Arguably, the most important goal currently for nuclear power plants is reduction of radiation source term values. These radioactive species increase manpower exposure, hamper optimized plant operation, and raise radwaste remediation and disposal costs. One of the most prevalent source term radionuclides is cobalt. Moreover, cobalt is particularly problematic due the presence of both soluble and colloidal (filterable) cobalt that often require separate means of treatment to achieve elimination, or at least reduction. The article below provides an introduction to colloidal cobalt.
Cobalt (Co) is a transition metal with the stable isotope 59Co. There are two common, relatively long half-lived activation products of cobalt (58Co with a half life of ~ 71 days and 60Co with a half life of ~5.3 years). Cobalt can exist in both the +2 and +3 oxidation states. In aqueous solutions under oxidizing and moderately reducing conditions (up to pH 9.5), the dominant cobalt species is the uncomplexed Co2+ ion. At higher pH’s, hydrolytic species of cobalt predominate(see Figure 1).
A colloid is a chemical mixture in which one substance is dispersed evenly throughout another. The particles of the dispersed material are suspended in the mixture rather than dissolved. As seen in Figure 2, the size of colloidal particles varies roughly between 5 nanometers (5 X10-9 m) and 5 microns (5 X 10-6 m). The largest colloidal particles are similar in size to finer suspended solids. Filtration can remove colloidal particles as long as the porosity of the filter is fine enough to operate effectively.
During refueling outages at nuclear power plants, initial reducing conditions favor release of both oxides and metals deposited on surfaces; subsequent oxidative conditions promote formation and migration of metal oxides into the coolant. These oxides subsequently are complexed with radionuclides such as 55Fe, 58Co, and 60Co. These reactions, in turn, result in so-called crud bursts of both colloidal particles and soluble ions. The colloidal metal oxides take on various physical forms such as flocked agglomerations, crystalline needles, or amorphous masses. Most colloidal particles formed in a crud burst are smaller than 0.45 microns and thereby not efficiently removed by filtration. Colloidal cobalt generally constitutes a larger fraction of the 58Co activity than of the 60Co activity, though the relative fractions of soluble and colloidal cobalt are generally unknown and vary from plant to plant.
Source term reduction is a universal goal in the nuclear industry. Such reduction requires control of both soluble (non-filterable) and colloidal (filterable) activity. Isotopes of cobalt, along with those of iron, copper, cesium, and antimony, are the most critical radionuclides in terms of source term reduction. Radioactive colloidal cobalt is pervasive in both pressurized water reactor (PWR) and boiling water reactor (BWR) plants. Consequently, specialized media are used in condensate polishers, reactor water cleanup (RWCU), fuel pool, blowdown demineralizers, chemical volume control systems (CVCS), and radwaste applications to reduce / eliminate colloidal cobalt.
Graver supplies specialized media for this purpose and is developing second generation products that eliminate some of the fragility of the current generation media. Additionally, Graver PowerGuard® with Poroplate®* in RWCU can be part of an overall strategy for source term reduction of cobalt.
*Poroplate® is a registered trademark of Purolator-Facet
New! The Aegis Tri-Slot Septa End Connector
Last year Graver introduced a new line of advanced stainless steel backwashable precoat septa for RWCU, fuel pool and other specialty applications in nuclear power generation. For replacements, existing installations and new designs, Aegis® PowerGuard® all metal septa fit original Graver bottom tube sheet installations with OEM supplied nuclear adapter end fittings.
Customers have requested a “Tri- Slot nuclear adapter” to minimize possible exposure while handling septa. Such an adaptor eliminates original hardware removal – and its associated possibility of internal vessel damage – and reduces the potential for personnel exposure during replacement.
Exceptional Reliability, Easy Use
We proudly announce the new Aegis ® Tri-Slot septa end connector, a precision-machined 316L stainless steel end connection. The Tri-Slot adapter offers the same reliability as the existing connection system in service since the start up of the original RWCU systems. Original OEM systems featured cast stainless cups mounted on the bottom tube sheet standpipes with a triple slot cutout for the tri-slot connector. On the new adaptor, three posts are precision-welded to a spring retainer assembly that is accepted by the cast cup in the vessel. The retained grade 302 stainless steel compression spring applies force to the sealing gasket to make a long-lasting, leak-proof seal on each element.
Constructed of 314L stainless steel, Aegis PowerGuard elements incorporate Poroplate®, an advanced sintered metal design that allows for precise control of porosity (from two to 200 microns) complemented by the exceptional durability, quality and reputation for which Graver is reknowned.
PowerGuard Features Include:
* Low pressure drop
* Virtually eliminates contaminant leakage
* High strength; Collapse strengths over 400 PSID
* Excellent resistance against corrosion and fatigue
* All low carbon 316L stainless steel construction
* Allows temperatures up to 1000°F
* Excellent cleaning and precoating characteristics
* Engineered and manufactured in the USA under ISO 2000
Aegis PowerGuard elements are also available for top tube sheet designs as well as replacements for all OEM designs including Delaval and Croll Reynolds. Graver supplies PowerGuard in any diameter, length and end-fitting configuration. They are available as individual elements or pre-assembled tube bundles.
Graver PowerGuard specialty metal products incorporating Poroplate sintered metal technology include stainless steel pleated resin trap elements, specialty under drains and strainers, and resin traps. Custom designs are available to retrofit equipment for power upgrades or to extend operating life.
Poroplate® is a registered trademark of Purolator Facet, Inc.
Email questions to email@example.com.
Question: What is ion exchange resin shelf life and why is it important?
Shelf Life – the length of time a resin may be stored prior to unacceptable degradation of properties or performance – varies by application and resin type.
Resins that will be re-generated by the enduser: these are often stored in “salt” forms; strong acid cation exchange resin in sodium form and strong base anion in chloride form are most common. Two-year shelf lives are often indicated for these forms, but they can be stored for many years and still function very well.
High purity condensate and nuclear grade resins: the shelf lives of these hydrogen and hydroxide form resins often depend on customer specifications, with most requesting a one or two-year shelf life. Sometimes nuclear plant customers maintain a safety stock that isn’t used before shelf life expires. How is this issue addressed?
Depending upon Graver’s sample test results, intended application and resin quality, shelf life recertifications may be possible. After recertification, the resin should be well rinsed prior to use in any system. Because most plant’s systems are not designed for processing resin or high-volume, high-flow-rate rinsing, Graver reprocesses or rinses the resins to improve quality and determine the probable success if the customer decides that processing is economically feasible.
Optimize resin quality and performance while minimizing recertification needs:
Purchase only the required quantity of resin for delivery four to six weeks before expected use.
Remember that most resins for high purity applications will need regeneration, additional processing and/or rinsing to meet ever more stringent specification requirements that can include super fines, leachable and extractable TOC, leachable and extractable post UV sulfates and maybe chlorides.
Order with ample lead-time, which can be lengthy. Resins are manufactured to set plans and schedules; some resins may be manufactured to order. Inventory levels throughout the supply chain are limited, so Graver requires lead-time for raw material resin manufacture, shipment, regeneration/processing/rinsing, final product testing and shipment to the customer. Graver will advise on optimum order timing.
Store in original packaging; don’t allow product to dry out. Maintain the product under temperature controlled conditions between 3°C (38°F) and 40°C (104°F). Do not allow resin to freeze; some suggest that frozen resin may be thawed naturally and used normally, but if handled improperly, frozen resin may crack or fragment, causing higher differential pressure and impaired performance.
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