Transient freezing of molten salts in pipe-flow systems: Application to the direct reactor auxiliary cooling system (DRACS). (15th January 2017)

• Record Type: Journal Article
• Authors: Le Brun, N.; Hewitt, G.F.; Markides, C.N.
• Is Part Of: Applied energy. Volume 186:Part 1(2017)
• Page Start: 56
• Page End: 67
• Publication Date: 2017-01-15
• Subjects: Molten salt -- Heat transfer fluid -- Freezing -- Solidification -- Phase change -- DRACS -- MSR; Power (Mechanics) -- Periodicals; Energy conservation -- Periodicals; Energy conversion -- Periodicals; 621.042
• DOI: 10.1016/j.apenergy.2016.09.099
• Languages: English
• ISSNs: 0306-2619

Abstract

Highlights: A thermo-hydraulic model has been proposed to simulate the transient freezing of molten salts in complex piping systems. The passive safety system DRACS in Generation-IV, molten salt reactor is susceptible to failure due to salt freezing. For the prototypical 0.2 MW reactor considered in this study considerable freezing occurs after 20 minutes leading to reactor temperatures above 900 °C within 4 hours. Conservative criteria for the most important/least known variables in the design of DRACS have been discussed. Over-conservative approaches in designing the NDHX should be used with caution as they can promote pipe clogging due to freezing. Abstract: The possibility of molten-salt freezing in pipe-flow systems is a key concern for the solar-energy industry and a safety issue in the new generation of molten-salt reactors, worthy of careful consideration. This paper tackles the problem of coolant solidification in complex pipe networks by developing a transient thermohydraulic model and applying it to the 'Direct Reactor Auxiliary Cooling System' (DRACS), the passive-safety system proposed for the Generation-IV molten-salt reactors. The results indicate that DRACS, as currently envisioned, is prone to failure due to freezing in the air/molten-salt heat exchanger, which can occur after approximately 20 minutes, leading to reactor temperatures above 900 °C within 4 hours. The occurrence of this scenario is related to an unstable behaviour mode of DRACS in which newly formed solid-salt deposit on the pipe walls acts to decrease the flow-rate in the secondary loop, facilitating additional solid-salt deposition. Conservative criteria are suggested to facilitate preliminary assessments of early-stage DRACS designs. The present study is, to the knowledge of the authors, the first of its kind in serving to illustrate possible safety concerns in molten-salt reactors, which are otherwise considered very safe in the literature. Furthermore, and from a broader prospective, the analytical tools developed in this study can also be applied to examine the freezing propensity of molten-salt flows in other complex piping systems where standard, finite element approaches are computationally too expensive.