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

Periodic Reporting for period 3 - NOMAD (Nondestructive Evaluation (NDE) System for the Inspection of Operation-Induced Material Degradation in Nuclear Power Plants)

Reporting period: 2020-06-01 to 2022-02-28

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. One of the irreplaceable parts that limits the lifetime of NPPs is the reactor pressure vessels (RPV). One of the operation conditions that affect the RPV lifetime is the neutron exposure. The evaluation of the neutron irradiation induced embrittlement allows for an estimation of the operational lifetime of NPPs. Currently, destructive tests are performed on surveillance samples in order to assess the progressive material degradation in RPVs. Non-destructive methods for materials characterization of the actual RPV body through the cladding can, thus, be very helpful to in-situ assess the actual material degradation.
NOMAD objective was to provide a tool for probing the RPV condition non-destructively and locally through the cladding, quantifying the actual material degradation. NOMAD thereby focuses on the non-destructive investigations of RPV steels in ageing reactors to better assess their integrity for lifetime management.
In order to develop a tool for probing the RPV condition non-destructively through the cladding, quantifying the actual material degradation a calibration on well-qualified materials including surveillance specimens is required. Within NOMAD, Charpy samples as well as larger blocks, partially with cladding, have been investigated in various irradiated conditions representing different degradation levels using multiple NDE methods. Charpy samples of Eastern and Western RPV design, base and weld materials have been provided in non-irradiated condition and at different neutron irradiation levels. One Western RPV material was represented at multiple samples scales with the challenging goal to study neutron-induced embrittlement across a variety of sample geometries reaching from Charpy samples to large cladded blocks emulating a near-service RPV condition. 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 order to characterize the neutron irradiation induced embrittlement non-destructively, multiple NDE methods were applied after preliminary optimization. They deliver in total 28 features, i.e. different but complementary information about the material properties.
In a second step, several Charpy sample sets of four RPV steels of Western design base and weld material at different irradiation conditions from previous irradiation programs have been measured by NDE methods. Charpy samples of two other RPV materials of Western and Eastern RPV design were investigated before and after irradiation.
In a third step, a set of six cladded and six non-cladded blocks of a Western RPV material haven been non-destructively investigated before and after irradiation.
In this way, for the first time the progression of the material properties induced by neutron irradiation was collected taking into account the initial condition of the materials (before irradiation). Several mechanical properties were measured for calibration purposes after the NDE measurements.
All data measured together with the corresponding standard deviations and data related to irradiation conditions were merged into the NOMAD database, separated in terms of their physical principles and correlated with the mechanical properties.
Studies on the sensitivity of the NDE methods to embrittlement using different algorithms showed that several measured NDE parameters do not have a clear correlation with the neutron irradiation induced embrittlement. A machine learning-based approach was used to create models that accurately predict the neutron irradiation-induced embrittlement, measured as the DBTT, even though the data set is small. These models predict the embrittlement of Charpy and/or block samples with a mean averaged error under 19°C, which is close to the approximate accuracy of the traditional Charpy impact test.
To determine 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. 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. Due to the limited number of blocks the validation focuses on data collected on Charpy samples only. A transfer of the findings to cladded blocks or cladded RPV material is per se not possible and requires a new validation.
The current NOMAD tool can be interpreted as a proof-of-concept for the non-destructive evaluation of DBTT by using a multi-parameter data science approach under laboratory conditions. Concerning the application of the NOMAD tool on cladded blocks further NDE measurements on statistically relevant samples set are needed. Recommendations have been made under the assumption of a continuation of the development towards a higher TRL. The considerations take into account the future application inside an RPV. From a field application point of view, the feasibility study regarding further development of the NOMAD tool towards a field system was successful. In the long term, the NOMAD tool can complement the Charpy test-based protocols. For new installations, the toolset can be applied from as early as the preparatory research phase of the installation. This future tool will have the potential to provide a complete map of the vessel.
The NOMAD website 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 NOMAD and ADVISE (www.advise-h2020.eu) and a joint symposium has been organized. The NOMAD results have been made public in 19 Publications and 53 Communication activities.
Progress beyond the state of the art
• Equivalence between destructive and non-destructive tests: Investigation and documentation how the mechanical properties are linked to and correlate with NDE results.
• Application of NDE to realistic RPV material with cladding, not only to Charpy samples: The multi-method tool can be applied to cladded RPV material. The tool was calibrated, evaluated and tested on Charpy samples and cladded material and validated on Charpy samples only.
• Combined NDT approach based on electrical methods, ultrasound and micromagnetics in a single tool: The reason for using more than one NDE technique for materials characterization is the increased accuracy and reliability against disturbing influences such as material variations and surface conditions. Ultrasound and electrical approaches deliver information complementary to the micromagnetic approach.
NOMAD provided an optional procedure to determine the degradation parameters additionally to the standardized methods needed for the assessment of LTO and contribute to the extension of the existing database containing materials data, degradation parameters and progression of the mechanical properties.
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