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XYLENE POWER LTD.

FNR STEEL SUPPORT LATTICE

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

FUEL TUBE BUNDLE MATERIAL AND DIMENSIONS:
The fuel tube bundle frame and shroud are fabricated from HT-9 steel (85% Fe, 12% Cr, 1% Mo, 0% C, 0% Ni).

The height allowances for the octagonal fuel bundle components from bottom to top are: legs (1.5 m), bottom grating (0.1 m), fuel tubes (6 m), lifting point (0.3 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 length of 8.0 m.
 

HORIZONTAL CLEARANCE:
The present design provides an ideal 0.25 inch clearance between a mobile fuel bundle and each of the adjacent fixed fuel bundles. With good dimensional tolerance control this clearance should be sufficient to allow reasonable core zone material swelling.
 

7) The fixed octagonal fuel bundle maximum outside face to outside face distance is:
23 X (5 / 8) inch = 14.375.0 inches.

9) The square fuel bundle maximum outside face to outside face distance is:
19 X (5 / 8) inch = 11.875 inches.

11) To prevent overall fuel bundle swelling in the core region in that region the diagonal reinforcing sheets are reduced in width and the fuel bundle shroud sheets contain vertical slots to allow shroud and diagonal sheet swelling in the core region without causing significant overall horizontal fuel bundle width swelling.

21) An important issue in earthquake protection is bolting the fixed fuel bundles together to form a rigid matrix. We do not want liquid sodium sloshing back and forth to change the fuel assembly geometry and hence its reactivity.
 

THERMAL EXPANSION:
Note that the open steel lattice near the bottom of the primary liquid sodium pool will thermally expand with increasing surrounding liquid sodium temperature. During normal reactor operation the open steel lattice is likely to be about 150 degrees C cooler than the liquid sodium temperature at the top of the fuel bundle. Hence the differential horizontal width thermal expansion per fuel bundle is approximately:
20 ppm / deg C X 150 deg C X 13.125 inch = 0.039 inch
The fuel bundle leg sockets must provide sufficient play to accommodate this differential thermal expansion.
 

STEEL SUPPORT LATTICE:
The 1.5 m high open steel support lattice supports the entire weight of the fixed fuel bundles and stabilizes the hydraulic actuators. This steel lattice provides sufficient distance separation between the fuel bundles and the bottom of the sodium pool to ensure that there is no long term deterioration of the stainless steel pool bottom due to neutron absorption. This open lattice also allows free circulation of liquid sodium beneath the fuel tubes. A 1.2 m long push rod maintains separation between each mobile fuel bundle and its hydraulic actuator. This separation extends the working life of the hydraulic actuators. The fixed fuel bundle legs keep the fixed fuel bundles 1.5 m above the open steel lattice to protect that lattice from neutron damage. The sockets mounted on the top of the open steel lattice correctly position the fixed fuel bundles. However, a fixed fuel bundle can be released from socket by removing its 4 top bolts and then lifting the fixed fuel bundle a few inches using the overhead gantry crane.
 

MECHANICAL RIGIDITY CONSIDERATIONS:
A major issue in fuel bundle design is horizontal mechanical stability and rigidity because the overall fuel bundle height of 8.0 m is much greater than its width (.3016 m or 0.3651 m). Hence, the mechanical design of the fuel bundles is important to ensure that during fabrication, transport, installation and operation the fuel bundles do not bend, warp or otherwise deform. Such bending or warping could potentially cause a jam in the sliding of a mobile square fuel bundle within the surrounding matrix of fixed octagonal bundles.

A fixed octagonal fuel bundle has corner girders which extend past the fuel tubes to also serve as support legs and attach to the diagonal sheets that provide a central lifting point. On installation the corner girders of fixed octagonal fuel bundles connect to adjacent fixed octagonal fuel bundles by through bolts at the top of each corner girder and by cast sockets at the bottom of each corner girder. The cast sockets are firmly attached to the open steel support lattice. The cast sockets are tapered at their tops to allow practical blind mating with the fuel bundle legs with +/- 6 mm position error. The axis of the cast sockets lies at 45 degrees to the axis of the fuel bundle grid.

The corner girders of every fixed fuel bundle extend downwards 1.5 m below the bottom of the fuel fuel tube support grating. At the top of the fuel bundle 0.3 m X 3/8 inch diagonal sheet extensions provide lifting points for fuel bundle installation and removal. Short corner girder upward extensions allow use of bolts for connecting together adjacent fixed octagonal fuel bundles.
 

The entire weight of the fixed octagonal fuel bundles is supported by the four fuel bundle legs. These legs extend 1.5 m below the fuel tube bottoms to allow mobile fuel bundle travel, to allow liquid sodium to easily flow into the bottom of the fuel bundles and to minimize long term fast neutron damage to the open steel lattice. These legs are stabilized by leg to leg diagonal members attached to the inside of the corner girders.

In operation each mobile fuel bundle's weight is borne by its hydraulic actuator which sets the amount of mobile fuel bundle insertion into the matrix of octagonal fuel bundles. The mobile fuel bundle travel is limited at the bottom by the probe length (1.2 m) and the hydraulic cylinder end piece and piston thickness (0.3 m) and height of the steel lattice (1.5 m) and at the top by a hydraulic actuator vent hole.

The hydraulic actuator for a square mobile fuel bundle consists of a 1.5 m long hydraulic cylinder 9.75 inch ID, 10.75 inch OD + 9.74 inch OD piston which moves the bottom of a mobile square fuel bundle probe up and down, and is located in the open steel lattice. Each hydraulic actuator has a bottom fitting which mates with the corresponding hydraulic pressure line. The mobile fuel bundle bottom probe OD matches the hydraulic cylinder ID to keep the mobile fuel bundle upright when the mobile bundle is fully retracted.

The fuel tube spacing within a fuel bundle is maintained using a spiral 20 gauge wire winding on each fuel tube and the diagonal plates.


 

HYDRAULIC ACTUATOR DETAIL:
Within the 1.5 m high open steel support lattice are vertical hydraulic piston actuators each formed from a 1.5 m length of 10.75 inch OD steel pipe with an internal piston. The fuel bundle has a 1.2 m long push rod from the filter adapter bottom to the piston. About 0.3 m is dedicated to supporting the fuel bundle bottom filter, leaving 1.2 m for mobile fuel bundle extraction. This push rod holds the mobile fuel bundle vertical when the push rod is fully retracted. The push rod OD (9.74 inches) matches the hydraulic cylinder ID (9.75 inch). The push rod has a bottom taper for smooth insertion into the hydraulic cylinder.

The hydraulic piston has sealing piston rings similar to those in a diesel truck or marine engine.rod. The actuator piston has a tapered bottom edges for easy insertion into the hydraulic cylinder.

The extent of insertion of a mobile square fuel bundle into the fixed fuel bundle matrix is determined by the volume of liquid sodium inside the fuel bundle's hydraulic actuator. There is fluid pressure feedback which indicates the approximate mobile fuel bundle vertical position due to the changing buoyancy of the indicator tube. The hydraulic fluid feed tube is routed through the open steel lattice. This hydraulic tube must be sufficiently flexible to allow for +/- 1 m ______earthquake induced movement of the primary sodium pool with respect to its concrete enclosure.

In the event of a complete hydraulic cylinder jam an entire line of such hydraulic cylinders and related steel lattice extending half way across the pool may have to be removed and replaced.

To cause a mobile fuel bundle to insert into the fixed fuel bundle matrix liquid sodium at up to 100 psi is injected under the hydraulic piston which gives up to 7466 lb of lifting force to raise the mobile fuel bundle and its indicator tube. If the piston attempts to move too high the high pressure liquid sodium behind the piston is released into the primary sodium pool via a vent hole in the hydraulic cylinder side wall. This arrangement provides a certain upper limit on the piston travel. An orifice located on each high pressure sodium feed tube limits the rate at which a mobile fuel bundle can be inserted into the matrix of fixed fuel bundles. For normal piston position control an orifice restricted hydraulic drain valve is used. However, note that the mobile fuel bundle hydraulic drain valve used for reactor safety shutdown is not orifice restricted.
 

FIXED FUEL BUNDLE MASS:
Forming the outer shroud on each fixed fuel bundle are 4 X (3 / 16) inch thick steel sheets 11.6875 inches X 6 m. At the bottom are diagonal reinforciung bars for an equivalent volume of:
4 X 11.6875 inch X 8 m X (3 / 16) inch X (.0254 m / inch)^2 = 0.04524 m^3

Note that at the bottom of the fixed fuel bundle the shroud is replaced by diagonal reinforcing bars.

The equivalent mass of four fixed fuel bundle outer shroud plates is:
+ (0.04524 m^3) X 7.870 X 1000 kg / m^3
= 356.05 kg

The mass of the fixed fuel bundle corner girders is:
4 X 1.8047 inch^2 X 8 m X (0.0254 m / inch)^2 X 7.870 X 10^3 kg / m^3
= 293.10 kg

The fixed bundles have diagonal reinforcing sheets 17.41 inches wide. The mass of each diagonal sheet and its bottom equivalent in reinforcing bars is:
17.41 inch X 3 / 16 inch X 8 m X (0.0254 m / inch)^2 X 7.870 X 10^3 kg / m^3
= 132.59 kg / octagonal fuel bundle diagonal sheet.

The bottom steel grating of a fixed fuel bundle is composed of 44 pieces of .125 inch X 4.0 inch X 14.375 inch steel, for a total volume of:
44 pieces / bundle X .125 inch X 4.0 inch X 13.75 inch = 302.5 inch^3

The mass of each fixed fuel bundle's bottom grating is:
302.5 inch^3 X (0.0254 m / inch)^3 X 7.870 X 1000 kg / m^3 = 39.012 kg

From FNR Fuel Tubes each active fuel tube has a mass of 7.40kg___________

Each fixed octagonal fuel bundle contains 416 fuel tubes, each with a mass of 7.4 kg________. Hence the mass of fuel tubes is:
416 tubes X 7.4 kg_______ / tube = 3078.4 kg______

TOTAL FIXED OCTAGONAL FUEL BUNDLE MASS:
416 loaded fuel tubes + 4 shroud plates + 4 fixed fuel bundle corner girders + 2 diagonal plates + octagonal bundle grating
3078.4 kg_______ + 356.05 kg + 293.10 kg + 132.59 kg ______+ 39.012 kg
= 3899.152 kg

Note that the fixed octagonal fuel bundle bottom grating must transfer the weight of 416 loaded fuel tubes onto the outer corner girders which form the fixed fuel bundle legs. Immediately below the fuel tube support grating is the entrance filter.
 

MOBILE SQUARE FUEL BUNDLE MASS:
The shroud plates of a square fuel bundle weigh:
4 X [10 + (5 / 8)] inches X (3 / 16) inch X 6 m X (.0254 m / inch)^2 X 7.870 X 10^3 kg /m^2
= 242.76 kg / shroud.

The square fuel bundles each have diagonal reinforcing plates weighing:
[14.496] inches X (3 / 16) inch X 6 m X (.0254 m / inch)^2 X 7.870 X 10^3 kg /m^3
= 85.54 kg /plate
 

The square fuel bundles each have four corner girders weighing:
4 X (11 / 16) inch^2 X 6.5 m X (.0254 m / inch)^2 X 7.870 X 10^3 kg / m^3
= 90.75 kg

From FNR Fuel Tubes each active fuel tube has a mass of 7.40kg_______.

The mobile fuel bundles each have 280 fuel tubes each weighing 7.4 kg_______ for a total fuel tube weight of:
280 fuel tubes X 7.4 kg_______ / fuel tube = 2072 kg.

The bottom steel grating of a mobile fuel bundle is composed of 36 pieces of 0.125 inch X 4.0 inch X 11.25 inch steel, for a total volume of:
36 pieces / bundle X .125 inch X 4.0 inch X 11.25 inch = 202.5 inch^3

The mass of each mobile fuel bundle's bottom grating is:
202.5 inch^3 X (0.0254 m / inch)^3 X 7.870 X 1000 kg / m^3 = 26.11 kg

Thus the total weight of mobile fuel bundle is:
242.76 kg + 85.54 kg + 90.75 kg + 2072 kg + 26.11 kg = 2517.16 kg

The mobile fuel bundle bottom grating must transfer the weight of 280 loaded fuel tubes onto the fuel bundle's diagonal plates, shroud and outer corner girders and then to the push rod while still permitting unobstructed primary liquid sodium flow. The diagonal plates connecting the piston push rod to the square fuel bundle must allow unobstructed primary liquid sodium flow while bearing the load imposed by the mobile fuel bundle and while supporting the fuel bundle entrance filter.
 

PASSIVE FUEL BUNDLES:
In order to achieve fuel bundle interchangability the passive fuel bundles are the same size and are mounted in the same manner as the active fuel bundles. However, the passive fuel bundles are supported so that their square bundles are not mobile and will not fall out of the fixed fuel bundle matrix.
 

HYDRAULIC ACTUATOR COMPONENTS:
There is a 0.15 m high X 9.75 inch OD piston that slides within the 10.75 inch OD X 1.5 m hydraulic cylinder to cause insertion of the square fuel bundle into the octagonal fuel bundle matrix.
 

Hydraulic Cylinder:
Mass = Pi (10.750^2 - 9.759^2) inch^2 / 4 X 1 m X (.0254 m / inch)^2 X 7.874 X 10^3 kg / m^3
= 81.79 kg

Hydraulic cylinder bottom disk:
Mass = Pi (9.750 inch / 2)^2 X 0.5 inch X (.0254 m / inch)^3 X 7.874 X 10^3 kg / m^3
= 4.8168 kg

Hydraulic Cylinder Piston:
Piston Mass = Pi (9.750 inch / 2)^2 X 3 inch ____X (.0254 m / inch)^3 X 7.874 X 10^3 kg / m^3
= 28.91 kg
 

MAXIMUM COMPONENT SIZE:
Sooner or later some component of the steel support lattice assembly will need replacement. All such components must fit into the heat exchanger airlock. The height from the basement floor to the primary sodium pool deck is 19 m, so that height sets the maximum possible component length. This air lock has to be about 2 m deep to allow for heat exchanger connecting pipe and for the hydraulic piston assemblies. This airlock width is limited to about 1 m by adjacent heat exchanger radial piping and by the intermediat heat exchanger diameter. This airlock is physically off-set to realize sufficient pool deck floor space for its trap door.
 

This web page last updated December 30, 2020

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