Downscaling a supercritical water loop for experimental studies on system stability M. Rohde ⇑ , C.P. Marcel 1 , C. T’Joen, A.G. Class 2 , T.H.J.J. van der Hagen Section of Physics of Nuclear Reactors, Delft University of Technology, Mekelweg 15, Delft 2629 JB, The Netherlands article info Article history: Received 22 February 2010 Received in revised form 28 September 2010 Accepted 29 September 2010 Available online 1 November 2010 Keywords: Fluid-to-ﬂuid modeling Experimental facility Supercritical ﬂuids SCWR Stability abstract In industry, supercritical water is being used as e.g. separation agent, solvent or coolant due to the unique ﬂuid properties near the critical point. This has lead to the proposal for a nuclear reactor based on super- critical water, operating at a pressure of 25 MPa and bulk temperatures between 280 °C and 500 °C. The large change of the water density in such a reactor may cause the system to become thermal-hydrauli- cally unstable. Numerical as well as experimental investigation of this phenomenon is therefore essential. The rather high pressure, temperatures and power signiﬁcantly push up the costs of an experimental facility. For this reason, we propose a scaling procedure based on Freon R-23 as the working ﬂuid so that (i) pressure, power and temperatures are signiﬁcantly reduced and (ii) the physics determining the dynamics of the system are almost completely preserved. Practical issues, such as the onset of deteriora- tion of heat transfer, are touched upon as well. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Supercritical water is applied in many industrial applications because of its unique ﬂuid properties near the critical point. Due to the strong density change near the critical point, for example, small temperature changes allow for controlled separation of dis- solved materials [1]. Another application of supercritical water can be found in modern, coal-ﬁred electricity plants (Japan, Den- mark, United States, Germany) [2]; the high heat capacity near the critical point allows for transferring dense amounts of energy from the heated section to the turbines. Moreover, the high core exit temperature results in high cycle efﬁciencies. Therefore, the international Generation-IV forum proposed a nuclear reactor based on supercritical water (the SCWR), which is planned to deli- ver energy in the near future (2025) [3]. Past research on loop systems with boiling water has shown that such systems can become unstable due to the presence of a large change of densities. In a nuclear boiling water reactor, for example, the density change within the core section runs from 740 kg/m 3 to 180 kg/m 3 . In the current US, European and Japanese designs of a nuclear reactor based on supercritical water, the den- sity range is even larger, being from 777 kg/m 3 to 90 kg/m 3 . One therefore can expect that, under particular conditions, a reactor based on supercritical water may become unstable as well. These density-related instabilities can be classiﬁed into two cat- egories, being static and dynamic instabilities [4]. The ﬁrst category deals with instabilities which can be explained by using the steady- state characteristics of the thermal-hydraulic system. An example is the ﬂow excursion [5]. The second category consists of time-depen- dent instabilities which are related to a delayed response to initial perturbations with a feedback response. The delay determines, among other things, whether perturbations will grow or diminish [6]. There are numerous interacting phenomena behind this delay such as the ﬁnite residence time of the water in the core, local fric- tion, heat transfer dynamics within the fuel and the feedback of the local coolant density on the reactivity of the reactor. The stability of the SCWR can be studied both numerically (e.g. [7–9]) as well as experimentally. The latter one has the advantage that a real physical system is studied; costs, however, might become unaffordably high. The costs can be signiﬁcantly reduced by down- scaling the reactor to smaller dimensions, by using lower pressures, lower temperatures and a lower power, thereby using a ﬂuid differ- ent from water. Examples of such experimental facilities are the R-12 based DESIRE facility [10] and the R-134a based GENESIS facility [11], both mimicking a natural-circulation boiling water reactor. For a supercritical loop, Marcel et al. [12] developed a scaling procedure and proposed to use a Freon-mixture of R-125 and R-32, which shows very good similarities to supercritical water at SCWR conditions at 6.2 MPa. This mixture, however, has severe restrictions in terms of ﬂammability and thermal stability and requires accurate monitoring of the mixture composition. Further- more, Marcel et al. [12] showed that the same scaling rule should be applied to the radial as well as the axial dimensions. By doing so, the radial dimensions of an experimental facility would become rather small, resulting in an undesirably high frictional pressure drop within the system. The purpose of this work is (i) to ﬁnd a less restrictive ﬂuid that mimics supercritical water under the relevant SCWR conditions, 0017-9310/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2010.09.063 ⇑ Corresponding author. Tel.: +31 15 278 6962; fax: +31 15 278 6422. E-mail address: m.rohde@tudelft.nl (M. Rohde). 1 Comision Nacional de Energia Atomica, 8400 S.C. de Bariloche, Argentina. 2 Karlsruhe Institute of Technology, Institute for Nuclear and Energy Technologies, P.O. Box 3640, D-76021 Karlsruhe, Germany. International Journal of Heat and Mass Transfer 54 (2011) 65–74 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt