XYLENE POWER LTD.

FNR INDICATOR TUBES

By Charles Rhodes, P.Eng., Ph.D.

INDICATOR TUBE DESCRIPTION:
An indicator tube is a sealed buoyant hollow metal tube filled with argon. The lower end of each indicator tube is hooked on to the lifting point of a movable fuel bundle after the movable fuel bundle has been installed. The upper end of each indicator tube projects above the liquid sodium surface. Each indicator tube has a metal top cap that is shaped nearly flat for use with an overhead monitoring system.
 

INDICATOR TUBE PURPOSE:
The purposes of an indicator tube are to indicate to the overhead FNR overhead monitoring system:
a) The actual vertical position of a particular movable fuel bundle in the matrix of fixed fuel bundles;
b) The liquid sodium discharge temperature for the particular movable fuel bundle.
 

INDICATOR TUBE USE:
There are 6.8 m high buoyant argon filled indicator tubes field attached to the lifting points of movable active fuel bundles. The vertical position of each active movable fuel bundle is visually indicated by the 0.2 m to 1.3 m height of the of the top of its indicator tube above the liquid sodium surface.

The temperature of the indicator tube, as indicated by its infrared emission, is a good indication of the temperature of the liquid sodium in the immediate proximity of the indicator tube.

The indicator tube provides sufficient positive buoyancy that when 1.3 m of the indicator tube is projecting above the liquid sodium surface the indicator tube maintains an upright vertical position due to its positive buoyancy in hot liquid sodium.

Note that the buoyancy of the indicator tube, when the indictor tube is fully immersed in liquid sodium, is not sufficient to lift the weight of the movable fuel bundle to which it is attached.
 

INDICATOR TUBE ATTACHMENT:
Indicator tubes are attached to the movable fuel bundle lifting points after the movable fuel bundles are installed and are removed before the movable fuel bundles are repositioned. The indicator tube attachment point is the movable fuel bundle lifting points. Each indicator tube has a dual J type bottom hook for attachment to the lifting point.
 

LIFTING POINTS:
A FNR fuel bundle lifting point is achieved by replacing the (3 / 16) inch thick diagonal plates with (3 / 8) inch thick diagonal plates in the upper portion of the fuel bundle where there are no fuel tubes. Two 3.0 inch diameter holes in each diagonal plate form the lifting points.________

The diagonal plates connecting each fuel bundle lifting point to the corresponding fuel bundle corner girders must allow unobstructed liquid sodium natural circulation and must not prevent individual fuel tube insertion or extraction.
 

SAFETY FUNNCTION
An important safety function of the indicator tubes is forcing a reactor cool shutdown if a heavy object falls on top of the FNR. That object will cause a high enough liquid sodium hydraulic pressure to force the movable fuel bundle to withdraw.
 

MOVABLE FUEL BUNDLE TRAVEL LIMIT:
The movable fuel bundle vertical travel is limited to 1.1 m by the FNR actuator design.
 

INDICATOR TUBE DESCRIPTION:
Each indicator tube consists of a 8.625 inch OD, 0.148 inch wall steel tube with a flat top cap. The tube is sealed and filled with argon at ambient pressure. This design ensures that the temperature of the indicator tube is approximately the same as the liquid sodium discharge temperature of the associated movable fuel bundle.

The steel tube, must withstand the external liquid sodium head pressure and the temperature dependent internal argon gas pressure.
 

INDICATOR TUBE TOP COVER:
At the top each indicator tube is a 8.625 inch diameter nearly flat top cover.
 

LASER TARGET:
Each indicator tube presents a 8.625 inch diameter round elevation target to a celing mounted laser raster scanner. If the distance from the laser to the apparent target position is too long it means that the laser is not looking at a valid target.

Note that when the movable fuel bundles are fully inserted into the matrix of fixed fuel bundles the indicator tube top covers are about 1.3 m above the liquid sodium coolant surface.
 

INDICATOR TUBE MATERIAL AND DIMENSIONS:
The indicator tube diameter should be minimal to minimize obstruction of the natural liquid sodium circulation, but must be sufficient to allow accurate steady state movable fuel bundle liquid sodium discharge temperature measurement.

The indicator tubes are fabricated from HT-9 steel (85% Fe, 12% Cr, 1% Mo, 0% C, 0% Ni). When the movable fuel bundle is fully inserted the length of the indicator tube plus hook places the top of the indicator tube 1.3 m above the liquid sodium surface. Hence the indicator tube itself is about:
5.5 m - 0.2 m + 1.3 m = 6.6 m long.

The fuel bundle air lock inside length is sufficiemt to accommodate indicator tubes.

Indicator Tube:
[(8.625 inch OD X 0.148 inch wall)] X 6.8 m long

Indicator Tube Mass:
Mass = Pi X (8.625 inch X 0.148 inch) X 6.6 m X 7.874 g / cm^3
= Pi [1.2765 inch^2 X 6.8 m X 7.874 X 10^3 kg / m^3 X (0.0254 m / inch)^2
= 138.53 kg + (2 X cap mass) + hook mass.

Displaced sodium Volume = Pi[(4.3125 inch)^2] X (6.6 - 1.3) m
= Pi [18.59766 inch^2] X 5.3 m

The density of hot liquid sodium is:
0.84 X 10^3 kg / m^3

Displaced sodium mass:
= Pi [18.59766 inch^2] X 5.3 m X 0.84 X 10^3 kg / m^3 X (0.0254 m / inch)^2
= 167.81 kg

Hence the indicator tube is sufficiently buoyant.
 

VERTICAL THERMAL EXPANSION:
Note that the open steel lattice and the fuel bundles will thermally expand vertically with increasing surrounding liquid sodium temperature. During normal reactor operation the open steel lattice is likely to be about 120 degrees C cooler than the liquid sodium temperature at the top of the fuel bundle. The thermal expansion will be significant and will affect the calculation of the movable fuel bundle insertion into the matrix of fixed fuel bundles unless temperature compensating measurements are performed. Hence the overhead laser scanner may need need a compensating fixed fuel bundle elevation measurement.

The differential vertical thermal expansion per fuel bundle is approximately:
20 ppm / deg C X 430 deg C X 16.5 m = 0.1419 m
Hence it is essential that the laser scanning system cancel out vertical thermal expansion.  

This web page last updated December 12, 2025.