Irradiation Induced Changes in Mechanical and Microstructural Properties of the High Flux Reactor Vessel: Update of the Results From 2014 and 2015 Surveillance Test Campaigns

The reactor vessel of the High Flux Reactor (HFR) in Petten has been fabricated from Al 5154 - O alloy grade with a maximum Mg content of 3.5 wt. %. The vessel experiences large amount of neutron fluences (notably at hot spot), of the order of 10²⁷ n/m², during its operational life. Substantial damage to the material’s microstructure and mechanical properties can occur at these high fluence conditions. To this end, a dedicated surveillance program: SURP (SURveillance Program) is executed to understand, predict and measure the influence of neutron radiation damage on the mechanical properties of the vessel material. In the SURP program, test specimens fabricated from representative HFR vessel material are continuously irradiated in two specially designed experimental rigs. A number of surveillance specimens are periodically extracted and tested to evaluate the changes in fracture toughness properties of the vessel as a function neutron fluence. The surveillance testing results of test campaigns performed until 2009 were already published by N. V. Luzginova et. al. [1]. The current paper presents results from the two recent surveillance campaigns performed in 2014 and 2015. The fracture toughness and tensile testing results are reported. Changes in mechanical properties of Al 5154-O alloy with an increase in neutron fluence are discussed in correlation with the irradiation damage microstructure observed in TEM and the fracture morphology observed in SEM. The HFR surveillance testing results are compared to the historically published results on irradiated aluminum alloys and conclusions about the evolution of embrittlement trends in relation with irradiation induced damage mechanisms in HFR vessel are drawn at the end.