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Technical Cooperation Projects Activities

Regional Workshop on Improved Reactor Pressure Vessel Structural Integrity Assessment, 8–12 September 2008, Kuznetsovsk, Ukraine

The RPVs are the highest priority key components in nuclear power plants (NPPs). The RPV houses the reactor core and because of its function it has direct safety significance. Unexpected age related degradation of the mechanical properties of the RPV steel can lead to safety concerns related to the mechanisms involved in ageing, which include irradiation embrittlement, thermal ageing, temper embrittlement, fatigue, and corrosion.

Structural integrity of RPVs should be assured for all normal operating, upset, faulted, and accident conditions, as well as non-design transients such as pressurized thermal shock (PTS) through the whole operating life. Neutron irradiation degrades the mechanical properties of RPV steels, and the extent of the degradation is determined by the type and structure of the steel and other factors such as neutron fluence, irradiation temperature, neutron flux, and chemical composition. The most sensitive location in the RPV is the region adjacent to the reactor core (termed the beltline region). Welds and their heat-affected-zones (HAZ) in this region are particularly important since these regions have a higher probability for flaws.

The workshop was jointly organised by the Agency and EC-JRC, Institute of Energy. The Agency invited three experts from Republic of Korea, Russian Federation and Germany. EC-JRC sponsored invitation of two experts - from Czech Republic and Hungary.

A total of 20 participants from 9 countries in Eastern Europe region – Armenia, Bulgaria, Czech Republic, Hungary, Lithuania, Russian Federation, Romania, Slovakia and Ukraine - participated in the workshop. The workshop consisted of four technical sessions besides the opening and closing sessions:

• Session 1: Elements of Material Degradation
• Session 2: Experience, Mechanisms and Mitigation
• Session 3: Regulatory Aspects, Industry, Case Studies
• Session 4: Monitoring RPV, Flow accelerated corrosion, Application of Leak-Before-Break concept

In the workshop closing session, it has been stressed that it should be considered:

• The scope and intervals of RPV non-destructive examination – there are differences between approaches applied in VVER and PWR nuclear power plants worldwide,
• Harmonisation of various approaches in order to assess the RPV integrity and compare the situation in various countries (transition temperature calculations, use of reference bound curves, size and position of postulated defect, PTS calculations).

References:

• IAEA-TRS-429, Guidelines for Application of the Master Curve Approach to Reactor Pressure Vessel Integrity, Vienna (2005)
• IAEA-TECDOC-1435, Application of Surveillance Programme Results to Reactor Pressure Vessel Integrity Assessment, Vienna (2005)
• IAEA-EBP-WWER-8, Guidelines on Pressurized Thermal Shock Analysis for WWER Nuclear Power Plants (Rev. 1), Vienna (2006)