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With over thirty years of experience in pressurized water reactors, Purolite is the ion exchange industry expert. CriticalResin™ can drastically improve pressurized water reactor circuits elevating throughput and decontamination without sacrificing quality.
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The pressurized water reactor (PWR) is the most widely used nuclear power generation design and has achieved great success worldwide. The reactor core heats the surrounding pressurized water (the primary circuit or coolant). The water circulates to a steam generator, where it transfers its heat to a secondary water system. Steam at approximately 900 psig is created and is sent to a steam turbine to generate electricity. The advantage of this system is that the radioactive sections (the reactor and primary circuit) are separate from the rest of the power plant. Such separation helps to control and minimize potential contamination risks.
Ion exchange treats the four circuits within the primary circuit in a PWR nuclear plant:
The primary circuit water is directly connected with the reactor fuel and acts as a coolant and moderator. Impurities from the makeup water and corrosion products exposed to the core become radioactive. Primary circuit water transports heat from the fuel bundles and helps to cool the fuel. This water must be clean and free of soluble and suspended corrosion matter collected on the fuel rods.
Operators must control the level of inorganic salts (sodium, sulfate and chloride) to minimize corrosion in the granular structure of the fuel rod sheaths and system metal surfaces. Two types can occur — intergranular corrosion (IGC) or stress corrosion cracking (SCC). Corrosion by-products will foul fuel and contribute to irregular burning known as axial anomalies or crud-induced power shift (CIPS). These corrosion by-products will become activated isotopes that release as crud bursts during cool-down periods of outages. These buildups and releases can damage fuel sheaths and contribute to fuel leaks.
PWR’s use enriched UO2 (between 2.0 and 4.95 wt.%) in pellet form as fuel. In a 900 MW power plant, the reactor uses up to 72 tons of uranium per year. These pellets are contained inside zirconium alloy sheaths (zircaloy) to form a rod. In a typical design, approximately 200 of these fuel rods create a bundle or element. A reactor core can contain 150 to 200 rod bundles that are arranged for optimum heat generation.
Resin capacity for radioactive isotopes is defined by the decontamination factor (DF), which is the influent radioactivity divided by effluent radioactivity. This capacity depends on the nature and concentration of radioactive isotopes being addressed and the density of functional groups on ion exchange resins used. The table below presents relative affinities (ion selectivity) for cobalt and cesium, compared with lithium and hydrogen, for different degrees of cross-linking.
The table shows that the affinity for cesium over H+ and Li+ ions increases with the cross-linking of the strong acid cation exchange resin, while the affinity for cobalt is slightly lower than cesium. An exchange resin with high divinylbenzene (DVB) cross-linking has higher removal rates. High cross-linked macroporous cation exchange resins have greater porosity, which allows higher molecular weight divalent ions greater access to the active sites within the bead.
By contrast, highly cross-linked gel-type exchanger resins have a tighter matrix and thus insufficient porosity. This results in lower operating capacity. However, certain higher cross-linked gel resins may have a greater capacity for smaller molecular weight ions, compared to comparable cross-linked macroporous resins. The table below presents the results of tests carried out on coolant purification (primary circuit) containing lithium and boric acid.
The tests compared a mixed bed resin with a gel-type strong acid cation NRW3260 (normal cross-linkage) and a mixed bed resin with a macroporous type strong acid cation NRW3560 (high cross-linkage). The average influent was Co58 1.0E-3 µCi/cm, Cs137 3.0E-1 µCi/cm. The DF increases when the concentration of activity increases in the solution. The DF will vary noticeably during the cycle as the influent concentration changes
|Ion Selectivity for Principal Soluble Cations on Varying Cross-linked Cation Resin|
|Decontamination Factors (DF)|
|Mixed Bed with Gel Type SAC NRW3260||Mixed Bed with Macroporous Type SAC NRW3560|
|Bed Volumes Treated||DF for Cs 137||DF for Co 58||DF for Cs 137||DF for Co 58|