(388a) Evaluation of Liquid Sodium Impurity Deposition Models for Sodium-Cooled Fast Reactor Heat Exchangers

In sodium-cooled fast reactors, liquid sodium is used to remove heat from the reactor core and transfer it, through properly designed heat exchangers, to a working fluid in a power generation cycle. The presence of dissolved impurities in liquid sodium, such as oxygen or hydrogen, can lead to the deposition of solid forms of these contaminants, especially in the heat exchangers due to the solubility reduction resulting from sodium cooling. A project recently awarded to Westinghouse Electric Company and Argonne National Laboratory (ANL), in support of the Department of Energyâ??s Advanced Reactor Concepts (ARC) program [1], seeks to couple an impurity deposition model with a computational fluid dynamics (CFD) simulation of liquid sodium flowing in fine channels comprising the heat exchanger [2]. The deposition of sodium oxide (Na2O) from the
accidental ingress of air into the system and the simultaneous failure of cold traps can reduce or block the flow
of liquid sodium in these narrow channels, compromising plant availability and, in some circumstances, safety. Moreover, the resulting increase in flow resistance might degrade heat transfer efficiency and increase the pressure drop in the equipment.
To predict this plugging phenomenon, the coupling of an impurity mass-transfer deposition model with a liquid sodium fluid dynamics model is necessary as the presence of a deposit on the wall of a heat exchanger channel affects the hydrodynamics within the channel, which in turn alters the deposition process due to localized changes in parameters such as fluid temperature and heat conduction through the channel wall. The formation of oxide or hydride crystal deposits in supersaturated liquid sodium is a function of crystal nucleation and growth and of the mass transfer of the impurity from the bulk liquid across a solid-liquid interface. Several approaches have been developed in the literature to describe this behavior.
Recently, a first principles plugging model was developed by researchers at ANL to describe Na2O plugging phenomena within semi-circular channels that were on the order of several mm in diameter [3]. In their work, the rate of Na2O growth was a function of the oxygen concentration gradient, the overall mass transfer coefficient, and the effective density of the oxide deposit. As a first principles model, this approach has several distinct advantages in that any channel geometry can be utilized, crystallization kinetic data are not required, and only a limited number of fundamental inputs are necessary. The model was validated against a limited set of Na2O plugging experiments performed at ANL, and it was shown to predict the channel plugging time within
~25% of the experimental value. In its current form, the model assumes uniform growth of the deposit along
the perimeter of the channel wall, which may not occur in practice due to localized temperature variations. Additionally, an assumption was made about the volume fraction of liquid sodium within the Na2O deposit. Further comparison of this model against experimental data is required; however the model represents an excellent candidate for inclusion into the CFD heat exchanger channel model.
In addition to the work described above, studies on the kinetics, reaction order, and crystal morphology of sodium oxide and hydride deposits in cold traps designed to purify liquid sodium had been conducted in the
1980â??s [4-6]. Recently, this work has been expanded to include aspects of CFD for flowing liquid sodium [7-9]. This collective work provides an additional set of experimental data and theoretical considerations that can eventually support the CFD model that describes plugging in a heat exchanger channel. The cold trap geometry and scale is different than that of a heat exchanger channel since it can contain mesh packing and typically has a larger internal diameter on the order of tens of centimeters. However, the cold trap model treats the deposition process in a fundamental manner by including thermodynamic and kinetic parameters of the crystallization process such as activation energy and reaction order. The incorporation of these parameters
into a mass-transfer deposition model may provide a more tailored description of the deposition process than the first principles model; however it may introduce unnecessary complication without providing more accurate results. The cold trap model is also subject to assumptions regarding the liquid sodium volume fraction of the deposit.
In this work, an evaluation of the first principles and cold trap models will be performed. First, an attempt will be made to replicate the results of each model reported in the literature using an appropriate computerized method. For a more rigorous evaluation, both models will be adapted in an attempt to predict the deposit growth rates reported in experimental oxide and hydride deposition studies for [3, 6, 8]. This cross- examination will permit a detailed assessment of model strengths and weaknesses and will support a final decision for the development of the CFD model. A summary paper will describe the results of this comparative evaluation.

Acknowledgement:

This material is based upon work supported by the Department of Energy, Office of Nuclear Energy, under
Award Number DE-NE0000611.

Disclaimer:

This summary was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
References:
1. US Department of Energy, Funding Opportunity Announcement - Advanced Reactor Research and
Development, DE-FOA-0000818. February 13, 2013.
2. Ferroni, P., Sienicki, J., 2013 Modeling and Validation of Sodium Plugging for Heat Exchangers in Sodium â?? cooled Fast Reactor Systems. Westinghouse internal presentation, WAAP-8730. Panel on Department of Energy Investments in Advanced Nuclear Power. American Nuclear Society, Winter Meeting, November 13, Washington D.C., USA.
3. Sienicki, J., 2012. â??Unified Analysis of Sodium Oxide Deposit Growth and Sodium Pluggingâ?, ANS Winter

Meeting and Nuclear Technology EXPO, Nov. 11-15, 2012, San Diego, CA.

4. Latge C., 1984. â??Study of Sodium Oxide Crystallization Mechanisms and Kinetics in Cold Traps, DE85752907, U.S. Department of Commerce, National Technical Information Service.
5. Latge C., Champeix, L., Laguerie C., 1984. â??Sodium Oxide Crystallization in Liquid Sodiumâ?, Industrial

Crystallization 84, Process Technology Proceedings, 2, Proceedings of the 9th Symposium on Industrialized Crystallization, The Hague, The Netherlands, September 25-28, 1984, Elsevier Science Publishers B.V., Amsterdam, pp. 125-130.

6. Latge C., Hulme, G., Jones D.G., Perret, F., 1988. â??Experimental Studies of Packless Cold Traps for Validation of the VICSEN Code for Prediction of Cold Trap Behaviorâ?, 4th LIMET Conference, October 17-21, Avignon, France.

7. Khatcheressian, N., et al, 2012, â??Modelling of Cold Traps for Sodium Purification in Fast Reactorâ?.

Proceedings of the 22nd European Symposium on Computer Aided Process Engineering, June 17 -20, London, England.

8. Khatcheressian, N., Latge C., Joulia X., Gilardi, T., Meyer X., 2013. â??Development of a Mass Transfer Model for Sodium Purification in a Fast Breeder Reactorâ? International Conference on Fast Reactors and Related

Fuel Cycles: Safe Technologies and Sustainable Scenarios FR13, March 4-7, Paris, France.

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9. Khatcheressian, N. 2013. "Developpement d'un modele de transferts couples pour l'aide a Ia conception et

a Ia conduite des systemes de purification du sodium des reacteurs a neutrons rapides" Ph.D. Thesis.

Universite de Toulouse.

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