XYLENE POWER LTD.

FNR ACTUATOR

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

INTRODUCTION:
The insertion depth of the movable fuel bundles into the matrix of fixed fuel bundles is set by liquid sodium hydraulic actuators. Each FNR Actuator is a piston and cylinder device that uses pressurized liquid sodium to vertically position a movable fuel bundle. On loss of sodium pressure a movable fuel bundle will default withdraw from the matrix of fixed fuel bundles, causing a reactor shutdown. The FNR Actutors are used to compensate for fuel aging, to reduce the sodium temperature for service or to achieve total fission shutdown.

The insertion depth is primarily set by pumping a controlled volume of liquid sodium into each actuator cylinder using a positive displacement pump. The purpose of using a controlled volume is to avoid piston stiction effects. However, if a pressurized sodium circuit contains a gas pocket, there is opporunity for using a small decrease in movable fuel bundle buoyancy to cause slight withdrawal and hence negative feedback if the sodium gets too hot.

Buoyancy force = (displaced liquid sodium volume) X (liquid sodium density) X (gravitational acceleration)

An advantage of a buoyancy based FNR Actuator force component is that the density of sodium decreases with increasing sodium temperature. Hence the buoyancy force decreases with increasing sodium temperature, causing movable fuel bundle withdrawal. This buoyancy force component is created by having an argon gas charge on top of the pressurized Na used for actuator operation.

The top of the indicator tube shows the actual movable fuel bundle position.

The hollow portion of the indicator tube ducts gamma radiation upward.

Note that sodium that rises through a maovable fuel bundle should be guided around the outside of the indicator tube.

The FNR Actuators are used to set the insertion depth of the movable fuel bundles so as to achieve the desired fuel temperature at low thermal power. After the required movable fuel bundle positions have been determined the FNR average fuel temperature is regulated by thermal expansion/contraction of the FNR fuel assembly.

Linear thermal expansion of steel is typically about 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 play, hysterisis and position error of the actuator must be small compared to 0.1 mm.

The actual insertion of each movable fuel bundle into the matrix of fixed fuel bundles is monitored using an overhead precision optical device. This optical measurement establishes an initial position set point. In normal operation any subsequent deviation of the actual position from this set point results in a control signal to the actuator to return it to its position setpoint.

A FNR Actuator is a piston-cylinder arrangement that raise 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 piston. The mass of a FNR movable fuel bundle is about 1.7 tonnes. A hysterisis free vertical position resolution of about:
0.01 mm = 10 um
is desired.

The actuator consists of two ~ 1.5 m long vertical pipes, one which loosely slides over the other to provide movable fuel bundle position stabilization at times when adjacent fixed fuel bundles are not present. The bottom of the inner pipe is closed to form a piston. The top of the inner pipe is firmly attached to the bottom of the movable fuel bundle.open steel lattice. The top of the outer pipe is open to accept the piston. The bottom of the outer pipe is closed to contain the pressurized liquid sodium and is firmly attached to the open steel lattice. The top of the piston cylinder is slightly flared to permit easy blind piston isertion. The bottom of the outer pipe has a pressurized sodium connection tube.

The outer pipe has an OD of 8.625 inch and a wall thickness of 0.406 inch.
The inner pipe has an OD of 7.625 inch and a wall thickness of 0.875 inch.
The outer pipe ID is:
8.675 - 2(.406) = 7.863 inch = 0.1951 m
and the inner pipe ID is:
7.625 inch - 2 (.875) = 5.875 inch.

The metal piston ring OD is 7.863 inch
and the largest possible piston ring ID is 7.625 inch. Hence the minimum piston ring radial thickness is:
(7.863 - 7.625) / 2 = 0.119 inch,
If half of the piston ring thickness is to go into a machined groove the groove depth is 0.119 inch and the piston ring thickness is 0.238 inch

This seal should be backed up with a teflon seal ring.

The weight of the movable fuel bundle support pipe is about:
1.5 m X 35.67 lb / ft X 1 ft / 12 inch X 1 inch / 0.0254 m X 0.454 kg / lb = 79.6 kg

Simple thermal expansion of a fuel rod over 5 deg C is about:
(20 X 10^-6 / deg C) x 5 deg C X 300 mm = 0.03 mm

The required resolvable change in trapped presurized sodium fluid volume is:
Pi (0.1951 m / 2)^2 (.03 X 10^-3 m) = 8.964 X 10^-7 m^3 = 0.896 cm^3 which is the required resolution of the liquid sodium pump.?

The inner pipe is closed at its bottom, the outer pipe is closed at its top. The inner pipe is outside machined near its top to accept an external teflon O ring. (This O ring is easy to access for s. The function of the teflon O ring is both mechanical stabilization and fluid containment.) Then a reactor shutdown can be achieved by lowerin. Then the controlled range of travel of individual fuel bundles can be much smaller. This arrangement requires a more complicated indicator tube float design to provide the necessary buoyancy.

The change in sodium displacement provided by the actuator is:
Pi (.1951 m / 2)^2 (1.5 m) = 0.0448 m^3

The volume of a movable fuel bundle is:
________.

At the top of the inner pipe are vertical fins that connect to bottom fuel bundle.

Both of these pipes must be firmly attched due to potential high bending torque when adjacent fixed fuel bundles are not present.

The amount of insertion must be constantly monitored. If the insertion depth signal is lost even briefly lost the movable fuel bundle must e withdrawn, which is the default condition.

If a large weight falls on the indictor tubes the weight will cause a large pressure transient that can be used to trigger withdrawal of the affected movable fuel bundles.

A feature of this arrangement is simplicity and a single sodium pressure control tube per movable fuel bundle.

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.

OPEN AREA FRACTION:
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 60 steel pipe that has an OD of 8.625 inch and an ID of 7.863 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 open steel lattice has a clearance hole that stabilizes the sliding actuator 1.5 m above the bottom of the opem steel lattice.

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
or
4 D / 8.625 inch = 0.5
or
D = 8.625 inch / 8
~ 9 / 8 inch

WORM GEARDRIVE ALTERNATIVE

Bidirectional rotation of the worm gear sodium pump causes the outer pipe to move up and down over a height range of about 1.1 m. The 8.625 inch OD, round movable fuel bundle support pipe slides loosely over the inner pipe. The rupture dic is chosen so that it 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 the rupture disc fails gravity will cause the movable fuel bundle to fall to its fully retracted position.

Normally, 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 inner pipe projects through and slightly above this hole to also assist in blind insertion of the movable fuel bundle support pipe.

The teflon O ring seal keeps the movable fuel bundle at its the last set elevation.

The worm gear is driven by a bidirectional electric motor. Each actuator O ring is field replaceable whereas replacement of the open steel lattice requires major work.

The liquid sodium pressure tube feeding each actuator is routed along the bottom of the open steel lattice.

The required hydraulic pressure is provided by a feed from two pressurized liquid sodium tank which feed a common high sodum pressure manifold.

Each actuator has a dedicated sodium feed/drain tube. To insert a movable fuel bundle a NC solenoid valve connects thehighpressuremanifold to the movable fuel bundle control tube.To withdraw the fuel bundle a NO solenoid valve connects ththis tube to the sodium pool. A second parallel NO solenoid valve provides discharge flow certainty.

Hence loss of station control power causes 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 movable fuel bundle support pipe must keep the movable fuel bundle vertical. The inner pipe and the movable fuel bundle support pipe must both be of sufficient diameter and thicknes that they will not fail due to worst case shear force. Hence the need for a 8.625 inch OD schedule 60 movable fuel bundle support pipe and a 7.625 inch OD schedule xx inner pipe.

An advantage of this actuator design is that there is almost no vertical movement hysterisis. When there is no hydraulic fluid flow to the actuator, apart from seal leakage the movable fuel bundle remains at its last set vertical position. This mechanical configuration provides good 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.
 

DIMENSIONS:
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 500 degrees C environment and are operated by oil cooled electromagnets. The electromagnets are thermally isolaed using teflon.

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] / [ (464 movable bundles)] = 0.118 m
That is sufficient space for 0.375 inch ID tubing with compression fittings.
 

HYDRAULIC ACTUATOR POSITION:
The hydraulic acuator should be supported and oriented so that its fluid port is near its bottom. There should be a provision for venting trapped gas from ech hydraulic tube.
 

ACTUATOR PERFORMANCE STABILITY:
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 or actuators have not been subject to swelling or other problems that might cause a movable fuel bundle to jam. The fuel bundle should hae traped all retingeeeeeeeeeeeeeeeeee
 

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 (.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 8.625 inch OD fuel bundle support pipe 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 1.1 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.
 

SUPPORT PIPE BENDING RESISTANCE:
W = pipe wall thickness
R = average pipe radius
Theta = angle about pipe center
dA = W R d(Theta)
Bending resistance = 2 Integral from Theta=0 to Theta = Pi of:
[R sin(Theta)] W R d(Theta) Pmax
 
=Integral from Theta = 0 to Theta = Pi of:
[2 W R^2 Pmax sin(Theta)] d(Theta) Pmax
 
= 2 W R^2 Pmax [(-cos(Pi)) - (-cos(0))]
 
= 2 W R^2 Pmax [1 - (-1)]
= 4 W R^2 Pmax

For the outside pipe:
W = 0.406 inch
R = [8.625 inch + 7.863 inch] / 4 = 4.122 inch
Pmax = 10,000 lb / inch^2

Hence:
4 W R^2 Pmax = 4 (.406 inch) (4.122 inch)^2 (10,000 lb / inch^2)
= 275,932 lb-inch = 275,932 lb-inch X (.0254 m / inch) x (.454 kg / lb)
= 3182 kg-m

For the inside pipe:
W = 0.875 inch
R = (7.625 + 5.875) / 4 = 3.375 inch

These bending resistances are barely sufficient to allow a crane to lift one end of a horizontal movable fuel bundle..

PASSIVE FUEL BUNDLES:
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.
 

HORIZONTAL FUEL ASSEMBLY MOVEMENT CLEARANCE:
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.
 

FNR ACTUATOR REPLACEMENT
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. 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.
 

ACTUATOR PLAN VIEW:
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

This web page last updated January 5, 2025.