1.7.1 Limits in Temperature, Fluence, etc.
There is a lack of high-fluence data to verify if the shape of the fracture toughness curve changes at a certain threshold or not. Furthermore, the possible occurrence of additional flux effects at high dose may require model modifications [JRC26623].
Irradiation data for higher temperatures (~320 °C) would be very useful because typical operating temperatures of pressurized water reactor (PWR) are close to and higher than 300 °C. Common irradiation temperatures, however, are around 270–290 °C [EUR23207].
The influence of the neutron spectrum and other radiation field parameters on the embrittlement process needs to be analysed in more detail to better characterise the portability of research reactor results on light-water reactor (LWR) environment. In addition, this would help to improve the semi-mechanistic model [JRC30648]. The influence of the neutron flux is another parameter of uncertainty which has been subject of discussions since the 1960s. A lack of surveillance data from long term operating plants makes an estimation of portability from test reactor results on power plants difficult. In future, a comparison between reactor pressure vessel (RPV) specimens from decommissioned plants and specimens irradiated in test reactors may give helpful hints [JRC63603] [Keim2012].
For further investigation of the general influence of the manganese content on the irradiation embrittlement process, further studies at higher doses, and with additional annealing experiments are necessary – for the experimental results by themselves as well as for modelling purposes [Lambrecht2008].
The influence of irradiation temperature is only based on data sets in a limited temperature range which, however, covers the typical temperature regimes in LWR. Thus, the semi-mechanistic model by Debarberis et al. [JRC30537] may also have shortcomings. Future studies may help to extend the available database and hence increase the applicable temperature range for the model [JRC31151]. Further measurements are necessary to better quantify the chromium effect and its integration in the model [JRC33283].
So-called multi-scale modelling of irradiation effects has made significant progress. The models, however, are still not ready for use for real RPV materials [Keim2012].
Re-embrittlement after annealing is still not fully understood. Different mechanistic approaches are discussed and experiments show inconsistent results. Thus, future investigations are to be performed to show which microstructural model fits best. The applicability of the semi-mechanistic model of Debarberis et al. [JRC30537], however, is not significantly reduced [JRC46534].
Although modelling made significant progress during the PERFECT [EUR24455] and the following PERFORM 60 [PERFORM2013] projects, some open issues still remain, which are currently under investigation, especially in the case of the FeNiMn system.
When analysing fracture toughness measurements, precise analysis of combined datasets for each technique / material combination is necessary to gain reliable correlations between temperature shifts, fracture toughness, measurement technique, and irradiation. Direct measurement of the fracture toughness transition using the Master Curve concept needs further evaluation [EUR19961].
Non-destructive techniques need further characterisation and sophistication in the process of assessing the industrial feasibility of their use to monitor irradiation embrittlement in RPV steels [Acosta2001] [EUR22282]. The state-of-the-art report revealed that none of the investigated non-destructive techniques is established well enough to be considered in codes and standards. Further development of these techniques is necessary [AMES-NDT2000].
The superposition of ductile-to-brittle transition temperature (DBTT) shifts due to thermal embrittlement and irradiation embrittlement is an open issue for the future needing more detailed insight into microstructural phenomena upon neutron irradiation and thermal ageing. In addition, aspects of non-hardening embrittlement need further investigation to reach a consistent view [EUR23207]. Investigations of the cross-influence of irradiation embrittlement and thermal ageing are still missing for long term operation questions (60 or 80 years) [Brumovsky2014b].