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

In normal circumstances an argon atmosphere is maintained over the primary sodium pool. Anything that either enters or leaves this argon filled space does so by way of a mobile airlock.

Each FNR has four airlock trap doors in the pool deck around the reactor perimeter. These trap doors open upwards. These trap doors are located near the corners of the enclosed argon filled space to avoid conflict with the secondary sodium pipes.

Two smaller diameter airlocks are intended for routine exchange of FNR fuel bundles. At each scheduled fuel change there are over 200 fuel bundle airlock cycles, so to minimize the argon requirement it is important that the fuel bundle airlocks be no larger than necessary.

The two smaller mobile airlocks must have an inside length of greater than 8 m. The two end ball valves increase the overall length to about 10 m.

The other two airlocks are used much less frequently, are much larger in size and have different bottom end access arrangements. These two airlocks provide occasional personnel access and allow occasional replacement of gantry crane components, intermediate heat exchange bundles, intermediate heat exchange gundle supports, indicator tubes and open steel lattice components.

The larger airlock height is limited by the ceiling height (19 m) less the combined rail car plus scissors jack height. The inside height must be greater than ~ 9 m to accommodate intermediate heat exchange bundle assemblies.
The larger airlock width is limited by the space between adjacent heat exchange galleries and by available pool deck space. The inside width must be at least 1.1 m to accommodate open steel lattice sections.
The larger airlock length is limited by available pool deck space. The inside length must be greater than 1.5 m to accommodate open steel lattice sections and must be greater than ~ 2 m to accommodate intermediate heat exchange bundle assemblies.

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.

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 and slightly higher than the bottom of the fuel bundle.

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 every core fuel bundle must be surrounded by a transportation safety material (eg gadolinium) that will prevent the fuel bundle going critical if the shielded transportation container is accidentally immersed in water.

When a new fuel bundle arrives at a reactor site it is moved with a winch from the shielded fuel bundle transportation container mounted on the truck into an equally sloped 24 inch OD mobile air lock tube located in the 1 m wide concrete shielded space between two adjacent heat exchange galleries.

Full port ball valves are used to close the opper and lower ends of the mobile air lock tube. The lower valve must be capable of supporting the weight of the fuel bundle.

The mobile air lock tube is then rotated to the vertical position and is lowered onto a rail car that runs the length of two heat exchange galleries at the basement level. When opposite an airlock port the lower air lock tube is transferred onto another rail car that takes it to a point directly beneath the airlock port and is then scissors jack lifted to form a gas seal with the port. This gas seal is possible because when the fuel bundles are being changed the primary sodium pool deck temperature is about 120 degrees C which is EPDM gasket compatible.

Air is sucked out of the air lock and is replaced with argon. Then the trap door in the pool deck is opened, the upper full port valve is openned and the new fuel bundle is lifted out of the air lock and clear of the adjacent secondary sodium pipes by the gantry crane located above the primary sodium pool space. The gantry crane then moves the new fuel bundle to its desired mounting position. The gantry crane then moves a used fuel bundle into the airlock.The pool deck trap door is then closed and the air lock top ball valve is closed. The procedure is reversed to move the used fuel bundle to the truck mounted transportation container.

During normal reactor operation the airlock trap door is gas sealed by a rim which rests in a trough containing a low melting point metal alloy (woods metal). This liquid metal provides a reliable continuing air seal as long as the space over the primary sodium pool remains at atmospheric pressure.

It is important to never let the fuel assembly accidentally 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 movable 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 movable fuel bundles that they surround.

The fuel bundles are designed so that in the reactor core zone individual fuel tubes, corner girders and diagonal 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 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 moving with respect to one another and hence prevent lateral rocking of the fuel assembly as might occur during a severe earthquake.

There are 7.5 m high buoyant indicator tubes field attached to the movable 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 attached to the movable active square fuel bundle lifting points after the movable active square fuel bundles are installed and are removed before the movable active square fuel bundles are relocated.

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 the inside the shielded transportation container. If the transportation route involves passing over water every core fuel bundle must be surrounded by a neutron absorbing material such as gadolinium.

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

The heat exchange bundles with their associated piping are taller than the fuel bundles. Hence the height of the gantry crane hook above the top of the secondary sodium pipes is set by the height of the heat exchange bundles. To mitigate this height issue the secondary sodium pipes are run horizontally as close as practical to the pool deck.

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 air lock trap doors. The required clearance for airlock rail car transport reduces available storage space under the heat exchange galleries. The access stairwells of these galleries must be located at the end of each gallery that is farthest from the adjacent truck load/unload point.

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 into a mobile airlock which is then is rotated to the upright position. The airlock is then lowered and then moved along rail tracks until it is directly under an airlock 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 so as to make these bundles 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.

Maximum height above pool deck:____
Maximum depth below pool deck:____
Overall height:_____
Height above pool deck to which top of heat exchange bundle must be lifted in order to clear other secondary sodium pipes and fittings:________

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.

The gantry crane has electronic components that cannot survive at the normal 500 degree C temperature of the argon above the primary sodium pool. Prior to use of the gantry crane the space temperature must be lowered to about 120 degrees C. Then the gantry crane electronic package can be temporarily installed.

once planned work with the gantry crane is complete the gantry crane electronic package is removed before the reactor is restarted. Note that personnel access to the space above the primary sodium pool must be possible without use of the gantry crane.

This web page last updated July 13, 2021

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