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Based on the standardized Mullen and Tensile tests, the nano-fiber filter media has exceptional robustness and strength advantages compared with the resin impregnated glass fiber filter media.

The nano-fibers’ physical and mechanical advantages include dry and wet, as well as not exhibiting any directional or orientation bias. After receiving 3 mega rad of dose, the nano-fiber filter media remains significantly stronger and more robust than a brand new, non-irradiated resin impregnated glass fiber media sample.

Operators of PHWR reactors identified four (4) distinct areas of research that the development of nano-fiber filter media for nuclear application had to address in order to be considered for use in their systems. These included;

a) Chemical Compatibility/Stability
b) Dirt Holding Capacity
c) Initial Retention Efficiency
d) Physical & Mechanical Characteristics
a) Chemical Compatibility/Stability

A number of full sized filter cartridges were sent to National Research Council of Canada – RPC Fredericton facility for testing. RPC staff installed the test filters in a recirculation test rig. The duration of each recirculation test was approximately 3 weeks. Samples of the lithiated demineralized water were collected periodically throughout the course of the trial. The elements eluted from the filter cartridges were analyzed using ICP MS/ES techniques.

The filters tested included resin impregnated 0.1 µm glass fiber, nano-fiber singly ply and 3 mega rad nano-fiber co-pleat. The Total Dissolved Solids (TDS) eluted from the cartridges over the course of approximately 3 weeks did not vary significantly.

Table 1

Resin Impregnated G/F

mg/L

Single-Ply Nano

mg/L

3M Rad Co-Pleat Nano

mg/L

Total Dissolved Solids

9.40

10.08

11.57

 

b) Dirt Holding Capacity
A number of full sized filter cartridges were sent to IBR Laboratories (Grass Lake, MI) for testing. Using ASTM 795-88 & 797-88 test protocols, the dirt holding capacity of the nano-fiber filter media was compared against the standard resin impregnated 0.1 µm glass fiber filter media. The testing conditions emulated those of the PHWR PHT system with pH approximately 10.4, temperature = 40o C/105o F, Flux = 4.5 USGPM/10” and black iron oxide differentiated to 0.1 µm as test contaminant.

Table 2

 

Resin Impregnated G/F

grams/10” @ 90 psid

Single-Ply Nano

grams/10” @ 90 psid

3M Rad Co-Pleat Nano

grams/10” @ 90 psid

Black Iron Oxide differentiated to 0.1 μm

468

285

650

 

The 3 mega rad co-pleat nano filter media significantly outperformed the other samples. It is important to note that the DHC results for the resin impregnated glass fiber media were skewed. The loading rate at 36.7 mg/L was only 25 % of loading rate for the nano-fiber filters tested at 147 mg/L.
Filtration specialists recognize the significance that the loading rate has on the absolute dirt holding capacity. Given equivalent filters, a higher loading rate results in shorter time to terminal DP and lower absolute dirt holding capacity.

c) Initial Retention Efficiency
A number of full sized filter cartridges were sent to IBR Laboratories (Grass Lake, MI) for testing. Using ASTM 795-88 & 797-88 test protocols, the initial retention efficiency of the nano-fiber filter media was compared against the standard resin impregnated 0.1 µm glass fiber filter media. The testing conditions emulated those of the PHWR PHT system with pH approximately 10.4, temperature = 40o C/105o F, Flux = 4.5 USGPM/10” and black iron oxide differentiated to 0.1 µm as test contaminant.

Table 3

 

Resin Impregnated G/F

@ 0.1 μm

Co-Pleat Nano (new)

@ 0.1 μm

3M Rad Co-Pleat Nano

@ 0.1 μm

Percentage % Removal Efficiency Black Iron Oxide

79.74

99.1

99.1

 

The initial retention efficiency of a filter is recognized by filtration specialists as the most valid procedure for evaluating the performance of a group of filters. For filters with fixed pore structures, the retention efficiency of a media will continue to improve up until the point where the terminal differential pressure (?PTERMINAL) set point is met.

Even after receiving 3 mega rad of dose, the co-pleat nano significantly outperformed the 0.1 µm rated resin impregnated glass fiber filter media. It is also interesting to note that the initial retention efficiency of the co-pleat filter media was not affected, even after receiving high levels of dose.
Physical & Mechanical Characteristics.

Given the manner it is produced, in many instances filter media has many of the same properties as textiles. The textile industry has developed numerous testing procedures and standards. The filter industry has adopted the use of testing instruments originally used for textiles in their QC/QA programs.

The two most commonly used QC/QA instruments are the Mullen and Tensile testers. The Mullen tester measures the burst strength of a 2” diameter sample. Pressure is hydraulically applied until the sample bursts. The sample’s burst strength is measured in psi. The tensile tester uses a standard 1” wide test coupon which is clamped between two jaws and the force is applied to the breaking point. The result is measured in lbs/in.

Table 4

 

Resin Impregnated Glass Fiber

Single Ply Nano-Fiber

1Kilo Rad Co-Pleat Nano Fiber

3 Mega Rad Co-Pleat Nano Fiber

Dry Mullen

35

100

110 – 120

110

Wet Mullen

30

118

120

100

Dry Tensile Machine Direction

9.6

36.4

37.8

33.6

Wet Tensile Machine Direction

8

17.6

17.2

13.9

Dry Tensile Cross Machine Direction

7.7

28.5

27

25.8

Wet Tensile Cross Machine Direction

8.6

12

11.9

9.8

 

Based on the standardized Mullen and Tensile tests, the nano-fiber filter media has exceptional robustness and strength advantages compared with the resin impregnated glass fiber filter media. The nano-fibers’ physical and mechanical advantages include dry and wet, as well as not exhibiting any directional or orientation bias. After receiving 3 mega rad of dose, the nano-fiber filter media remains significantly stronger and more robust than a brand new, non-irradiated resin impregnated glass fiber media sample.

There were four distinct areas of prime research in the development of the nano-fiber filter media for the RCS:

a) Chemical Compatibility/Stability
b) Dirt Holding Capacity
c) Initial Retention Efficiency
d) Physical & Mechanical Characteristics
Chemical Compatibility/Stability

During the development of the nano-fiber filter media for RCS, the mantra of “Do No Harm to the Reactor” was adopted as the guiding principle.

In terms of chemical compatibility a series of tests was conducted by RPC-Fredericton. The tests incorporated a recirculation loop which included a full size filter. The parameters for the test as well as conditions of the test emulated the PHT system in a PHWR. The recirculating test liquid was deionized water that was pH adjusted using LiOH to pH > 10. Figure 4 provides an isometric view and key component list of the recirculation test rig.

Figure 1

Using the test apparatus identified in Figure 4, RPC-Fredericton conducted an extended series of recirculating tests, each lasting approximately 3 weeks. Samples of the recirculating water were periodically withdrawn and analyzed using ICP MS/ES procedures. The tests compared the extractable ion concentration eluted from various filters including co-pleat nano, single-ply nano and the resin impregnated glass fiber media. Chart 1 below, demonstrates the differences in Total Dissolved Solids between the three different filter media. Note the blue line identifies the TDS performance of the nano co-pleat media after receiving 3 mega rad dose.

Chart 1

There were some minor differences between the species and concentrations eluted from the various media. Table 1 below identifies the most significant variation and differences between the 3 mega rad irradiated Ahlstrom co-pleat and the Pall U001Z after approximately 3 weeks recirculation.

Table 5

Extractable Element

3M Ahlstrom Co-Pleat

Pall U001Z

Concentration Al, mg/L

3.27

0.31

Concentration Si, mg/L

0.74

2.77

Concentration Na, mg/L

1.22

0.76

Concentration Ca, mg/L

0.06

0.44

 

It is recognized that the higher rates of elution of Al and Na from the 3 mega rad irradiated Ahlstrom co-pleat media could potentially lead to neutron activation and the production of increased radiation fields in the PHT. Since the primary Al activation product (Al-28) has a relatively short half-life (2.2 minutes), operators and station chemists agree that this is unlikely to pose any additional safety concerns at the low levels that would be present in the RCS.
Al is also activated to Na-24 and Mg-27 and Na-23 to Na-24 (1368 keV 100% and 2574 keV 99.85%) which has a half-life of approximately 15 hours. The level of sodium eluted from the 3mega rad co-pleat nano fiber filter media is comparable to that eluted from the 0.1 µm resin impregnated glass fiber filter. Scaling the test results and after dilution into the full volume of the RCS there is expected to be minimal impact on system chemistry and radiation fields.

Dirt Holding Capacity:

DHC testing was conducted by Inter Basic Resource (IBR) Laboratories of Grass Lake, MI. IBR Laboratories has been the exclusive agency used by Ontario Power Generation (and previously Ontario Hydro) for all filter performance related challenges since the mid-1980’s.

Prior to commencing both DHC and initial retention efficiency testing, operators of Canada’s PHWR’s were canvassed for their preference of test surrogates. The choice lay between:

a) black iron oxide
b) latex spheres
c) virus

The operators’ unanimous choice for test contaminant was black iron oxide. Their choice was unchanged even when they acknowledged that black iron oxide could only be differentiated to 0.1 µm. The 0.1 µm for black iron is compared to latex spheres that can be differentiated to 0.03 µm and virus even smaller. The rationale for the consistent choice of magnetite was that the preferred surrogate was the most representative of the contaminants the CANDU operators expected to find in the PHT system.

The following chart provides a comparison of DHC performance between conventional 0.1 µm resin impregnated glass fiber, single ply nano and co-pleat nano filter media. All tests were conducted by IBR Laboratories following ASTM F795-88 and F797-88 protocols. The test parameters were designed to emulate the conditions in PHWR PHT systems and used full size filter cartridges flowing at 4.5 USGPM/10”.

In order to produce reasonably similar retention capacities, the test protocol for the 0.1 µm resin impregnated glass fiber media was intentionally altered. For the resin impregnated glass fiber media the loading rate of the black iron surrogate contaminant at 36.7 mg/L was only 25 % of the loading rate to the nano-fiber filters at 147 mg/L.

The relationship between loading rate and total retention in dirt holding capacity testing is a well understood phenomena. It relates to the permeability of the media, total number of available voids and increased pressure losses associated with increased loading factors.

Chart 2

 

There are two additional pieces of information that can be derived from the dhc chart:

a) All three samples successfully meet and exceed the collapse/rupture specification (75 psid) for filter cartridges that has been adopted by PHWR operators.

b) Under the stated test conditions all three samples exceed the OPG-Nuclear DHC specification for 120 grams/10” at 40 psid.

Initial Retention Efficiency:

IBR Laboratories conducted series of Initial Retention Efficiency tests on a number of full-sized filter cartridges according to ASTM 795-88 and 797-88 protocols. The parameters of the test were adjusted to emulate the conditions of PHWR PHT systems. Black iron oxide differentiated to 0.1 µm was the surrogate contaminant for all tests. Flux at 4.5 USGPM/10” was constant for all tests.

Filtration specialists have determined that the best metric for evaluating the performance of a filter is its initial retention efficiency. This recognizes the reality that for a filter with a fixed pore structure, its best removal efficiency occurs immediately before reaching its ?PTERMINAL. In process filtration over time, as ?P increases, the filter is building a filter cake, which itself acts as a filter. To avoid biasing results, all tests performed by IBR Laboratories were based on the initial retention efficiency. A graphic of the components in an initial retention efficiency test rig are detailed below in Figure 5.

Figure 2:

For any filter test there are multiple ways to describe the result.

a) Beta Efficiency at a given removal rating under specified test conditions
ßeta ValueX = No. ParticlesUPSTREAM
No. ParticlesDOWNSTREAM

b) Per Cent Removal Efficiency at a given removal rating under specified conditions
% RemovalX = ß – 1 * 100
ß

For nuclear process filtration applications the benchmark for performance is removal efficiencies at 99.98 % with a specified (X) micron rating (most often X = 0.1 µm). The 99.98 % removal efficiency equates to a Beta efficiency BETAX = 5000. It can also be presented as ß5000 = X

The table below compares the Initial Removal Efficiency of the co-pleat nano media (new, irradiated and used) to the performance of the resin impregnated glass fiber filter media.

Note that the initial removal efficiency of the co-pleated nano fiber media is unaffected by 3 mega rad irradiation. Also as predicted by theory, the used sample at > 99.99 % increased its efficiency over both the new and irradiated samples (99.1 %).

Table 6

 Physical & Mechanical Properties:

The textile industry has developed numerous testing procedures and standards. In many instances filter media has many of the same properties and characteristics as textiles. The filter industry has adopted the use of testing instruments originally used for textiles for use in their QC/QA programs. Figures 6 & 7 show images of the equipment most often used to measure strength and ruggedness of filter media.

Figure 3

Figure 4

The Mullen Tester measures the burst strength of the media. A 2” diameter sample is cut and placed in the machine and pressure is applied until the sample ruptures. The sample’s burst strength is measured in psi.

The tensile tester measures the force required to the breaking point of a standard 1” wide sample coupon in lbs/in.

The table below provides a comparison of the physical/mechanical characteristics between nano fiber filter media (single ply and co-pleat) and the conventional 0.1 µm resin impregnated glass fiber media. Note that even after receiving 3 mega rad irradiation, the co-pleat nano media is still significantly stronger and more robust than the conventional resin impregnated glass fiber media in a new, non-irradiated state.