**2. Literature on irradiation effects in Al alloys**

A substantial amount of literature is published on the irradiation behavior of Al alloys [2–11]. The available dataset on 6*xxx* series alloys is considerably larger due to their widespread use in several research reactors and cold-neutron sources [2, 4–10]. On the other hand, only limited data were found on 5*xxx* series alloys [3, 9, 11, 12]. The published data from the SURP of the HFR vessel are also included in this review [13]. Although 5*xxx* and 6*xxx* series alloys are

a Hotspot is the location on vessel wall where highest neutron fluence is received.

b Assuming that the irradiation conditions at the HFR hotspot are kept unchanged as they are in 2015.

fundamentally different in their microstructure and properties in unirradiated condition, the data on irradiated 6*xxx* series Al alloys have great relevance to 5*xxx* series data, because 5*xxx* series alloys slowly transform into 6*xxx* series alloys in the course of neutron irradiation due to the transmutation-produced Si content [9]. Therefore, this study is focused on both the 5*xxx* and 6*xxx* series types of Al alloys. The emphasis is on understanding the influence of neutron irradiation on mechanical and microstructural properties up to very high fluence values and, in particular, the effect of transmutation-produced Si content on the mechanical properties.

To help the discussion on differences in irradiation damage mechanisms in different Al alloys, a brief review on the differences in chemical composition and microstructure of 5*xxx* versus 6*xxx* series Al alloys is presented here. The microstructure of 6*xxx* series alloys is carefully engineered by a suitable age-hardening treatment to form coherent precipitates (GP zones and β") within the matrix to obtain the required mechanical properties [14]. On the other hand, the as-produced 5*xxx* series has no precipitates within the matrix. The Mg solute atoms present in the solid solution provide the required strength properties. The composition of Al alloys of interest for the current discussion is shown in **Table 1**.


**Table 1.** Chemical composition of different as-produced Al alloys in wt.%.
