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XYLENE POWER LTD.

FNR FUEL BUNDLE

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

INTRODUCTION:
This web page deals with FNR fuel bundles. A fuel bundle is an assembly of vertical fuel tubes that is totally replaced when the contained fuel is reprocessed. The fuel tubes are located on a square grid (5 / 8) inch center to center. The grid is square to allow adequate primary liquid sodium flow in the presnce of 15% linear fuel tube swelling. The fuel tube center to center distance in the fuel tube bundle was established by itterative liquid sodium flow and fuel reactivity calculations. The fuel bundle length and width were established by weight and size constraints relating to practical road transport of irradiated fuel bundles.

An active fuel bundle contains 532 active fuel tubes. An active fuel tube contains a core fuel rod where nuclear fission occurs. Active fuel tubes also contain four blanket fuel rods where fuel breeding occurs.

A passive fuel bundle contains 556 passive fuel tubes. A passive fuel tube contains five blanket fuel rods and hence supports fuel breeding but does not support nuclear fission.

A practical FNR consists of a 0.35 m high pancake shaped core zone surrounded by 1.2 m thick blanket zones.
 

DEFINITIONS:
Active fuel tube: contains 4 X 0.6 m blanket rod plus 1 X 0.35 m core rod.

Passive fuel tube: contains 5 X 0.6 m blanket rod

Active Fuel bundle = vertically sliding control portion with 244 active fuel tubes + fixed position surround portion with 288 active fuel tubes

Passive Fuel Bundle = fixed position fuel bundle with 556 passive fuel tubes

Active fuel bundle control portion = vertically slidiing protion of fuel bundle which is used to set fuel bundle discharge temperature. The control portion withdraws about 1.0 m into the fuel bundle support tubes to cause a reactor shutdown. Consists of 244 active fuel tubes, 4 corner girders 7 m long, bottom plate, push rod, lower control portion grid, upper control portion grid.

Active fuel bundle surround portion = fixed position portion of an active fuel bundle consisting of 288 active fuel tubes, four outside corner girders 9 m long, four inside corner girders 6 m long, surround top grid, surround botton grid

An actuator consisting of a 1 m long hydraulic cylinder + piston + 1 m push rod moves control bundle up and down, is located in bottom 1 m of fuel bundle support tube.

Fuel bundle support tube = 12 inch X 12 inch X 2 m long, holds active fuel bundle surround portion or passive fuel bundle in correct fixed position and contains the control portion hydraulic actuator
 

FUEL BUNDLE DETAIL:
Every active fuel bundle has a dedicated control portion insertion actuator. Each surround fuel bundle has a maximum horizontal length allocation of 0.400 m (15.748 inch) and a maximum horizontal width allocation of 0.400 m (15.748 inch).

The nuclear reactions take place within the fuel bundles. Each fuel bundle in plan view is square and contains:
24 X 24 = 576 potential vertical fuel tube positions. The fuel tubes are positioned on a square grid with (5 / 8) inch center to center spacing. However the 4 X 5 outer corner fuel tube positions are required for shroud structure reinforcement girders, and 2 (4 X 3) fuel tube positions are required for control bundle corner girders leaving:
576 - 20 -12 - 12 = 532
actual fuel tubes per fuel bundle.

The nominal face to face size of a fuel bundle exclusive of its shroud is:
(24 tubes X (5 / 8 ) inch / tube) = 15.00 inch

The fuel tube bundles are designed so that individual fuel tubes can linearly swell without the external length or width of the fuel bundle materially changing. Fuel bundles are intended to be replaced after 15% linear swelling of the most intensely neutron irradiated sections. The fuel tube array center-to-center spacing is established by end gratings that are located out of the main fast neutron flux and hence are protected from fast neutron induced swelling.
 


 

INDICATOR TUBES:
Each fuel bundle has an indicator tube that projects about 4 m above the tops of the fuel tubes when the control portion is fully inserted intop the surround portion. The indicator tube must be strong enough to allow it to be used for lifting the entire fuel bundle.

The indicator tube shows the vertical position of the control bundle, allows measurement of gamma flux, and allows measurement of fuel bundle discharge temperature. The temperature is indicated by a Bourdon tube wound on a vertical plane. The tube has a mirror attached to it that reflects a laser beam. The reflected beam forms an image. The position of that image moves with the internal pressure in the Bourdon tube. That pressure is the vapor pressure of the internal sensing fluid such as mercury.
 

INDEPENDENT FUEL BUNDLES:
In plan view each active fuel bundle consists of a 15.25 inch X 15.25 inch square fixed fuel bundle surround portion containing 288 active fuel tubes and a 9.875 inch X 9.875 inch square central fuel bundle control portion containing 244 active fuel tubes. The discharge temperature of the active fuel bundle is determined by the distance of insertion of the control portion into the surround portion.

In plan view each passive fuel bundle is a 15.25 inch X 15.25 inch square containing:
576 - 4(5) = 556 passive fuel tubes. Since there is little heat released by the passive fuel tubes there is little liquid sodium natural circulation past these fuel tubes.

The outer corner girders of every fuel bundle extend downwards ~ 3 m below the bottom of the fuel bundle fixed support grid and are outside the 12 inch X 12 inch square surround bundle support tubes.

In plan view the entire fuel bundle is a 24 X 24 array containing 576 theoretical fuel tube positions. Inside the active fuel bundle surround portion is the sliding fuel bundle control portion which has a 16 X 16 array containing 256 theoretical fuel tube positions.

At the outer corners of every fuel bundle are 4 outer corner girders, each of which occupies 5 fuel tube positions. At the inner corners of the active fuel bundle surround portion are 4 inner corner girders each of which occupies 3 fuel tube positions. At the outer corners of the active fuel bundle control portion are 4 more outer corner girders each of which occupies 3 fuel tube positions. Hence the active fuel bundle surround portion contains:
576 - 4(5) - 4(3) - 256 = 288 real fuel tubes
and the active fuel bundle control portion contains:
256 - 4(3) = 244 real fuel tubes.

The outside of the fuel bundle control portion has dimensions of [16 (5 / 8)] inch X [16 (5 / 8)] inch = 10 inch X 10 inch.

Supported by the outer corner girders of the fuel bundle surround portion is the shroud which has a thickness of (1 / 16) inch. The shroud must have large triangular openings in the core region to allow for shroud material swilling due to the high fast neutron flux. The shroud provides each fuel bundle structural stability. The shroud should form diagonal crosses on each fuel bundle face.

When the fuel bundle control portions are fully inserted into the fuel bundle surround portions the reactor is at maximum power and should provide a liquid sodium discharge temperature of over 440 degrees C. When the fuel bundle control portions are 1 m withdrawn from the surround bundles the reactor must be subcritical. The fuel bundle geometry limits the range of acceptable Pu concentrations in the fuel rods.

From a power control perspective each fuel bundle can almost be regarded as an independent FNR. Each fuel bundle has its own discharge temperature which is determined by the extent of insertion of the fuel bundle control portion into the fuel bundle surround portion. Under forced cold shutdown conditions gravity causes the fuel bundle control portion to drop 1 m into the 12 inch X 12 X 0.5 inch X 2 m support tube.

The outer corner girders on the fuel bundle control portion and the inner corner girders on the fuel bundle surround portion are each about 6 m long to act as slide guides for the fuel bundle control portion. There are stops that prevent the fuel bundle control portion travelling either too far up or too far down.

The corner girders of the fuel bundle surround portion transfer the weight of the fuel bundle surround portion onto the 12 inch X 12 inch X 0.5 inch X 3 m support tubes. These support tubes have 1 m high side apertures to allow liquid sodium to flow into the bottom entrance of the fuel bundle control portion cooling channels and to pass through the fuel bundle support tubes.

When the fuel bundle control portion is fully inserted into the fuel bundle surround portion there is a 1 m high nearly open space under the fuel bundle that allows both horizontal and vertical liquid sodium flow.

At the bottom of the fuel bundle control portion corner girders is a 9.875 inch X 9.875 inch X 0.5 inch steel plate fitted with a 1 m long piston push rod. The actuator piston has a tapered bottom for easy blind insertion into the hydraulic cylinder. This piston slides inside a 10.75 inch OD, 9.75 inch ID X 1 m high steel pipe that acts a a hydraulic cylinder. This hydraulic cylinder is located at the bottom of the 12 inch X 12 inch X 0.5 inch X 2 m square support tube. As the control bundle withdraws its corner legs sink up to 1 m into this 12 inch X 12 inch support tube. To insert the control bundle into the surround bundle liquid sodium at up to 60 psi is injected under the piston which raises the piston and hence raises the fuel bundle control portion. If the piston lifts too high the high pressure liquid sodium behind the piston is released into the primary sodium pool. This arrangement acts as a piston travel upper stop.

On the corners of the 12 inch X 12 inch X 0.5 inch X 2 m support tubes are slide guides that mate with a lower extension of the fuel bundle outer corner girders. These slide guides are tapered at their tops to allow practical blind mating. These slide guides are vertically short to allow liquid sodium to easily circulate in the lower 2 m of height under the tube bundle.

The support tubes are welded to 14 inch X 6 inch X 0.50 inch horizontal tubes each of which carries a line of 12 inch X 12 inch X 3 m support tubes that are spaced at 0.400 m intervals. The 14 inch X 6 inch X 0.5 inch tubes are themselves spaced 0.400 m center to center. This center to center spacing is set by 1.748 inch wide spacers between the horizontal tubes. The spacers must be mounted to give the whole assembly of fuel tubes structural stability in a earthquake.

There are 3.5 m high closed end indicator tubes attached to the upper four corners of the active fuel bundle control portion with diagonal rods.

The thermal power output from a fuel bundle is proportional to the gamma flux propagating up its closed end indicator tube.

The fuel bundle discharge temperature is indicated by the vapor pressure inside the closed end indicator tube.

The vertical position of the control bundle is indicated by the height of the top of the indicator tube.

Inside the 12 inch X 12 inch X 0.50 inch support tube is a hydraulic actuator formed from 1 m of 10.75 inch OD steel pipe with an internal piston. This actuator has a piston travel of about 1 m and is used to position the active fuel bundle control portion within the active fuel bundle surround portion.

The extent of insertion of the control portion into the surround portion is determined by the volume of liquid sodium inside the hydraulic actuator. The hydraulic tube is routed through the 14 inch X 6 inch horizontal steel tube on the bottom of the primary sodium pool. Small holes are cut in the 14 inch X 6 inch steel tube to allow hydraulic tube installation. In the event of a hydraulic cylinder jam an entire line of such cylinders is removed and replaced.

There is a 1 m high push rod from the piston to the bottom plate on the active fuel bundle control portion.
 

FUEL TUBE BUNDLE DIMENSIONS:
The fuel tube bundle frame and shroud are fabricated from HT-9 steel (85% Fe, 12% Cr, 3% other, 0% C, 0% Ni).

The height allowances for the main fuel bundle components from bottom to top are: support tube (2.0 m), contained piston cylinder 1.0 m, active fuel bundle control portion travel = 1 m, bottom grating (0.1 m), fuel tubes (6 m), top grating (0.1 m), upper diagonals (0.5 m), indicator tube (3.5 m), lifting eye (0.2 m) for an overall length of about 13 m.

The surround bundle length and width are:
24 X (5 / 8) inch + 2 (1 / 16) inch for shroud + 2 (1 / 16) inch for shroud fastners = 15.25 inch

Each FNR fuel bundle, including tolerance allowance, is nominally 15.25 inches long X 15.25 inches wide X 20 feet and has a mounting space allowance 0.400 m long X 0.400 m wide. There is approximately:
(0.400 m X 1 inch / 0.0254 m) - 15.25 inch = 0.498 inches tolerance in length and width to allow for 0.10 inches of differential thermal expansion + 0.20 inches of off ideal axis tolerance in any horizontal direction due to all causes.

The fuel tube height is limited by availability of 0.500 inch OD X 0.065 inch wall thickness steel tube

The fuel bundle length and width are limited by the weight of the lead shielded container needed to move a fuel bundle by truck.
 

FUEL BUNDLE DESIGN:
The purpose of the shroud is to prevent fuel tubes in adjacent fuel bundles rubbing against each other as the fuel bundles are installed and removed and to provide structural stability. The shroud walls are (1 / 16) inch thick sheet steel with spot welded fasteners that project at most (1 / 16) inch from the steel outer surface. To allow for fuel bundle shroud material swelling without buckling the shroud has triangular opennings.

Each fuel tube bundle contains:
a 24 X 24 = 576 array of potential tube positions on 0.625 inch square centers. Each fuel bundle has a (1 / 8) inch thick steel shroud allowance consisting of 2 X (1 / 16) inch for the metal thickness and 2 X (1 / 16) inch for the fastener and distortion thickness. Hence tube bundle's outside length and width is nominally:
24 (5/ 8) inch + 2 (1 / 8) inch = 15.25 inch.

The mounting space is 0.400 m X (1 inch / 0.0254 m) = 15.748 inches

Hence there is an additional:
(15.748 inch - 15.25 inch) / 2 = 0.249 inch
of fuel bundle length and width allowance to accomodate differential thermal expansion and off axis fabrication tolerances.

Each fuel bundle surround portion is structurally stabilized by its rigid outer corner angle girders, which allow fuel bundles to vertically slide past one another during fuel bundle installation and removal. These girders are:
(14 / 8) inches X (14 / 8) inches X (9 / 16) inches X 9 m. The cross sectional area of these girders is:
[ 9(49) / 256] inch^2

Thirty-two corner fuel tube positions on the fuel bundle surround portion are lost due to corner girders. The cross sectional area of the corner girders is:
(7 / 8) inch^2

24^2 - 4(5) - 4(3) - 4(3) = 576 - 44
= 532 real fuel tube positions
 

FUEL BUNDLE GRATINGS:
The fuel tubes are held in position by the steel gratings at the top and bottom of each fuel bundle. The gratings transfer the weight of the fuel tubes onto the fuel bundle corner girders. It is of paramount importance that the gratings never structurally fail. Hence every fully fabricated grating must be tested at 3X its normal maximum load.

The top and bottom steel gratings of a fuel bundle are composed of 2 (48) pieces of .125 inch X 4.0 inch X 15.000 inch steel, for a total volume of:
2 (48) pieces / bundle X .125 inch X 4.0 inch X 15.000 inch = 720.0 inch^3

Of this amount the grating metal volume used on the fuel bundle control portion is:
2 (32) pieces / bundle X .125 inch X 4.0 inch X 10.000 inch = 320.0 inch^3

Hence the grating metal volume on the fuel bundle surround portion is:
720 inch^3 - 320 inch^3 = 400 inch^3

The mass of the active fuel bundle two control portion gratings is:
320 inch^3 X (0.0254 m / inch)^3 X 7.870 X 1000 kg / m^3 = 41.269 kg

The mass of the active fuel bundle two surround portion gratings is:
400 inch^3 X (0.0254 m / inch)^3 X 7.870 X 1000 kg / m^3 = 51.586 kg

The mass of each passive fuel bundle gratings pair is:
720 inch^3 X (0.0254 m / inch)^3 X 7.870 X 1000 kg / m^3 = 92.855 kg

Between the active fuel bundle control portion and the active fuel bundle surround portion is a (1 / 8) inch gap to provide reliable sliding clearance.

Note that the active fuel bundle surround portion bottom grating must transfer the weight of 288 loaded fuel tubes onto the outer corner girders while still permitting unobstructed primary liquid sodium flow.

The active fuel bundle control portion bottom grating must transfer the weight of 244 loaded fuel tubes onto its outer corner girders and hence to the actuator piston while still permiting unobstructed primary liquid sodium flow.

The diagonals connecting the indicator tube to the active bundle control portion corner girders must also allow easy primary liquid sodium flow.

The diagonals connecting the indicator tube to the passive fuel bundle outer corner girders must also allow easy primary liquid sodium flow.

The 12 inch X 12 inch X 0.5 inch X 2 m support tubes are welded to the steel floor frame. The 10.75 inch OD X 1 m hydraulic cylinder fits inside the bottom of the 12 inch X 12 inch X 2 m support tube. There are 8 tapered slide guides welded onto the outside corners of the 12 inch X 12 inch X 2 m support tube. The tapers assist in blind fuel bundle positioning. Each support tube also has 4 precisely positioned welded corner girder stops. There is a piston that slides within the 10.75 inch OD X 1 m hydraulic cylinder to cause insertion of the active fuel bundle control portion into the corresponding surround portion.

Forming the shroud are 4 (1 / 16) inch thick steel sheets 15 inches X 240 inches for a volume of:
4 X 15 inch X 240 inch X (1 inch / 16) X (.0254 m / inch)^3 = 0.014748 m^3

Due to triangular cutouts this volume is cut approximately in half to:
0.014748 m^3 / 2

The mass of this shroud cover per fuel bundle is:
+ [(0.014748 m^3) / 2] X 7.870 X 1000 kg / m^3
= 58.03 kg

There are 4 fuel bundle surround portion outer corner angle girders, each with a cross sectional area of 9 (49) / 256 = 1.7266 inch^2:
4 X (1.7226 inch^2) X 9 m X (.0254 m / inch)^2 X 7.874 X 10^3 kg / m^3
= 315.04 kg

There are 4 fuel bundle surround portion inner corner angle girders each with a cross sectional area of (7 / 8) inch^2:
4 X (7 / 8) inch^2 X 6 m X (.0254 m / inch)^2 X 7.874 X 10^3 kg / m^3
= 106.68 kg

From FNR Fuel Tubes each active fuel tube has a mass of 5.7397 kg.
 

TOTAL FUEL BUNDLE SURROUND PORTION MASS
288 loaded fuel tubes + 2 surround bundle gratings + 4 fuel bundle surround portion outer corner girders + 4 fuel bundle surround portion inner corner girders + shroud cover
= 288 (5.7397 kg) + 51.586 kg + 315.04 kg + 106.68 kg + 58.03 kg
= 2184.37 kg
 

FUEL BUNDLE CONTROL PORTION:
Note that the indicator tube must be able to safely lift the entire weight of the fuel bundle.
Indicator Tube: 5.563 inch OD X 0.258 inch wall X 3.5 m long
Mass = Pi X 5.563 inch X 0.258 inch X 3.5 m X (.0254 m / inch)^2 X 7.874 X 10^3 kg / m^3
= 80.169 kg

Control bundle corner girders:
4 X (7 / 8) inch^2 X 7 m X (0.0254 m / inch)^2 X 7.874 X 10^3 kg / m^3
= 124.46 kg
 

Indicator tube connectors:
Volume = 4 X 0.5 inch^2 X 30 inch = 60 inch^3

Mass of indicator tube diagonal connectors = 60.0 inch^3 X (0.0254 m / inch)^3 X 7.874 X 10^3 kg / m^3
= 7.742 kg

Active fuel bundle control portion bottom plate:
10 inch X 10 inc X 0.5 inch X (.0254 m / inch)^3 X 7.874 X 10^3 kg / m^3 = 6.451 kg

Active fuel bundle piston rod (push pipe):
Pi X 5.563 inch OD X 0.258 inch wall X 1.0 m long X (0.0254 m / inch)^2 X 7.874 X 10^3 kg / m^3
= 22.905 kg
 

TOTAL MASS OF CONTROL BUNDLE:
244 loaded fuel tubes + 2 control portion gratings + 4 control bundle corner girders + indicator tube + indicator tube diagonals + control portion bottom plate + piston push rod
= 244 (5.7397 kg) + 41.269 kg + 124.46 kg + 80.169 kg + 7.746 kg + 6.451 kg + 22.905 kg
= 1683.486 kg
 

TOTAL ACTIVE FUEL BUNDLE MASS:
Surround bundle + control bundle
= 2184.37 kg + 1683.486 kg
= 3867.85 kg
 

PASSIVE FUEL BUNDLES:
In order to achieve fuel bundle interchangability the passive fuel bundles are made and installed in the same manner as the active fuel bundles.
 

SUPPORT ASSEMBLY MASS:
The mass of each complete fuel bundle assembly is significant. The major mass components consist of:

Support Tube 12 inch X 12 inch X 0.500 inch wall X 2.0 m long
Mass = (12^2 - 11^2) inch^2 X 2.0 m X (.0254 m / inch)^2 X 7.874 X 10^3 kg / m^3
= 233.673 kg

Support tube wings:
Each support tube has 4 welded wings 3.74 inch long X 1.5 m high X 0.5 inch thick which stabilize the support tube against adjacent support tubes. The wings are located slightly off center so that adjacent wings overlap and are bolted together to give the overal structure stability.

The minimum support tube wing mass is:
4 X 3.74 inch X 0.5 inch X 1.5 m X (0.0254 m / inch)^2 X 7.874 X 10^3 kg / m^3
= 56.997 kg

Guides:
There are 8 guides to contol the sliding fit of the surround bundle outer corner girders over the support tube. Each guide is an L shaped piece 1 inch thick X 2 inches X 3 inches.

Guide Mass = 8 X 3 inch^2 X 3 inch long X (.0254 m / inch)^3 X 7.874 X 10^3 kg / m^3
= 9.29 kg

Hydraulic Cylinder:
Mass = Pi (10.750^2 - 9.759^2) inch^2 / 4 X 1 m X (.0254 m / inch)^2 X 7.874 X 10^3 kg / m^3
= 81.79 kg

Hydraulic cylinder bottom:
Mass = Pi (9.750 inch / 2)^2 X 0.5 inch X (.0254 m / inch)^3 X 7.874 X 10^3 kg / m^3
= 4.8168 kg

Piston:
Piston Mass = Pi (9.750 inch / 2)^2 X 3 inch X (.0254 m / inch)^3 X 7.874 X 10^3 kg / m^3
= 28.91 kg
 

BASE:
The 12 inch X 12 inch X 0.5 inch support tubes are welded to a base made from 14 inch X 6 inch X 5 / 8 inch rectangular tube. Note that access holes are cut into the base material to allow running 13 hydraulic lines per base tube. The support tubes are spaced 0.400 m center to center using 1.748 inch spacers and connected to vertical members at the pool walls.

Base mass = 0.4 m X 40 inch X (5 / 8) inch X (.0254 m / inch)^2 X 7.874 X 10^3 kg / m^3
= 50.800 kg
 

SURROUND BUNDLE GIRDER STOPS:
The surround bundle girder stops transfer the weight of the surround bundle from the surround bundle girders onto the support tubes. The 4 surround bundle girder stops are fabricated from 1.75 inch X 1.75 inch X 0.5 inch angle and are each 4 inches long.
Surround bundle girder stop mass = 4 X 3.0 inch X 0.5 inch X 14 inch X (.0254 m / inch)^3 X 7.874 X 10^3 kg / m^3
= 10.83 kg
 

SUPPORT SYSTEM MASS PER ACTIVE FUEL BUNDLE:
Mass of support = base + support tube + support tube wings + 8 guides + Hydraulic cylinder + hydraulic cylinder bottom + Piston + surround bundle girder stops
= 50.800 kg + 233.67 kg + 56.997 kg + 9.29 kg + 81.79 kg + 4.8168 kg + 28.91 kg kg + 7.45 kg + 10.83 kg
= 483.92 kg
 

TOTAL MASS PER FUEL BUNDLE OF CONTROL PORTION + SURROUND PORTION + SUPPORT:
Surround bundle + Control Bundle + support
= 2184.37 kg + 1683.486 kg + 483.92 kg
= 4351.78 kg
 

The reactor floor load not including the liquid sodium is about:
[4351.78 kg X 1 pound /0.454 kg] / [14 inch X 15 inch] = 45.64 psi
 

This web page last updated July 3, 2017

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