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International Students Workshop on High Performance Light Water ReactorsMarch 31

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rd, 2008, Karlsruhe, Germany

CFD Analysis of feedwater flow in the HPLWR pressure vessel

Helena Foulon1)

, Alexander Wank2)

, Thomas Schulenberg2)

1) Universitt Karlsruhe (TH)Kaiserstrasse 12

D 76131 [email protected]

2) Institute for Nuclear and EnergyTechnologies, Forschungszentrum KarlsruheP.O. Box 3640, 76021 Karlsruhe, Germany

Phone +49-(0)7247-82-5781, Fax -6323

I. Introduction

The High Performance Light Water Reactor (HPLWR) is a new concept of light water reactors,cooled and moderated with water at supercritical pressure conditions of around 25 MPa. It is one ofthe six concepts of the Generation IV International Forum. According to Schulenberg et al. [1] water

shall enter the core at a temperature of 280C and exit as superheated steam of 500C, leading to athermal efficiency higher than 40%, which is the key advantage of this concept.

Millet et al. [2] have presented a FEM analysis for the reactor pressure vessel (RPV) of theHPLWR. The objective in their work has been to optimize the wall thickness of the RPV to stay withinforging limits. The mechanical strains have been calculated by [2]. For the thermal stresses, the crucialheat transfer coefficients have been analytically estimated. High temperature differences, resulting inhigh thermal stresses and maybe even in the failure of a component, have to be avoided and thus, aCFD analysis has become necessary to calculate the heat transfer coefficients more accurately.

This paper presents the CFD analysis of the feedwater flow in between the reactor pressurevessel (RPV) and the core barrel containing the internal components of the reactor. The flowvelocities, the heat transfer coefficients, and the temperature distribution have been calculated in orderto obtain boundary conditions for upcoming stress analyses.

II. Flow characterist ics in the annular gap

The feedwater entering the reactor is split up. According to Waata et al. [3] 25 % of the coolingwater flows upwards, passes the closure head before it flows downwards and passes the steamplenum, thus entering the core as moderator and gap water. The cooling water flow, however, is onlymodeled up to the gaps in the control rod guide assemblies before the water passes the closure head.The other 75% of the cooling water flows downwards in the annular gap between the core barrel andthe RPV; it flows around the steam outlet pipe to cool it and to shield the RPV from high temperatures.Figure 1 shows the RPV with its internals. The feedwater flow is indicated with the blue arrows,whereas the red arrows indicate the superheated steam exiting the RPV, which is taken into accountas a boundary condition as pointed out below. The dashed lines indicate the limit of the analyzedregion.

The large Reynolds number of Re = 2,2 x ,calculated in the exit cross section of the supply

pipe of the backflow limiter, which has been designed and optimized by Fischer et al. [4] and isimplemented in the four inlets of the RPV, shows that the feedwater flow is highly turbulent. On thebasis of the first conceptual design of the RPV from Fischer et al. [5], the annular gap between theRPV and the core barrel has been isolated in order to build a CAD Model of the analyzed fluid volume.

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III. Numerical Model

All calculations have been carried out with the CFD software package STAR-CD version 3.26.Only steady state analyses have been performed and the standard high-Reynolds k--model has beenchosen. Due to symmetry, only of the total RPV has been modeled as shown in figure 1 and the cutsurfaces of the fluid domain were modeled as cyclic boundary conditions. The complex geometry of

the back flow limiter has been simplified to obtain a mesh with reasonable complexity and areasonable number of cells for the simulation. The Mesh has been generated with ProAmm, a subprogram of STAR-CD and consists of 570000 hexahedral cells.

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Fig.1 Feedwater flow in the RPV of the HPLWR and corresponding analyzed fluid volume

The inlet boundary, given by the backflow limiter described in [4], is simplified with an annularsurface. The inlet velocities are defined according to the results obtained by Fischer et al. [4] .For theinlet boundary, the value of the velocity has been defined with a radial component (7,7 m/s) and acircumferential component (9,8 m/s). For the external surface of the steam outlet pipe, a constanttemperature has been adopted according to [2]. At the outlet boundaries, an outlet condition has beenapplied with a defined flow split condition for the two outlet surfaces at the top (25%) and bottom(75%). Outlet boundary in this case means the assumption of zero gradient for the velocity.

IV. Results

First results have been obtained for the cooling water flow in the annular gap as seen in figure 2.The distance between the backflow limiter and the inner wall of the RPV is very small, which is shownin figure 2(B). The highest velocities are obtained near the inlet due to the high inlet velocity and thesmall gaps in this region. Here the velocity reaches values over 12 m/s in the gaps between the backflow limiter and the reactor pressure vessel. Also, near the outlet steam pipe, where the geometry isvery complex and the gaps get small, higher velocities are obtained. In the region around the steamoutlet pipe, the velocities are very small reaching values only in between 0,03 m/s and 0,25 m/s andthe flow has no uniform direction.

Fig.2 (A) Fluid velocity in the analyzed fluid volume

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Fig.2 (B) Velocities in cross section of the inlet region (A) and in the region around the steam outlet pipe (B)

The highest values of the heat transfer coefficients are obtained near the inlet where the inlet flowcollides with the wall of the RPV reaching values up to 50000 W/mK as shown in figure 3. In the flowaround the steam outlet pipe, the heat transfer coefficients are significantly lower due to the very smallvelocity of the cooling water. At the surface of the RPV, the heat transfer coefficients are almosthomogenous except near the steam outlet pipes where they reach a maximum value of 20000W/mKdue to the high velocities in the small gaps as described before. Millet et al. [2] have estimated a heattransfer coefficient of 2020 W/mK in the region around the steam outlet pipe and of 3020 W/mK forthe surface of the RPV. Table 1 shows a comparison between the estimated values by Millet et al. andthe heat transfer coefficients calculated with CFD.

Fig.3 Heat transfer coefficients of feedwater in the annular gap of the RPV and particularly in outlet steam pipe.

The temperature distribution is shown in figure 4. As predicted, the highest temperatures (maximum330C) are in the region of the steam outlet pipe. For the rest of the analyzed volume, an almosthomogenous temperature of around 280C is obtained. A slight temperature deviation of about 5C isfound in the recirculation zone behind the steam outlet pipe, which is shown in figure 4.

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Table 1 Heat transfer coefficients in the steam outlet flanges and in the inside surface of RPV.

Fig.4 Temperature distribution of the cooling water flow in the annular gap of the RPV

IV. Conclusions and Outlook

First results for the flow velocity, the temperature distributions, as well as for the heat transfercoefficients in the analyzed volume between the RPV and the core barrel have been obtained. It isfound that there is no uniform flow direction in the region around the steam outlet pipe. But theanalytical estimations of heat transfer could be confirmed. Since the velocities are very small in theheated region, heat transfer is small thus, which helps to minimize thermal stresses of the reactorpressure vessel.

Acknowledgements

This work has been funded by the European Commission as part of their project HPLWR Phase 2,contract number 036230.

References

1. T. SCHULENBERG, J. Starflinger, J. Heinecke Three Pass Core Design Proposal for a High

Performance Light Water Reactor, Proc. INES-2 Conf. 2006, Yokohama, Japan (2006); publishedin Progress in Nuclear Engineering (2007).

2. G. MILLET, K. Fischer, T. Schulenberg FEM-Simulation fr den Reaktordruckbehlter einesReaktors mit berkritischen Dampfzustnden, IKET FZK Karlsruhe, Germany (July 2007).

3. WATAA, T. Schulenberg, X.Cheng, J. Starflinger, Result of a coupled neutronics andthermalhydraulics analysis of a HPLWR assembly, Paper 5690, Proc. ICAPP 05, Seoul, Korea(2005).

4. K. FISCHER, E. Laurien, A. G. Claas, T. Schulenberg, Design and optimization of a backflowlimiter for the high performance light water reactor, Proc. GLOBAL 07, Boise, USA (2007).

5. K. FISCHER, J. Starflinger, T. Schulenberg, Conceptual design of a reactor pressure vessel andits internals for a HPLWR, Proc. ICAPP 06, Reno, USA (2006).

6. K. FISCHER, T. Schneider, T. Redon, T. Schulenberg, J. Starflinger, Mechanical design of corecomponents for a high performance light water reactor with a three pass core, Proc. GLOBAL 07,Boise, USA (2007).

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