XYLENE POWER LTD.

FNR OPEN STEEL LATTICE

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

INTRODUCTION:
The FNR Open Steel Lattice (OSL) is an octagonal shaped corner connected assembly of fixed fuel bundle support modules 16.6 m diameter X 3.1 m high.

The functions of the OSL are:
a) To position, support and orient the fixed fuel bundles;
b) To orient and support the actuators of the square movable fuel bundles;
c) To establish the mpvable fuel bundle guide channels;<
d) To default position movable fuel bundles to the fully withdrawn position;
e) To contain, protect and route the FNR actuator hydraulic pressure tubes with provisions for earthquake induced fuel assembly movement;
f) To allow natural circulation of liquid sodium from the intermediate heat exchange bundes to the lower ends of the fuel tubes.
g) To ensure enough below fuel tube sodium to protect the bottom of the sodium pool from long term neutron damage;
h) To firmly position the fuel bundles such that in a severe earthquake the maximum vertical displacement of the movable fuel bundles with respect to the fixed fuel bundles is less than 1 mm;
i)To provide for fuel bundle shuffling and open steel lattice assembly/disassembly as required for FNR long term service.
 

VERTICAL DIMENSION ASSUMPTIONS:
a)Level reference is the top surface of the sodium pool bottom on the top surface of the levelling plates.
b) Bottom of fixed fuel bundle fuel tubes is at 3.5 m above the sodium pool bottom;
c) The overall length of the fuel bundle support pipes is 1.4 m;
d) The fuel bundle support pipe fin length is 0.3 m;
e) When the movable fuel bundles are fully withdrawn the movable fuel bundle fins rest on the actuator cylinder top. f) The projection length of the fuel bundle support pipe is:
1.4 m - 0.3 m = 1.1 m
g) (Actuator cylinder top)+ 1.0 m + 0.3 m + 0.1 m + 6 m + 0.5 m = 10 m
giving:
actuator cylinder top) = 2.1 m
h)Fixed fuel bundle cylinder top = 3.5 m - 0.1 m - 0.3 m
= 3.1 m
=top of fixed fuel bundle support module

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 that rest between the weld reinforcement s(trips. The weld reinforcement strips and levelling plates protect the inside floor of the sodium pool from wear.

The hydraulic tube conduits and the shared fixed fuel bundle support module feet rest on the weld reinforcement strips and levelling plates.

The fixed fuel bundle support modules are corner connected to form a matrix that bridges the hydraulic tube conduits.

The weight of the fixed fuel assembly is transferred to weld reinforcement strips and levelling plates via shared feet under the fixed fuel bundle support legs. The weight of the movable fuel bundles is transferred to the weld reinforcement strips and levelling plates via the inverted U conduits that protect the FNR Actuator hydraulic control tubes.

Short lengths of the same type of inverted conduit serve as spacers for orthogonal position stabilization of the shared feet of the fixed fuel bundle support modules.

The FNR actuators fit into square spaces between fixed fuel bundles and fixed fuel bundle support modules within the open steel lattice. The actuator cylinders connect to the hydraulic control tubes via bottom center taper fittings.

The fixed fuel bundle support modules and condut sections are prefabricated using precise numerical machining equipment and then are field assembled. Each fixed fuel bundle support module is precisely:
[21 X (9 / 16) inch] x [21 X (9 / 16)inch] X 3.1` m high.

The top 1.1 m of each fixed fuel bundle support module contains a central 8.625 inch OD pipeheld in place by 1.1 m long vertical fins. The corner legs are 3.1 m long out of which 1.1 m is welded to the fins.

Between adjacent fixed fuel bundles is a movable fuel bundle channel:
[19 X (9 / 16) inch] X [19 X (9 / 16) inch]
which provides a 1.0 m deep cavity into which moveable fuel bundles slide when they are retracted without blocking underneath natural sodium circulation.

The OSL modules have the same footprint as do the fuel bundles that they support. The OSL modules are held together by an eight section perimeter belt that has attachments designed for holding the OSL modules in position. The OSL modules are assembled from the center line going outwards to enable proper posioning of the hydraulic tubing.
 

OPEN STEEL LATTICE:
The open steel lattice and the hydraulic conduits support the entire weight of the fuel assembly and the FNR actuators. 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 1.4 m long fuel bundle support pipe of which 0.3 m are taken up by fins. For movable fuel bundles that pipe maintains separation between the movable fuel bundle and its hydraulic actuator piston. 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 piston 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 bolts 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:
The OSL is composed of modules, each with corner support legs that bridge the hydraulic tubing conduit that runs horizontally along the sodium pool bottom. The sodium pool has a bottom with a nearly flat upper surface that is covered by weld reinforcement strips.

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 provision for 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: support pipe probe (1.1 m), bottom grating and grating clearance (0.1 m), support pipe fins (0.3 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 diagonal plates for coupling to the fins.

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:
The actuator for a movable fuel bundle consists of a piston in a 2.1 m long cylinder. The piston moves the bottom of a movable fuel bundle support pipe up and down.

The piston cylinder top is axis stabilized to 0.1 m high,
[2 x (9 / 16) inch] X [2 x (9 / 16) inch]
corner girders with 0.1 m high vertical fins.

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).

About 0.3 m of movable fuel bundle length is dedicated to the fuel bundle gratings, frontspace and side fins.

The movable fuel bundle bottom support piston 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 support pipes 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 underneath the open steel lattice. These hydraulic tubes must be sufficiently flexible to allow for earthquake induced movement of the pen 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 constantly default to the fully withdrawn position.
 

This web page last updated November 29, 2025.