XYLENE POWER LTD.

FNR INDICATOR TUBES

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

INDICATOR TUBES:
Indicator tubes are 6.8 m high buoyant vertical, coaxial thin wall steel tubes that project 0.2 m to 1.3 m above the surface of the primary liquid sodium pool. The outer tube is 8.625 inch OD, 0.148 inch wall (Schedule 10). The inner tube is 1.315 inch OD, 0.109 inch wall (Schedule 10). The annular space between the two tubes is argon filled. A standard FNR has 464 indicator tubes visible above the core zone of the reactor. In addition there are about 4700 buoyant 9 inch diameter stainless steel spheres floating on the primary liquid sodium.
 

INDICATOR TUBE PURPOSE:
The purpose of the indicator tubes is to indicate to the overhead FNR monitoring system:
a) The insertion depth of each movable fuel bundle into the matrix of fixed fuel bundles;
b) The gamma flux originating from each movable fuel bundle;
c) The liquid sodium discharge temperature of each movable fuel bundle.

Note that the floating sphere OD is selected to be similar to the Indicator Tube OD so that on average over the reactor core zone there are three floating spheres for each indicator tube to provide good surface coverage of the primary liquid sodium.
 

INDICATOR TUBE DESCRIPTION:
There are positively buoyant indicator tubes attached to the movable fuel bundles. These indicator tubes extend vertically 0.2 m to 1.3 m above the primary liquid sodium coolant surface. The indicator tubes are field attached to the lifting points on the movable fuel bundles.

Each indicator tube consists of a central 1.315 inch OD with 0.109 inch wall steel tube open at the bottom with a flat top hat containing a pin hole and a surrounding thermally isolating concentric sealed buoyant argon gas filled annular region defined by a 8.625 inch OD, 0.148 inch wall steel tube. The annular region thermally isolates the central region ensuring that the temperature of the liquid sodium in the central tube is approximately the same temperature as the liquid sodium discharge temperature of the associated movable fuel bundle.

The outside steel tube, which is thermally isolated from the inside steel tube, must withstand the external primary liquid sodium head pressure and the internal argon gas pressure. The bottom metal spacer and the top thermally insulating spacer between the inner and outer tubes keeps the annular region gas sealed.

The annular argon gas filled region, in combination with the movaable fuel bundle fuel tube plenum regions, provides a low density path for gamma rays to pass through the primary liquid sodium bath. The gammma ray flux indicates the relative fission power of each movable fuel bundle.

The central tube top cover has a small hole to vent gases trapped in the central tube. This top cover is made of a material that has a high near infrared radiation emissivity.

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

Note that the buoyancy of the annular region is not sufficient to lift the net weight of a movable fuel bundle when the indictor tube is fully immersed in liquid sodium.
 

THE CENTRAL TUBE TOP COVER:
The inner tube of each indicator tube assembly extends slightly above the outer tube. At the top each central tube is a 8.625 inch diameter flat thermally conductive top cover which is thermally bonded to the inner steel tube. This top cover has a small central hole that keeps the top pocket gas pressure inside the inner tube equal to the outside ambient gas pressure.

The bottom of this top cover is thermally insulated. Its flat upper surface emits infrared radiation which indicates its temperature and hence the temperature of the movable fuel bundle liquid sodium discharge. The precision and reproducibility of this temperature indication is an important aspect of FNR temperature monitoring.

In operation,the liquid sodium level in the indicator tube central tube is approximately the same as the liquid sodium level outside the indicator tube.
 

LASER TARGET:
Each indicator tube presents a 8.625 inch diameter round elevation target to an overhead laser 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 primary liquid sodium coolant surface.
 

TEMPERATURE INDICATORS:
This top cover emits IR radiation indicating its temperature.
 

GAMMA RAYS:
Gamma radiation passes up inside the fuel tube plenums, through the fuel tube top plugs, through the liquid sodium near the indicator tube attachment point, through the Indicator Tube bottom annular spacer, up the annular gas space inside the indicator tube, through the top annular thermal insulator and then through the indicator tube top hat. Note that the top annular insulator is normally out of the liquid sodium so its argon gas seal is not challenged by continuous direct exposure to hot liquid sodium. Note that the bottom annular spacer can be metal because at the bottom of the indicator tube the internal sodium and the external sodium are at the same temperature.
 

INDICATOR TUBE:
There are 6.8 m high buoyant 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 exposed height of the top of its indicator tube above the primary liquid sodium surface.
 

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 points are the movable fuel bundle lifting points. Each indicator tube has a dual J type bottom hook for attachment to the lifting points of a movable fuel bundle.
 

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 mo 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 also allow unobstructed primary liquid sodium flow and must not prevent individual fuel tube insertion or extraction.
 

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 7.4 m above the top of the movable fuel tubes. Hence the indicator tube itself is only about:
6.0 m - 0.5 m + 1.3 m = 6.8 m long.

The height allowances for the fixed fuel bundle components from bottom to top are: legs including bottom grating (1.5 m), fuel tubes including end plugs (6 m), top of fuel bundle (0.4 m), swelling allowance 0.1 m. Hence the fuel bundle shipping container and the air lock tube must be able to accommodate a fuel bundle with an overall fuel bundle length of 8.0 m.

This same air lock is long enough to accommodate indicator tubes.

Indicator Tube:
[(8.625 inch OD X 0.148 inch wall) + (1.315 inch OD X 0.109 wall)] X 6.8 m long

Indicator Tube Mass:
Mass = Pi X [(8.625 inch X 0.148 inch) + (1.315 inch X 0.109 inch] X 6.8 m X 7.874 g / cm^3
= Pi [1.2765 inch^2 + 0.14335 inch^2] X 6.8 m X 7.874 X 10^3 kg / m^3
= Pi [1.41985 inch^2] X 6.8 m X 7.874 X 10^3 kg / m^3

Displaced Volume = Pi[(4.3125 inch)^2 - (.5485 inch)^2] X 6.8 m
= Pi [18.59766 inch^2 - 0.30085 inch^2] X 6.8 m
= Pi [18.2968 inch^2] X 6.8 m

Average density = Mass / displaced volume
=[1.41985 inch^2] X 7.874 X 10^3 km /m^3 / 18.2968 inch^2
= 0.61103 X10^3 kg / m^3

When 1.3 m of the tube is projecting above the liquid sodium surface the mass stays the same but the displaced length decreases to:
6.8 m - 1.3 m = 5.5 m.
In effect the average density increases to:
0.61103 X 10^3 kg / m^3 X 6.8 m / 5.5 m = 0.7554 X 10^3 kg / m^3

The density of hot liquid sodium is:
0.84 X 10^3 kg / m^3
so that buoyancy is maintained. However, the indicator tube buoyancy is barely sufficient because in the above calculation there is no allowance for the weights of the indicator tube end spacers, the hook or the insulated top.
 

VERTICAL THERMAL EXPANSION:
Note that the open steel lattice near the bottom of the primary liquid sodium pool 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.  

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

The hydraulic fluid feed tubes are routed through the open steel lattice. These hydraulic tubes must be sufficiently flexible to allow for +/- 0.5 m earthquake induced movement of the fuel assembly with respect to the primary pool walls.
 

BUOYANT SPHERES:
In a cartesian coordinate system the Indicator tubes sharing the same X abcissa value have acenter to center separation of 26.25 inches. In a cartesian coordiants system Indicator tubes sharing the same ordiante value have a center to center separation of 26.25 inches. Along a45 degree angle indicator tubes have acenter to center separation of:
[2 (26.25 inch / 2)^2]^0.5 = (1 / 2^0.5)(26.25 inch)
= 18.56155 inch

Hence the maximum sphere diameter is:
18.56 inch / 2 = 9.28 inch.

For adequate Na surface coverage the minimum sphere diameter is 8.6 inch.

A well filled pattern consists of a sphere on the 45 degree angle between Indicator tubes and a sphere inthe biddle of this diamond shape. The spheres an the 45 degree angles are each shared betwen two adjacent cells. The Indicator tubes are each shared between four adjacent cells. Thus each cell consists of:
4 indicator tubes X (1/4)
+ 4 spheres X (1 / 2)
+ 1 sphere X 1
= 3 spheres / indicator tube.
 

TYPICAL COVERAGE FRACTION:
Each cell has an area of:
[26.25 inch / 2^0.5] = [26.25 inch]^2 / 2 = 344.532 inch^2

Assume that indicator tubes and spheres all have ODs of 8.625 inch. Each whole element occupies:
Pi (8.625 inch / 2)^2

Hence the Na surface coverage fraction is:
233.705 inch^2 / 344.532 inch^2 = 0.6783BR>  

CELL PATTERN PLAN VIEW:

I
O O
I O I
O O
I
where:
I = indicator tube
O = sphere
 

SPHERE BUOYANCY:
The sphere surface area is 4 Pi R^2.
The sphere mass = 4 Pi R^2 T Rho
where:
T = surface thickness
Rho = steel density

Sphere volume = (4 / 3) Pi R^3

Hence average sphere density is:
[4 Pi R^2 T Rho] / [(4 / 3) Pi R^3]
= [3 T Rho / R]

After allowing for weld weight the spheres should float about half immersed in liquid sodium. Note that the floating spheres will project about 5 inches above the liquid Na surface whereas the minimum projection of the indicator tubes is 0.2 m or 8 in‌ches above the liquid Na surface. This issue is important in laser scanning of the indicator tube top elevations.
 

SPHERE FABRICATION:
Each floating sphere is assembled from two hemispheres. Each hemisphere is made from a sheet stainless steel disk initially about Pi R in diameter. Radial slots with the desired width as a function of radius are cut using a laser cutter and then the sheet is bent around a hemispherical form. The slot edges are welded together to make the sheet steel hemisphere. Then two hemispheres are welded together to make a sphere. The sphere must then be sealed and leak tested. The finished spheres must have a maximum diameter of 9.2 inches and a minimum diameter of 8.6 inches.
 

This web page last updated August 14, 2022.