In the EU, currently used pressurised thermal shock (PTS) analyses are reaching their limits in demonstrating the safety for power plants facing long-term operation (LTO). A PTS assessment is a very complex process involving multiple disciplines, and the prediction of uncertainties propagation and quantification is a very challenging task previously solved through a simplified conservative approach.

APAL set out to develop advanced deterministic as well as probabilistic PTS assessment methods and to quantify safety margins for LTO improvements. The consortium addresses multi-disciplinary and multi-physics challenges related to RPV safety assessment and PTS mitigation. All recommendations and conclusions gathered in the course of the project are used to define best practices covering all aspects of advanced PTS analyses for LTO. The probabilistic approach to be applied in APAL offers the possibility to handle the uncertainties without acting overly conservative, as well as the capacity to supply quantitative data for the risk-based lifetime management approaches, which are also used by a wide range of other industrial fields.

In APAL, PTS analysis includes thermal hydraulic, structural and fracture mechanics assessments. For the identification of LTO improvements relevant for PTS analysis, an extensive literature review in combination with a collection of experience results to be used for validation of advanced methods is needed. APAL starts with a literature review and a collection of the partners’ experience to define the state-of-the-art for LTO improvements (hardware and software) relevant in PTS analysis. This includes LTO improvements intended for PTS mitigation. The definition of the state-of-the-art includes a collection of existing solutions, approaches and assessments for LTO improvements together with the identification of technology gaps and possible solutions. The LTO improvements might either have an impact on thermo-hydraulic (TH) analysis (e.g. heated emergency cooling water) or on methods and solutions used for structural and fracture mechanics assessment (e.g. weld residual stresses). The most influencing or relevant LTO improvements are further assessed as benchmark cases. In addition, APAL will identify the human interaction during a PTS event based on operational procedures, followed by the quantification of the human factor on boundary conditions for TH analysis. Besides the uncertainties connected with computer code models, initial boundary conditions, and parameters of NPP systems, the work also focuses on uncertainties connected with human factor – an area not very much elaborated and evaluated in PTS analysis so far. This includes:

• Uncertainties in the computer code models will be determined by comparison of calculation results with available experimental data
• Uncertainties in plant data will be determined by expert judgement or operational experience
• Uncertainties related to human factor will be based on definitions from pre-requisite work of state-of-the-art identification

The recommendations and conclusions from all work performed in the project are gathered to define best practices for an advanced PTS analysis for LTO.