**Author details**

.

,

). This is because

. Finally, the plateau in regime 3 starts

. It should be noted that the estimated

) is only two-thirds of the

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

For the 5154-O alloy at the hotspot irradiation conditions, regime 1 ends at ~2 × 1026 n/m2

at ~4 × 1026 n/m2 and is expected to continue up to very high thermal fluences, that is, greater

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

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

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

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

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

The work presented in this article is performed as a part of SURveillance Program (SURP) of HFR vessel with the financial support of NRG. The author thanks Dr. O. Wouters and Ir. T.O. van Staveren for useful discussion and critical review of this work. The author also acknowl‐

edges Dr. C. Li for helping in the extraction of data from the literature.

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

. The corresponding fast fluence of this data point is 2 × 1026 n/m2

) and Si (~4.3%) content of the HFR hotspot by the

until ~31 × 1026 n/m2

than the estimated hotspot thermal fluence by the end of 2025 (~20 × 1026 n/m2

HFR hotspot thermal fluence by the end of 2025 (~20 × 1026 n/m2

to enter into the brittle regime (regime 4).

a thermal fluence of ~4 × 1026 n/m2

estimated thermal fluence (~20 × 1026 n/m2

studied 5052-O alloy.

410 Radiation Effects in Materials

plateau regime.

at the end of 2025.c

**Acknowledgements**

**Export control note**

c

338 K, up to 42 × 1026 n/m2

Regime 2 is observed between ~2 × 1026 and ~4 × 1026 n/m2

Murthy Kolluri

Address all correspondence to: kolluri@nrg.eu

Nuclear Research and Consultancy Group (NRG), The Netherlands
