Preliminary results of quality inspections of pipeline components at Unit 3 of Mochovce NPP
In its press release of 4 May 2020, Nuclear Regulatory Authority of the SR (ÚJD SR) informed the public about the ongoing inspection of pipeline components at Unit 3 of Mochovce NPP. This statement is available to the public on the ÚJD SR website.
In this press release, ÚJD SR has informed the public about the following facts:
- the reason which led to the need to carry out inspections of pipeline components,
- the methods used during the inspections,
- the expected scope of inspections and the application of a graded approach to inspections, which shall be carried out according to: “… the importance of the components from the point of view of nuclear safety. For the most important components, every single piece will be inspected. In the case of less important components, a percentage random sample of each delivery shall be inspected to cover each delivery and batch, and in a case of negative findings, the scope of the inspection shall be extended.
- the current status of ongoing inspections,
- and on the inspections of the entire material testing process carried out by ÚJD SR and the approach of ÚJD SR to non-conforming piping components (if it will not be possible to sufficiently reliably prove the quality and properties of the materials used … such components will be replaced).
In another statement of 2 September 2020, ÚJD SR informed, among other things, about the course of the process of material inspections at Unit 3 of Mochovce NPP. This information is available to the public on the ÚJD SR website.
The material inspections and evaluation of the results of these inspections at Unit 3 of Mochovce NPP reached the final stage in mid-January 2021. The company Slovenské elektrárne in cooperation with its suppliers has prepared a preliminary Summary Report “Verification of the quality of specific deliveries of pipeline components used on classified equipment at Unit 3 of Mochovce NPP”. With the aim to keep openness and transparency in communication with public in this process, ÚJD SR published this report on 21 January 2021 (the report is available in Slovak version only).
For the final completion of inspections in accordance with the Inspection Methodology proposed by Slovenské elektrárne and its suppliers, and accepted by ÚJD SR, it remains to complete a relatively small scope of inspections (compared to the inspections performed up to now). It is necessary to complete tests as follows:
- parts of low-alloy carbon steel pipeline components, which are categorised in safety class SC III (these are mainly pipeline components installed into the essential service water system, which are during operation loaded by a relatively small internal overpressure),
- parts of stainless-steel pipeline components that are not categorised as the safety-relevant from the point of view of nuclear safety.
For such components, a sensitivity analysis of material exchange was performed, which takes into account the results of already performed tests. Based on the results of the sensitivity analysis, it is expected with high probability that no significant non-conformities will be found during these inspections.
Nevertheless, ÚJD SR insists on completion of these inspections in accordance with the set methodology. It is assumed that these inspections will be completed during February 2021. Their results will be included in the final Summary Report “Verification of the quality of specific deliveries of pipeline components used on classified equipment at Unit 3 of Mochovce NPP”, which will be also published on the ÚJD SR website.
The preliminary Summary Report is provided in the following structure:
- Reasons for inspecting of the pipeline components at Unit 3 of Mochovce NPP (Chapter 6).
- Methodology of the performed inspections, including the explanation the scope of inspections and the use of a graded approach to inspections (Chapter 7).
- The methods used to verify the quality of the materials at the various stages of verification (Chapter 8). It is possible to assume that this chapter may be mainly of interest of the professional public. For the sake of completeness, it should be added that the testing was time consuming process. The testing itself was preceded by the preparation of the tested equipment – dismantling of the insulation and possibly also the construction of scaffolding, surface preparation, sampling for the given type of test and after testing – re-installation of the insulation. From this point of view, the scope of work performed is extremely large.
- Summary of results of inspections / tests performed (Chapters 9 to 11).
- Evaluation of the results of the performed tests from the point of view of the applicability of the given pipeline components at Unit 3 of Mochovce NPP (Chapters 12 to 17).
- Final evaluation of results (Chapter 18).
Summary data on the performed tests of pipeline components at Unit 3 of Mochovce NPP are as follows:
- Total number of tested pipeline components: 2960
- Number of pipeline components for which insignificant deviations in their chemical composition were found from the recommended values defined in the technical conditions for the given steel trade mark: 288
Each such deviation was evaluated individually and it was confirmed that it does not affect the final properties of the pipeline component. The probable cause of these insignificant deviations could be the process of further processing of metallurgical semi-finished products, uneven distribution of alloying elements in the production process, different method of measurement, statistical error of used measurements or use of secondary raw materials in metallurgical production (e.g. slightly increased content of Co and Cu). The measured values of the parameters of these pipeline components became part of their accompanying technical documentation. These parts were left installed in the Unit 3 technology after proper evaluation.
- Number of pipeline components for which a material trade mark other than that declared in the documentation (in the certificates) has been identified: 52
The most common case was the substitution of steel type according to GOST 08Ch18N10T with steel according to EN X6CrNiTi18-10 with comparable characteristics and similar chemical composition. Analysis of the chemical composition and mechanical characteristics of these pipeline components were performed in accredited laboratories. The identified parameters were included in the accompanying technical documentation. After a detailed evaluation, these pipeline components were left installed in the technology of Unit 3 of Mochovce NPP as satisfactory. These pipeline components represent approximately 3.3% of the inspected pipeline components.
- Number of pipeline components that were replaced (*): 8
All replaced pipeline components were made of low-alloy carbon steel. Five of these pipeĺine components were replaced due to the finding of the substitution of the steel type with a type which is not certified for use on the Unit 3 of Mochovce NPP (material of the type 21MnCr5 or 33NiCrMo14-5). In three cases, the use of an inappropriate material type found (12 050 instead of P265GH). All 8 mentioned pipeline components were replaced by pipeline components made of certified materials. Replaced pipeline components represent approximately 0.27% of the tested pipeline components. (*): the number of replaced pipeline components does not include those that had to be replaced due to the use of destructive testing methods.
ÚJD SR inspectors monitored the testing of the quality of pipeline components at Unit 3 of Mochovce NPP on a daily basis. They inspected compliance with the testing procedures and were regularly informed on its results. In some cases, they performed their own reference measurements using ÚJD SR technical support and they adjusted the scope of testing in order to achieve its maximum conservative results.
The evaluation of the results of the quality tests of the material of the pipeline components of Unit 3 of Mochovce NPP was provided by reputable experts in the field of materials of nuclear installations:
- for the Slovenské elektrárne: experts from the Institute of Materials and Machine Mechanics of the Slovak Academy of Sciences, from the Faculty of Materials Technology of the Slovak University of Technology, internal specialists of Slovenské elektrárne, material specialists and suppliers of Slovenské elektrárne, suppliers of Železiarne Podbrezová and Authorized Legal Persons (technical inspection),
- for the ÚJD SR: experts of the University of Žilina and internal specialists of the ÚJD SR.
The final Summary Report “Verification of the quality of specific deliveries of pipeline components used on classified equipment at Unit 3 of Mochovce NPP” will be made available to the public after the full completion of all inspections at Unit 3 of Mochovce NPP. The necessary data on the results of inspections will be provided to the operators of VVER nuclear installation for the application of lesson learned in their daily practice.
The recent results point out to the obvious fact that measures will need to be taken and they will focus to:
- to improve inspections in the process of manufacturing and processing of metal products before their delivery to the customer; with the aim to assure that the issued documents proving quality and properties (certificates) of the components are reliable, and that the properties of the pipeline components are identical to those declared,
- for the introduction of additional inspections of pipeline components before their receipt at the customer and also before their installation in technological units.
Any questions regarding the quality inspections of pipeline components at Unit 3 of Mochovce NPP and the preliminary Summary Report “Verification of the quality of specific deliveries of pipeline components used on classified equipment at Unit 3 of Mochovce NPP” can be sent via e-mail to firstname.lastname@example.org or via traditional mail to ÚJD SR, Bajkalská 27, 82007 Bratislava, Slovakia.
The preliminary Summary Report “Verification of the quality of specific deliveries of pipeline components used on classified equipment at Unit 3 of Mochovce NPP” (available in Slovak version only).