Analysis of Flow Patterns in the I.R.I.S Reactor Downcomer and Lower Plenum during Regular Operation

Chapter 7

Conclusions and Development

A qualitative analysis of the thermal-hydraulic on downcomer and lower plenum phe-nomena was performed. The result of this phase of the work is the PIRT illustrated in section 4.3. Phenomena occurring in this portion of the cooling system were identified and ranked. A deeper study of the phenomena having the greatest relevance is requested in future activities. The purpose of this activity will be:

• to validate the PIRT and to better quantify the importance of phenomena; • to address design issues;

• to provide insight for proper scaling of the test facility to be set up at the

Uni-versity of Pisa, in order to properly select the instrumentation and run meaningful experiments.

On the basis of the present analysis some design modifications have been already proposed; some of them have been accepted and included in the latest design.

The CFD has been identified as a suitable tool to get quantitative information. Most important phenomena can be simulated separately in different sub-regions. Moreover the whole downcomer and lower plenum region can be simulated to get some insights into phenomena interaction. The attention has been focused on the entrance of the coolant flow into the downcomer from the steam generators.

Before modeling the downcomer inlet, the commercial code adopted has been verified on some experimental data available in literature for parallel confined jets (see references [7] and [8]). Six runs have been completed. In the first five runs a mesh sensitivity analysis has been performed: five different, step by step refined meshes, have been implemented and the

k−  turbulence model was adopted. The first mesh improvement actually lead to a better

agreement with experimental results; further mesh improvements are not effective. The obtained results can mimic qualitatively and quantitatively the axial velocity contour plots. Concerning the transversal velocity contour plots worse results were obtained; though the

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same order of magnitude of variables was achieved, their local distribution was found quite different. The CFD is also able to predict the flow structure; vortices in a plane perpendicular to the flow direction were detected, where the experimental data showed similar flow arrangement. The adopted CFD models substantially failed in the prediction of jet mixing; the jet decay is greatly under-estimated. The asymptotic trend of relevant variables was caught, but there is a large difference between experimental and calculated values. In order to improve the poor prediction of the jet decay, a more complex turbulence model, the SST model, was adopted in the sixth run. Results from this run show an even worse agreement with experimental data than k−  runs. Particularly, this run failed even in the prediction of the axial velocity profile in the inlet pipe. In a further code assessment, a turbulence models screening could be done; meshes with a very high resolution in the first portion of the confinement could be implemented. The possibly unsteady nature of the phenomenon could be investigated; other simulation approaches could be adopted.

The awareness achieved in this code verification, was utilized in the downcomer inlet simulation. Based on the benchmarking, it could be concluded that the CFD under-predicts the jet decay. Thus in the case of IRIS CFD simulations the jet has a reduced spreading in both radial and circumferential directions. In simulating the downcomer inlet four runs have been completed. The first two runs would have investigated the effects of a different inlet axial velocity profile; unfortunately, in both cases, in spite of the different inlet pipe length, a plug inlet flow profile has been achieved. The axial velocity contour plots show that the jet structure dissipates along the cylindrical portion of the downcomer and should not affect the velocity field in the lower plenum. Recirculation zones are visible in both circumferential and radial planes. The jet shows bigger diffusion towards the vessel wall than towards the barrel wall. The analysis of the velocity profiles shows that the jet has a non uniform influence on the surrounding water inventory. The flow within two adjacent steam generators is more involved; its velocity increases due do the momentum transfer. The fluid filling the region below the steam generator header is less influenced by the presence of the jet. It is likely that the shape of the steam generator outlet has a role in that. The steam generator outlet, and its downstream as well, could be modeled in such a way as to achieve a more uniform flow. The other two runs have been done with reference to a down scaled model, with a scale ratio of 1/5; the same scale will be adopted in the test facility. The comparison of the results for the full scale and the down scaled model will provide insight for proper scaling. In fact, two different scaling approach have been adopted. In the first of these two runs, the residence time was preserved; inlet velocity was taken equal to one fifth of the prototypical velocity. In the second run the inlet prototypical velocity was conserved in order to get a high Reynolds number (the preservation of the Reynolds number would imply such a high velocity that it could not be achieved in a test facility). The comparison of the obtained contour plots shows that the second approach has a better agreement with the full scale case. The axial velocity takes negative values,

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whereas in the residence time approach the axial velocity minimum value is equal to zero. Such a kind of analysis should be done for all the most important phenomena identified in the PIRT, in order to:

• review the IRIS downcomer and lower plenum design; • improve phenomena understanding;

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List of Acronyms

ASME American Society of Mechanical Engineers CFD Computational Fluid Dynamic

CRDM Control Rod Drive Mechanisms ECCS Emergency Core Cooling System

EMDAP Evaluation Model Development and Assessment Process FNS Full Navier-stokes

FRC Fractional Rate of Change FSA Fractional Scaling Analysis H2TS Hierarchical Two-Tiered Scaling

IRIS International Reactor Innovative and Secure IRR Internal Rate of Return

ISIS Inherently Safe Immersed System LOCA Loss Of Coolant Accident

LOFA Loss Of Flow Accident LWR Light Water Reactor

MIT Massachusetts Institute of Technology MOX Mixed OXide

MRX Marine Reactor X NPP Nuclear Power Plant PCJ Parallel confined Jets

BIBLIOGRAPHY 150

PNS Parabolized Navier-Stokes PWR Pressurized Water Reactor RCP Reactor Coolant Pump RCS Reactor Coolant System RPV Reactor Pressure Vessel RV Reactor Vessel

SARP Severe Accident Research Program SASM Severe Accident Scaling Methodology SG Steam Generator

SGTR Steam Generator Tube Rupture SLB Steam Line Break

SST Shear Stress Transport TLS Three Level Synthesis

UCB University of California at Berkeley