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Source Term Reduction Using Specialty Macroporous Resins
Technical Paper
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Figure 1: MHI Study on Filterability of CriticalResin NRW5010 at pH 5 and 7
Source Term Reduction Using Specialty Macroporous Resins
Abstract
Specialty macroporous ion exchange resins are well-established in primary systems for cleanup and full-power nuclear operations. New macroporous resin development, which will be discussed, has expanded applications of their use. Nuclear plants using specialty macroporous resins have far greater dose retention on cleanup beds and lower activity on post filters, among other benefits previously reported. In addition, the loading of cobalt, cesium and other metals is reviewed. Performance of the macroporous anions CriticalResin™ NRW5010 and the new orthoporous CriticalResin NRW5070 in relation to colloidal particulate removal is discussed.
Introduction
Ion exchange resins continue to be the most cost-effective media to control low-level impurities in nuclear coolant and boiler water. Demands on resins, however, continue to be pushed by ALARA (As Low As Reasonably Achievable). Resins must maintain lower impurities in system water by minimizing the release of organic and inorganic contaminants and controlling ionic leakage. Resin capacities increase to maximize loading, extend resin life and minimize waste generation. Performance is achieved while encountering degradation from oxidative chemistries and radiation present in the systems. Additionally, some resins are used for operations considered unique to resin technology. Specifically, the filtration of extremely fine corrosion products that conventional filtration will not remove. Macroporous resins are now commonly used to attain these increased expectations in nuclear operations. Achievements reported with macroporous resins have resulted in these products being considered among the best-accepted practices for nuclear plant operations.
Mechanism for Macroporous Anion and Cation Layering
Layering resin in cleanup beds, spent fuel pool demineralizers and radwaste systems have become standard practice. This process contributes greater flexibility in addressing impurities such as colloids and higher levels of divalent isotopes such as Cobalt 58.
The logic justifying a layer of macroporous anion over a layer of cation followed by the mixed bed was to capitalize on the specialty anion resin’s ability to filter colloidal particulates, primarily metal oxides of iron, cobalt, nickel and chromium and to maintain the stability of these oxides in the alkaline environment of the anion matrix. Additionally, an electrostatic charge associated with the anion works to assist the movement of the fine particles into the large anion pores.
A study by MHI (Mitsubishi Heavy Industry, Ltd)1 indicates that CriticalResin NRW5010 improved the filterability of iron if the influent solution pH was 7 rather than 5 (Figure 1). This is supported by solubility studies of metals such as Fe and Ni in water2. Having the anion on top of metal oxides encounters this alkalinity-filtering layer maintains the stability of the oxide particle. If the cation resin were on top free mineral acidity would reduce the solution pH reducing the effectiveness of the CriticalResin NRW5010.
Another independent report3 has confirmed CriticalResin NRW5010 is superior to other macroporous and gel anions in removing colloidal silica. A New England plant today operates multiple beds of Purolite A501POH (non-Nuclear CriticalResin NRW5010) to remove colloidal silica from municipal drinking water. These beds have been in service for over 30 years and are regenerated regularly with caustic. Bead attrition is approximately 5% annually.
Figure 2: Diagram for a Capped CVCS Cleanup Bed
Placing the layer of CriticalResin NRW160 between CriticalResin NRW5010 and the mixed bed is supported by three independent studies4,5,8 that document greater loading capacity and greater removal (DF) of soluble cations with specific emphasis on cesium and cobalt when compared to other high-capacity gel cation resins. The data justified layering this high-capacity cation before the polishing mixed bed resin to address the high levels of soluble cobalt and reduce loading on the polishing mixed bed (Figure 2). This improves overall mixed bed performance and effluent quality.
Figure 3: Diagram of a Radwaste Application With the Capped Mixed Bed
Use of the macroporous resins in radwaste systems has a different configuration than in RFO beds. The lead bed is loaded with CriticalResin NRW160 addressing significant cation loading of 58Co and 60Co (Figure 3). The second bed is layered with the macroporous anion, CriticalResin NRW5010, that addresses particulate material (although less efficiently) passing the lead bed. If the lead cation bed exhausts before the mixed bed, which is generally the case, the macroporous anion service life will be extended. When the mixed bed is replaced, the macroporous anion will also be replaced. Typically, radwaste systems will have ultra-filtration or reverse osmosis equipment ahead of the ion exchange equipment. In these systems, the need for CriticalResin NRW5010 is greatly reduced, as minimal particulate material is present. However, the CriticalResin NRW160 will continue to offer a significant benefit.
Macroporous Cation Resin CriticalResin NRW160
CriticalResin NRW160 is the highest capacity (2.1 eq/l) macroporous strong acid cation resin available to the nuclear market. For over 20 years, this cation resin has been successfully used in nuclear operations. It is primarily known for its ability to selectively remove and hold ionic species such as cesium and cobalt over lower cross-linked cation resin (Table 1). The benefit of this macroporous cation is a high affinity for isotopes and ion accessibility to functional sites within the bead.
This macroporous cation, as a standalone resin, is used where high ionic loading and crud are encountered7, including outage clean-up beds, spent fuel pool, and radwaste treatment. Currently, over 44 PWR (Pressurized Water Reactor) and 3 BWR (Boiling Water Reactor) sites are using this cation.
Table 1: Selectivity of Ions in Relation to Cation Cross-linking
% DVB
4
8
12
16
Li
0.9
0.85
0.81
0.74
H
1
1
1
1
Co
2.65
2.8
2.9
3.05
Cs
2
2.7
3.2
3.45
Figure 4: Layered Bed Arrangement for a RFO (Refueling Outage) Cleanup Bed
Macroporous cation in a cleanup bed, layered between and in the mixed bed placed on the bottom (Figure 2), provides a cation-rich bed with approximately 1.5:1 equivalence of SAC to SBA. Total SAC equivalence for a 30 ft3 cleanup bed is 714 eq compared to 612 eq for a gel mixed bed. This is approximately a 17% higher total cation capacity over a gel mixed bed. Cation selectivity by the macroporous cation also offers a greater DF for Cesium and Cobalt over gel cations (Figure 5).
Figure 5: Decontamination of Cs137 and Co58 When Treated With Mixed Bed Resins Containing Macroporous CriticalResin NRW160 Cation and 8% Cross-linked Gel Cation Decontamination Factors (DF) are averaged over total treated volume
Figure 6: Dose (mR/hr) on a Cleanup Bed After a Refueling Outage at Seabrook Nuclear
The use of CriticalResin NRW160 in cleanup beds has allowed greater loading of soluble cationic isotopes such as Co58 in PWR’s and Co60 in BWR’s, which are principal isotopes released during the reducing and oxidation steps of the cleanup before the outage. Figure 6 represents bed loading during an outage at Seabrook Nuclear plant. The macroporous cation removes high levels of soluble ions. In contrast, the corresponding anionic component, which is generally a hydroxide of the metal, contributes limited loading on the anion in the mixed bed. Testing by an independent source4 referenced CriticalResin NRW160 as “a preferred upstream media to optimize the performance of downstream polishing media that requires an acid environment.” Additionally, this media had 3-5 times the DF for removing divalent metals Mn54, Co57, Co58, Co60, Co134 and Cs137 compared to several other cationic media.
The most significant use of the macroporous cation resin is in mixed bed applications. This application addresses low-level ionic polishing by cleanup CVCS beds during full power spent fuel pool demineralizers and radwaste vessels. Full power CVCS and spent fuel pools mixed beds are challenged by low-level impurities and a high background of boric acid buffered with lithium to adjust pH. In these mixed bed applications, the macroporous cation resin provides high operating selectivity for impurities and lower selectivity for lithium over lower cross-linked resins.
Prior to and during an outage, spent fuel pool and coolant water is more aggressive to cation resins due to the presence of peroxides. Testing with CriticalResin NRW160 showed slightly elevated TOC release compared to higher cross-linked gel resins. When tested in an additional independent study against a 14% crossed-link gel, CriticalResin NRW160 was slightly lower TOC release. Change in total capacity and moisture was negligible for all resins tested. Evaluating macro cation and high cross-linked gel cation resins in a spent fuel pool sulfate excursion were similar.
Macroporous Anion Resins
There are several macroporous anion resins offered for use in the nuclear industry. The two resins receiving the greatest attention address fine colloidal isotopes. The best known of these anions is CriticalResin NRW5010. However, the second-generation anion, CriticalResin NRW5070, has been introduced, offering superior crush strength compared to CriticalResin NRW5010.
These macroporous anions effectively remove colloidal particulates estimated to be under 0.1µ in size. These very fine particulates are primarily released from steam generator and piping surfaces during the outage cleanup reducing step, particularly at the point of peroxide addition or oxidizing step. Colloids pass through the void of the gel resin bed and contribute to small micron post-filter plugging. Additionally, they collect on fuel bundles and system surfaces where they contribute to deposits, source term and contamination events. They also contribute to pump seal wear and pass through to waste treatment complicating final processing and are the main cause of the decontamination of casks used for fuel transfer to dry storage. Basic characteristics for the two macroporous anions are presented below (Table 2).
Figure 7: Electron micrograph of CriticalResin NRW5010 Macroporous Anion
CRITICALRESIN NRW5010
The macroporous anion CriticalResin NRW5010 (Figure 7) has gained worldwide recognition for its ability to filter colloidal isotopes. The high number of very large pores, along with the positive charge characteristic of the quaternary amine functional group, supports the subtle attraction and entraps the fine particles. Additionally, the alkaline nature of an anion environment allows any particulate metal, such as iron, to form stable complexes, which become favorable binding sites for other colloids and metal ions, such as, cobalt. These complexes adhere tightly to the resin, which minimizes the formation of hot spots in resin vessels and allows resin and activity to be slurried with minimal wash water to waste resin storage.
Figure 8: Activity on a Cleanup Bed Where CriticalResin NRW5010 Layer is Compared to Gel Anion
CriticalResin NRW5010 has a crush strength under 40 g/bead. Due to this characteristic, it is used explicitly on layered bed applications where a 12-inch (30 cm) layer is added on top of the cleanup beds or other beds that address colloids. Systems that have used CriticalResin NRW5010 include refueling outage cleanup beds (primary application), spent fuel pool beds, and radwaste beds. Many PWR units have successfully loaded CriticalResin NRW5010 on CVCS beds for use during full power. Diablo Canyon, however, encountered post filter pluggage after nine months of service with CriticalResin NRW5010 layered on the CVCS bed, and the resin was removed.
Interestingly, this plant experienced its cleanest outage at the end of that cycle. TVA Sequoyah has also reported an increase in post-filter changeouts but felt other factors contributed and continues using the overlay. Other plants using this macroporous anion during full power have reported no issues.
CriticalResin NRW5010 performance is consistently documented to remove very fine isotopes that commonly pass-through clean-up beds and cause extended cleanup run time (Figure 8). Other benefits reported include:
High dose loading on cleanup beds
Smaller micron post filters
Fewer filter change-outs during cleanup
Less dose on steam generators and associated piping
Faster cleanup times
Fewer contamination events were reported
Generally, more efficient treatment of radwaste from an outage
These benefits account for:
Time and cost savings during outages
Less total waste generation
Reduction in unit source term
It must be noted that although all these benefits are reported by plants employing this technology, macroporous resins alone cannot be credited solely. Good chemistry, proper training and operating efficiency are essential to achieve the benefits associated with CriticalResin NRW5010.
Figure 9: Electron Micrograph of CriticalResin NRW5070
CRITICALRESIN NRW5070
CriticalResin NRW5070, the next generation macroporous anion resin (Figure 9), was developed to provide greater crush strength over CriticalResin NRW5010 yet continues to support the removal of fine colloidal isotopes. Data in Table 2 above detailed crush strength at being above 200 g. Total capacity has also increased with a minimum of 1.0 eq/l in the OH form. The lower pore diameter and pore volume, accounting for improved mechanical durability, was initially a concern for lowering filtering efficiency. However, multiple installations and successful repetitive use at Exelon have supported the effectiveness of this product. The key performance factor has been using 0.1 µm post filters during cleanup with a noted lower dose on these filters.
Conclusion
The use of macroporous cation and anion resins in cleanup beds continues to gain recognition due to their performance and the fact that this technology is recognized as a best-accepted practice. These macroporous resins are used in more than 50% of US PWR’s and three BWR’s.
The use of the macroporous cation resin allows operations to achieve high loading of soluble impurities and isotopes both in cleanup environments and polishing applications. Additionally, this product has demonstrated stability in the presence of oxidants such as peroxide resulting in low TOC release.
The macroporous anion CriticalResin NRW5010 continues to show excellent performance in removing fine colloidal activity that contributes to clean up issues – handling this resin after the service cycle has been favorable as activity remains bound with the resin minimizing, transfer and handling issues. There also has been no performance issue associated with the low crush strength of this resin.
The next generation macroporous anion CriticalResin NRW5070 is installed at 14 locations, and performance data has been favorable from reporting stations.
The specialty macroporous resins developed for the nuclear industry have shown tremendous benefits, including operating savings and increased efficiencies without compromising performance quality.
With continued demand for ALARA, the macroporous resins are processed to meet industry standards. This is a challenge for the entire ion exchange resin industry, which Purolite is and will continue to address.
References
1. Mitsubishi Heavy Industries, LTD. Water Chemistry Technology Group, Water Reactor Engineering Department, Nuclear Energy Systems Engineering Center, Nuclear Energy Systems Headquarters and Nuclear Development Corporation Nuclear Environment Research & Development Department.
2. Plating Waste Treatment, Cherry, K., Ann Arbor Science, 1982, p. 46.
3. Bob Rohan 860-528-6512 US Filter in South Windsor CT. Personal communication.
4. Enhanced Liquid Radwaste Processing Using Ultrafiltration and Chemical Additives Results of Pilot Scale and Media Testing, EPRI Study Product ID #1009562, November 2004.
5. Centec XXI Evaluation of Cation Resin for Cobalt Removal, April 1999.
7. Wolff, J.J., “Purolite Resins for use in Nuclear Power Plants,” Purolite International, Paris, 1991.
8. Heller, T., “Macroporous Resin Impact on Radionuclide Cleanup,” EPRI LLW Conference, June 2005.
9. Heller, T., “Latest Operating Experience with Macroporous Resins in the Nuclear Industry,” Society of Chemical Industry, Cambridge, England, July 2008.
10. EPRI #1009562 “Enhanced Liquid Radwaste Processing using Ultrafiltration and Chemical Additives,” September 2004.
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