W. J.M. De Kruijf, T. H.J. J. Van Der Hagen, R. Zboray, A. Manera

Interfaculty Reactor Institute, Delft University of Technology

R. F. Mudde

Kramers Laboratorium voor Fysische Technologic, Delft University of Technology Netherlands

Abstract. At the Delft University of Technology two thermohydraulic test facilities are being used to study the characteristics of Boiling Water Reactors (BWRs) with natural circulation core cooling. The focus of the research is on the stability characteristics of the system. DESIRE is a test facility with freon-12 as scaling fluid in which one fuel bundle of a natural-circulation BWR is simulated. The neutronic feedback can be simulated artificially. DESIRE is used to study the stability of the system at nominal and beyond nominal conditions. CIRCUS is a full-height facility with water, consisting of four parallel fuel channels and four parallel bypass channels with a common riser or with parallel riser sections. It is used to study the start-up characteristics of a natural-circulation BWR at low pressures and low power. In this paper a description of both facilities is given and the research items are presented.

1. INTRODUCTION

Natural circulation is a key item in the design of innovative natural-circulation-cooled Boiling Water Reactors (BWRs). Instead of using recirculation pumps to provide the cooling flow for the core, the core flow is driven by the density differences between the two-phase mixture in the core and the essentially single-phase flow in the downcomer. The natural-circulation core flow is enhanced by using a riser section on top of the core. Because the core flow cannot be controlled by means of a pump, the recirculation core flow is an internal variable of the system. Natural circulation has been used in the early stages of reactor development. Both the Experimental Boiling Water Reactor (EBWR) [1] and the Vallecitos Boiling Water Reactor [2] could be used with natural circulation. Later, the Humboldt Bay atomic unit and the Dodewaard plant have been operated as commercial BWR/1 plants with natural circulation. More recently the interest in natural circulation as a possibility for the core cooling has been renewed. This can be seen in the design of the Simplified Boiling Water Reactor, based on which new designs such as the ESBWR have been proposed [3]. The trend in these designs with respect to the reactor core is towards larger cores and higher power, combined with larger risers to enhance the natural-circulation core flow.

Because the core flow responds to changes in power the stability of a natural-circulation BWR is somewhat different from the stability of a forced-circulation BWR. Therefore, the stability of a natural-circulation BWR requires special attention. It has been shown that two different instability types exist for such a reactor, denoted by type-I and type-II [4]. Type-I oscillations are typical for natural-circulation BWRs and are driven by the gravitational pressure drop over the core and riser. Type-II oscillations are driven by the interplay between single-phase and two-phase friction in the core. This division in different types is not sharp. The transition from one type to the other occurs gradually. Although the character of both types of oscillations is different one could describe both of them as density-wave oscillations.

The type-I oscillations may occur during the start-up phase of the reactor, because this unstable region broadens as the pressure decreases and because it is associated with low — power operating conditions. The flashing of water in the riser at low pressures induces this type of oscillation. The neutronic feedback is not important for this type of instability. The core region is essentially single-phase and thus the power oscillations will be small. Moreover, at such a low level power oscillations cannot cause any damage to the fuel. However, large flow oscillations should be avoided in view of their possible effect on structural materials. Different types of oscillations could be possible in view of the different possible configurations of parallel fuel bundles with common or parallel riser sections.

Type-II oscillations may occur during high-power/low-flow conditions in both natural — circulation or forced-circulation BWRs. Because the density in the core region fluctuates, the nuclear feedback is essential in the analysis of these types of oscillations. A division is made between core-wide, regional, and local oscillations. In core-wide oscillations the total power in the core will vary and all fuel bundles oscillate in-phase. This mode is favoured neutronically because of the subcriticality of the regional modes. In regional oscillations the total power will remain nearly constant and the power distribution will vary periodically. This mode is favoured thermohydraulically because the total flow will be nearly constant. A third type of oscillation is a local thermohydraulically unstable fuel bundle which induces small changes in the power and the power distribution.

The pressure dynamics and the feedwater (inlet subcooling) dynamics of the system should also be taken into account; this might especially be important for type-I oscillations in natural — circulation BWRS for which water flashes into large volumes of steam giving rise to large flow oscillations.

At the Delft University of Technology two thermohydraulic test facilities are being used to study the instability types in natural-circulation BWRs: DESIRE for type-II oscillations and CIRCUS for type-I oscillations. A description of DESIRE is given in Section 2, a description of CIRCUS is given in Section 3. Both facilities can be used to produce valuable experimental data needed for further model development in the system codes applied for nuclear power plant analyses[5],