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**INTRODUCTION:**

This web page deals with FNR geometrical constraints.

**FNR PLAN VIEW:**

The following FNR geometrical calculations are based on a FNR with a rated thermal power of 1000 MWt. The following diagram shows the plan view of the fuel bundle array.

In the above diagram only one quadrant of active fuel bundles is fully populated and only 4 of 32 intermediate heat exchange bundles are shown.

**ACTIVE FUEL BUNDLE ARRAY:**

In order to realize 1000 MWt there must be 640 active fuel bundles. These fuel bundles are positioned in an octagonal array formed from a 28 bundle X 28 bundle square with 36 bundles clipped off each corner. Thus the total nimber of active fuel bundles is given by:

(28 X 28) - 4 (36) = 784 - 144 = **640 active fuel bundles**

Each of the straight faces of the active fuel bundle octagon is 12 active bundles long. The length of each of the diagonal faces is:

[8^2 + 8^2]^0.5 = [128]^0.5 = 11.31 active fuel bundles long.

The active fuel bundle octagon straight face to straight face distance is 28 active fuel bundle widths of 0.4 m.

The longest radius of the active fuel bundle assembly is:

{[14^2 + 6^2]^0.5} X 0.4 m

= 232^0.5 X 0.4 m

= **6.0926 m**

**ENCIRCLING PASSIVE FUEL BUNDLE ARRAY:**

Outside the active fuel bundles are 3 fully populated rings of passive fuel bundles. This array can be viewed as a 34 fuel bundle X 34 fuel bundle square with 12 bundle long straight faces indcating that 66 bundles have been clipped off each corner. Hence the number of passive fuel bundles is:

(34 X 34) - 4 (66) - 640
= 1156 - 264 - 640

= **252 passive fuel bundles**

The total number of (active fuel bundles + passive fuel bundles) is:

1156 - 264 = **892 fuel bundles**

The maximum (active + passive fuel) bundle array radius is:

{[17^2 + 6^2]^0.5 X 0.4 m

= 18.0278 X 0.4 m

= **7.211 m**

**ARRAY INCLUDING COOLING ACTIVE, PASSIVE AND COOLING FUEL BUNDLES:**

Outside the 3 fully populated rings of passive fuel bundles is one nearly full ring of cooling active fuel bundles and one only partially populated ring of cooling active fuel bundles. Neither of these two rings of cooling active fuel bundles have chimneys or indicator tubes attached. Hence in terms of external dimensions the cooling active fuel bundles are the same as passive fuel bundles. Thus, ignoring the partially populated outer cooling fuel bundle ring the fuel bundle array can be regarded as being formed from a 36 bundle X 36 bundle square with 14 bundle long straight faces.
However, each straight face is missing 1 bundle off each end. In addition each corner of this array is missing 66 bundles. Hence the number of bundles inside this 36 X 36 array is:

(36 X 36) - 4(66) - 4(2)

= 1024 bundles.

Hence the number of cooling active fuel bundle positions inside this array is:

1024 - 892 = 132

There are further cooling active fuel bundles that would fit inside a 38 X 38 array. Outside the middle of each straight face there are 4 such bundles. Outside each diagonal face there are 7 such bundles. Thus the total number of additional positions for cooling active fuel bundles is:

4 (4 + 7) = 44

Thus the total number of positions for cooling active fuel bundles is:

132 + 44 = **176 positions**.

Thus the total fuel bundle array has:

640 active fuel bundle positions

+ 252 passive fuel bundle positions

+ 176 cooling positions

= **1068 fuel bundle positions**

The cooling positions are in two groups. There is the inner ring of 132 positions that doubles as a fourth passive fuel bundle ring. There is the outer ring of 44 positions which during normal reactor operation are left vacant, allowing fuel bundle position flexibility for unscheduled maintenance.

**FUEL BUNDLE PERIMETER PATH:**

Around the perimeter of the fuel bundle array there must be a clear path for moving fuel bundles. Thus including the perimeter path the sodium pool inside the intermediate heat exchange bundles contains a subset of a 40 X 40 array. The radius of this array in the middle of its straight faces is:

20 X 0.4 m = 8.0 m

The maximum radius of this array at the end corners of the straight faces is:

{[19^2 + 7^2]^0.5} X 0.4 m

= 20.2485 X 0.4 m

= **8.0994 m**

The maximum radius of this array along the diagonal faces is:

{[17^2 + 11^2]^0.5} X 0.4 m

= 20.2485 X 0.4 m

= **8.0994 m**

**PRIMARY SODIUM POOL DIAMETER:**

Assume that the radial length allowance for the intermediate heat exchange bundles is 2.2 m. Hence the theoretical minimum primary sodium pool radius is:

8.0994 m + 2.2 m = 10.3 m

However, this dimension provides now allowance for fabrication and construction tolerances and for fuel bundle movement clearance. It is prudent to design in 0.2 m on each side of the pool to allow for tolerance and movement clearance. Hence the final pool inside radius is:

10.3 m + 0.2 m = **10.5 m**

resulting in a primary sodium **pool inside diameter of 21.0 m**

** INTERMEDIATE HEAT EXCHANGE BUNDLE REPLACEMENT:**

In order to replace an intermediate heat exchange bundle it must be moved around the fuel bundle array perimeter. However, the intermediate heat exchange bundles can be moved over the top of cooling fuel bundles and passive fuel bundles but not over the top of active fuel bundle chimneys. Without relocating active fuel bundle chimneys the maximum permissible intermediate heat exchange bundle width is:

8.1 m - 6.0926 m = **2.0 m**

which is not a serious constraint.

If we are to have perimeter space for 34 fuel bundles (32 bundle locations + 2 airlocks) the maximum heat exchange bundle width is limited to:

[Pi (2)(10.5 m - 2.2 m)] / 34 = 1.5338 m

However, this dimension provides no provision for fabrication tolerance or movement clearance. It is prudent to **restrict the maximum heat exchange bundle width to 1.500 m** to allow for tolerance and movement clearance.

Note that the heat exchange bundle manifolds need to have about a 1.45 m end radius to fit in the air lock. Hence the intermediate heat exchange bundle design will need further refinement. Similarly the pipe flange connections to the intermediate heat exchange bundles will need design refinement to fit within the air lock.

**AIR LOCK ASSUMPTION:**

Assume that the facility has two air locks, each 3.0 m ID to permit exchange of either several fuel bundles or one intermediate heat exchange bundle. The air locks should be designed for complete evacuation, and hence must have an external safe working pressure rating of 101 kPa. It may be helpful to fit the airlock with a thin metal liner such that the liner alone can maintain gas isolation after it is filled with argon.

**FUEL BUNDLE EXCHANGE:**

1)The 1st step in fuel bundle exchange is to remove all members of the cooling fuel bundles for reprocessing. Hence there are then 176 cooling fuel bundle positions available.

If there is any need for intermediate heat exchange bundle replacement this is an opportune time for this replacement.

2) The 2nd step is to clear a radial path toward the center of the fuel bundle assembly. That may require interim moving of as many as 34 active and passive fuel bundles into vacant cooling positions. Thus there are as many as: 176 - 34 = 142 positions available for cooling active fuel bundles. Typically 132 of these positions are used. The remaining 10 are kept available to support unplanned reactor maintenance.

3) The 3rd step is to move:

up to 132 used fuel bundles from the fuel bundle assembly interior to the vacant cooling positions.

4) The 4th step is to replace the 132 moved interior fuel bundles with new fuel bundles brought in via an airlock.

5) The 5th step is to replace the up to 34 fuel bundles along the access path.

6) The 6th step is to reposition the 132 bundles removed from the reactor interior to fill positions on the inner cooling ring. There are now 44 vacant cooling positions in the outer cooling ring. It is prudent to leave these positions vacant to allow unplanned access to interior fuel bundles.

The above procedure is followed once every six years so that each lot of 132 active fuel bundles has six years to cool for reprocessing while immersed in liquid sodium and **all the active fuel bundles are recycled once every 30 years**. Every 6 years the active fuel bundles are repositioned so that over a 30 year period all the fuel bundles receive approximately equal fast neutron exposure.

This web page last updated March 17, 2019.

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