Apparatus for filtering water containing radioactive substances in nuclear power plants

An apparatus for filtering waste water containing radioactive solid particles or produced in a nuclear reactor power plant and wherein the water is filtered with a plurality of U-shaped hollow fibers each having a sponge like mesh structure containing numerous pores for trapping radioactive substances, the hollow fibers being made of flexible polyethylene and each pore having a diameter of about 0.1 micron. An air pipe is provided beneath the U-shaped hollow fibers for bubbling air into water to be filtered surrounding the fibers, thereby removing solid particles trapped in the pores.

BACKGROUND OF THE INVENTION 
This invention relates to apparatus for filtering water containing 
radioactive substances, for example radioactive waste water produced in a 
nuclear electric power generating plant, or primary water of a boiling 
water type nuclear reactor. 
As filtering apparatus of radioactive waste liquid produced in a nuclear 
electric power generating plant, it has been used a precise filtering 
device utilizing a filtering aid. But as ion exchange resin has been used 
as the filtering aid, a large quantity of used resin is produced thereby 
increasing the quantity of radioactive waste to be discarded. Since 
radioactive waste is dangerous, it has been necessary to sink it in deep 
seas or to store it in underground storage. 
Recently, improved filtering apparatae which do not produce any secondary 
waste and can obtain treated water of better quality have been developed. 
Examples are an ultrafilter (hereinafter termed UF), and a filter 
comprising a flat film having numerous pores each having a diameter of 1 
micron. However, since the ultrafilter must circulate a large quantity of 
water, not only the construction of the circulation system is complicated 
but also the cost of installation increases, whereas the flat film has a 
large tendency of clogging the minute pores with solid substances. In 
addition, it requires a large installation area. 
In a nuclear electric power generating plant too, the same type of filters 
are used in a condensate feed system as well as primary water feed system 
for a nuclear reactor. 
The particle size distribution of insoluble impurities (CRUD) contained in 
radioactive waste liquid and condensate or primary water fed into a 
boiling water type nuclear reactor is shown in FIG. 1. Although the 
particle size distribution varies more or less depending upon the type of 
the liquid or water to be filtered, a substantive portion of the CRUD has 
a particle size of about 1 micron, and the percentage of CRUD having a 
particle size of 0.1 micron or less is very small. 
Consequently, in a UF having pores having a diameter much smaller than 0.1 
micron, substantially no solid particles enter into the pores so that 
there is no tendency of clogging the pores with the solid particles 
because trapped solid particles can readily be removed by backwashing. 
Since in the UF, the pore size is small, the quantity of water that can be 
filtered is small which not only increases filtering area but also 
requires a high filtering pressure which also increases the cost of 
installation. 
In a flat film type filter having a pore size of about 1 micron, since the 
waste liquid contains relatively large quantity of solid particles having 
a diameter of less than 1 micron, the tendency of clogging the pores 
increases whereby the film must be renewed frequently. 
Recently, hollow fibers made of polyvinyl alcohol or copolymers thereof 
with styrene, etc., and having a sponge-like mesh structure were developed 
as disclosed in Japanese Laid-Open Patent Specification No. 73390/1980. 
The hollow fiber disclosed therein has a diameter of 0.5-5 mm and contains 
numerous pores interconnected to form a complicated mesh structure. Each 
pore has a diameter of 0.005-1.0 micron, preferably 0.01-0.5 micron. This 
polyvinyl alcohol hollow fiber has an excellent water permeability, and 
high acid resistant property. Usually, 1,000-5,000 fibers are bundled in a 
U-shape and water containing iron is passed through the U-shaped bundle of 
fibers. Then iron is trapped in the pores and high purity water suitable 
for use in boilers or preparation of foodstuffs or drinks can be obtained 
on the outside of the fibers. 
But these polyvinyl alcohol fibers are solid having low mechanical strength 
so that they fracture or damage when subjected to a relatively small force 
or shock. Furthermore, clogged fibers cannot be regenerated with 
backwashing unless a special chemical agent is used. Presence of a 
chemical agent in nuclear reactor water, especially in a boiling water 
type reactor, is not permissible. In a nuclear power plant, water utilized 
in the plant is desired to have much higher purity than other 
applications. In other words, the concentration of solids in water should 
be extremely small. 
When filtering water used in a nuclear power plant with hollow fibers 
having a sponge like mesh structure of the type described above, not only 
dense films of solids are formed on the surfaces of the fibers but also a 
large quantity of the solids flows into the pores thus clogging the same. 
In other industrial applications, since the concentration of the solids is 
high, they aggregate into large particles which close the openings of the 
pores, whereby even when solids of small particles are present in the 
water to be filtered, such small particles are prevented from entering 
into the pores, thus preventing clogging thereof. For this reason, in the 
case of a nuclear power plant, the pore diameter must be selected 
carefully for preventing clogging of the pores. 
SUMMARY OF THE INVENTION 
It is the principal object of this invention to provide an efficient 
apparatus for filtering water containing radioactive substances in nuclear 
power plants utilizing hollow fibers having a sponge-like structure which 
can be regenerated with water, have high mechanical strength and are 
flexible so as not to be broken by external force or shock. 
Another object of this invention is to provide an improved filtering 
apparatus capable of efficiently trapping solid particles of radioactive 
substances and efficiently removing trapped solid particles with water 
thereby reducing the frequency of renewal of the hollow fibers. 
According to this invention, there is provided apparatus for filtering 
waste water containing radioactive substances in a nuclear power plant 
comprising a casing, a plurality of synthetic resin hollow filters 
installed in the casing, each fiber having a sponge-like mesh structure 
containing a number of pores extending between inner and outer walls of 
the hollow fiber, means for admitting water containing radioactive solid 
particles into the casing for trapping the solid particles in the pores, 
thereby filtering the water, a collection chamber located opposite from 
the casing across a head plate and being in communication with the 
interior of the hollow fibers for collecting filtered water passing 
through the pores into the interior of the fibers; and means for 
withdrawing filtered water from the collection chamber, wherein the hollow 
fibers are made of flexible polyethylene and the pores have a diameter of 
about 0.1 micron.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The filtering apparatus of this invention has a construction as shown in 
FIG. 2. As shown, the apparatus comprises a cylindrical casing 5 
containing cylindrical partition walls 7 and a head plate 12 from which a 
plurality of U-shaped bundles 3 of hollow fibers having a sponge-like mesh 
structure are suspended. 
Cap 20 is located on the opposite side of head plate 12 from casing 5 and 
defines collection chamber 22 which is in communication with the hollow 
interior of the fibers in bundles 3. Water containing radioactive 
impurities (CRUD) is admitted into the casing 5 through a pipe 8 and 
filtered while passing through numerous pores extending in the transverse 
direction of each hollow fiber. Cleaned water passes from the hollow 
interior of the fibers into collection chamber 22, from which it is 
discharged to the outside through pipe 9. The casing 5 is supported by a 
plurality of pedestals 13. To effect backwashing, pressure is applied to 
the interior of the casing 5, by pressurized air, for example. Then clean 
water is forced to pass through the pores to remove small solid particles 
trapped therein or large solid particles closing the outside openings of 
the pores. The solid particles thus removed are discharged to the outside 
of the casing 5 through a drain pipe 14 at the bottom thereof. At the time 
of backwashing, it is advantageous to blow air bubbles into the water in 
the casing 5 through air nozzles 6 provided for an air inlet pipe 4 near 
the lower ends of respective U-shaped bundles to vibrate the hollow fibers 
in various directions as shown in FIG. 4 to facilitate removal of the 
solid particles. Air collected beneath the head plate 12 is discharged to 
the outside through a discharge pipe 11. 
According to this invention, the hollow fibers are made of flexible 
polyethylene. The following Table shows comparison of various mechanical 
characteristics of hollow fibers made of flexible polyethylene and 
polyvinyl alcohol. 
TABLE 
______________________________________ 
Sample 
Polyvinyl 
Item Polyethylene 
alcohol 
______________________________________ 
pull strength 336 253 
(g/fiber) 
elongation 45.2 7.6 
(%) 
knot strength 319 impossible 
(g/fiber) to measure 
elongation 48.3 because PVA 
(%) fiber is 
hang strength 568 brittle 
(g/fiber) 
elongation 32.0 
(%) 
______________________________________ 
Remarks: 
As diagrammatically shown in the Table, in the pull test one end of a fiber 
was secured and the other end is pulled; in the knot test a knot is formed 
at an intermediate point of the fiber; and in the hang test one end of one 
U-shaped fiber is fixed, the other U-shaped fiber is hung from the one 
U-shaped fiber. The tested flexible polyethylene had a pore size 
sufficient to pass solid particles having a molecular weight of about 
300,000. 
As can be noted from this Table the flexible polyethylene hollow fibers 
utilized in this invention have higher mechanical strength than 
conventional polyvinyl alcohol hollow fibers. It should be particularly 
noted that the polyethylene hollow fibers utilized in this invention are 
pliable or flexible, so that there are such advantages that they would not 
be broken by external force so that they can be handled readily and that 
as shown in FIG. 4 they can bend in various directions during backwashing 
due to a turbulent flow caused by air bubbles thereby increasing removal 
of trapped solid particles. In contrast, since polyvinyl alcohol hollow 
fibers are solid and brittle they can not manifest these advantages. 
As the filtering operation proceeds, the pressure difference between the 
inlet pipe 8 and the discharge pipe 9 increases as a result of trapping or 
clogging of the solid particles in the pores, so that when the pressure 
difference reaches a predetermined value, backwashing is carried out in a 
manner described above. 
FIGS. 5 and 6 are graphs showing the relation between the number of 
backwashings and the variation in the pressure difference when hollow 
fibers having pore diameters of 1 micron and 0.1 micron respectively are 
used. Comparison of FIGS. 5 and 6 shows that, in the case of pore diameter 
of 1 micron, the rise in the pressure difference after backwashing is 
large (about 3.0 Kg/cm.sup.2 or more), whereas in the case of pore 
diameter of 0.1 micron, the rise in the pressure difference is not so 
large (only about 1 Kg/cm.sup.2). This means that with hollow fibers 
having a pore diameter of 1 micron, clogging is liable to occur but with 
hollow fibers having a pore diameter of 0.1 micron, the tendency of 
clogging is small with the result that the frequency of renewal of the 
hollow fibers can be reduced. As a consequence, hollow fibers having a 
pore diameter of about 0.1 micron are suitable for filtering water 
containing radioactive solid particles.