Luciano Burgazzi

Thermal–hydraulic passive system reliability-based design approach

Abstract In order to enhance the safety of new advanced reactors, optimization of the design of the implemented passive systems is required. Therefore, a reliability-based approach to the design of a thermal–hydraulic passive system is being considered, and a limit state function (LSF)-based approach elicited from mechanical reliability is developed. The concept of functional failure, i.e., the possibility that the load will exceed the capacity in a reliability physics framework, in terms of performance parameter is introduced here for the reliability evaluation of a natural circulation passive system, designed for decay heat removal of innovative light water reactors. Water flow rate circulating through the system is selected as passive system performance characteristic parameter and the related limit state or performance function is defined. The probability of failure of the system is assessed in terms of safety margin, corresponding to the LSF. Results help the designer to determine the allowable limits or set the safety margin for the system operation parameters, to meet the safety and reliability requirements.

Failure mode and effect analysis on ITER heat transfer systems

The complexity of the ITER (International Thermonuclear Experimental Reactor) plant and the inventories of radioactive materials involved in its operation require a systematic approach to perform detailed safety analyses during the various stages of the project in order to demonstrate compliance with the safety requirements. The failure mode and effect analysis (FMEA) methodology has been chosen to perform the safety analysis at system level for ITER. The main purposes of the work are: to identify important accident initiators, to find out the possible consequences for the plant deriving from component failures, identify individual possible causes, identify mitigating features and systems, classify accident initiators in postulated initiating events (PIEs), define the deterministic analyses which allow the possible accident sequences to be quantified, both in terms of expected frequency and radiological consequences, and consequently, to ascertain the fulfillment of ITER safety requirements. This paper summarises the FMEA performed for the heat transfer systems (HTSs).

State of the art in reliability of thermal-hydraulic passive systems

In order to address the issues posed by the development of advanced nuclear technologies, this article endeavours to analyse the current state of the art in reliability of passive systems, for their extensive use in future nuclear power plants. Inclusion of failure modes and reliability estimates of passive components for all systems is recommended in probabilistic safety assessment (PSA) studies. This has aroused the need for the development and demonstration of consistent methodologies and approaches for their reliability evaluation, within the community of the nuclear safety research. This report provides the insights resulting from the survey on the technical issues associated with assessing the reliability of passive systems in the context of nuclear safety, regulatory practices and probabilistic safety analysis. Special emphasis is placed on the reliability of the systems based on thermal-hydraulics, for which methods are still in a developing phase. The main achievements of these studies are presented and a viable path towards the implementation of the research efforts is delineated as well.

About time-variant reliability analysis with reference to passive systems assessment

Abstract This study addresses the time-variant reliability assessment in relation to systems presenting a non-stationary random process during their operation, such as thermal-hydraulic passive systems for advanced reactors, resting on natural circulation. The reliability assessment efforts conducted so far do not address this specific aspect: the dependence upon time is usually ignored, or at most the system unavailability is intended to be assessed per mission time, during which the parameter values, as t – h parameters for instance, are assumed as constant quantities. The study endeavours to present a consistent approach to model the natural circulation passive systems, in terms of time-variant performance parameters, as for instance mass flow-rate and thermal power, to cite any.

Reliability studies of a high-power proton accelerator for accelerator-driven system applications for nuclear waste transmutation

The main effort of the present study is to analyze the availability and reliability of a high-performance linac (linear accelerator) conceived for Accelerator-Driven Systems (ADS) purpose and to suggest recommendations, in order both to meet the high operability goals and to satisfy the safety requirements dictated by the reactor system. Reliability Block Diagrams (RBD) approach has been considered for system modelling, according to the present level of definition of the design: component failure modes are assessed in terms of Mean Time Between Failure (MTBF) and Mean Time To Repair (MTTR), reliability and availability figures are derived, applying the current reliability algorithms. The lack of a well-established component database has been pointed out as the main issue related to the accelerator reliability assessment. The results, affected by the conservative character of the study, show a high margin for the improvement in terms of accelerator reliability and availability figures prediction. The paper outlines the viable path towards the accelerator reliability and availability enhancement process and delineates the most proper strategies. The improvement in the reliability characteristics along this path is shown as well.

Reliability Methods for Passive Safety Functions

This paper presents the study performed within the framework of a European Project called Reliability Methods for Passive Safety Functions (RMPS). Its objective is to propose a specific methodology to assess the passive system thermal-hydraulic (T-H) reliability. The methodology is tested on an example of industrial passive system: the Isolation Condenser System (ICS). The T-H calculations are performed using the RELAP5, ATHLET and CATHARE computer codes. In this paper, the present state of the methodology and its application to the example are described. Owing to the recent start of the project, only results concerning the first objective of the project (identification and quantification of the sources of uncertainties and determination of the important variables) are presented. A sensitivity analysis was carried out on 69 computation results performed with the RELAP5 code. This first analysis highlights the significant parameters influencing the performances of the ICS and shows a non-monotonous behavior of the system.Copyright

Latest liquid lithium target design during the key element technology phase in the international fusion materials irradiation facility (IFMIF)

Abstract International Fusion Materials Irradiation Facility (IFMIF), being jointly developed by EU, JA, RF and US, is a deuteron–lithium (Li) stripping reaction neutron source for fusion materials testing. In 2002, a 3 year Key Element technology Phase (KEP) to reduce the key technology risk factors was completed. A liquid Li target has been designed to produce intense high energy neutrons (2 MW/m 2 ) up to 50 dpa/year by 10 MW of deuterium beam deposition which corresponds to an ultra high heat load of 1 GW/m 2 . This paper describes the latest design of the liquid Li target system reflecting the KEP results and future prospects.

Activities on IFMIF lithium target at ENEA

Abstract The status of R&D activity ongoing at ENEA on the lithium target system of the international fusion materials irradiation facility (IFMIF) is reported. The activity has been launched in year 2000 in the frame of IFMIF key element technology phase in order to reduce the key technology risk factors and to guarantee the required availability and reliability of the IFMIF liquid Li target system. The items discussed in the paper are related to the Li jet flow stability numerical analysis, water flow simulation experiment, backplate remote handling simulation and Li target safety analysis.

Status of activities on the lithium target in the key element technology phase in IFMIF

Abstract This paper describes the status of the liquid lithium (Li) target facility in the key element technology phase (KEP) of the International Fusion Materials Irradiation Facility (IFMIF). The IFMIF is being jointly developed by the European Union (EU), Japan (JA), Russian Federation (RF) and US to provide an accelerator-based D–Li neutron source for testing the candidate materials for fusion reactors. A key issue of the Li target is to obtain a stable liquid Li flow with a speed of 20 m/s under a deuterium beam deposition of 10 MW. In the KEP, 19 tasks for the Li target are proposed and shared by EU, JA and RF. These tasks are a Li simulation experiment by water jet, Li flow experiment, corrosion/erosion, remote handling of the target assembly, and safety analysis. In addition to the KEP tasks, detailed design of the target is being performed.

Passive system reliability modeling of the combination of failure modes

Innovative probabilistic models to extend the reliability analysis of passive systems under different modes of failure are proposed.The prevailing failure mode on the system can be predicted through the failure probability assessment on each specific mode. A realistic case is presented to analyze a passive system with two kinds of major failure modes — natural circulation stoppage due to e.g., isolation valve closure (a catastrophic failure) and heat transfer process degradation due to e.g., deposit thickness on component surfaces (a degradation failure). Modeling of each individual failure mode together with system reliability analysis is presented and results are discussed.Copyright