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Nondestructive Evaluation (NDE) System for the Inspection of Operation-Induced Material Degradation in Nuclear Power Plants

Periodic Reporting for period 2 - NOMAD (Nondestructive Evaluation (NDE) System for the Inspection of Operation-Induced Material Degradation in Nuclear Power Plants)

Reporting period: 2018-12-01 to 2020-05-31

The long-term operation (LTO) of existing NPPs has already been accepted in many countries as a strategic objective to ensure adequate supply of electricity over the coming decades. To estimate the remaining useful lifetime of NPP components, LTO requires reliable tools [1]. In this context, the objective of the H2020 project NOMAD is the development, demonstration and validation of a non-destructive evaluation (NDE) tool for the local and volumetric characterisation of the embrittlement in reactor pressure vessel (RPV) materials. Consequently, the following steps will be taken:

• Development and demonstration of a software-based NDE tool for the characterisation of RPV embrittlement, accounting for material heterogeneities, complementing and exceeding the existing information from surveillance programs.
• Extension of the existing database of RPV material degradation by adding correlations of mechanical, microstructural and NDE parameters, including quantification of reliability and uncertainty.
• Application of the developed tool to cladded RPV material.

NOMAD takes into account the priorities of reactor operation, responding to stringent safety requirements from regulators, and seeks to foster convergence of nuclear safety approaches. The approach to be developed within NOMAD will deliver information complementary to and exceeding the information obtained by destructive tests of surveillance samples, which are currently assumed to represent the whole component and do not take into account possible local material variations. NOMAD aims to fulfil requirements for nuclear safety in the framework of assessment of lifetime operation.

[1] The Economics of Long-term Operation of Nuclear Power Plants, Nuclear Development 2012, Nuclear energy agency and organization for economic cooperation and development, ISBN 978-92-64-99205-4.
In order to reach the objective of the project a complete sample matrix of representative European RPV steels of multiple sample scales from full Charpy over non-cladded blocks to cladded blocks has been provided. Part of them has been irradiated in the frame of previous irradiation campaigns (CHIVAS) and another part has been manufactured and irradiated in the frame of this reporting period of the NOMAD project. Samples of Eastern and Western RPV design materials have been irradiated at different neutron irradiation levels. The Western RPV steel represented at multiple sample scales reaching from Charpy samples to large cladded blocks has been irradiated at three neutron irradiation levels with the goal to study neutron-induced embrittlement simulating an almost realistic RPV condition across a variety of sample geometries. In case of the Eastern RPV steel Charpy samples have been irradiated at three neutron irradiation levels.
To ensure that all defined samples serve the goal of a validated NDE tool, the suitability of the selected materials and sample geometries has been verified successfully.
In addition, basic mechanical tests, fracture toughness tests and Charpy tests have been carried out for all provided materials.
Moreover, multiple NDE technologies including micromagnetic, electrical and acoustic techniques have been successfully tested on all provided materials and sample geometries. To determine the capabilities of the individual NDE techniques and the performance of the NDE tool regarding the future application in the field a study has been carried out assessing the field conditions as well as their effects on the different NDE techniques. This includes important inspection technique parameters as well as possible difficulties regarding the conduction of inspections in an NPP and in similar environments. The information collected has been analysed and compiled, providing an overview of the most relevant field conditions to be addressed in the validation process. Based on this information and on the assessment of the samples available, the design of experiments was supported by the development of a validation plan.
According to the validation plan non-destructive tests on neutron-irradiated Charpy samples have been completed and evaluated in term of irradiation conditions and mechanical properties. Trends of different non-destructively determined parameters have been identified.
For the first time, Charpy samples of two representative European RPV steels of Western and Eastern design have been characterized non-destructively before and after neutron irradiation in order to capture the initial condition of the material before irradiation. The results of non-destructive measurements are currently being evaluated in terms of irradiation conditions and material properties.
All data measured non-destructively and destructively together with the corresponding standard deviations and data related to irradiation conditions are being collected and merged into the NOMAD Database. The development of the NOMAD Tool has started. The basic structure of the NOMAD Tool is an ordinary regression algorithm and uses only open-source libraries. The primary objective is to predict the ductile-to-brittle transition temperature caused by neutron irradiation. The first results obtained on Charpy samples show that it is possible to estimate neutron-induced embrittlement by using a combination of different non-destructive measurement methods.
Moreover, non-cladded and cladded blocks have been non-destructively investigated before irradiation. Additionally, cladding material has been investigated in order to check whether the non-destructively obtained signals originate from the cladding or from the material underneath the cladding. The non-destructive measurements on irradiated cladded and non-cladded blocks are under preparation.
Addressing possible synergies between related H2020-EURATOM funded initiatives the preparation of a joint symposium has been launched (https://www.nomad-horizon2020.eu/events/european-symposium-on-non-destructive-evaluation-for-npp).
For the first time, a systematic study is carried out on a well-characterized set of multiple sample scales that correlates the microstructure, mechanical properties, neutron irradiation conditions and non-destructive properties. A variety of NDE methods is applied to provide information complementary to those generated by destructive tests, which can be performed on current surveillance samples only. NOMAD will provide an optional procedure to quantify the neutron-induced degradation on surveillance samples and cladded RPV material additionally to the existing standardized methods and will contribute to the extension of the existing knowledge concerning degradation phenomena and progression of the material properties.
NOMAD will foster the development of NDE for the characterisation of ageing materials, going along with increasing innovation capacity and integration of new knowledge for the surveillance of a safe operation of RPV. After validation, a credible and reliable tool for the quantification of material degradation from results of non-destructive evaluation shall become available. The proposed concept will furnish supporting information or confirmation, when there is no monitoring of material properties by destructive testing during periodical safety reviews.
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