The APAL (Advanced Pressurized Thermal Shock (PTS) Analysis for Long-Term Operation (LTO)) project was launched in October 2020 for four years with funding from the European Union’s HORIZON 2020 program.

Within APAL, an extensive literature review was performed and experience with defining the state-of-the-art of the Warm Pre-Stress (WPS) effect, which has an impact on the reactor pressure vessel (RPV) brittle fracture margin in both deterministic and probabilistic terms, was collected. To gather the worldwide experience of the WPS approaches and models, a comprehensive questionnaire was developed followed by each APAL partner response. It mainly focused on the following aspects: collection of existing WPS approaches and models implemented in standards and rules for RPV brittle fracture assessment; identification of the WPS issues; collection and analysis of the existing experimental data.

The work presents a short description of the existing WPS national approaches and models along with standards or rules for RPV brittle fracture assessment under PTS events. Attention is focused on the applicability of WPS and its restrictions for constraint effect and crack arrest.

The second part of this work is devoted to the identification of the open issues connected to the practical WPS application in RPV integrity assessment together with the APAL partners’ views on this topic. The following problems are discussed:

- Overall view regarding including WPS in the RPV PTS assessment (applicability, benefit in practical applications);

- thermal-hydraulic aspects of a WPS approach (WPS benefits depending on the transient history);

- WPS and probabilistic RPV brittle fracture assessment (applicability);

- WPS and ductile fracture (applicability);

- WPS and residual stress (RS), (treatment of RS in regard to WPS effect).

The last part is dedicated to the analysis of experimental data for defining the WPS benefit. For this purpose, the Czech and Ukrainian experimental data (WWER materials), and the SMILE’s project data (heat-treated 17MoV8-4mod steel and 18MND5 steel) were analysed. The WPS effect on fracture toughness (FT) was evaluated using the ratio of enhanced FT KFrac (at re-load, after WPS) to FT of virgin material KIC (without pre-load), as obtained from the experimental data. It was shown, that relatively to the KIC,50% and KIC,95%, the WPS effect led to an increase of FT in 97.6% and 80.9% of cases respectively, which quantifies and confirms the benefit of WPS.

This work describes worldwide experience and best practice of the WPS and its application for the RPV integrity assessment. The paper’s conclusions are also focused on the recommendations for dealing with WPS issues.

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