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

Abstract

The possibility of molten salt freezing in piping 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. 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 the preliminary assessments of DRACS designs. Such study is, to the knowledge of the authors, first in its type and it serves to illustrate the possible safety concerns in molten salt reactors, which are otherwise considered very safe in the literature. On a broader prospective, the analytical tools developed in this study can be used to model the freezing of molten salts in complex piping systems where standard, finite element approaches are computationally too expensive.

Transient freezing of water between two parallel plates: A combined experimental and modelling study

“…The values of these two heat transfer coefficients have a significant impact on the accuracy of the model and their accurate estimation is important for freezing predictions. The same method described by Le Brun et al [15,17,45] was adopted here, following some adjustments for our specific case, in order to model the transient freezing of the liquid in the test section. In more detail, we converted the referenced model equations from a cylindrical geometry (pipe) to a channel configuration (flat plate) and modified all terms relating to the geometrical features of the channel and copper plate.…”

“…Freezing in MSRs can lead to dangerous scenarios, one example being the sudden solidification inside the steam generator or the Direct Reactor Auxiliary Cooling System (DRACS) as shown by Le Brun et al [15,16] and Le Brun [17]. A sudden drop in the wall temperature, which can be induced by an unwanted depressurisation of the steam generator with consequent lowering of the water boiling point, can result in the solidification of the liquid inside the channel and lead to clogging.…”

Fluid flow during phase transition: From viscous fluid to viscoelastic solid via variable-order calculus

“…Flows of fluids undergoing phase change due to physical or chemical processes are a set of phenomena which are highly relevant in the context of both theoretical and applied problems, and have been examined and analyzed in works pertaining to the fields of classical fluid mechanics, metallurgy, reactor physics, biomechanics, geophysics, and more. Examples of these include flows of thin films during solidification as presented by Myers et al, 1 flows of water freezing into ice in the vicinity of ships' hulls or aircraft wings as given by Sultana et al 2 and Moore et al, 3 magma flows as presented by Boukar e and Ricard, 4 molten metal flows during casting processes as given by Wu et al, 5 flows of molten salts in advanced nuclear reactors as presented in Le Brun et al, 6 and flows of organic tissue during organogenesis as presented by Jones and Chapman, 7 etc.…”