Abstract
Worldwide there are more than 449 nuclear power plants (NPPs) in operation among which 329 reactors are older than 25 years and 94 will be operating for more than 40 years in 2020. Lifetime extensions are requested up to 50–60 years and sometimes even up to 80 years of operation for many existing NPPs. Long-term operation (LTO) of existing NPPs has therefore been accepted in many countries as a strategic objective to ensure supply of electricity for the coming decades. Within this strategy, the European Commission launched the NOMAD project, among others, through the Horizon-2020 programme.
The reactor pressure vessels (RPVs) cannot be tested destructively in a direct way, neither can it be replaced. An indirect way is the use of Charpy samples from the so-called surveillance programs. The general strategy on the long term should focus on the ability to perform direct non-destructive evaluation (NDE) of the embrittlement of the vessel. NDE can be used to confirm that the data obtained by surveillance programs are being representative of the real state of the vessel for LTO. Moreover, a generic concern of large nuclear components such as the reactor vessel is the possible material heterogeneity such as macro-segregated regions which could eventually be located in the component but not in the baseline material used as surveillance material. Local non-destructive material inspection and comparison to reference materials in similar irradiation conditions would lead to a better assessment of the properties of the materials at any location of the vessel.
The objective of NOMAD is to develop a tool that is capable of non-destructively evaluate the embrittlement of the vessel wall. The final system should be capable of inspecting the microstructure of the materials through the cladding. The tool that will be developed, will use existing and proven nondestructive testing techniques (NDT) with optimized and adjusted sensors. A combination of several techniques based on micro-magnetic, electrical and ultrasonic methods are investigated. Within NOMAD, they are calibrated and validated on a set of existing and newly irradiated samples consisting out of the most common RPV steels from Eastern and Western design, such as 22NiMoCr37, 18MND5, A533-B, A508 Cl.2, A508 Cl.3 and 15kH2NMFA.
For the first time, a systematic study on a well-characterized set of samples that correlates the microstructure, mechanical properties, neutron irradiation conditions and non-destructive properties will be carried out. It will not only extend the existing database, but will include issues such as reliability, and uncertainty of the techniques as well as on material heterogeneity. The focus is laid on unbroken Charpy samples and large blocks with and without cladding to “simulate” the actual RPV inspection scenario.
This paper gives an overview of the present status of the NOMAD project with focus on the outcome in WP1. The first preliminary NDE results from 6 set-ups and 28 parameters were compared with DBTT results from Charpy impact tests. They are very promising. Final results and detailed analysis will however only be available at the end of the project.
