Precoat Filters:

A precoat filter eliminates the problem of physically handling radioactive materials, since the filter material (known as the medium) could be installed and erased remotely. Within the filter housing is a bundle of septums (vertical tubes, on that the filter medium is deposited). The septums in a few filters are around 1 inch in diameter and 3 feet long and are commonly made of perforated or porous metal (generally stainless steel). There might be various hundred of these septums within a filter. Septums in other filters are around 3 inches in diameter and 3 feet long and are made of porous stone or porous ceramic material. There are commonly less than 100 of these larger septums within a filter.

The filtering medium fibers might be finely separated perlite, diatomite, asbestos, or cellulose. Diatomite, a least expensive medium, is used to filter liquid waste which will be discharged from the plant. Cellulose is commonly used for processing water which will be returned to a reactor, since diatomite could peremit silica leaching.

While a precoat filter is in use, water which enters the filter vessel passes by the filter medium which is deposited on the septums and then leaves by the outlet. Before the filter could be placed into operation, therefore, the filter medium must be installed; that is, the filter must be precoated.

A first step in precoating the filter is to near the inlet and outlet valves to the filter. The filter medium used is mixed along with demineralized water within an external mixing tank to form slurry that is pumped by the filter. A few of the filter medium deposits on the septums and is held there through the pressure of water on the outside of the septums. At the starting of the precoating procedure, a few of the fibers of the filter medium pass by the septums, either since they are smaller than the openings or since they pass by lengthwise. Therefore, there is still a few filter medium within the water as it leaves the filter, then the slurry is recirculated again and again until the water is clear. Clear water denotes that the entire filter medium is deposited on the septums or the filter is precoated.

One features of the precoating procedure is in which an extremely even layer of filter medium (around 1/8 inch thick) is deposited on the septums. That occurs since the circulating slurry follows the path of least resistance. While the coating at one point reaches a certain thickness and the slurry takes the fibers to another point, and this procedure continues until precoating is done.

Since water pressure contains the filter in place, flow must be manages by the recirculating loop to keep the medium from falling off. This is known as a holding flow. Since the inlet and outlet valves are opened for general usage, known as service flow, a holding flow is gradually cut off.