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.

4.3.1. CAPCN test facility

A high-pressure natural convection loop (CAPCN) was constructed and operated to produce data in order to verify the thermal hydraulic tools used to design the CAREM integral LWR design, mainly the dynamical response. This is accomplished by the validation of the calculation procedures and codes for the rig working in states that are very close to the operating states of CAREM reactor. CAPCN resembles CAREM in the primary loop and steam generators, while the secondary loop is designed just to produce adequate boundary conditions for the heat exchanger. Water enters the heated section from the lower plenum. The nuclear core is simulated by electric heaters. The heated water flows up through the riser to the upper plenum where a liquid-vapor interphase exists. The water exits this plenum through an outer volume in contact with the steam generator. The steam generator has two coils, once through, secondary inside. The sub-cooled water flows down through a downcomer or cold leg to the lower plenum. Natural circulation flow may be regulated by a valve in the cold leg and a bypass to the bottom of the riser. This rig was constructed according to ASME for the following primary parameters: 150 bar and 340°C. The primary loop may operate in saturated or sub-cooled regimes, with a heating power up to 300kW and different hydraulic resistance.

The circuit configuration allows the study of stationary states similar to CAREM conditions of pressure, specific flow and enthalpy. Height was kept in a 1:1 scale.

Many experiments were performed in order to investigate the thermal-hydraulic response of the system in conditions similar to CAREM operational states. The influence of different parameters like vapor dome volume, hydraulic resistance and dome nitrogen pressure was studied. Perturbations in the thermal power, heat removal and pressure relief were applied.

The dynamic responses at low pressure and temperatures, and with control feedback loops were also studied. It was observed that around the operating point self-pressurized natural circulation was very stable, even with important deviation on the relevant parameters.

The data obtained is being used to test numerical procedures and codes. A sensitive analysis was performed and most sensitive parameters were identified. A representative group of transients were selected, in order to check computer models. Simulation models are in current development against a reference transient, without adjustment. When this contrast is clear, models will be compared against the representative group of transients. The information on specific models should be incorporated into CAREM modelling.

4.3.2. Driving forces of NC in the containment

In many passive safety systems natural circulation is an essential part of the system. The main aim is to use density differences in a pool or in a closed loop to transfer thermal energy from a source to a heat sink without using other energies than gravitational energy. This process can take place with or without phase changes.

Passive systems with phase changes can develop considerable driving forces. The pressure differences often are in the range from 1 kPa–100 kPa. Examples are the passive containment

37 cooling system, developed by General Electric, or the Emergency Condenser, developed by

Siemens.

Passive systems without phase changes have much smaller driving forces with pressure differences in the range from 1 Pa to 1000 Pa. An example is the building condenser, developed by Siemens. The natural circulation with single-phase flow is valid both on the primary and on the secondary side, as long as the temperature differences are small. But small driving pressures are not equivalent with ïneffective”. For example: heating up water from 5°C to 10°C gives a density difference of 0.3% kg/m³ (or 0.03%).

If the pool height is 5 m, then the maximum driving pressure is about 15 Pa. This corresponds to a vertical flow velocity of 0.17 m/s. Suggesting that the rising plume has a diameter of 2 m, the mass flow is about 500 kg/s and the thermal flux is about 10.5 MW.

In other passive systems natural circulation with pressure differences in the range <<1 Pa can determine whether a component can reach the right operating conditions or not. Assuming that radiolytic gas is accumulated from the original concentration 10^-5 by a factor 100in a “bull eye” of a conventional RPV level measurement, then the density difference is about 0.03 kg/m³. In a “bull eye” with a gas space of 5 cm height the driving pressure results to 0.015 Pa.

Nevertheless in a “bull eye” that was properly formed this small pressure difference was sufficient to initiate a counter-current flow (a special form of natural circulation) of the enriched gas mixture against the fresh steam inside the connecting pipe. So in this “bull eye”

no accumulation of radiolytic gas above the ignition concentration was possible. In another

“bull eye”, which was not properly designed, the opposite was true.

So the effectiveness of systems with natural circulation is not primarily a question of high driving forces but of the proper design. Normally a simple calculation is sufficient for the designer to calculate the driving forces in a balance with flow resistances. It is not necessary to know each detail of the flow fields like degree of turbulence, exact velocity profiles in different regions of the circuit or the exact form and flow velocity of a raising plume in a pool.

Rough calculations normally are sufficient to decide whether a system using natural circulation is properly designed or not.

Fundamental research seems to be necessary in all cases where components normally are tested with pure steam instead with nuclear steam, which always has a small content of radiolytic gas. To avoid handling pure hydrogen and pure oxygen in a laboratory it could be possible to generate “radiolytic gas” by an electorolytical apparatus continuously and in small quantities. Such experiments can be very instructive about the accumulation of the radiolytic gas, about the natural circulation effects in components with stagnating steam atmosphere and about the possibilities to get a passive transport of the enriched steam back to the RPV or to another line with intensive steam flow.

The opposite of natural circulation is stratification. In most cases where stratification takes place, it is neglected both by the designer and by the computer code. Experiments in the PANDA facility showed that stratification could become the essential effect in containment's atmosphere. So a deeper knowledge about the counterparts “natural circulation” and

“stratification” seem to be unavoidable to do the right calculations with mixed or with separated media in a containment or in a pool.

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4.3.3. Experiments for Indian AHWR

The proposed Indian AHWR is a vertical pressure tube type, heavy water moderated, boiling light water cooled reactor employing two-phase natural circulation as the normal mode of coolant circulation. It also relies on natural circulation for heat removal during shutdown and accident scenarios such as LOCA. Both the passive shutdown heat removal system and the passive containment cooling system employ condensers, which reject heat to large pools of water. To test the adequacy of the natural circulation systems employed and to validate the performance of the various computer codes developed several experimental facilities have been proposed. Some of them are already in operation and others are in various stages of implementation. A brief description of the various test facilities is given in the following to subsections.

The integral test loop is being set up to simulate the primary system thermal hydraulic behavior of AHWR during normal operating and accident conditions. The facility has been designed based on a 3-level approach. Primary system scaling is based on the power-to-volume scaling philosophy (level-1). Important local phenomena, which can potentially influence the integral system behavior (e.g. CHF, flow pattern transition, etc.), have been simulated (level-2). Boundary flows (accumulator injection, break flow, etc.) of energy and mass, which can significantly modify the integral system behavior have also been simulated (level-3). It is a full elevation and full pressure facility simulating the AHWR with a volume scale ratio of 400. The facility incorporates all the essential components of AHWR with velocity simulated to 1:1 scale. It can be used to generate experimental data on the steady state and stability behavior of AHWR. It also incorporates safety systems such as the shutdown heat removal system and emergency core-cooling system, which includes an advanced accumulator and the gravity driven cooling system (GDCS).

The high-pressure natural circulation loop is a simple rectangular loop, having similar geometry as one of the parallel channels of AHWR. It is a full pressure test facility set up to investigate the steady state and stability behaviour of natural circulation. The facility is already in operation. Experiments conducted in this facility include steady state two-phase natural circulation and stability of two-phase natural circulation. The parametric influences investigated include the effect of pressure, power and inlet sub-cooling. The facility has provisions to investigate the geometry effects such as loop height, pipe diameter, inlet and exit orificing. The facility is designed to operate up to 115 kg/cm² pressure, at 315^oC.

An experimental facility to study the flow pattern transition instability under 2-phase natural circulation conditions has been designed. The design pressure corresponds to 125 bar. It is proposed to carry out experiments on flow pattern transition instability with four different diameter tubes. This will also facilitate investigation of the effect of scaling on two-phase natural circulation. The facility has been set up and commissioned with instruments. To investigate the flow pattern transition, visualization of the flow patterns is possible using neutron radiography. This technique will also facilitate the measurement of the cross-sectional average void fraction and its profile, which will also be compared with similar measurements, obtained by conductance probes.

4.3.4. Experiment for AC600/1000 in China

When a station blackout accident occurs, the decay heat of AC600/1000 reactor core can be removed to the atmosphere as the ultimate heat sink through natural circulation flows established by the primary coolant loop, the secondary side cycle of SG and air cycle

39 respectively. Nuclear Power Institute of China (NPIC) has built a passive emergency residual

heat removal (ERHR) experiment facility in order to research the natural circulation flow characteristics for AC600/1000 ERHR system and to verify the computer code ERHRAC. The facility parameters are as the following:

Heating power: 80–400 kW

Design pressure in the secondary side of SG: 8.6 MPa Elevation difference between air cooler and SG: 7.2 m Equivalent diameter of chimney: 0.7 m

Height of chimney: 12 m

The core make-up water tanks are used in the design of AC600/1000 so as to eliminate the high-pressure safety injection pumps and increase reliability of the safety system. The function characteristics and coolant flow transient of core make-up water tank, accumulator and automatic depressurization system can be tested using NPIC core make-up water tank experiment facility to simulate a small LOCA. The facility parameters are as the following:

Design pressure: 17.2 Mpa

Design temperature: 310°C Core make-up tank volume: 1 m³ Accumulator volume: 1 m³

RPV volume: 0.76 m³

Pressurizer volume: 0.7 m³

Break sizes: 0.5,1,2,5,10 mm