Contribution to radiation damage modeling of reactor pressure vessel materials in the ductile upper shelf regime (From Charpy impact upper shelf energy to crack resistance curve)

Ren Wei

Promoter

Pardoen Thomas, thomas.pardoen@uclouvain.be

SCK•CEN Mentor

Chaouadi Rachid
rachid.chaouadi@sckcen.be
+32 14 33 31 76

SCK•CEN Co-mentor

Uytdenhouwen Inge
inge.uytdenhouwen@sckcen.be
+32 14 33 31 71

PhD started

2018-01-01

Short project description

All current regulatory assessment procedures of reactor pressure vessel (RPV) rely essentially on the Charpy impact test, in particular the two parameters characterizing the ductile-to-brittle transition temperature (DBTT) and the upper shelf energy (USE). Such parameters are derived from the surveillance programs aiming at monitoring the vessel materials ageing and embrittlement under irradiation. During the last decade, thanks to the enhanced surveillance strategy developed at SCK•CEN, a large number of crack resistance measurements were collected. Contrary to the upper shelf energy concept which is rather rudimentary, the crack resistance curve including initiation toughness and tearing resistance relies on fracture mechanics. Today, the available database of crack resistance properties includes various RPV materials including forgings and welds irradiated to different fluence levels.

Similarly to the DBTT, a regulatory trend curve established on an empirical basis is used for the vessel integrity assessment. On the other hand, the most important requirement for operation is that the USE should not below an arbitrary threshold energy level of 68J. With the development of fracture mechanics on small size samples, it becomes possible to assess the evolution of the RPV materials properties at upper shelf temperatures with modern concepts based on fracture mechanics. As a result, the embrittlement trend curve at upper shelf should also be adapted accordingly and substitute empirical by physical insight.

Objective

The objective of the this work is to collect all available data that were produced in the last decade including both tensile and Charpy impact as well as crack resistance curves to provide a physically-guided engineering model that can be used as a trend curve for RPV materials. The ultimate goal is to be able to predict initiation fracture toughness and tearing resistance of RPV steels based on the material variables (Cu, Ni, P, ...) and irradiation variables (irradiation temperature, neutron flux and fluence).

This subject combines an experimental part aiming to collect and classify all available data on irradiation effects on the RPV materials properties with an analytical part consisting in physical understanding of the underlying mechanisms of both radiation damage and ductile fracture. It is of prime importance to understand how irradiation modifies the microstructure (nano-size irradiation defects) and this translates into the changes of the crack resistance properties. The phenomenology of the radiation damage model that should be developed can rely on the same concepts used for the DBTT.

The outcome of this work is the development of an improved radiation damage tool that can be very helpful in assessing the fracture properties of vessel materials at high temperatures.