CSNI1016 THETIS.

1. NAME OF EXPERIMENT:  THETIS 80% BLOCKED CLUSTER EXPERIMENT.

3. DESCRIPTION OF TEST FACILITY

The THETIS facility consists of the main rig, the steam separator and holding vessel, power supplies, pressure control, reflood water supply, balloon pressure balance. The main pressure vessel houses the test cluster in its shroud tube. A vertical pressure tube may be connected in parallel with the main pressure vessel to simulate the downcomer in gravity reflood and boil down tests. The third vessel, which is connected for level swell boil up tests, contains an adjustable central weir which provides a constant head to control the liquid level in the test section. All three vessels are connected at their top ends and the bottom ends may be connected as required for the type of experiment to be performed.

The steam-water separator consists of an extension to the shroud tube surmounted by a liquid deflector, the whole being enclosed in a thin cylindrical housing. The liquid phase collecting in the bottom of the housing is passed out through a penetration of the pressure vessel wall into a holding vessel. The steam passes out through a 300 mm long 55 mm diameter chimney in the top of the housing which is fitted with an orifice plate in its upper end to measure the steam mass flow rate.

The test section power is supplied by four 200 kW rectifier units connected in series.
Two pressure transducers are used to measure the rig pressure, one connected to the upper part of the main pressure vessel and one to the bottom. The outputs from both transducers are recorded on the data logger.

Water can be drawn from one of two storage tanks on the rig which are filled with demineralised, deoxygenated water. One of the tanks may be heated to a temperature of up to 95 degrees C. Two pumps are installed each capable of delivering up to 0.75 kg/sec. The pumps may be used either singly or in parallel to deliver water from either tank.

4. DESCRIPTION OF TEST

A series of single phase cooling, forced reflood, gravity reflood and level swell experiments has been performed on a full length, electrically heated 7 x 7 rod fuel cluster containing a blockage simulating very severe clad ballooning.

The blockage was formed by fitting thin-walled Inconel balloons of length 455 mm to a 4 x 4 group of rods. These reduced the subchannel flow area by 80% over a length of 200 mm.

The single phase cooling experiments provided data on the level of heat transfer within the cluster and the enhancement produced by turbulence created by the spacer grids.

The forced reflood experiments have led to a better understanding of the rewetting processes during bottom reflooding, the very important influence of spacer grids in two-phase flow and the complicated heat transfer processes within the blockage. The insight obtained has been used to develop a mechanistic model which gives quantitative support to the explanations which have been put forward.

The gravity reflood experiments used a very simplified representation of the reactor system to investigate steam binding and inlet flow oscillation effects. They indicate that essentially the same phenomena are important in both forced reflood and gravity reflood. The variation in inlet flow produced by the variations in system parameters was the dominant influence. The inlet flow oscillations which occurred in gravity reflood appeared not to influence overall rewetting or heat removal performance.

The level swell experiments investigated the relationship between void fraction and superficial steam velocity at pressures up to 40 bar and compared the data with various correlations. The correlation of Gardner was found to be most satisfactory.

5. RESTRICTIONS ON THE COMPLEXITY OF THE PROBLEM: EXPERIMENTAL LIMITATIONS OR SHORTCOMINGS

6. PHENOMENA TESTED

Thermal hydraulic behaviour and coolability of a PWR core which has suffered clad ballooning in a LOCA, forming severe partial blocking.

- K.G. Pearson, C.A. Cooper and D. Jowitt:
The THETIS 80% Blocked Cluster Experiment
Part 1: Description of THETIS Rig, the 80% Blocked Cluster and
Experimental Procedures (September 1984)
- K.G. Pearson, C.A. Cooper and D. Jowitt:
The THETIS 80% Blocked Cluster Experiment
Part 2: Single-Phase Cooling Experiments (September 1984)
- K.G. Pearson, C.A. Cooper and D. Jowitt:
The THETIS 80% Blocked Cluster Experiment
Part 3: Forced Reflood Experiments (September 1984)
- K.G. Pearson, C.A. Cooper and D. Jowitt:
The THETIS 80% Blocked Cluster Experiment
Part 4: Gravity Reflood Experiments (September 1984)
- K.G. Pearson, C.A. Cooper and D. Jowitt:
The THETIS 80% Blocked Cluster Experiment
Part 5: Level Swell Experiments (September 1984)
- K.G. Pearson, C.A. Cooper and D. Jowitt:
The THETIS 80% Blocked Cluster Experiment
Part 6: Overview (September 1984)
- K.G. Pearson, C.A. Cooper, D. Jowitt and J.H. Kinneir:
Reflooding Experiments on a 49-Rod Cluster Containing a Long 90%
Blockage AEEW-R 1591 (January 1983)

11. TEST DESIGNATION:  The THETIS rig and the 80% blocked cluster test assembly

15. ESTABLISHMENT

AEA Technology
Winfrith Technology Centre
Dorchester DT2 8DH
United Kingdom

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