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

A FNR Actuator is a linear actuator that inserts or withdraws a movable fuel bundle from the matrix of fixed fuel bundles. This insertion must be smooth and hysterisis free. The inserion occurs against the force of gravity. The withdrawal is assisted by the foce of gravity. A repeatable vertical position resolution of the order of 1 mm is required.

A FNR actator consists of a hydraulic motor driven worm gear which drives a flat gear mounted on a strong vertical threaded shaft, which is supported by its bottom end by a thrust bearing and is stabilized about 1.5 m from its bottom by a rotating bearing. Bidirectional rotation of the hydraulic motor causes a lifting nut on the threaded shaft to move up or down over a range of about 1.1 m which in turn causes the movable fuel bundle to move up or down. The bottom end of the movable fuel bundle support pipe has a hex socket end that slips over the lifting nut and prevents it from rotating as the threaded shaft rotates. The travelling nut keeps the movable fuel bundle at its the last set elevation. The threaded shaft fits loosely inside the movable fuel bundle support pipe. When the movable fuel bundle is fully withdrawn the threaded rod holds the movable fuel bundle upright.

The axis of the hydraulic motor is horizontal. This axis is at 45 degrees to the axis of the fuel bundle grid and is on the diagonal of the adjacent fixed fuel bundle space.

A movable fuel bundle has a mass of over two tonnes, so this assembly must be sufficiently robust to dependably support and stabilize the movable fuel bundle. When the movable fuel bundle is withdrawn the threaded rod inside the movable fuel bundle support pipe must hold the movable fuel bundle vertical. The threaded rod and the movable fuel bundle support pipe must both be of sufficient diameter that they will not fail due to worst case shear force. eg Likely need at least a 4 inch diameter threaded shaft.

A sketch shows that the OD of the flat gear needs to be about:
19 X (5 / 8) inch = 11.875 inch. That is tangent to the sides of the movable fuel bundle space alloction. The flat gear radius may need to be reduced by the radius of the worm gear.

The matching worm gear minimum threaded length is about:
3 X (5 / 8) inch X (2)^0.5 = 2.65 inch

The hydraulic motor OD is about:
12 X (5 / 8) inch X (2)^0.5 = 10.60 inch

The hydraulic motor fluid chamber thickness is:
4 X (5 / 8) inch X (2)^0.5 = 3.53 inch

The hydraulic turbine radius at the fluid jet is:
4.5 X (5 / 8) inch X (2)^0.5 = 3.977 inch

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 jets the movable fuel bundle remains in its last set position. This mechanical configuration provides excellent FNR fuel geometry stability.

Each hydraulic motor has two dedicated hydraulic lines, one for causing movable fuel bundle insertion on one for causing movable fuel bundle extraction.

The threaded shaft and its nut must be sufficiently robust to support the movable fuel bundle. This shaft is about 1.8 m tall and is stabilized both at its bottom by the thrust bearing and by a rotor bearing located about 0.6 m above the open steel lattice bottom. The top of the moving nut travels downwards from about 1.8 m above the open steel lattice bottom to about 0.7 m _____above the open steel lattice bottom. We need to allow 0.1 m for the thickness of the moving nut.

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 actuator threaded shaft is located directly under the center of the corresponding movable fuel bundle. The flat drive gear and its companion hydraulic motor are located near the bottom of the vertical threaded shaft and underneath the adjacent fixed fuel bundle.

The actuators are integral to the open steel lattice that supports the FNR fuel assembly.

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

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

Hence number of flat gear teeth:
= Pi (19)(5 /8) inch X 6 teeth / inch
= 223.83 teeth ~ 224 teeth

Hence the number of flat gear teeth = 224

Assume that the jackscrew threaded rod is 4 inch diameter and has 2.25 threads per inch.

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

One rotation of the hydraulic motor delivers a shaft energy of:
Fj 2 Pi (3.977 inch)= Fj 24.988 inch

Let Fl be the load force (~ 2 tonne weight of movable fuel bundle)

One rotation of the worm gear lifts the load [1 inch / (2.25 x 224)]

Neglecting friction conservation of energy gives:
(Fj 24.988 inch) = [Fl /(2.25 X 224)] inch
Fl / Fj = 224 (2.25) (24.988)
= 12,593.9
~ 12,594

Fl = 2000 kg X 9.8 m / s^2,
which sets the minimum value of Fj at:
Fj = (2000 kg X 9.8 m / s^2) / 12,594
which is equivalent to a fluid jet which delivers sufficient momentum / unit time to support a 0.159 kg mass.

Consider the required cross sectional area A of the fluid stream: Assume a fluid pressure of 0.2 MPa. 0.2 MPa A = 0.159 kg X 9.8 m / s^2
A = 0.159 kg X 9.8 m / s^2 / (0.2 X 10^6 kg m / s^2 m^2)
= (0.159 X 9.8 / 0.2) X 10^-6 m^2
= 7.79 mm^2

Consider a tube with (1 / 4) inch ID. Its open cross sectional area is:
Pi (1 inch / 8)^2 X (0.0254 m / inch)^2
= 31.669 X 10^-6 m^2
= 31.7 mm^2

Thus allowing for various friction losses we need to use a (3/ 8) inch ID tubing for the hydraulic lines. These lines can be (1 / 2) inch OD stainless steel tubing in sections connected together by stainless steel compression fittings. Note that perfect leak proof connections downstream of the control valves are not essential but reliable mechanical connections are essential.

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 primary sodium pool walls up to 0.5 m in any direction without the hydraulic lines sustaining physical damage. It is probably prudent to route this hydraulic tubing through a conduit for physical protection where it crosses over the pool deck.

The hydraulic fluid control valves should be normally closed but openned by application of argon control pressure. These valves exist in a 460 degrees C environment. The argon control pressure is turned on by normally closed solenoid valves which open when energized and which are located outside the primary 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 primary 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 0.50 inch OD tubing with compression fittings.

A practical pressurized hydraulic fluid source may be an immersed pump that pumps to a pressure tank used for actuator activation.

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 primary 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 will be on the bottom.

Each FNR Actuator relies on smooth meshing of the worm gear with the flat gear. Both the bottom of the threaded shaft and the hydraulic motor 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 moveable 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 (.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 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 diagonal plates that provide lower central support and 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 and by cast fittings at the bottom of each corner girder. These cast fittings are firmly attached to the open steel support lattice. The cast fittings fit inside the fixed fuel bundle legs and are tapered at their tops to allow practical blind mating with the fuel bundle supports with +/- 6 mm position error.

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 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 and the reinforced diagonal plate extensions. These supports 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 fast neutron damage to the open steel lattice.

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

About 0.3 m of height is dedicated to the fuel bundle grating and fuel bundle bottom filter.

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 by fixed studs so that they are not movable and can not withdraw from the fixed fuel bundle matrix.

In an earthquake it is important for the fuel assembly to be able to slide horizontally about half way under the intermediate heat exchange bundle bottom manifold before there is a collision between the fuel assembly and the 12 inch diameter bottom manifold feed pipe. This clearance issue requires careful control the height of the open steel lattice, the height of the fuel bundles and the depth of the intermediate heat exchange bundles.

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 must have an internal height of about 2.5 m to allow for intermediate heat exchanger connecting pipe.

This airlock internal width is limited to about 1.5 m by adjacent intermediate heat exchanger radial piping and by the intermediate heat exchanger outside diameter.

Hence the open steel lattice is fabricated in 48 slices, the largest of which is 7.7 m long X 1.5 m high X 0.6 m wide. For other open steel lattice slices the height and width are the same but the slice lengths are shorter. Each slice supports two half rows of fuel bundles. Each slice has its own hydraulic pressure connections for its movable fuel bundle actuators.

The slices fit horizontally together with locating pins and horizontal bolts which fix the slices in the correct relative horizontal positions. Each slice hs a flat bottom for low friction sliding over the underlying layer of ball bearings.

This web page last updated May 5, 2022.

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