Fabio Pasti’s research while affiliated with NRG, Nuclear Research & consultancy Group and other places

This paper presents a leakage prediction study for Leak Before Break (LBB) assessments. The LBB concept has emerged as an essential tool for ensuring the safety of nuclear installations and pressurized piping systems. The approach is backed by accurate estimation of leakage rates from postulated cracks. In this study the methodology is based on the UK procedure for assessing structures containing defects (R6). The research is part of the ongoing in-house development of a software for LBB analyses and it aims to better understand and improve leak rate prediction methodologies for LBB assessments. The software is comprised of structural integrity modules and thermal hydraulic modules. This study presents the implementations of different coolant leak rates flow models, the Henry-Fauske model for flashing subcooled fluids and as single-phase Bernoulli model for lower temperatures. The thermal-hydraulics Henry Fauske model is evaluated against available experimental data and the differences are discussed. The Bernoulli based model is compared with another software. The implemented procedure is compared with two different software results for the structural modules. The structural calculations show coherence between the implemented procedure and the benchmark software that is based on the same methodology. In contrast the results are considerably different respect to another available software. The data show the significance and limitations of each model in various cases.

The Double Ended Guillotine Break (DEGB) is a pipe failure mode in nuclear power plant installations which is the postulated root cause of a Loss Of Coolant Accident (LOCA). Pipe whip restraints and jet impingement shield are safety systems that mitigate the effects of this postulated accident. The Leak Before Break (LBB) failure mode occurs when a small leak from a cracked pipe is detected prior to the DBEG failure. The application of this failure mode and the detection of cracked pipes would increase defense in depth and justify the removal from the plant of the pipe whip restraints and the jet impingement shield, decreasing costs and maintenance time. In this study a probabilistic assessment is performed with an in-house developed LBB software. The assessment models the complexity of the physical model and the diversity of the systems with stochastic input variables for the material properties and crack parameters. The developed deterministic code is based on the UK procedure for assessing the integrity of structures containing defects (R6). The Detectable Leak Before Break (DLBB) procedure is used, which is based on the Failure Assessment Diagram (FAD) Option 1 assessment procedure. The structural integrity assessment is then coupled with the Henry-Fauske two-phase critical flow model for the evaluation of the leakage rate. A sensitivity analysis is performed to reduce the number of probabilistic variables. The output from the assessment is the probability of defect detection prior to structural failure. The Second Order Reliability Method (SORM) is the probabilistic method used. The probabilistic results are then compared with the safety factors currently used for deterministic LBB assessments.

Pressurized thermal shock (PTS) may cause a quick, catastrophic cleavage fracture in a reactor pressure vessel (RPV) of a pressurized water reactor (PWR). Low temperatures, thermal strains, and radiation embrittlement can all combine to create dangerous situations for structures, specifically thick-walled reactor pressure containers with fractures and welds as weak areas. A thorough picture of the temperature and stress intensity is required to determine the likelihood of the onset and spread of a cleavage crack. The ductile to brittle transition temperature affects the critical stress intensity for brittle cleavage fracture. This complicated combination of loads, absolute temperatures, and temperature gradients is combined with radiation damage to evaluate the likelihood that cleavage fracture will occur. In earlier works, simulations were carried out using combined computational fluid dynamics (CFD) and finite element method (FEM) simulations to get the most realistic picture of this issue. However, due to the complexity of the problem, the thermal mixing of the fluid and its effects on the RPV wall are simulated by models that are simplified in terms of geometric complexity and physics. This study investigates the effect of the interaction between multiple emergency core cooling (ECC) plumes on the thermal response of the RPV wall by considering a full (360 degree) RPV geometry with two loops for the ECC fluid injection. We first perform a transient conjugate heat transfer CFD simulation to compute the spatial and temporal evolutions of RPV wall temperature. The unsteady Reynolds-averaged Navier-Stokes equations are solved on the fluid side, and the unsteady heat transfer equation is solved on the solid side. Next, a static structural analysis using FEM is conducted using the temperature profile obtained from CFD analysis on a one-loop reactor as input. The goal of the FEM analysis is to investigate the link between the depth, length, and ratio of the crack and the probability of failure. A probabilistic approach is used to evaluate the possibility of failure. The ultimate goal of these studies is the generation of a code that can implement hydraulic models that replace the time and resource-demanding CFD and FEM analysis.