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Alternatives for the development of more reliable secondary heat exchangers for sodium fast reactors


Autore: Lenci G., Cumo M., Driscoll M.J., Todreas N.E.

Collana: CA - 65 - Cagliari 2010

The achievement of a high capacity factor and reliability for sodium fast reactors is an important issue for increasing their economic competitiveness while lowering the related capital risk. Lessons learnt from sodium fast reactor operation have shown that breaks in steam generator tubes represent a significant cause of capacity factor reduction, requiring long shutdown periods. Proposed solutions to this problem include the possibility of improving the reliability of secondary heat exchangers (sodium-to-water steam generators or sodium-to-gas heat exchangers) through an informed selection of innovative design configurations, materials, leak-detection systems and inspection techniques. This work studies different possible alternatives to fulfill the following fundamental requirements: frequency and magnitude reduction of leak accidents and rapid detection of those accidents in order to mitigate their consequences.
Two or three solid boundaries can be designed to separate sodium from the power conversion fluid channels, either in shell-and-tube, rectangular-channel or printed circuit heat exchangers. A gap between the two main heat exchanging channels can be designed to contain a liquid or gaseous (e.g. Na or He) fluid, for leak-detection purposes. In the unlikely event of a break which puts sodium and water in contact, hydrogen detectors installed in the sodium circuit rapidly reveal small leaks before the break size is increased by wastage. The optimal time between two programmed inspections can be evaluated using a proposed method based on statistical study of crack formation and propagation, so the probability of having undetected cracks which propagate throughout the channel material can be determined. The time required for checking cracks can be reduced using semiautomatic inspection devices, based on ultrasonic and eddy current technologies. The aforesaid improvements could lead to the development of design solutions where the intermediate heat transfer loop could be eliminated, improving the overall thermodynamic efficiency, reliability and economic competitiveness of sodium fast reactors.

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