In the most countries that operate the nuclear power plants with reactor pressure vessels a safety margin accounting a data scatter is applied for a conservative evaluation of a radiation shift of the ductile to brittle transition temperature for RPV metal. This scatter is to a significant extent due to material inhomogeneity and errors in determining the temperature shift and neutron fluence. In the regulatory practice of Ukraine, the obsolete approaches are used that can lead to an underestimation or overestimation of the transition temperature shift depending on the number of test data points.
In order to use the updated regulatory approaches that will be consistent with international practice, it is necessary to know the magnitude of the data scatter on the transition temperature shift which is characterized by a standard deviation. Therefore, the aim of the research work was to estimate the data scatter for WWER reactor pressure vessel materials using statistical methods.
The paper presents the results of a statistical analysis for a large array of surveillance test data for WWER-1000 reactor pressure vessels of NPP units which are operated in Ukraine. The data scatter for RPV base and weld metal has been estimated using a statistical treatment for the dependencies of a transition temperature shift, ΔTF, on the fast (Е > 0,5 MeV) neutron fluence. The ΔTF values have been derived from the Charpy impact tests. The Charpy V-notch specimens have been irradiated in the nuclear power reactors within a neutron fluence range of (3,0 ÷ 92,2)·1022 m-2 in the frame of a national surveillance program.
The analysis has shown the data scatter relative to the average regression line for RPV materials is characterized by a standard deviation of 5,5 °С. Based on the results obtained, it was suggested to use a double standard deviation of 11 °С as a safety margin to provide a conservative estimate for the radiation shift of the transition temperature of the WWER-1000 reactor pressure vessel materials.
2. Standards for Strength Calculations of Components and Piping of Nuclear Power Plants [Normyi rascheta na prochnost oborudovaniya i truboprovodov atomnyih energeticheskih ustanovok]: PNAE G-7–002-86. Energoatomizdat, Moscow, 1989. 525 p.
3. Standard Program of the Inspection for WWER-1000 Reactor Pressure Vessel Metal Properties by Surveillance Specimens [Tipovaya programma kontrolya svoystv metalla korpusov reaktorov VVER-1000 po obraztsam-svidetelyam]: PM-T.0.03.120-04. — NNEGC Energoatom, Kyiv, 2008. 36 p.
4. Revka, V.M., Chyrko, L.I. (2016), Regulatory aspects of material science support for the WWER-1000 RPV safe operation [Normatyvni aspekty materialoznavchoho suprovodu bezpechnoi ekspluatatsii korpusiv reaktoriv VVER-1000] // XXIII annual science conference, Institute for Nuclear Research NAS Ukraine, 01–05 February 2016, Kyiv, Ukraine / Abstracts, P. 105
5. Regulatory Guide 1.99, Revision 2. Radiation Embrittlement of Reactor Vessel Materials / U.S. Nuclear Regulatory Commission, 1988. 9 p.
6. 10 CFR 50.61 Fracture toughness requirements for protection against pressurized thermal shock events / NRC 10, Code of Federal Regulations, Part 50 — Domestic Licensing of Production and Utilization Facilities, U.S. Nuclear Regulatory Commission, 2007, P. 699—704
7. Guidelines on pressurized thermal shock analysis for WWER nuclear power plant [Rukovodstvo po analizu termicheskogo udara dlya AES s reaktorami tipa VVER]: IAEA-EBP-WWER-08. Vienna: IAEA, 2005. 73 p.
8. Approbation of the normative document SOU NAEK 087:2015 “Method for the determination of radiation embrittlement of the RPV metal by the surveillance test data” [Aprobatsiya normativnogo dokumenta SOU NAEK 087:2015 “Metodika opredeleniya radiatsionnogo ohrupchivaniya metala korpusov reaktorov po rezultatam ispyitaniy obraztsov-svideteley”]: technical report, № 300/26—257 / Institute for Nuclear Research NAS Ukraine, Kyiv, 2018. 35 p.
9. Zaidel, A.N. (1985), Errors in the measurements of physical quantities [Pogreshnosti izmereniy fizicheskih velichin], Nauka, Leningrad. 112 p.