THE PANDA FACILITY

The PANDA general experimental philosophy, facility design, scaling, and measurement concepts were defined in early 1991 [13]. On-site construction began in 1993 with the delivery of the major PANDA components. The facility is now complete and fully instrumented. The

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instrumentation includes numerous temperature sensors (including "floating thermocouples"

capable of measuring the temperature of the surface of water pools), pressure as well as dif-ferential pressure and water level difference sensors, flow rate measurements (over wide ranges), non-condensable fraction sensors (oxygen probes), phase (liquid or gas) detectors, electric power measurement, and sensors detecting the positions of valves; a total of some 620 channels. The computer-based data acquisition system (DAS) is capable of sampling all chan-nels continuously with a frequency of 0.5 Hz. For short periods of time, sampling with a "burst"

frequency of 5 Hz is also available. The DAS displays the data in a variety of "screens" repre-senting schematics of various parts of the facility [14]. The time histories (trends) of selected channels can be displayed at will. The facility is operated and controlled remotely and inter-actively by a computer-screen-based system.

The very first series of PANDA experiments have been steady-state tests of PCCS con-denser performance. These have been counterpart tests to similar tests conducted at the PANTHERS facility in Italy [7]. Extensive facility characterization tests will follow: the heat losses from the facility, as well as the actual pressure-drop-flowrate characteristics of the var-ious lines will be obtained. These are needed for the accurate description of the facility by computer codes. The actual transient system behavior tests will follow.

In relation to the SBWR certification effort, the PANDA facility was primarily designed to examine system response during the long-term containment cooling period. The overall objectives of the transient PANDA tests are to demonstrate that:

- The containment long-term cooling performance is similar in a larger scale system to that previously demonstrated with the GIRAFFE tests.

- Any non-uniform spatial distributions in the Drywell or Wetwell do not create significant adverse effects on the performance of the PCCS.

- There are no adverse effects associated with multi-unit PCCS operation and interactions with other reactor systems.

- The tests will also extend the database available for computer code qualification.

The initial test series at PANDA will include two Main Steam Line Break (MSLB) tests.

One test will be similar to a GIRAFFE MSLB test with uniform Drywell conditions, while a second is planned in a manner maximizing the influence of Drywell asymmetries on the oper-ation of the PCCS condensers. Uniform and asymmetric Drywell conditions can be created in PANDA via the capability to vary the fraction of steam flow which is injected into each of the interconnected Drywell vessels, Fig. 2. The steam condensing capacity directly connected to each vessel (i.e., the number of condenser units) can also be varied. Such tests and several systematic variations of the experimental conditions will help identify any scale and multi-dimensionality effects that may not have been present in the smaller scale facilities. The test matrix for further tests is dictated by the needs of certification and code assessment and is still evolving. Steam and/or non-condensables can be injected at various locations hi the Drywell vessels to study mixing phenomena and to provide envelope information for the corresponding SBWR conditions.

3.1. Scaling Considerations

A rigorous scaling study [15] covering all the SBWR related tests describes the scaling rationale and details of the PANDA facility. Other documents cover particular aspects of scal-ing [10,16,17,18].

The SBWR containment is particularly complex and thermal-hydraulically coupled to the primary system. In addition to the complexity of the usual BWR pressure suppression system and its various components, such as the main vents, one must now also consider the operation of the particular PCCS system and its components. Both "top-down" and "bottom-up" scaling considerations [19] and criteria were developed.

According to the "top-down" approach, general scaling criteria are derived by considering the processes controlling the state of classes of containment sub-systems (e.g., containment volumes, pipes, etc.). Close examination of the specific phenomena governing the operation of certain system components (e.g., vents immersed in the Pressure Suppression Pool of the SB-WR) leads to "bottom-up" scaling rules.

3.1.1. Top-Down Scaling

Generic scaling criteria for thermal-hydraulic facilities, such as those proposed by ISHn and KATAOKA [20], are not specific to the combined thermodynamic and thermal-hydraulic phenomena taking place inside containments. Thus, specific scaling criteria for the design of facilities simulating the dynamic operation of BWR containments such as PANDA were de-rived [15]. For example, mass and energy transfers take place between containment volumes through their junctions. Heat may also be exchanged between volumes by conduction through the structures connecting them. These exchanges lead to changes in the thermodynamic con-dition of the various volumes; this, in particular, leads to changes of the volume pressures. The junction flows (flows between volumes) are driven by the pressure differences between volumes. Thus, the thermodynamic behavior of the system (essentially, its pressure history) is linked to its thermal-hydraulic behavior (the flows of mass and energy between volumes).

Fig. 2: Schematic Representation of the PANDA Facility.

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The "top-down", generic scaling criteria were derived by considering generic processes, including:

(1) The effects of the addition of heat and mass to a gas or liquid volume (namely, the resulting rates of change of the pressure).

(2) The rates of phase change at interfaces such as pool surfaces.

(3) Hows of mass between volumes.

Prototypical fluids, i.e. those pertaining to the actual reactor system, under prototypical thermodynamic conditions, are used in PANDA. The fact that the fluids are expected to be in similar thermodynamic states, and have similar composition in the prototype and the model, can be used to simplify the analysis and scaling of the facility. The first two processes listed above (1 and 2) confirm the validity of the (familiar) scaling of all the following variables with the "system scale", R:

(power)R = (volume)R = (horizontal area in volume)R= (mass flow rate)R = R

where the subscript R denotes the ratio between the corresponding scales of prototype and model. Although other choices are also possible, the system scale R can be defined as the ratio of prototype to test facility power input. For PANDA, R = 25. A time scale of 1:1 between prototype and model has been adopted for the presently planned PANDA tests; however, this is not a necessity. Under certain conditions, the choice of a scale for the volumes different from the system scale R will lead to accelerated (or decelerated) tests in time.

Process (3) leads to the determination of the pressure drops and of the hydraulic charac-teristics of the junctions between volumes. In the BWRs, certain pressure drops and the corresponding junction flows are controlled by the submergence depth of vents in the Pressure Suppression Pool. The analyses of these processes justify the choice of 1:1 scaling for the vertical heights in general and for the submergence depths in particular [15].

The pressure evolution resulting from the thermodynamics of the system and the pressure drops between volumes must clearly scale in an identical fashion. Considering the fact that prototypical fluids are used, this requirement links the properties of the fluid (in particular the latent heat and the specific volumes of water and steam) to the pressure differences between volumes (and to the submergence depths of vents), resulting in 1:1 scaling for pressure drops.

Thus, the above considerations result in:

1:1 scaling for pressure differences, elevations and submergences.

This scaling rule determines the pipe diameters, lengths and hydraulic resistances, and the transit times between volumes. These transit times should, in principle, have the same (1:1) time scale as the inherent time constants of the system considered in the analysis of process (1).

This matching cannot be perfect, but is shown not to be important [15].

The criteria derived are combined to arrive at general scaling laws for the PANDA model of the prototype SBWR. Several non-dimensional numbers, three time scales and certain geo-metric ratios must be matched.

The three time scales produced by the analysis are the scales for the rates of volume fill, of inertial effects, and of pipe transfers. Clearly, the systems considered here are made of large volumes connected by piping of much lesser volumetric capacity. The pressure drops between these volumes are not expected to be dominated by inertial effects. Thus, the inertia and transit times, which are of the same order of magnitude, are much smaller than the volume fill times.

Consequently, the important time scale that must be considered in scaling is the volume fill time scale. The other two time scales are clearly of lesser importance. In relation to this

con-elusion, the lengths of piping connecting containment volumes and the velocities in these pipes do not have to be scaled exactly. Usually (and fortunately), the total pressure drops in the pip-ing are dominated by local losses, so that the total pressure drops hi the scaled facilities end up being somewhat smaller. They can therefore be matched by introducing additional losses by local orificing.

3.1.2. Scaling of Specific Phenomena - Bottom-Up Approach

Bottom-up scaling for phenomena that were selected as being of particular importance by a Phenomena Identification and Ranking Table (PIRT) exercise were examined in detail to arrive at then: proper simulation in the PANDA facility [15]. Several documents cover in detail such particular aspects of scaling [10, 16,17,18,21], as already noted.

The scaling of thermal plumes, mixing and stratification phenomena in the pool, as well as in the Drywell volume, heat and mass transfers at liquid-gas interfaces, the heat capacity of containment structures, and heat losses were examined in detail in [15]. Of particular impor-tance is the scaling of the various vents, discharging mixtures of steam and non-condensable gases into the Pressure Suppression Pool. This is important in relation to the possibility of steam "bypassing" this pool and entering directly into the Wetwell gas space. Heat and mass transfer in the PCCS condensers must also be properly scaled, considering both condensation inside the tubes and heat transfer on the secondary pool side [21]. The latter may be affected by any induced natural circulation.