Advanced Assessment Methods for Nuclear Power Plant Safety

Latest insights into possible impacts on the results of pressurised thermal shock analyses

The APAL consortium has just released a summary of one of the major project outputs: the definition of the state-of-the-art of long-term operation (LTO) improvements that may have an impact on the results of pressurised thermal shock (PTS) analyses. The reviews include a collection of existing solutions/approaches for assessment of LTO improvements, a collection of existing assessments and the identification of gaps and possible improvements. This work was led by Carlos Cueto-Felgueroso from TECNATOM.

The process was carried out through the preparation of technical questionnaires covering relevant issues. The APAL partners had to fill in the questionnaires based on their experience and approaches in the analyses carried out in their countries.

In addition, some partners also contributed both information and knowledge to a better understanding of specific assessments or LTO improvements.

Four topics were defined a-priori to be reviewed/investigated in more detail:

  1. Residual stress distributions for welds (WRS) and cladding: Values and distributions of residual stresses (RS) for PTS analyses are generally taken from standards and technical literature. These are derived from measurements on specimens or from a combination of measurements and detailed Finite Element Model (FEM) calculations. A topic of further investigation regarding PTS analyses for LTO includes the possible RS relaxations during operation. For this purpose, RS measurements, ideally on materials from decommissioned reactor pressure vessels (RPVs), require further research.
  2. Warm pre-stress (WPS) approach applied in PTS: Most partners take WPS into account in their PTS assessments. Different standards or methodologies are used in participating countries. Some open issues were identified, such as possible non-conservative estimation of the WPS effect when the most severe load is estimated in the thermal-hydraulic (TH) analysis, and the possibility that important information from the transient is lost (e.g., existence of load disturbances) when an envelope of the results obtained from the TH analyses is used.
  3. Thermal-hydraulic (TH) analyses (including definition of human factor): Literature reviews and partners’ experience highlight the need to move towards best estimate plus uncertainty (BEPU) methods instead of applying conservative assumptions. Regarding human interactions, most partners do include them in their PTS analyses, however, these are generally at fixed times. The impacts of failure of the operator to successfully complete an action or the time required to complete the action are relatively unexplored in Europe. Therefore, further research is needed on this topic.
  4. Probabilistic PTS analyses: In the EU the use of probabilistic PTS analysis in the scope of structural integrity assessment has become of interest in the last two decades. The currently used tools and software for probabilistic PTS analysis by the European and International partners is described. Recommendations and conclusions were drawn as well as possible improvements identified for use of probabilistic PTS analysis.

In addition, further potential LTO improvements relevant for PTS analyses in participating countries are investigated. This covers the improvements relating to the PTS transients, RPV materials, plant procedures and software, as well as methods of PTS analyses.

Full report - Deliverable No. 1.6 “Public summary report of WP1” - on the above-mentioned results