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

The FNR Open Steel Lattice (OSL) is an octagonal shaped open steel lattice assembly that is about 16 m diameter X 1.5 m high. The OSL is normally positioned centrally on the bottom of the primary sodium pool. The functions of the OSL are:
a) To support the fuel assembly;
b) To support the fuel assembly skirt;
c) To contain and position the actuators for the movable fuel bundles;
d) To contain and position the actuator hydraulic pressure tubes;
e) To allow cool liquid sodium to flow freely into the bottom of each fuel bundle;
f) To protect the bottom of the primary liquid sodium pool from long term neutron damage;
g) To provide low friction bearings between the upper surface of the bottom of the primary liquid sodium pool and the lower surface of the OSL such that in a seveere earthquake the fuel assembly and the OSL remain in position while the primary sodium pool walls and floor move with the surrounding ground.
h) In the event of fuel melting to prevent accumulation of acritical mass.

The 1.5 m high open steel lattice supports the entire weight of the fuel assembly 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 fuel bundle diagonal plate bottom extension maintains separation between each movable fuel bundle and its hydraulic actuator. This separation extends the working life of the hydraulic actuators. The fixed fuel bundle diagonal plate bottom extentions 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.

The fuel bundles are supported by an open steel lattice. In order to make the open steel lattice components replaceable this lattice is composed of 30 slices, such that each slice supports 3 X (~ 1 / 2) lines of fuel bundles. Hence the width of each open steel lattice slice is:
15.034 m / 15 = 1.002267 m

The length 0f the longest ~ half slice is:
11 X [19 X (5 / 8) inch] + 12 X [23 X (5 / 8) inch]
= 130.625 inch + 172.5 inch
= 303.125 inch
= 7.699375 m
= 7.7 m

Hence to allow for tolerance issues the inside dimensions of the equipment transfer airlock must be greater than:
1.1 m X 1.6 m X 7.8 m

The OSL has a nearly flat bottom surface. The primary sodium pool has a bottom with a nearly flat upper surface. Between these two surfaces is a layer of one inch diameter steel ball bearings. Thus, there is low friction connection between the OSL and the bottom of the primary liquid sodium pool.

In a severe earthquake the inertia of the OSL, the fuel assembly and the surrounding primary liquid sodium will tend to keep these masses in position while the walls of the primary sodium pool move with the earthquake vibrations. Note that the walls can move horizontally up to 0.8 m relative to the OSL before an impact between the OSL and the intermediate heat exchanger support columns can occur. If these columns fail the relative movement can be up to 1.7 m before the OSL impacts the primary sodium pool wall. That distance is believed to be sufficient to endure public safety during the largest recorded earthquakes.

Even if this severe movement damages the fuel bundles in the outer rings the interior movable active fuel bundles should still be free to slide vertically.

Provided that all the movable fuel bundles czn fully withdraw the FNR should be safe against a severe earthquake.

Note that the high pressure sodium for the movable fuel bundle actuators must be delivered to the OSL via thin hydraulic tubes that can safely flex 0.8 m in any direction when these tubes are fixed to the pool deck 16 m above. Note that in an earthquake these thin tubes are subject to considerable drag forces.

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.

The present design provides an ideal initial 0.25 inch clearance between a movable 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.

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

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

To prevent overall fuel bundle swelling in the core region in that region the shroud plates are eliminated.

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.

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 120 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 120 deg C X 13.125 inch = 0.0315 inch
The fixed fuel bundle leg sockets must provide sufficient play to accommodate this differential thermal expansion.

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 down below the fuel tubes to also serve as support legs and attach to the diagonal sheets that provide lower central support and an upper 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 supports 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 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 supports extend 1.5 m below the fuel tube bottoms to allow movable 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.

In operation each movable fuel bundle's weight is borne by its actuator which sets the amount of movable fuel bundle insertion into the matrix of fixed fuel bundles. The movable fuel bundle travel is limited at the bottom by its support length (1.2 m).

About 0.3 m of height is dedicated to the fuel bundle gratings and bottom filters.

The actuator for a movable fuel bundle consists of a 1.5 m long threaded rod with a nut which moves the bottom of a movable fuel bundle support up and down, and is located in the open steel lattice. Each actuator rod has a bottom thrust bearing and a worm gear drive driven by a bidirectional hydraulic motor. The hydraulic fluid is primary sodium. The movable fuel bundle bottom support ID matches the threaded rod OD to keep the movable fuel bundle upright when the movable bundle is fully retracted and there are no adjacent fixed fuel bundles. The bottom tube has a bottom taper for smooth blind insertion onto the threaded rod.

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

The extent of insertion of a movable fuel bundle into the fixed fuel bundle matrix is determined by the number of rotations of the hydraulic motor. The position feedback is bia the indicator tube. The hydraulic fluid feed tubes are routed through the open steel lattice. These hydraulic tubes must be sufficiently flexible to allow for +/- 2 m earthquake induced movement of the primary sodium pool with respect to the open steel lattice.

In the event of a hydraulic motor/drive failure a portion of the open steel lattice may have to be removed and replaced. The open steel lattice portions must be engineered to fit through the available airlock(s). These portions are bolted together to maintain their relative positions.

In this respect the open steel lattice consists of sections, each 1.5 m high X ~ 1 m wide X 8 m long.

To cause a movable fuel bundle to insert into the fixed fuel bundle matrix liquid sodium at up to 100 psi is injected to the hydraulic motor. This motor rotates the worm gear which drives the flat gear attached to the threaded rod which raises or lowers the movable fuel bundle and its indicator tube. An orifice located on each high pressure sodium feed tube limits the rate at which a movable fuel bundle can be inserted into the matrix of fixed 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.

Sooner or later some component of the open steel lattice assembly will need replacement. All such components must fit into an equipment transfer airlock. This air lock has to be about 2.5 m high to allow for heat exchanger connecting pipe and for the hydraulic piston assemblies. This airlock internal width is limited to about 1.5 m by adjacent intermediate heat exchanger radial piping and by the intermediate heat exchanger diameter.

Hence the open steel lattice is fabricated in 32 slices, the longest of which is 7.7 m long X 1.5 m high X 1.0 m wide. For other slices the hight and width are the same but the lengths are shorter. Each slice supports three half rows of fuel bundles. Each slice has its own hydraulic pressure connections for its movable fuel bundle actuators.

This web page last updated April 21, 2022.

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