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

Periodic Reporting for period 1 - NOMAD (Nondestructive Evaluation (NDE) System for the Inspection of Operation-Induced Material Degradation in Nuclear Power Plants)

Reporting period: 2017-06-01 to 2018-11-30

In almost all major industrial countries worldwide, today, nuclear power plants (NPPs) are used to generate electricity. The long-term operation (LTO) of existing nuclear power plants (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 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 re-quirements 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 has been set up of multiple sample scales from half Charpy over full Charpy samples to non-cladded and cladded blocks. Samples of Eastern and Western RPV design, base and weld material have been provided in non-irradiated condition and at different neutron irradiation levels (neutron fluence and temperature). One Western RPV material is represented at multiple samples scales with the goal to study neutron-induced embrittlement across a variety of sample geometries reaching from Charpy samples to large cladded blocks simulating an almost realistic RPV condition. Some of the samples are available from previous irradiation programs, some will be irradiated within NOMAD. To this, the preparation of the irradiation of Charpy samples, cladded and non-cladded blocks is ongoing. 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 properties (yield and ultimate tensile strengths) and fracture toughness tests (master curve approach) have been determined for several provided materials. Metallographic analysis on selected samples have confirmed the observations resulting from the mechanical tests.
Moreover, multiple NDE technologies including micromagnetic, electrical and acoustic techniques have been developed and successfully tested on all provided materials and sample geometries. Setting optimization and round-robin tests have been carried out on several sets of non-irradiated Charpy samples and large non-cladded and cladded blocks made from representative Western and Eastern RPV steels. Furthermore, non-destructive tests on neutron-irradiated Charpy samples of four Western RPV steels have been completed and are currently being evaluated. For the first time, samples, which will be irradiated, have already been characterized non-destructively in as-received condition in order to capture the initial condition of the material before irradiation.
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 initiated 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 a NPP and in similar environment conditions. 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.
An internal project management platform has been set up and is in use since the beginning of the project, June 2017. The platform is constantly updated and, if necessary, revised in order to reflect the progress in the project. The NOMAD website (www.nomad-horizon2020.eu) as a major tool of project dissemination, has been generated to reach the broad public raising awareness of the NOMAD project activities, objectives and results. Addressing possible synergies between related H2020-EURATOM funded initiatives a plan has been compiled for the collaboration between NO-MAD and ADVISE (www.advise-h2020.eu).
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 surveillance samples only. NOMAD will provide an optional procedure to quantify the neutron-induced degradation 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 becomes 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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