss of Coolant Accident (LBLOCA) had for arriving at the current layout of the Reactor Coolant System (RCS). Furthermore avoiding the severe accidents in 1979, 1987 and 2011, is at the basis of the proposal.
In the latter part (sections 3 and 4), the elements of the proposed technological safety barrier are discussed: the As-Low-As-Reasonably-Achievable (ALARA) principle, the Best Estimate Plus Uncertainty (BEPU) approach, the Extended Safety Margin Detection (E-SMD) hardware, the Emergency Rescue Team (ERT) strategy (or a virtual entity for the reactor) and the Independent Assessment (IA) concept. The additional safety barrier, although not demonstrated in the paper, is expected to reduce for a factor in the range 10–1000 the probability of core melt and to have a cost in the order of 1% the cost of a nuclear reactor unit.
Reference is made to large Water-Cooled Nuclear Reactors now under operation, construction or design. New reactor concepts including SMR may benefit of the argumentations in the paper. The activity is triggered from noting the weaknesses of nuclear fuel primarily at high burn-up and the increasing complexity of NPP. Key related conclusion is the characterization of selected fuel failure mechanisms: this is also connected with the need of regulatory countermeasures. Similarly, selected I&C features are described to outline the NPP complexity.
The difficulty to finalize the analysis of complex transient scenarios (e.g., in case of LBLOCA, connected with the nuclear fuel weaknesses, the propagation of pressure waves, and the presence of debris in the coolant in case of sump recirculation), might suggest dropping LBLOCA from the mandatory list of events to be analyzed within the licensing process. Therefore, design related motivations to keep LBLOCA as a key accident in safety analyses have been re-stated: this might imply a relaxation in the application of licensing criteria and the (obvious) full consideration of the role of containment.
The key result from the activity is the proposal and the characterization of a ‘new technological’ safety barrier which seems unavoidable in view of the identified issues. The new barrier has the capability:
To deal with the nuclear fuel weakness and the NPP complexity.
To reduce the core melt probability down to the value corresponding to the fall of a large meteorite around the concerned NPP site.
To possibly restore the public confidence towards nuclear technology.
The concept for the new barrier is based on the As-Low-As-Reasonably-Achievable (ALARA) principle, the Best Estimate Plus Uncertainty (BEPU) approach, the Extended Safety Margin Detection (E-SMD) hardware, the Independent Assessment (IA) requirement and the Emergency Rescue Team (ERT) strategy.
A rough cost has been performed ending with evaluation of an overall cost for the barrier given by 1% the cost of a unit reactor. The main difficulty for the implementation of the barrier appears to be associated with the rigorous satisfying of the IA requirement.
The human factors as key part of nuclear reactor safety and the worldwide global political and economic strategies are not considered within the present framework: those topics shall have a role as far as the implementation of the safety barrier and the public acceptance are concerned.
The industry and/or the Government of responsible Countries where applicable, become the main players for the possible implementation of the ideas in this paper. A strategy is needed in this connection: academia and research institutes willing to be engaged into practical applications of the nuclear technology should become actors.
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