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

In order to enable practical Fast Neutron Reactor (FNR) siting in urban areas fully assembled FNR fuel bundles are shipped to and from FNR sites. Practical shipping by road requires that fuel bundles in transport containers be rated for working lateral accelerations of (1 / 2) g. A Fast Neutron Reactor (FNR) is made modular and economic through the use of 1149 to 1369 such fuel bundles. Each fixed octagonal fuel bundle contains 416 fuel tubes. Each square fuel bundle contains 280 fuel tubes.

Each FNR has four airlocks around the reactor perimeter. Two opposite smaller diameter airlocks are of 23 inch ID and are intended for routine exchange of FNR fuel bundles. The other two airlocks are much larger in diameter to provide occasional personnel access and to allow occasional replacement of intermediate heat exchange bundles and steel support lattice components.

A fuel bundle is transported in a near horizontal position inside a shielded transport container mounted on a flat deck truck, with the top of the fuel bundle near the back of the truck.

The fuel bundle overall length and width are limited by the weight of the shielded container needed to safely move a radioactive fuel bundle along city streets by truck. The fuel bundle and its transportation container are supported at an slight angle from horizontal to ensure that the fuel tubes, fuel rods and contained sodium all remain near the bottom of the fuel bundle.

It is contemplated that the truck trailer will have 4 rear axels each with 4 of wheels so as to be rated for 4 axles X 4 wheels / axle X 5 tons per wheel = 80 tons at the rear of the truck triler. The truck tractor will have additional 2 load bearing axles each with 4 wheels as well as forward wheels for steering.

During truck transport the fuel bundle must be surrounded by a removeable transportation safety material (eg a neutron absorbing powder) that will prevent the fuel bundle going critical if the shielded transportation container is accidentally immersed in water. At the reactor site the transportation safety material is removed and then the fuel bundle is pulled with a winch from the shielded container mounted on the truck into an equally sloped 24 inch OD air lock tube located in the 1 m wide concrete shielded space between two adjacent heat exchange galleries.

This air lock tube is then rotated to the vertical position and then is moved under the pool deck using a small rail car for bottom support. This airlock tube is then jack lifted to form a gas seal to the bottom of the pool deck directly underneath a trap door. This gas seal is possible because when the fuel bundles are being changed the pool deck is at a temperature of about 120 degrees C. Air is sucked out of the air lock tube and is replaced with argon. Then the trap door in the pool deck is opened and the fuel bundle is lifted out of the air lock tube and clear of the pool deck by the gantry crane located above the primary sodium pool space. The pool deck trap door is then closed. The gantry crane moves the fuel bundle to its desired mounting position. This trap door is gas sealed by a rim which rests in a trough containing a low melting point metal alloy (woods metal).

It is important to never let the fuel assembly accidentaly go critial. In loading fuel bundles into the primary sodium pool each mobile fuel bundle should be installed in its fully withdrawn position before installing the surrounding fixed fuel bundles. Similarly the fixed fuel bundles surrounding a mobile fuel bundle should be extracted from the primary sodium pool before extracting the mobile fuel bundle. That strategy ensures that the fuel assembly will not become critical due to pulling a mobile fuel bundle right through a group of adjacent fixed fuel bundles.

Thus the fuel bundle insertion and extraction order is important. In assembly of the fuel bundle array the mobile fuel bundles are ALWAYS installed in their fully retracted positions before their surrounding fixed fuel bundles are installed. Similarly, in disassembly of the fuel bundle array the fixed fuel bundles are ALWAYS removed before removing the retracted mobile fuel bundles that they surround.

The fuel bundles are designed so that in the reactor core zone individual fuel tubes, corner girders, diagonal plates and shroud plates can all linearly swell due to fast neutron bombardment without the external width of the fuel bundle significantly changing. In the core zone the edges of the shroud sheets are designed to slide sideways over the corner girders as the material swells. In the core zone the fuel bundle diagonal plates are reduced in width to provide a dimensional swelling allowance.

Fuel bundles are intended to be replaced before 15% linear swelling of the most intensely neutron irradiated sections occurs. The fuel tube array center-to-center spacing is established by the fuel bundle top and bottom geometries which are not exposed to fast neutron irradiation and by the fuel tube spiral wire winding. Note that the fuel tube array geometry is stabilized by the fuel bundle diagonal plates.

At the upper corners of the fixed octagonal fuel bundles are through bolts which are used to corner connect adjacent fixed octagonal fixed fuel bundles. These bolts prevent adjacent fixed fuel bundles sliding axially with respect to one another and hence prevent lateral rocking of the fuel assembly as might occur during a severe earthquake.

There are 4.5 m high buoyant indicator tubes field attached to the mobile active fuel bundles. The vertical position of each active square fuel bundle is visually indicated by the 0.3 m to 1.5 m exposed height of the top of its indicator tube above the primary liquid sodium surface.

Indicator tubes are hook attached to the mobile active square fuel bundles after the mobile active square fuel bundles are installed and are removed before the mobile active square fuel bundles are relocated. The indicator tube attachment point is the fuel bundle lifting point.

The fuel bundle removal procedure is simply the reverse of the fuel bundle installation procedure. A winch is required on the flat deck truck to slide a fuel bundle from inside the air lock tube to inside the shielded transportation container.

The heat exchange bundle installation and removal procedures are similar to the fuel bundle installation and removal procedures except that different air locks are used that have different dimensions, different trap doors and different auxiliary equipment. The heat exchange bundle airlocks also provide occasional worker access to the space above the primary sodium pool and allow occasional replacement of steel support lattice and gantry crane components.

In plan view the required pool deck trap doors are 45 degrees ahifted from the truck dock in order to obtain sufficient pool deck space for the larger trap door. The required clearance for the airlock transport rail car reduces available space under the adjacent heat exchange galleries. The access stairwells of these galleries must be located at the end of each gallery that is farthest from the adjacent air lock.

Heat exchange bundles are transported in the horizontal position on flat deck trucks with their tops near the back of the truck. From a truck deck a heat exchange bundle is winched onto a horizontal platform and then is rotated to the upright position. The heat exchange bundle is then lowered and moved along a track until it is directly under a trap door in the pool deck. Directly beneath that trap door but at the basement floor level is an apparatus for washing potentially radioactive surface sodium off removed heat exchange bundles. The surface radioactivity must be removed from a heat exchange bundle before it is safe for truck transport. Any water used in this wash facility must be carefully controlled to ensure that it never leads to basement flooding and that any radioactive particles that it acquires are carefully removed.

The height allowances for the octagonal fuel bundle components from bottom to top are: legs (1.5 m), bottom grating (0.1 m), fuel tubes (6 m), lifting point (0.3 m), swelling allowance 0.1 m. Hence the fuel bundle shipping container and the air lock tube must be able to accommodate a fuel bundle with an overall length of 8.0 m.

The fixed octagonal fuel bundle maximum outside face to outside face distance is:
23 X (5 / 8) inch = 14.375.0 inches. Hence the diagonal length allowance before swelling is:
2^0.5 X 14.375 inch = 20.329 inch.

After 13% swelling this diagonal may be as large as:
1.13 X 20.329 inch = 22.972 inch,

After 15% swelling this diagonal may be as large as:
1.15 X 20.329 inch = 23.3787 inch.

The square fuel bundle maximum outside face to outside face distance is:
19 X (5 / 8) inch = 11.875 inches.

Thus it appears that a fuel bundle airlock can be fabricated using 24 inch OD o.500 inch wall steel pipe.

To prevent overall fuel bundle swelling in the core region in that region the diagonal reinforcing sheets are reduced in width and the fuel bundle shroud sheets contain vertical slots to allow shroud and diagonal sheet swelling in the core region without causing significant overall horizontal fuel bundle width swelling.

Note that the primary sodium pool deck will thermally expand, so the alignment between the air lock tubes and the pool deck must allow for such thermal expansion.

This web page last updated January 13, 2021

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