Page 85 - Neutrons for Sciences and Society

Chapter 4 - The negotiations
final decision was taken at the following meeting of the steering committee on 16 March 1967. Achieving 35 years of operation [49 years in 2019] without any incidents linked to the coexistence of heavy water with ordinary water showed this was a wise decision. The ability to remove beam tubes, as requested by the users has proved very useful as it was found that, under the influence of radiation for a decade, the aluminium alloy used to fabricate beam tubes became embrittled. It was then necessary to replace them, an operation foreseen in the original design. This embrittlement had been taken into account in the project. Dautray had actually asked for an analysis of the vessel of the Pegasus reactor operating at Caderache, and an extrapolation to predict the lifetime in the RHF. The alloy used for the RHF was aluminium-magnesium AG3Net. The studies showed the necessity for a replacement of the core assembly68 every five years, and this was incorporated into the reactor design. This fragility of heavily irradiated aluminium is well explained in the report at the Santa Fé conference. At present the ILL is studying the possible use of Zircaloy for this assembly. [2018 addition: More than 10 years after this sentence was written the ILL still uses aluminium. Thanks to the experience gained, the lifetime of all the heavily irradiated aluminium parts is now well known and thanks to some improvements to the already excellent design (see chapter 8), they can all be replaced during scheduled maintenance operations. As a result, it has been possible to extend the lifetime of the reactor considerably while remaining in full compliance with the safety rules.]
68 This assembly comprises the fuel element, the reflector tank containing heavy water, the beam tube nose-pieces close to the fuel element, and the integrated sources (cold source and hot source).
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