4.
ACTIVATED CORROSION PRODUCTS

4.1 INTRODUCTION

Activated corrosion products are produced by neutron activation of either the corrosion product deposit on the fuel surface or the in-core structure materials. The activation cross sections and specific activities for some major corrosion products are given in Table 2–5 and Figure 2–12, respectively. The activated corrosion products are released from fuel surface deposits by erosion and spalling caused by hydraulic shear forces in some cases and by dissolution in other cases. Some activated products are released from in-core materials by dissolution and wear. The activation products in the coolant can be soluble or insoluble, and they are transported by water to all parts of the primary system. This presents problems with regard to accessibility and safe maintenance of various components because of radiation fields. Among those activated corrosion products, the γ-emitting activities (Co-60, Co-58, Zn-65, Mn-65 and Fe-59) are more important in creating the radiation field problems. The longer-lived species (Fe-55, Ni-63 and Co-60) are of more concern with the problems in the radioactive waste handling and disposal. Radioactive waste production and handling will not be the major subject in this monograph, but some important aspects of radioassays of radioactive waste will be discussed separately in Section 7.

Since the coolant chemistries in BWRs and PWRs are totally different, the behavior of corrosion product transport and radiation field buildup in the two reactor systems should be expected to be different, and they will be discussed separately in the following sections.

4.2 ACTIVATED CORROSION PRODUCTS IN BWRs

4.2.1 Activation of Corrosion Products on Fuel Surfaces

A great number of fuel deposit samples have been analyzed and the results can be found in the literature(1,2,3). Some typical data of corrosion product and activity distribution along the length of the fuel rod are shown in Figures 4–1 and 4–2. The corrosion product deposits are generally found to be heaviest at ~50–100 cm from the bottom of the fuel rod, where the boiling starts. The specific activities are generally proportional to the exposure along the fuel length.



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Radiochemistry in Nuclear Power Reactors 4. ACTIVATED CORROSION PRODUCTS 4.1 INTRODUCTION Activated corrosion products are produced by neutron activation of either the corrosion product deposit on the fuel surface or the in-core structure materials. The activation cross sections and specific activities for some major corrosion products are given in Table 2–5 and Figure 2–12, respectively. The activated corrosion products are released from fuel surface deposits by erosion and spalling caused by hydraulic shear forces in some cases and by dissolution in other cases. Some activated products are released from in-core materials by dissolution and wear. The activation products in the coolant can be soluble or insoluble, and they are transported by water to all parts of the primary system. This presents problems with regard to accessibility and safe maintenance of various components because of radiation fields. Among those activated corrosion products, the γ-emitting activities (Co-60, Co-58, Zn-65, Mn-65 and Fe-59) are more important in creating the radiation field problems. The longer-lived species (Fe-55, Ni-63 and Co-60) are of more concern with the problems in the radioactive waste handling and disposal. Radioactive waste production and handling will not be the major subject in this monograph, but some important aspects of radioassays of radioactive waste will be discussed separately in Section 7. Since the coolant chemistries in BWRs and PWRs are totally different, the behavior of corrosion product transport and radiation field buildup in the two reactor systems should be expected to be different, and they will be discussed separately in the following sections. 4.2 ACTIVATED CORROSION PRODUCTS IN BWRs 4.2.1 Activation of Corrosion Products on Fuel Surfaces A great number of fuel deposit samples have been analyzed and the results can be found in the literature(1,2,3). Some typical data of corrosion product and activity distribution along the length of the fuel rod are shown in Figures 4–1 and 4–2. The corrosion product deposits are generally found to be heaviest at ~50–100 cm from the bottom of the fuel rod, where the boiling starts. The specific activities are generally proportional to the exposure along the fuel length.

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Radiochemistry in Nuclear Power Reactors Figure 4–1. Axial Distribution of Iron and Cobalt on Fuel Surface (Reproduced with Permission, J. Nucl. Sci. Tech., Ref. 4)

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Radiochemistry in Nuclear Power Reactors Figure 4–2. Axial Distribution of Ca-60 and Co-60 Specific Activity on Fuel Surface (Reproduced with Permission, J. Nucl. Sci. Tech., Ref. 4)

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Radiochemistry in Nuclear Power Reactors The deposit samples are taken from the fuel cladding surface under water in the fuel pool using a special sampling device similar to that described by Uchida et al.(4) (Figure 4–3). An area of ~2 cm2 is first “brushed” to remove the loosely-adherent deposit (outer layer). The same area is then “scraped” with a scraping stone to remove the tenacious deposit (inner layer). While the brushing or scraping process is in progress, the deposit removed from the fuel cladding surface is sucked in with water in the sampling device, transported through the sampling tube out of the fuel pool, and collected on a membrane filter. As shown in Figure 4–1, the activated corrosion products in the outer layer are only a small fraction of the total activities in the deposit. The specific activities in the inner layer are also generally higher, as expected, because of longer residence time on the cladding surface. The levels of fuel deposit and its chemical composition depend on the corrosion product input from the condensate and feedwater systems, and the levels of feedwater input vary significantly, depending on the materials used in the condensate system and the type of condensate treatment system. With the deepbed demineralizer system, the ionic species in the condensate are more effectively removed than the insoluble particulate species. On the other hand, the removal efficiencies for the ionic and insoluble species are just reversed in the filter-demineralizer system. Thus, the fuel deposit in the deepbed system contains a higher percentage of iron than that in the filter-demineralizer system. This fundamental difference may cause some differences in the activity concentration and chemical behavior of activated corrosion products in the coolant (Section 4.2.2). The corrosion product (mainly represented by iron, customarily called “crud”) deposition on a heating surface can be described by a simple equation: (4–1)

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Radiochemistry in Nuclear Power Reactors Figure 4–3. Fuel Cladding Deposit Sampling Device (Reproduced with Permission, J. Nucl. Sci. Tech., Ref. 4)

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Radiochemistry in Nuclear Power Reactors where: D = deposition rate constant, kg/cm2/day W = weight of crud deposit at time t, g/cm2 C = crud concentration in reactor water, g/kg Q = heat flux on fuel surface, kcal/cm2/day L = latent heat of water vaporization, kcal/kg p = probability of deposition, dimensionless k = release constant, day−1 At steady state, Equation 4–1 can be integrated as: (4–2) Initially when W is small, and the initial linear deposition rate (4–3) If k is large, and/or t is long, W may reach an equilibrium value, (4–4)

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Radiochemistry in Nuclear Power Reactors In the boiling region with steam, the deposition rate constant has to be modified by a factor of (1—V), where V is the steam void fraction in the region. The release constant k may vary as a function of water flow velocity and the thickness and chemical composition of the crud deposit. A mathematical model has also been developed to describe the activation of corrosion products in the fuel deposits. The assumptions for model development, the derivation of the model and a comparison of experimental data with the results of model calculations are given below: Assumptions: The parent atoms in the deposit increase linearly with time, and no release of the parent nuclide from the deposit is assumed. (This is generally true for a BWR with low crud input) The active atoms carried in by feedwater are insignificant. The active atoms are uniformly distributed in the fuel deposit. The release of active atoms is a first-order reaction regardless of the true mechanism. From these assumptions, the rate of formation of active atoms in the deposit on a fuel unit surface is described by Equation 4–5: (4–5) where: N = number of active atoms in the deposition unit surface area (atoms) No = number of parent atoms deposited on unit surface area at time t (atoms), No=nt n = constant rate of deposition of parent atoms from water to unit surface area on the fuel (atoms/s) P = reactor power and geometric factor k = a pseudo net release rate constant of active atoms from deposit into water (s−1)

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Radiochemistry in Nuclear Power Reactors   t = time of irradiation (s) λ = decay constant (s−1) = effective neutron flux (n/cm2/s), assumed constant σ = activation cross section (cm2) σb = burnup cross section (cm2) Replacing No with nt, Equation 4–5 becomes: (4–6) Equation 4–6 can be rearranged to become: (4–7) where Multiplying both sides of Equation 4–7 by eCt and rearranging the equation, one obtains (4–8) Integrating Equation 4–8 gives (4–9) where K is an integration constant.

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Radiochemistry in Nuclear Power Reactors Putting K back in Equation 4–9. or Since activity A=Nλ, and No=nt, therefore, (4–10) and the specific activity can be calculated by Equation 4–9: (4–11) Based on some earlier reactor data(2), the values of k and C are empirically determined for the activities in BWR fuel deposits (Table 4–1). Also given in Table 4–1 are estimated equilibrium specific activities for some common activated corrosion products. A comparison of the calculated specific activities with the experimental data* obtained in several reactors is shown in Figure 4–4. It is apparent that the specific activities of the shorter-lived isotopes become saturated (or reach equilibrium) much sooner than the longer-lived isotopes. As shown in Table 4–1, Co-60 has the highest specific activity, and it is also the dominate activity in the deposit in most cases. *   Each data point represents the average of all samples taken from a fuel bundle.

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Radiochemistry in Nuclear Power Reactors 4.2.2 Concentrations and Chemical Behavior of Activated Corrosion Products in Reactor Water The concentrations of activated corrosion products in reactor water vary from reactor to reactor, depending largely on the materials used in the entire reactor and condensate systems. Some activated corrosion products (mainly Co-60 and Co-58) may be released from the in-core structure materials, but most of the other activities are released from the fuel deposit. Some selected American National Standard activation products concentrations in reactor coolants are given in Table 4–2. More recent data have shown much lower activity concentrations, as shown in Table 4–3, in which the activated corrosion products are determined as “soluble” and “insoluble” species separately. Table 4–1 EMPIRICAL RELEASE CONSTANTS AND EQUILIBRIUM SPECIFIC ACTIVITIES FOR ACTIVATION PRODUCTS IN BWR FUEL DEPOSITS Nuclide k(day−1) λ(day−1) C (day−1)* Equilibrium Specific Activity** Co-60 1.26×10–3 3.6×10–4 0 1.62×10–3 80 Ci/g Co-58 5.9×10–3 9.8×10–3 4.8×10–3 2.05×10–2 0.65 Ci/gNi Fe-55 8.56×10–4 7.03×10–4 0 1.56×10–3 0.6 Ci/g Fe-59 7.09×10–3 1.55×10–2 0 2.26×10–2 25. mCi/g Mn-54 2.28×10–3 2.2×10–3 0 4.48×10–3 35. mCi/gFe Zn-65 2.42×10–3 2.8×10–3 0 5.22×10–3 1.8 Ci/g Ni-63 1.03×10–3 19×10−5 0 1.05×10–3 90 mCi/g * **Based on average thermal neutron flux=3.5×1013 n/cm2/s and fast neutron flux=7×1013 n/cm2/s

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Radiochemistry in Nuclear Power Reactors Figure 4–4. Bundle Average Specific Activities of Activated Corrosion Products in Fuel Deposits

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Radiochemistry in Nuclear Power Reactors Figure 4–15. Iron Solubility for Magnetite as a Function of Hydroxide Concentration at 250°C and 300°C, at Hydrogen Partial Pressure of 1 atm (Reproduced with Permission, EPRI NP-3463, Ref. 8)

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Radiochemistry in Nuclear Power Reactors 4.3.2 Radiochemical Composition in PWR Coolants The radiochemical composition of a typical PWR reactor coolant given by American National Standard Radioactive Source Term(5) is compared with the BWR data in Table 4–2. The average activity concentrations of selected nuclides measured in two reactors are compared in Table 4–8. If one can extrapolate the measurement at ambient to operating temperatures in the coolant, the data given in Table 4–8 suggest that Cr-51 tends to occur in reactor coolant predominately in solids, while others have been found in both soluble and insoluble forms. However, Mn-54 seems to be more soluble and Co-58, Co-60 and Fe-59 are more insoluble. At shutdown, the spiking activities are dominated by the solubles (Table 4–8). Typical spiking data for Co-58 and Co-60 are shown in Figure 4–16. The cobalt activities increased dramatically when boration was essentially completed and the coolant temperature was reduced below 300°F. Another spike occurred after hydrogen peroxide had been added. One must be cautious when the reactor coolant corrosion product data are evaluated. Problems in withdrawing representative samples from a PWR primary coolant through long sample lines are well recognized.(10,12,13) Unlike the high-purity water in the BWR coolant, the PWR coolant chemistry is rather complex, and it presents a special environment for some radiochemical species which may easily change their solubilities and undergo certain interactions with the oxide film on the sample line when the coolant is cooled down and water pH is changed. Experience at Ringhals(13) suggests that, in general, the measured concentrations of some activated corrosion products do not bear any useful relationship to the concentrations of corrosion products in the coolant at operating temperatures. The measured concentrations of some corrosion products (e.g., soluble Co-60 and Mn-54) are strongly dependent on sampling flow rate and the boron concentration. (See sampling procedures in Appendix B.)

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Radiochemistry in Nuclear Power Reactors Table 4–8 AVERAGE ACTIVITITES OF SELECTED NUCLIDES IN REACTOR COOLANTS FROM THE BEAVER VALLEY AND TROJAN PLANTS (Activities in nCi/kg) (Reproduced with Permission, EPRI NP-3463, Ref. 8)   Beaver Valley Trojan   Operating Shutdown Operating Shutdown Nuclide Solids Solutes Solids Solutes Solids Solutes Solids Solutes Cr-51 159±209 3.2±1.2a 62 NDc 232±362 41b 466 1364 Mn-54 8.9±15.8 69±41 6.3 2994 17±18 21±40 55 113 Fe-59 12±17 6.8±5.3 281 ND 17±21 10±10 37 52 Co-58 162±240 183±403 1135 1.7×105 437±657 210±249 680 4253 Co-60 45±82 30±35 96 5823 22±27 9±11 86 384 Co-58/Co-60 3.9±3.0 11±20 12 29 15±6 34±17 8 11 Error indications are standard deviations. Entries without error indications are single measurements. aAverage of four measurements; nine additional measurements were ND. bOne real value, the remaining measurements were ND. cND=Not detected.

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Radiochemistry in Nuclear Power Reactors Figure 4–16. Concentrations of Co-58 and Co-60 Activities in a PWR Primary Coolant During Shutdown Operation (RCAMS=Reactor Coolant Activity Monitoring System (Reproduced with Permission, EPRI NP-3463, Ref. 8)

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Radiochemistry in Nuclear Power Reactors 4.3.3 Corrosion Product Deposition On Out-Of-Core Surfaces The corrosion product activity transport processes and radiation buildup on the main coolant piping and steam generator have been the main subject of a recent investigation.(8,9) Coolant chemistry in PWRs plays an important role in the radiation field buildup. The primary long-term source of radiation fields in either BWR or in PWR plants is Co-60, but in PWR Co-58 can also be a significant source, especially early in life. Cobalt-58 has a half-life of 71 days and hence reaches an equilibrium level in a few months. A review of the data obtained from the various out-of-core locations shows that the specific activity of the crud deposits varies, depending upon location and the base material. A comparison of the surface activity concentration on the piping, steam generator tubing, and fuel surfaces of a Westinghouse PWR plant is shown in Figure 4–17. The difference in activity deposition could be caused by the corrosion mechanism of Inconel tubing versus that of stainless steel piping, the effects of the smooth surface of the tubing versus that of the rougher piping surface, or the amount of activity and flow rates of the coolant passing through a tube compared with that in a section of piping. In terms of Co-60 activity, inventories on the various surfaces are approximately 1000 Ci on the core, 500 Ci on the steam generator tubing and 100 Ci on the piping after several years of operation.(10) In the primary coolant system, the corrosion product activities (mainly Co-60 and Co-58) can be transported throughout the system in either soluble forms or insoluble crud. Because of the complexity of the coolant chemistry, it is almost impossible to quantitatively identify which form of the activity is more important. A mathematical model (CORA) has been developed to describe the transport of corrosion products in the PWR system(10). The computer code nodal diagram is shown in Figure 4–18. Figure 4–19 shows the widely spread dose rate data measured on the steam generator channel head and the range of CORA results. Recent studies on the role of coolant chemistry on PWR radiation field buildup(9,14,15) have concluded that control of pH is an important factor in reducing the radiation buildup. Lower radiation fields are generally observed at plants which were operated with constant, or higher, pH, and the fuel crud deposits were thinner. The effect of pH on the solubility and the mobility of the crud is obvious. The key in reducing the radiation field is to minimize the deposition of corrosion products on the fuel surfaces, where they are

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Radiochemistry in Nuclear Power Reactors Figure 4–17. Comparison of Unit Surface Activities at Three Different Locations and Materials of Construction (Reproduced with Permission, EPRI NP-4246, Ref. 10)

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Radiochemistry in Nuclear Power Reactors Figure 4–18. CORA Model Nodel Diagram (Reproduced with Permission, EPRI NP-4246, Ref. 10)

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Radiochemistry in Nuclear Power Reactors Figure 4–19. Measured Steam Generator Channel Head Dose Rates and Range of CORA Results (Reproduced with Permission, EPRI NP-4246, Ref. 10)

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Radiochemistry in Nuclear Power Reactors being activated by the neutron flux and subsequently being released and transported to other parts of the primary system. Laboratory measurements(16) of the solubility of several nickel ferrite stoichiometries as a function of pH, temperature and dissolved hydrogen indicate that solubilities tend to increase at higher temperatures when the solution pH is higher. Most recently, initial results from the high pH tests at Ringhals-4 in Sweden(17) have shown that the radiation fields are significantly lower than the typical plants at the comparable operation time. Ringhals-4 has operated at a pH of 7.3 (maximum LiOH concentration of 3.3 ppm) for much of its ~2 years of operations. It must be cautioned that operating at higher LiOH concentrations (Li≥3.5 ppm) in the coolant may result in higher fuel surface oxidation. 4.4 REFERENCES (1) L.D.Anstine, et. al., “BWR Corrosion-Product Transport Survey”, EPRI NP-3687 (September 1984). (2) A.Strasser, et. al., “Corrosion-Product Buildup on LWR Fuel Rods”, EPRI NP-3789 (April 1985). (3) L.D.Anstine, “BWR Radiation Assessment and Control Program: Assessment and Control of BWR Radiation Fields”, Volume 2, EPRI NP-3114 (May 1983). (4) S.Uchida, et. al., J. Nucl. Sci. Tech., 24, 385–392 (May 1987). (5) American Nuclear Society, “American National Standard Radioactive Source Term for Normal Operation of Light Water Reactor” ANSI/ANS-18.1–1984. (6) C.C.Lin, et. al, “Corrosion Product Transport and Radiation Field Buildup Modeling in the BWR Primary System”, 2nd Int. Conf. Water Chemistry of Nuclear Reactor System, BNES, October 1980, Paper 46; Nuc. Tech. 54, 253 (1981). (7) C.C.Lin and F.R.Smith, “BWR Cobalt Deposition Studies, Final Report”, EPRI NP-5808 (May 1988). (8) C.A.Bergmann and J.Roesmer, “Coolant Chemistry Effects on Radioactivity at Two Pressurized Water Reactor Plants”, EPRI NP-3463 (March 1984). (9) C.A.Bergmann, et. al., “The Role of Coolant Chemistry in PWR Radiation-Field Buildup”, EPRI NP-4247 (October 1985).

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Radiochemistry in Nuclear Power Reactors (10) S.Kang and J.Sejvar, “The CORA-II Model of PWR Corrosion-Product Transport”, EPRI NP-4246 (September 1985). (11) F.H.Sweeton and C.F.Bass, J. Chem. Thermodynamics, 2, 479 (1970). (12) N.R.Large, et. al., “Studies of Problems of Corrosion Problem Sampling from PWR Primary Coolant”, Water Chemistry of Nuclear Reactor System 5, Vol. 1, P. 63, BNES, London (1984). (13) M.V.Polley and P.O.Andersson, “Study of the Integrity of Radioisotope Sampling from the Primary Coolant of Ringhals 3 PWR”, ibid, P. 71, (1989). (14) M.Yamada and Z.Ojima, “Correlation Between Deposition Fuel, pH Control and Radiation Field Trend”, 1988 JAIF Int. Conf. Water Chemistry in Nuclear Power Plants, Vol. 1, P. 162 (April 1988), Japan Atomic Industrial Forum, Tokyo. (15) H.Takiguchi, et. al., “Radiation Buildup Control by High pH Chemistry in the Tsuruga-2 Primary Coolant System”, ibid. P. 168. (16) R.H.Kunig and Y.Sandler, “Solubility of Simulated PWR Plant Corrosion Products”, EPRI NP-2448 (1985). (17) K.Egner, “Getting the Dose Down at Sweden’s PWR Plants”, Nucl. Engng. Int. 31 49 (1986). (18) C.C.Lin, “Foreign Approaches to Controlling Radiation Field Buildup in BWRs”, EPRI NP-6942-D (August 1990). (19) C.C.Lin, “Optimum Water Chemistry in Radiation Field Buildup Control”, Third International Workshop on Implementation of ALARA at Nuclear Power Plants, Paper 1–2, May 8–11, 1994, Hauppauge, New York. (20) C.C.Lin, F.R.Smith and R.L.Cowan, “Effects of Hydrogen Water Chemistry on Radiation Field Buildup in BWRs”, International Conference on Chemistry in Water Reactors: Experience and New Developments, Vol. 1, 271 (April 24–27) 1994, Nice, France.

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