What is NOMAD about?
The overall concept aims at a synergetic combination of NDE methods as a hybrid tool for through-cladding degradation assessment of reactor pressure vessels (RPVs), taking into account local heterogeneities. All methods concerned indirectly determine material properties, therefore a calibration is required in order to link measured parameters to material degradation. In order to deal with the fact that a universal calibration of the hybrid method is not possible and many RPV base and weld materials exist, in NOMAD a common trend of NDE data regarding neutron-induced embrittlement will be identified and described, based on result parameters optimised for this procedure. The advantage of having a common trend in NDE data is the possibility to re-adjust it for practical application with few reference measurements instead of a large calibration sample set. The NOMAD application will rely on a database of material behaviour, which is built during the project. Mathematical-statistical algorithms and a deep understanding of the physical method basics will be the key to accomplish this task. Taking on this mission, NOMAD pursues the following strategic goals:
Description of sample sets
A thorough study will be undertaken to define a matrix of materials to be selected as well as the neutron irradiation conditions such as fluence and temperature. Hence neutron-irradiated non-cladded and cladded specimens of relevant RPV materials will be allocated for further investigations in hot cells. Destructive examinations of the microstructural and mechanical properties of the neutron-irradiated material will be carried out and reported along with existing data to be compiled. Reference measurements (mechanical/destructive tests and microstructural investigations) will be performed in order to confirm results (DBTT shift, upper shelf energy, hardness) obtained by the NDE methods.
Non-destructive materials characterisation and evaluation
To characterise the changes of microstructure and mechanical properties induced by neutron irradiation on the aforementioned materials, optimal sensors will be designed and measuring settings for each non-destructive evaluation method will be determined in a first step. In a second step cladded and non-cladded specimens will be characterised using several NDE methods with proven basic feasibility. In the next step, the methods illustrating the best sensitivity for neutron irradiation-induced embrittlement in non-cladded material will be adopted to and applied on cladded material. Finally, the resulting data will be compiled in a materials database. Correlations between mechanical, microstructural and NDE parameters, including quantification of reliability and uncertainty, will be analysed and documented with respect to initial (non-irradiated) microstructure, material variability and other influencing factors.
The main goal is the development of a software-based multi-parametric NDE tool for the inspection of cladded RPV material under similar operaion conditions, taking into account microstructure heterogeneities in the RPV. The NDE techniques showing the best sensitivity for neutron irradiation-induced damage will be selected and classified on a physical and microstructural basis.
Proof of principles (Application and Validation)
Finally, the reliability of the NDE tool, taking into account the most relevant parameters that might affect the measurements and its application in the field will be determined on the basis of existing European qualification standards and recommendations for the application of the developed NDE tool provided.
We will develop a standardised non-destructive evaluation tool for locating and characterising material damage in reactors operating beyond their design lifetime.
This tool shall be used in periodic safety reviews of the reactor pressure vessel complementary to the standardised destructive methods.