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

FNR power is regulated by thermal expansion of the fuel assembly. Thermal expansion is typically 10 ppm / degree C. If the reactor setpoint is being regulated to within +/-10 degrees C and if the insertion depth of the actuator length is typically 2.5 m then the equivakent range of thermal expansion is about:
+/-10 degrees C X 10 X 10^-6 / deg C X 2.5 m = +/- 2.5 X 10^-4 m = +/- 0.25 mm

Hence the hysterisis and other error in the actuator must be small compared to 0.1 mm.

A FNR Actuator is essentially a screw jack that raises or lowers a movable fuel bundle over a range of 1.1 m for insertion or withdrawal of that movable fuel bundle from the matrix of fixed fuel bundles. This insertion must be precise, smooth and hysterisis free. The weight of the movable fuel bundle less its buoyancy force in liquid sodium less the buoyancy force caused by the Indicator Tube is constantly acting downwards on the actuator. The mass of a FNR movable fuel bundle is about 1.7 tonnes. A hysterisis free vertical position resolution of about 0.01 mm is desired.

If the jack screw has 4 threads / inch or one thread / 6 mm, each 360 degree rotation of the jack screw advances the relative position of the movable buel bundle with respect to the fixed fuel bundle by 6 mm. Hence the jack screw rotation corresponding to a 0.01 mm advancement is:
(0.01 mm / 6 mm) X 360 degrees = (360 / 600) degree = 0.6 degree.

Assume that the jackscrew is fitted with a flat gear having 60 teeth. Then a one tooth advancement of this flat gear causes an actuator advancement of:
6 mm X (1 / 60) = 0.1 mm

Then we can obtain a 0.01 mm actuator advancement by use of a worm gear with at least 10 resolvable positions. Note that the worm gear shaft must be free of axial play.

The reactor setpoint temperature is controlled by the amount of movable fuel bundle insertion into the matrix of foxed fuel bundles. This net insertion is a function of the difference in length between the actuator and the fixed fuel bundle supports, which consist of the fixed fuel bundle legs plus the height of the open steel lattice.

A condition that we need to avoid is zero power setpoint drift. If at zero power the temperature of the sodium causes the movable fuel bundle to net insert further into the matrix of fixed fuel bundles then the setpoint will spontaneously rise. To avoid this problem:
(TCE of the actuator) X (actuator length) < (TCE fixed fuel bundle support) X (fixed fuel bundle support length)

If both are formed from the same material this condition is normally met because generally:
(actuator length) < (fixed fuel bundle support length)

If these conditions are met a rising sodium pool temperature tends to reduce the reactor temperature setpoint which provides negative feedback for setpoint stability.

For the same reason Earthquake induced changes to the vertical displacement of movable fuel bundles with respect to fixed fuel bundles must be kept less than 1 mm to prevent an earthquake from initiating a sodium void instability prompt neutron critical event. This requirement suggests use of screw jack actuator with a worm gear drive.

Consider the open area for sodium rising through a movable fuel bundle between the fuel tubes. At most this area is:
[(9 inch / 16)^2 - Pi (.375 inch / 2)^2] / tube X 248 tubes
= [0.3164 - 0.11044] inch^2 X 248
= 0.20596 X 248 inch^2
= 51.078 inch^2

This area will be slightly reduced by the spiral wire winding and by cumulative fuel tube swelling.

To obtain the required bending resistance we choose a movable fuel bundle support pipe which is 8 inch schedule 160 steel pipe that has an OD of 8.625 inch and an ID of 6.813 inch.

The open area potentially available around the outside of this pipe is:
[19 (9 / 16) inch]^2 - Pi [8.625 inch / 2]^2
= 114.222 inch^2 - 58.4262 inch^2
= 55.7958 inch^2

Hence the 8.625 inch OD pipe cross section should not significantly impede the vertical sodium flow provided that sodium can flow laterally over the top of the movable fuel bundle support pipe.

The actuators fit into the open steel lattice that supports the FNR fuel bundles.

The dedicated area available for the actuator mechanism is:
19 (9 / 16) inch X 19 (9 / 16) inch
= 10.6875 inch X 10.6875 inch

Further space for the actuator hydraulic motors is obtained by overlapping adjacent fixed fuel bundle area. Note tha at the bottom of the open ssteel lattice this space is available because the fixed fuel bundles are supported by the open steel lattice more than 1 m above the actuators.

The movable fuel bundle grating area potentially obstructed by the movable fuel bundle support pipe is:
Pi (8.625 inch / 2)^2

The entry area for sodium flowing over the top of the movable fuel bundle support pipe is:
Pi (8.625 inch) D
where D is the axial distance between the top of the support pipe and the bottom of the grating. In order to maintain the same cross sectional area for sodium flow:
[Pi (8.625 inch) D] / [Pi (8.625 inch / 2)^2] = 0.5
4 D / 8.625 inch = 0.5
D = 8.625 inch / 8
~ 9 / 8 inch

These parameters indicate that the threaded shaft should be 6.0 inches OD which on the diagram is approximated by 49 / 8 inch

The actuator shown contains a (49 / 8) inch diameter 1.5 m high vertical threaded steel shaft located directly under the center of the movable fuel bundle. The bottom of this shaft is supported by a thrust bearing. This shaft has a:
6.625 onch OD = 53 / 8 inch OD axial drive gear mounted close its bottom end. This axial drive gear is realized by machining teeth onto the outside of a piece of 6 inch schedule 80 pipe (6.625 inch OD, 5.761 inch ID). The bottom end of the threaded shaft is machined down to 5.761 inches (146.329 mm) to accommodate the axial drive gear and is further reduced to 140 mm diameter to fit the bottom thrust bearing. This axial drive gear is driven by a hydraulic motor acting through a 1.5 inch diameter horizontal axis worm gear.

Bidirectional rotation of the worm gear causes a non-rotating 3 inch high hex lifting nut on the 6 inch diameter threaded shaft to move up and down over a height range of about 1.1 m. The 8.625 inch OD, 6.813 inch ID round movable fuel bundle support pipe slides loosely over the threaded shaft. The bottom end of the movable fuel bundle support pipe fits over the lifting nut with a socket that prevents the lifting nut from rotating as the threaded shaft rotates. The vertical forces on the lifting nut are born by a set of small diameter radial screws. These screws are chosen so tha they will fail in shear if the vertical force becomes too large, as in a vertical impact to the FNR fuel assembly that might affect its relative fuel geometry. Such an impact might result from a ground penetrating bomb dropped from a high altitude.

When these screws fail in shear gravity will cause the movable fuel bundle to fall to its fully retracted position.

Rotation of the worm gear causes the movable fuel bundle to slowly move up or down.

The 8.625 inch OD movable fuel bundle support pipe slides through a matching slightly larger diameter hole in a guide plate or ball bearing mounted on the top surface of the open steel lattice. This hole provides the movable fuel bundle lateral stability when the movable fuel bundle is totally withdrawn (at its lowest position) and the adjacent fixed fuel bundles are not present during fuel loading and unloading periods. The top of this hole is slightly conical to assist in blind insertion of the movable fuel bundle support pipe into this hole. The 6 inch diameter threaded shaft projects through and slightly above this hole to also assist in blind insertion of the movable fuel bundle support pipe.

The hex lifting nut keeps the movable fuel bundle at its the last set elevation.

The worm gear fits between the axial drive gear and one side of the dedicated movable fuel bundle space allocation.

The worm gear is driven by a bidirectional hydraulic motor. The hydraulic motor assembly shares the same shaft as the worm gear. Each actuator is field replaceable whereas replacement of the open steel lattice requires major work. Note that for field replacement the threaded shaft and its axial drive gear (6.625 inch OD) can be lowered through the upper guide plate or ball bearing (8.625 inch ID).

The open steel lattice components are too large to fit through an airlock. Like the sodium pool, the open steel lattice is not designed for field replacement.

Each hydraulic motor has two dedicated hydraulic lines, one for causing movable fuel bundle insertion into the matrix of fixed fuel bundles and one for causing movable fuel bundle withdrawal from the matrix of fixed fuel bundles.

The two hydraulic pressure tubes feeding each hydraulic motor are routed along the bottom of the open steel lattice.

The required hydraulic pressure is provided by gravity feed from two overhead liquid sodium tanks. One tank feeds one set of shutdown movable fuel bundles. The other tank feeds the other set of movable fuel bundles. The feed valves to the hydraulic motor connections that cause fuel bundle withdrawal are normally open. The remaining feed valves are normally closed. Hence loss of station control power causes drain down of the contents of the overhead Na tanks through the hydraulic motors which cause movable fuel bundle withdrawal and hence reactor cold shutdown.

A movable fuel bundle has a mass of about 1.7 tonnes. Hence the actuator must be sufficiently robust to dependably support, position and stabilize this mass. When the movable fuel bundle is fully withdrawn the threaded rod and the movable fuel bundle support pipe must keep the movable fuel bundle vertical. The threaded shaft and the movable fuel bundle support pipe must both be of sufficient diameter that they will not fail due to worst case shear force. Hence the need for a 8.625 inch OD thick wall movable fuel bundle support pipe and a 6.0 inch diameter threaded shaft.

The OD of the axial drive gear is:
6.625 inch = 53 / 8 inch

The axial drive gear is heat shrunk on to an unthreaded end portion of the threaded shaft.

Assume that the worm gear is realized using commercial threaded rod 1.5 inch diameter. That rod has 6 threads / inch.

The matching worm gear minimum threaded length is about:
3 X (5 / 8) inch = 15/8 inch

The hydraulic motor parts must fit under (1 / 4) of the adjacent fixed fuel bundle space allocation so as to permit hydraulic motors associated with other movable fuel bundles to overlap the same fixed fuel bundle space allocation.

An advantage of this actuator design is that there is almost no vertical movement hysterisis. When there is no hydraulic fluid flow to the hydraulic motor the movable fuel bundle remains at its last set vertical position. This mechanical configuration provides excellent FNR fuel geometry stability.

The actual vertical position of each movable fuel bundle is monitored via an overhead scan of the elevation of the corresponding indicator tube top.

The axial drive gear tooth spacing is the same as the worm gear tooth spacing at 6 threads / inch.

Hence number of axial drive gear teeth:
= Pi (53/8) inch X 6 teeth / inch
= 124.878 teeth ~ 124 teeth

Assume that the threaded shaft is 47 / 8 inch outside diameter and has 8 / 3 threads per inch. Then:
124 rotations of the worm gear will cause the movable fuel bundle to rise or fall by (3 / 8) inch. Hence one worm gear rotation causes a change in movable fuel bundle insertion of:
3 inch / 8 (124) = .003024 inch
= .003024 inch X .0254 m / inch
= 7.68 X 10^-5 m
= 76.8 um

Hence to precisely set the FNR temperature setpoint we need to control the worm gear rotation to a fraction of one revolution. That rotation control is achieved via pulse width control of the hydraulic fluid pressure lines that feed the hydraulic motor.

Let Fj be the fluid jet tangential force in the hydraulic motor.

One 360 degree rotation of the worm gear delivers a lifting energy of:
1.7 tonne X 2200 lb / tonne X (3 / 8 inch) / 124 = 11.31 lb-inch.

Then the hydraulic motor must provide this much shaft energy per rotation. In one rotation the hydraulic motor paddle centers travel about:
Pi ( 42 / 8 inches) = 16.49 inches.
Hence the hydraulic fluid must provide a tangential force on the hydraulic motor paddles of at least:
11.31 lb-inch / 16.49 inch = 0.686 lb.

Assume that the hydraulic tubing is round within an inside diameter of 0.375 inch. Then the hydraulic jet area is:
Pi (0.375 inch / 2)^2 = 0.11044 inch^2

Then the minimum hydraulic jet pressure is:
0.686 lb /0.11044 inch^2 = 6.21 lb / inch^2

In reality, due to stiction and imperfect energy transfer in the hydraulic motor we probably need 3X this pressure. In order to provide for rapid emergency movable fuel bundle withdrawal this pressure should be provided by two gravity feed tanks mounted near the ceiling of the primary pool space. The downflow liquid sodium pipes need to each be of the order of 8 inch ID to provide a sufficient surge flow of liquid sodium when all the actuators are simultaneously demanding service.

The hydraulic tubing must be routed behind and between the intermediate heat exchange bundles such that in an earthquake the fuel assembly can move horizontally with respect to the sodium pool walls at least 0.5 m in any direction without the hydraulic lines sustaining any physical damage. This hydraulic tubing should be routed through conduits for physical protection where it crosses over the pool deck.

The hydraulic fluid withdrawal control valves should be normally open and the insertion contrl valves should be normally closed so as to default to the withdrawn position on loss of station power. These valves exist in a 460 degrees C environment and are argon pressure operatred. The argon control pressure is turned on by normally closed solenoid valves which open when energized and which are located outside the sodium pool space. The argon control pressure lines can be teflon to provide a thermal break.

Assume that the hydraulic tubes are evenly spaced around the perimeter of the sodium pool. Then the average tube to tube center to center distance is:
[Pi (20 m) - 8 m] / [2 (464 movable bundles)] = 0.059 m
That is sufficient space for 0.375 inch ID tubing with compression fittings.

A practical pressurized hydraulic fluid source is an immersed pump that pumps to an overhead gravity pressure tank used for actuator operation.

The hydraulic motor should be supported and oriented so that its CW and CCW fluid input ports are on top and its fluid discharge port is on the bottom but still well above the sodium pool floor. The issue is that we want any dirt that finds its way into the hydraulic system to fall out via the hydraulic motor fluid discharge port, which should be on the bottom.

Each FNR Actuator relies on smooth meshing of the worm gear with the flat drive gear. Both the bottom of the threaded shaft and the worm gear shaft must be firmly supported so that this smooth gear meshing is sustained far into the future. It might be prudent to have an automatic test sequence that from time to time sequentially runs every movable fuel bundle up and down by a controlled amount to demonstrate that the actuators continue to work as designed and that the fuel bundles have not been subject to swelling or other problems that might cause a movable fuel bundle jam.

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 (.2715 m or 0.3286 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 might potentially cause a jam in the vertical sliding of a movable fuel bundle within the surrounding matrix of fixed fuel bundles.

A fixed fuel bundle has corner girders which extend down below the fuel tubes to also serve as support legs and attach to the internal diagonal plates that provide central support and to an upper central lifting point. On installation the corner girders of fixed fuel bundles connect to adjacent fixed fuel bundles via diagonal bolts at the top of each corner girder. The fixed fuel bundles are held in place on top of the open steel lattice by cross fittings. The cross fittings fit inside the fixed fuel bundle legs and are tapered at their tops to allow practical blind mating with the fuel bundle support legs.

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 0.4 m diagonal sheet extensions provide lifting points for fuel bundle installation and removal. Short corner girder upward extensions allow use of diagonal bolts for horizontally connecting together adjacent fixed fuel bundles.

The entire weight of the fixed fuel bundles is supported by the four fuel bundle legs which are stabilized below the gratings by diagonal members. These legs extend 1.5 m below the fuel tube bottoms to allow liquid sodium to easily flow into the bottom of the fuel bundles and to minimize long term neutron damage to the open steel lattice.

In operation each movable fuel bundle's weight is borne by the 3.0 inch high lifting nut on the threaded shaft which sets the amount of the movable fuel bundle's insertion into the matrix of fixed fuel bundles. This lifting nut engages 8 threads. The movable fuel bundle travel is limited at the bottom by its 1.1 m of projecting support pipe length after an allowance of 0.4 m for the grating, support pipe to grating clearance and support pipe adapter plates.

In order to achieve fuel bundle mounting interchangability the passive fuel bundles are the same size and are mounted in the same manner as the active fuel bundles. However, the otherwise movable passive fuel bundles are supported by fixed studs so that they are not movable and remain in a fixed position with respect to the fixed fuel bundle matrix.

In an earthquake it is important for the fuel assembly to be able to slide horizontally at least one m before there is a collision between the fuel assembly and an intermediate heat exchange bundle assembly. The actuator hydraulic tubes must have sufficient slack to permit this relative motion.

Sooner or later a particular FNR Actuator will need replacement. All such actuators should fit into an equipment transfer airlock.

This airlock internal width must be about 1.5 m to accommodate intermediate heat exchanger radial piping and the intermediate heat exchange bundle manifold outside diameter.

The open steel lattice is fabricated in sections that are field assembled. The open steel lattice has a flat bottom for low friction sliding over the underlying layer of ball bearings. The open steel lattice bottom plates must smoothly fit together. On top of the bottom plates, directly under the fixed fuel bundles, must be trays containing neutron absorbing gravel. This gravel must not be permitted to foul the movable fuel bundle actuators.

In plan view the maximum size of the dedicated actuator spcce is:
[19 X (9 / 16) inch] X [19 X (9 / 16) inch]
= 0.2714 m X 0.2714 m

Notee that the hydraulic motors must overlap the adjacent fixed fuel bundle space without interfering with each other.

This web page last updated November 21, 2023.

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