The integrity of the reactor pressure vessel (RPV) is a necessary condition for safe operation of nuclear power plants (NPP) throughout the project life. It is well known that as a rule RPV service life assessment is based on brittle strength calculation for pressurized thermal shock (PTS) scenarios.

In the WWER countries the classical fracture mechanics calculations under PTS are commonly used with the thermal-hydraulics (TH) edge conditions (EC) consideration at inner RPV surface only. Outer surface is always assumed as without heat transfer at all (of course, except of scenarios with outer flooding).

The article presents the procedure for the formation of thermal EC for the RPV (i.e. lower nozzle forging, cylindrical part and elliptical bottom), reactor supports, as well as for the reactor thermal insulation. The procedure takes into account the heat transfer due to radiation in the GA-301 room and the presence of forced supply of “cold” air by the TL05 ventilation system.

The results of fracture mechanics calculations for the most severe PTS scenarios of the Unit 3 RPV of South-Ukrainian NPP are presented. Besides, both type of results are demonstrated: with and without consideration of the RPV outer surface heat transfer due to the air-cooling ventilation system.

A brief history of Ukrainian warm pre-stress (WPS) approach development is presented. Also, the comparative analysis of the other approaches (such as Russian, old VERLIFE and new VERLIFE, USA and French) is made. And finally, the impact of the different WPS techniques as well as classic tangent point approach on the RPV safety margin against fast fracture is analyzed.

It is shown that the RPV air cooling consideration:

-       leads to conservatism decreasing of RPV lifetime assessment in case of using tangent point, American or Russian WPS approach;

-       provides satisfaction of cladding integrity criterion according to VERLIFE methodology, which allows Utilities not to perform additional monitoring of the cladding in the RPV nozzle region;

-       can justify resource of the RPV nozzle region, without any compensatory actions (such as staff emergency actions during PTS scenario).

At the same time, taking into account the RPV air cooling alongside with the Ukrainian WPS approach, can lead to the formally prohibition of the WPS application for the most severe PTS scenarios of OTHER group, and consequently – to the safety margin decreasing in comparison to the classic calculations.

Also concluded that Ukrainian WPS approach needs to be refined more carefully or harmonized with Russian or American ones.


Impact of the Outer Surface Air Cooling and Wps Approaches on the Brittle Fracture Margin of Wwer Rpv

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2020/volume/83815

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