editorial integrated deterministic and probabilistic...

EditorialIntegrated Deterministic and Probabilistic Safety Analysis forSafety Assessment of Nuclear Power Plants

Francesco Di Maio,1 Enrico Zio,1,2 Curtis Smith,3 and Valentin Rychkov4

1Energy Department, Politecnico di Milano, Via La Masa 34, 20126 Milano, Italy2Chair on System Science and Energetic Challenge, Fondation EDF, Centrale Supelec, Chatenay-Malabry Cedex, 92295 Paris, France3Idaho National Laboratory, Idaho Falls, ID 83415, USA4EDF R&D, 92141 Clamart Cedex, France

Correspondence should be addressed to Francesco Di Maio; [email protected]

Received 28 June 2015; Accepted 6 July 2015

Copyright © 2015 Francesco Di Maio et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

The present special issue provides an overview of the researchin the field of Integrated Deterministic and ProbabilisticSafety Assessment (IDPSA) of Nuclear Power Plants (NPPs).

Traditionally, safety regulation for NPPs design andoperation has been based onDeterministic SafetyAssessment(DSA) methods to verify criteria that assure plant safetyin a number of postulated Design Basis Accident (DBA)scenarios. Referring to such criteria, it is also possible toidentify those plant Structures, Systems, and Components(SSCs) and activities that aremost important for safety withinthose postulated scenarios. Then, the design, operation, andmaintenance of these “safety-related” SSCs and activities arecontrolled through regulatory requirements and supportedby Probabilistic Safety Assessment (PSA).

On the other hand, compliance with the evolving reg-ulatory requirements and the drive for enhancements insafety and economics calls for the development of innovativedeterministic and probabilistic approaches of assessment forthe existing NPPs and their life extension. In this respect, amedium-term challenge is to combine the use of determinis-tic and probabilistic methodologies in an IDPSA framework,which includes the tools and methods that couple probabilis-tic and deterministic approaches.

In this special issue, this coupling is shown to(i) improve the computational approach of NPPs Ther-

mal-Hydraulic (TH) models, thanks to the employ-ment of metamodels for multiparameter and nonlin-ear modelling and for the throughout exploration ofthe space of combinatorial plant scenarios. Gaussian

Process- (GP-) based surrogate models are proposedfor incorporating new information from tests or oper-ating experience into TH codes, for reducing theuncertainty in the output of the code and for dealingwith the computational burden required for cali-brating parameters of the TH codes used for thedeterministic simulation of the NPPs response inaccident scenarios,

(ii) treat aleatory (stochastic aspects of accident scenar-ios) and epistemic (model andparameters) uncertain-ties in a consistent manner. A Risk Informed SafetyMargin Characterization (RISMC) approach is pro-posed for treating the uncertainties (e.g., stochasticfor the safety system recovery times and epistemic forthemodel parameters) for a joint quantification of thefrequency of core damage probability and of the safetymargin for a relevant NPP accident,

(iii) identify and characterize a priori unknown vulnera-ble scenarios. Two approaches are proposed: a charac-terization of a failure domain based on scenario clus-tering by decision trees, with the goal of highlightingthe influence of timing and order of events in thescenario grouping thereby obtained and a risk-basedclustering method for the identification of PrimeImplicants (PI) and of Near Misses (NM) scenarios,that is, minimum vulnerable combinations of failureevents that lead the system to fault or quasi-fault states(a condition close to accident), respectively,

Hindawi Publishing CorporationScience and Technology of Nuclear InstallationsVolume 2015, Article ID 136940, 2 pageshttp://dx.doi.org/10.1155/2015/136940

2 Science and Technology of Nuclear Installations

(iv) describe the time-dependent and spatial interactionsbetween physical phenomena, equipment failures,safety and nonsafety systems interactions, controllogic, and operator actions. An improved model todescribe the time-dependent interactions betweenfire and firefighting by simultaneously accountingfor stochastic and epistemic uncertainties by a FireDynamics Simulator (FDS) embedded into a MonteCarlo Dynamic Event Tree (MCDET) is proposed.Also, a combined modelling approach based on theTheory of Probabilistic Dynamics (TPD) for thedynamic reliability modelling and simulation of anaccident under uncertain events is proposed for theassessment of the probability of a NPP containmentfailure due to hydrogen combustion,

(v) reduce the reliance on expert judgment and simplify-ing (or overly conservative) assumptions about inter-dependencies. A deterministic analysis is integratedwith a probabilistic analysis for the estimation of aNPP core damage frequency, with as little as possibleinclusion of expert judgment into the assumptionadoption. Also, highlighting the effects of decoupling,at different level of interdependence, the analysis ofa NPP liquefaction from a ground vibration motionanalysis due to different expert judgment on theirmutual interaction is given,

(vi) reduce the overall costs of the analysis and reducethe maintenance costs of the integrated models com-pared to separate deterministic and probabilisticmodels. The benefit of an explicit treatment of time-dependencies and priorities of defects/failure occur-rences by integrated deterministic and probabilisticmodels is shown for an improved planning of themaintenance activities on NPP components.

As this journal aims to promote development in the areaof nuclear sciences and technologies, it has been natural topresent this special issue reporting on recent research in thefield of IDPSA in support to the sustainability and safety ofthe nuclear option and the renaissance of nuclear technologyin the world and especially in those countries where nuclearprograms have not yet been developed.

Acknowledgments

Finally, we wish to point out that this special issue wouldhave not been possible without the outstanding contributionsof all the authors and reviewers. Our sincere professionalappreciation and personal gratitude go to all these people.

Francesco Di MaioEnrico Zio

Curtis SmithValentin Rychkov

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