4.1. INTRODUCTION

Although a large amount of thermal hydraulic data exist from the beginning of nuclear reactor development there is still some need for additional experimental data. This is on one hand due to the focus on an area not well studied in the past — in this case natural circulation in nuclear reactor application — and on the other hand due to additional and more detailed requirements from modern CFD code developments. Because of the high costs for good experimental data it should be evident that new experiments should only be performed if existing data do not meet the requirements with respect to quality and degree of detail. If new data are required the very best instrumentation should be used, and, if necessary, new instrumentation should be developed. International cooperation is encouraged.

4.2. CURRENT STATUS OF THE EXPERIMENTAL WORK

Natural circulation phenomena have been investigatedto a large extent. There are a large number of experiments and facilities. As an example the experiments performed with APEX (US), SPES (Italy) and ROSA/LSTF facilities have given data of the same design (AP-600) with different scaling. Instead of studying the operational performance of the whole concept, the core make-up tank (CMT) behaviour was investigated with PACTEL facility. PANTHERS, PANDA and NOKO represent facilities which simulate different systems of the same design.

As an example of coordination between different facilities with different scaling and code work is the NACUSP research project. One of the main research items that have been identified for natural-circulation flow is the stability of two-phase natural-circulation flow. The former studies have been usually financed by national sources. Within the Euratom 5th framework programme, the NACUSP project is dealing with this research item. The goal of NACUSP is to improve the economics of operating and future plants through improved operational flexibility, enhanced availability, and increased confidence level on the safety margins regarding the stability issues in boiling water reactors (BWRs). The experimental part of this project focuses on natural-circulation loops being of specific value for the ESBWR.

cale

Experiments will be performed in 4 experimental facilities, which complement each other, ranging from small scale to large scale, and from low-pressure, low-power operating conditions, to nominal and high-power operating conditions. Figure 1 gives an overview of the facilities used.

The DESIRE (at Delft University of Technology) and CLOTAIRE (at Commissariat l’Energie Atomique) facilities will be used to study the natural circulation and stability characteristics at nominal pressures. This is important in view of the study of coupled neutronic/thermohydraulic stability, which is an issue for large BWRs. A good understanding and validation effort of the thermal hydraulic stability is essential as a basis for other work performed within NACUSP on reactor stability and in preparation for future work on regional reactor stability. Because of its flexibility the DESIRE facility will mainly be used to perform parametric studies [1], whereas the CLOTAIRE facility will be used to study the effects of upscaling. The CLOTAIRE facility was originally built to study PWR steam generators [2].

At low pressures the CIRCUS (at Delft University of Technology) and the PANDA (at Paul Scherrer Institute) facilities will be used to study the natural circulation and stability characteristics. It is known that at low-pressure, low-power operating conditions (e. g. start-up) natural circulation flow is susceptible to gravity-driven instabilities. Because of the high sensitivity of the flow to perturbations, these conditions are very challenging for computer codes to predict. CIRCUS will be used for parametric studies [1], whereas specific tests will be performed in the large-scale PANDA test facility. PANDA has already been used to study passive decay heat removal in the ESBWR [3].

The experimental work described above will be performed in close collaboration with computational work using a selection of computer codes. Table I gives an overview of the NACUSP partners and the computer codes used for analyzing the experimental data of the facilities.

TABLE I. NACUSP PARTNERS AND COMPUTER CODES TO ANALYZE TEST — FACILITY DATA.

4.3. EXAMPLES OF NC EXPERIMENTS

This section gives some examples of the NC problems being considered in new reactor concepts and relevant experimental works both performed earlier and to be performed in future.