**5. Summary and conclusions**

**Figure 7.** Fracture details of 5154-O alloy with a crack-tip thermal fluence of 9.81 × 1026 n/m2

CRDF A-2 tubes of HFBR produced from 6061-T6 alloy have shown a fracture toughness value of ~8 (MPa)·m1/2 after irradiation to a much higher thermal neutron fluence of ~42 × 1026 n/m2 at 338 K (see **Figure 4**). The decrease in fracture toughness from an unirradiated value of 21.75 (MPa)·m1/2 for this alloy is primarily attributed to the following: (i) formation of very fine (~8 nm) Si-rich precipitates in the grains due to high TFR of 21 (as explained in Section 4.4) and (ii) large silicon flakes occupying about one-fifth of the grain boundary area at this high transmutation-produced Si content of 8 wt.% [5]. Fracture surface of this alloy revealed substantial intergranular separation with some residual ductility indicating that the contribu‐ tion of grain boundary fracture mechanisms is increased at such high fluence values to enter

From the above discussion, no differences in the evolution of irradiation damage at high fluences (in regime 3 and 4) are expected between 5*xxx* and 6*xxx* alloys due to differences in their initial chemical composition and microstructure. Moreover, the difference in the TFR of HFR SURP data and literature data is conservative as explained in the next section. This allows the use of published fracture toughness literature data of 6061-T6 Al alloys to predict the

It is known from the literature that a high difference in TFR can have substantial effect on irradiation hardening and embrittlement behavior of the same material [5]. It was highlighted in [3] that both the thermal and fast neutrons play independent and important roles leading to microstructural damage and corresponding property changes. A very high TFR, ranging from 80 to 500, could explain the observed craze-cracking in AG3-NET alloy (Al–3% Mg) beam tubes in the Reactor Haut Flux (RHF) at Grenoble [23]. Lijbrink et al. [12] pointed out that fast neutron flux reduces the effectiveness of the Si precipitation hardening process. A possible explanation for this behavior (as given in [5, 12]) is as follows. Fast flux has two opposite effects

**i.** The kinetic energy supplied by fast flux temporarily increases the solubility limit of Si in the matrix and opposes the condensation requirements for the precipitation.

fracture toughness behavior of 5154-O Al alloy of HFR vessel at high fluences.

surface characterized by dominant microdimples and some cleavage facets [13].

into the brittle regime (regime 4).

408 Radiation Effects in Materials

on precipitation:

**4.4. Effect of thermal-to-fast flux ratio (TFR)**

. Figure shows fracture

A literature review on highly irradiated 5*xxx* and 6*xxx* series Al alloys is conducted to understand the expected changes in mechanical properties of HFR vessel material in relation with microstructural aspects beyond the current surveillance data to support the HFR SURP program. It was found that the irradiation swelling in 5*xxx* series alloys is not a crucial degradation mechanism for the expected hotspot fluence values of HFR vessel by the end of 2025. Dislocation damage is expected to reach a saturation limit at relatively low fast-fluence values. The damage caused by precipitation of transmutation Si is found to be the dominant mechanism affecting the fracture toughness properties of irradiated 5*xxx* and 6*xxx* series Al alloys at high thermal fluence values. Tensile and fracture toughness data collected from the literature up to very high thermal fluences is analyzed in comparison with the available HFR surveillance data. The observed changes in mechanical properties are classified into four different regimes.

The contribution of various irradiation damage mechanisms to the evolution of microstructure and mechanical properties is discussed in all four regimes for 5*xxx* and 6*xxx* series alloys. A rapid hardening regime characterized by a sharp drop in ductility and fracture toughness is observed at the onset of irradiation. In this regime, a higher degree of embrittlement is observed in 5*xxx* series due to the formation of Mg2Si precipitates in addition to the contribution from dislocation damage. On the other hand, the hardening and embrittlement observed in regime 1 of 6*xxx* series alloys is primarily due to dislocation damage. The contribution from the precipitation of transmutation Si is estimated to be minor for 6*xxx* alloys in this regime. The contribution of both mechanisms continues in the transition regime (regime 2) for both alloy types, until both dislocation damage and precipitate density evolve toward a saturation value. Due to this, a lowering in irradiation hardening rate and correspondingly a slow decrease in ductility and fracture toughness toward a stable value are observed in this regime. Regime 3 is characterized by a plateau in ductility and fracture toughness values, due to no further increase in dislocation and precipitate density. A slow hardening observed in this regime is primarily due to the growth of existing precipitates. The behavior of 5*xxx* alloys is found to be similar to 6*xxx* series alloys when irradiated under similar conditions in this regime. A final regime (regime 4) with an increasing hardening rate and a decreasing ductility indicates that the contribution of grain boundary fracture mechanisms increases at such high fluence values to enter into the brittle regime (regime 4).

For the 5154-O alloy at the hotspot irradiation conditions, regime 1 ends at ~2 × 1026 n/m2 . Regime 2 is observed between ~2 × 1026 and ~4 × 1026 n/m2 . Finally, the plateau in regime 3 starts at ~4 × 1026 n/m2 and is expected to continue up to very high thermal fluences, that is, greater than the estimated hotspot thermal fluence by the end of 2025 (~20 × 1026 n/m2 ). This is because for a 5052-O alloy, which was irradiated at similar conditions as HFR hotspot and resembles the alloy microstructure and composition of 5154-O, a plateau in ductility was observed from a thermal fluence of ~4 × 1026 n/m2 until ~31 × 1026 n/m2 . It should be noted that the estimated HFR hotspot thermal fluence by the end of 2025 (~20 × 1026 n/m2 ) is only two-thirds of the studied 5052-O alloy.

Additionally, high fluence fracture toughness data is found from the CRDF A-2 tubes of the HFBR in Brookhaven National Laboratory, USA, produced from 6061-T6 alloy, irradiated at 338 K, up to 42 × 1026 n/m2 . The corresponding fast fluence of this data point is 2 × 1026 n/m2 , which gives a high TFR of 21 compared to the HFR hotspot TFR value of maximum 1.4. The reported thermal fluence and Si content of this data point are approximately two times the estimated thermal fluence (~20 × 1026 n/m2 ) and Si (~4.3%) content of the HFR hotspot by the end of 2025. Knowing that the transmutation-produced Si induces major damage to the microstructure of irradiated Al alloys, this high fluence data point from CRDF A-2 of HFBR is likely to give a conservative estimation of the fracture toughness value under HFR conditions due to irradiation of this alloy at much higher TFR (leading to high embrittlement) and negligible differences in the embrittlement behavior of 5*xxx* and 6*xxx* series alloys in the plateau regime.

From the above observations of literature tensile and fracture toughness data on irradiated Al alloys, one can conclude that the probability of the fracture toughness of HFR hotspot to fall below the design limit is negligible up until the currently estimated hotspot thermal fluence at the end of 2025.c
