XYLENE POWER LTD.

FNR OPEN STEEL LATTICE

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

INTRODUCTION:

The FNR Open Steel Lattice (OSL) is a rectangular lattice of braced vertical 8.625 inch OD steel pipes of alternating heights that support the fuel bundles.

The functions of the OSL are:
a) To durably support, position, vertically orient and rotationally orient the fuel bundles;
b) To incorporate the FNR Actuators for controlling the insertion of movable fuel bundles into the matrix of fixed fuel bundles;
c) To contain, protect and route the FNR actuator hydraulic pressure tubes with provision for earthquake induced fuel assembly movement;
d) To allow natural circulation of cooler liquid sodium to the lower ends of the fuel tubes.
e) To provide a sufficient below fuel tube sodium thickness to protect the bottom of the sodium pool from long term neutron damage;
f) To provide for future open steel lattice assembly/disassembly as might be required for FNR long term service.
g) To provide sufficient fuel bundle support lattice reinforcement to resist minor earthquakes ;
 

VERTICAL DIMENSION ASSUMPTIONS:
a) Level reference is the horizontal top surface of the bottom of the sodium pool;
b) The bottom of the sodium pool is covered with a 1 cm thick leveling plates to provide a level surface unaffected by bottom butt welds. Thee levelling plates also provide long term bottom protection;
c) the levelling plate are covered by a 2 cm thick positioning plate which has a precisely positioned hole for each 8.625 inch OD support pipe. The support pipe hole centers form a 11.8125 inch square grid;
d) The bottoms of each fuel bundle support pipe end cap is 1 cm above the sodium pool bottom top surface;
e) The top of each fuel bundle support pipe end cap is 4 cm above the sodium pool bottom upper surface.
f) The grating gap is 0.15 m;
h) The fin length parallel to the probe axis is 0.25 m;
i) At full insertion the bottom of all fuel tubes is 3.0 m above the top surface of the sodium pool bottom;
i) Via 4 machined notches for rotational orientation the support pipes can overlap the fins by 2 cm. j) Hence the large fuel bundle support pipe length exclusive of the end cap but inclusive of the notches is:
3.00 m - 0.15 m - 0.25 m + 0.02 m - 0.03 m - 0.01 m = 2.58 m;
k) The fuel bundle probes extend 1.1 m beyond the fins.
l) The length of the probe piston is:
0.11 m;
m) The piston travel is restricted to:
1.1 m - (piston length) - (notch depth)
= 1.1 m - 0.11 m - .02 m
= 0.97 m
n) Hence the small fuel bundle support pipe length is:
2.58 m - 0.97 m
= 1.61 m;
o) When fully withdrawn the piston bottom tip is:
1.61 m - 0.02 m - 1.10 m
= 0.49 m
above the bottom of the small fuel bundle support pipe.

The bottoms of the 8.625 inch OD pipes are positioned by holes in a plate that lies flat on the FNR sodium pool bottom

There are (1/ 4) inch X 1.5 inch steel strips that act as horizontal and angled braces to stabilize the vertical orientations of the 8.625 inch OD pipes.

The lattice center to center grid spacing is:
[42 X (9 / 16) inch ]/ 2 = 11.8125 inch.

The center to center distance along a horizontal diagonal is:
1.4142 X 11.8125 inch = 16.7052 inch

A horizontal diagonal length between taller pipe surfaces is:
16.7052 inch - 8.625 inch = 8.080 inches

Assume that this distance is reduced by 2.00 inches by 1 inch of fixing point protrusion at each end.

For 60 degree reinforcing diagonal braces the brace length between hole centers is:
(8.080 inch - 2 inch) X 2 = 12.16 inch. Hence the diagonal brace overall length is 13.66 inch

The pipes have 1.5 inch high X (1 / 8) inch deep circumferential groves machined on their surfaces.

The groves take surface ring fasteners, each 8.625 inch - 0.125 inch = 8.50 inch ID.

The inside perimeter length of each such fastener is:
8.5 inch X Pi = 26.703 inches

Each such fastener is fabricated from four strips of mild steel, 1.5 inch wide X (1 / 8) inch thick with length:
[(26.703 inch) / 4] + 4 inch = 10.676 inch.

Each such fastener piece has a 90 degree bend at each end to cause the last 2 inch of length to be normal to the pipe. This last 2 inch has a (1 / 2) inch hole to take a (1 / 2) inch bolt X 1.5 inch long, 2 washers and a nut. The diagonal stabilizing braces are 1.5 inch X (1 / 4) inch mild steel, 13.16 inch long, with (1 / 2) inch holes at each end and rounded corners.

The 2.58 m pipes are stabilized to each other using diagonal bracing near the top.

The 1.61 m high pipes are stabilized to the 2.58 m high pipes using similar grooves and fasteners with horizontal bracing near the top and bottom each:
11.8125 inch - 8.625 inch - 2.00 = 1.1875 inch long hole to hole or 2.6875 inch long overall.

Thus associated with each 2.58 m high pipe are 4 groves, 4 ring fasteners, 8 shared 13.16 inch long diagonal braces, 8 shared 2.1875 inch long horizontal braces.

Thus associated with each 1.61 m high pipe are two grooves, 2 ring fasteners, 8 shared 2.1875 inch long horizontal braces.
 

ASSEMBLY DETAIL:
The top surface of the sodium pool floor is made almost flat by use of plates of the same thickness as the weld reinforcement strips. The weld reinforcement strips and levelling plates protect the inside floor of the sodium pool from wear.

The hydraulic tube conduits and the 8.625 inch OD pipe caps rest on the weld reinforcement strips and levelling plates.

The hydraulic tubes run across the pool bottom at an amgle to utilize the available space under the fixed fuel bundles.

The FNR actuator cylinders fit into square spaces between fixed fuel bundles within the open steel lattice. The actuator cylinders connect to the hydraulic control tubes via bottom radial compression fittings.

The 8.625 inch OD pipe sections are prefabricated using precise numerical machining equipment and then are field assembled.

The top 0.25 m of each fuel bundle probe has four vertical fins. The fuel bundle corner legs aextend 0.4 m below the bottoms of the fuel tubes of which 0.25 m are welded to the fins.

Between each pair of adjacent fixed fuel bundles is a movable fuel bundle channel:
[19 X (9 / 16) inch] X [19 X (9 / 16) inch]
which provides a 1.1 m deep cavity into which moveable fuel bundles slide when they are withdrawan without blocking underneath natural sodium circulation.

The OSL is held together by the OSL base plate and diagonal bracing. The OSL is assembled from the center line going outwards to enable proper installation of bracing and positioning of the hydraulic tubing.
 

OPEN STEEL LATTICE:
The open steel lattice supports the entire weight of the fuel assembly. This steel lattice provides sufficient sodium separation between the core fuel rods 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.

Attached to each fuel bundle is a 1.35 m long probe of which 0.25 m are taken up by fins. For movable fuel bundles the probe is lifted by the underneath sodium hydraulic pressure. The piston is fabricated such that the supported weight of the movable fuel bundle causes the piston outside diameter to slightly increase, forming a sliding seal between the probe and the inside wall of the hydraulic cylinder.
 

FIXED FUEL BUNDLE SHUFFLING:
A fixed fuel bundle can be released from its socket by removing its 4 top diagonal clips and then lifting the fixed fuel bundle about 8 m using the overhead gantry crane so that its support pipe clears adjacent fuel bundles while the lower 5 m of overall module length remains immersed in liquid sodium.
 

EARTHQUAKE PROTECTION:
In a severe earthquake the OSL and its assembly of fuel bundles will tend to remain at a fixed distance from the horizontally moving walls of the sodium pool while the inertia of the liquid sodium causes that liquid to slosh about relative to the sodium pool walls.

The stainless steel hydraulic tubes have compression elbows at the bottom of the sodium pool side walls. The tubing then goes up the sodium pool side wall to the pool deck. The tubing then runs within another flat conduit to a wall mounted control valve panel. At each tube elbow there is clearancefor tube thermal expansion and contraction.

Note that the high pressure sodium for the movable fuel bundle actuators is delivered to the OSL via thin hydraulic tubes (0.25 inch ID) that have sufficient slack to safely flex. Provided that the movable fuel bundles and the rising hydraulic tubing can slide freely, the FNR should be safe against a severe earthquake.
 

FUEL BUNDLE DIMENSIONS:
The height allowances for the fuel bundle components from bottom to top are: projecting support pipe probe (1.1 m), bottom grating and grating clearance (0.15 m), fins (0.25 m), fuel tubes (6 m), lifting point (0.4 m), vertical swelling allowance 0.1 m. Hence the fuel bundle shipping container and the fuel bundle air lock must be able to accommodate fuel bundles with an overall length of 8.0 m.

The fuel bundle corner girders extend 0.4 m below the fuel tube bottoms for coupling to the fins.

The minimum fuel bundle airlock inside diameter is:
24 X (9 / 16) inch X (2)^0.5
= 19.092 inches
suggesting that it would be prudent to use a 24 inch pipe to form this airlock.
 

OPEN STEEL LATTICE MATERIAL AND DIMENSIONS:
The open steel lattice (OSL) is fabricated from HT-9 steel (85% Fe, 12% Cr, 1% Mo, 0% C, 0% Ni).The horizontal dimensions are the same as for fuel bundles.

HORIZONTAL CLEARANCE:
The present fuel bundle assembly design provides an ideal initial:
2(9 / 16) inch - 2 (3 / 16)inch = 0.75 inch clearance between an ideally fabricated movable fuel bundle and each of the adjacent fixed fuel bundles. With good fabrication dimensional tolerance control this clearance should be sufficient to allow for reasonable core zone material swelling.
 

The fixed octagonal fuel bundle maximum outside face to outside face distance is:
23 X (9 / 16) inch = 12.9375 inches.

The square movable fuel bundle maximum outside face to outside face distance is:
19 X (9 / 16) inch = 10.6875 inches.

To allow core region material swelling the bolted connections between adjacent fixed fuel bundles should have spacers of thickness:
2 {2 [(9 / 16)inch]^2}^0.5
= (18 / 16)(2)^0.5 inch
= 1.591 inch

An important issue in earthquake protection is proper bolting of the fixed fuel bundles and their supports 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 liquid sodium pool will thermally expand with increasing surrounding liquid sodium temperature. During normal reactor operation the bottom of the open steel lattice is likely to be about 60 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:
15 ppm / deg C X 60 deg C X 13.125 inch = 0.0118 inch
The fixed fuel bundle leg sockets must provide sufficient play to accommodate this differential thermal expansion.
 

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 (~0.3 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 movable square fuel bundle within the surrounding matrix of fixed octagonal bundles.

A fixed fuel bundle has corner girders which extend down 0.4 m below the fuel tubes to mate with support pipe fins and attach to the diagonal sheets that provide central stabilization and an upper central lifting point. On installation the corner girders of fixed octagonal fuel bundles connect to adjacent fixed octagonal fuel bundles by diagonal through bolts and spacers at the top of each corner girder and by sockets at the bottom of center. The sockets are integral to the open steel support lattice. The cast sockets are tapered at their tops to allow practical blind mating with the fuel bundle supports when the fueol bundle central pin is in place.

The corner girders of every fixed fuel bundle extend downwards below the bottom of the fuel fuel tube support grating. At the top of the fuel bundle 0.3 m 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 and the reinforced diagonal sheet extensions. These legs extend 0.4 m below the fuel tube bottoms to allow liquid sodium to easily flow into the bottom of the fuel bundles.
 

MOVABLE FUEL BUNDLE ACTUATOR:
To cause a movable fuel bundle to insert into the matrix of fixed fuel bundles liquid sodium at a presure of up to ___ psi is injected into the piston cylinder underneath the piston, which raises or lowers the movable fuel bundle and its indicator tube. The long 0.25 inch diameter orifice formed by each high pressure sodium feed tube limits the maximum rate at which a movable fuel bundle can be inserted into or withdrawn from the matrix of fixed fuel bundles.

In operation each movable fuel bundle's weight is borne by its actuator. the volume of the hydraulic fluid in the cylinder sets the amount of movable fuel bundle insertion into the matrix of fixed fuel bundles. The movable fuel bundle travel is limited by the movable fuel bundle support pipe length that projects beyond its fins(1.1 m). After allowing for the piston length the actual movable fuel bundle travel is only 1 m.

The movable fuel bundle bottom probe OD approximately matches the piston cylinder ID to keep the movable fuel bundle upright when the movable bundle is fully retracted and there are no adjacent fixed fuel bundles.

The fuel bundle probe pistons have bottom tapers to ease blind insertion onto the piston cylinders.

The vertical position feedback for a movable fuel bundle is via the indicator tube.

The hydraulic fluid feed tubes are routed along the bottom of the open steel lattice. These hydraulic tubes must be sufficiently flexible to allow for earthquake induced movement of the open steel lattice with respect to the sodium pool walls.

In the event of a hydraulic actuator failure that actuator may need to be replaced. The OSL must be designed to permit this replacement.
 

PASSIVE FUEL BUNDLES:
In order to achieve fuel bundle interchangability the passive movable fuel bundles are the same size and are mounted in the same manner as the active movable fuel bundles. However, the passive movable fuel bundles have no connected hydraulic tubing and hence default to the fully withdrawn position. since they liberate minimal inert gases part of their plenums cna be filled with depleted uranium.
 

MATERIAL REQUIREMENT:
The web page titled: FNR Geometry shows that a complete FNR has provision for 861 fixed fuel bundles and 828 movable fuel bundles. Hence the 8.625 inch OD pipe requirement is about:
861 (2.58 m) + 828 (1.61 m)
= 2221.4 m + 1333.1 m
= 3554.5 m

The web page titled: FNR Actuator indicates that the inside diameter of this pipe is:
6.875 inch

This pipe alone weighs:
48.8546 m^3 X 7.85 tonnes / m^3 = 383.5 tonnes

The mass of the levelling plates is:
Pi (10 m)^2 (.01 m) X 7850 kg / m^3 = 24.66 tonnes

The mass of the positioning plates is:
Pi (10m)^2 (.02 m) X 7.850 tonnes / m^3 = 49.32 tonnes

The mass of the circular fittings is:
[861 (3) + 828 (1)] (4) [1.5 inch X 0.125 inch X 10.676 inch) (.0254 m / inch)^3 x 7.85 tonne / m^3 = 3.51 tonnes

The mass of horizontal braces is:
[828 X 4 / 2] [1.5 inch X 0.25 inch X 2.6875 inch] (.0254 m / inch)^3 X 7.85 tonnes / m^3 = 0.2147 tonnes

The mass of diagonal braces is: [(861 X 4 X 2) / 2] X (1.5 inch X 0.25 inch X 13.66 inch)(.0254 m / inch)^3 (7.85 tonnes / m^3 = 2.27 tonnes

Hence exclusive of nuts and bolts the Open Steel Lattice has a mass of about:
383.5 tonnes + 24.66 tonnes + 49.32 tonnes + 3.51 tonnes+ 0.2147 tonnes + 2.27 tonnes = 463.47 tonnes

The open steel lattice will displace about:
(463.47 tonnes) / (7.85 tonnes / m^3) = 59.04 m^3 of liquid sodium.
 

This web page last updated March 19, 2026.