5.
WATER AND IMPURITY ACTIVATION PRODUCTS

5.1 INTRODUCTION

There are numerous water activation products and activated water impurities in the coolant (Table 2–4). Among those nuclides, only a few nuclides are radiologically significant in reactor operation. The following activities have been studied in detail: H-3, N-13, N-16, F-18, Na-24 and Cl-38. The chemistry of nitrogen activities and their transport behavior in the steam/condensate systems in a BWR has become an interesting and important subject of investigation under hydrogen water chemistry conditions(1).

5.2 TRITIUM IN PWRs

The study of tritium in PWRs is important because tritium’s long half-life (12.3 years) permits long-term buildup within the plant systems. Since reactor coolant is recycled, tritium is retained within the plant as tritiated water and release may occur as liquid, water vapor or gaseous tritium. The primary sources of tritium in the reactor coolant system in a PWR are: (1) diffusion of tritium from the fuel through the zircaloy cladding; (2) neutron activation of boron in the burnable poison rods and subsequent tritium diffusion through the stainless steel cladding; and (3) neutron activation of boron, duterium and 6Li in the reactor coolant. The fission yield of tritium for U-238 is ~0.01%.

Two major neutron reactions with boron resulting in tritium production are:

  1. 10B (n,2α)3T

  2. 10B (n,α) 7Li (n,nα)3T

It has been estimated(2) that approximately 10% of the tritium produced in stainless-clad burnable poison rods is released into the coolant, which contributes ~2/3 of the observed tritium buildup.

The production of tritium from the 6Li (n,α)3T reaction in the coolant is controlled by limiting the 6Li impurity in the 7LiOH used in the reactor coolant and in the lithium from demineralizers with 99.9% 7Li. However, it has been experienced that an increase in ambient reactor coolant system tritium levels may be used as an indicator of inadequate 7Li enrichment. The tritium produced by the neutron activation of deuterium in water (~0.015%) is less than 5 Ci/year, a few percent of total production



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Radiochemistry in Nuclear Power Reactors 5. WATER AND IMPURITY ACTIVATION PRODUCTS 5.1 INTRODUCTION There are numerous water activation products and activated water impurities in the coolant (Table 2–4). Among those nuclides, only a few nuclides are radiologically significant in reactor operation. The following activities have been studied in detail: H-3, N-13, N-16, F-18, Na-24 and Cl-38. The chemistry of nitrogen activities and their transport behavior in the steam/condensate systems in a BWR has become an interesting and important subject of investigation under hydrogen water chemistry conditions(1). 5.2 TRITIUM IN PWRs The study of tritium in PWRs is important because tritium’s long half-life (12.3 years) permits long-term buildup within the plant systems. Since reactor coolant is recycled, tritium is retained within the plant as tritiated water and release may occur as liquid, water vapor or gaseous tritium. The primary sources of tritium in the reactor coolant system in a PWR are: (1) diffusion of tritium from the fuel through the zircaloy cladding; (2) neutron activation of boron in the burnable poison rods and subsequent tritium diffusion through the stainless steel cladding; and (3) neutron activation of boron, duterium and 6Li in the reactor coolant. The fission yield of tritium for U-238 is ~0.01%. Two major neutron reactions with boron resulting in tritium production are: 10B (n,2α)3T 10B (n,α) 7Li (n,nα)3T It has been estimated(2) that approximately 10% of the tritium produced in stainless-clad burnable poison rods is released into the coolant, which contributes ~2/3 of the observed tritium buildup. The production of tritium from the 6Li (n,α)3T reaction in the coolant is controlled by limiting the 6Li impurity in the 7LiOH used in the reactor coolant and in the lithium from demineralizers with 99.9% 7Li. However, it has been experienced that an increase in ambient reactor coolant system tritium levels may be used as an indicator of inadequate 7Li enrichment. The tritium produced by the neutron activation of deuterium in water (~0.015%) is less than 5 Ci/year, a few percent of total production

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Radiochemistry in Nuclear Power Reactors rate. A typical example of tritium activity buildup in reactor water is shown in Figure 5–1. The tritium inventories in a typical PWR plant components are given in Table 5–1. Table 5–1 TRITIUM INVENTORIES IN PLANT COMPONENTS IN A PWR (Ref. 2) Component Water Volume (g) Tritium Concentration (μCi/g) Tritium Inventory (Ci) Reactor Coolant 1.17×108 0.416 48.7 CVCS Holdup Tank 9.5×107 0.315 33.3 Primary Water Storage Tank 2.84×108 0.194 55.0 Component Cooling System 1.1×107 0.240 2.6 Refueling Water Storage Tank 9.6×108 0.11 106.0 Spent Fuel Pit 2.26×109 0.09 204 Boric Acid Storage Tanks 7.56×106 0.184 1.4 Waste Holdup Tanks 1.09×107 0.132 1.4

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Radiochemistry in Nuclear Power Reactors Figure 5–1. Tritium Level in A PWR Primary Coolant (Ref. 2)

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Radiochemistry in Nuclear Power Reactors 5.3 Na-24 AND Cl-38 IN BWRs Na-24 and Cl-38 activities are two major activated products from water impurities. The main source of NaCl is believed to be the inleakage of the secondary coolant in the condensate-feedwater system. However, a significant “leaching-out” of sodium from the deep bed demineralizer system may also be an important contributor during normal operation. It is rather difficult to calculate directly the activity production from the core average neutron flux and the average coolant residence time in the core region (Section 2.7). Instead, based on a number of measurements in operating BWRs, it has been estimated that the equilibrium production rates of Na-24 and Cl-38 in the coolant during normal operation are, respectively: The equilibrium activity concentration per unit target material in the coolant can be calculated from: where Ceq = equilibrium activity concentration per unit target material, μCi/Kg/ppb R = activity production rate, μCi/kg/ppb W = reactor coolant mass, kg λ = decay constant, s−1 βc = reactor cleanup time constant, sec−1   If the equilibrium activity concentration is measured, the equilibrium target material concentration as well as the constant target source input rate (n) can be easily estimated. An example of calculation is shown in Table 5–2. A detailed model calculation

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Radiochemistry in Nuclear Power Reactors of activity buildup in the coolant during a no-cleanup test will be presented in Section 8.2. Table 5–2 EQUILIBRIUM CONCENTRATIONS AND SOURCE INPUT RATES OF Na+ (Na-24) AND Cl− (Cl-38) DURING NORMAL OPERATION IN A BWR*   Na+ Na-24 Cl− Cl-38 Production rate, R, μCi/s/MWt/ppb — 1.9×10–3 — 5.9×10–3 Equilibrium conc., μCi/kg/ppb   — 0.14 — 0.125 Equilibrium conc., μCi/kg (Observed) — 2.6 — 2.9 Equilibrium conc.,   18.6   23.2   Constant source input rate, ppb/hr   8.04   10   *Reactor power=1820 MWt Reactor water mass, W=2×105kg Feedwater flow rate, F=920 kg/s Cleanup flow rate, f=24 kg/s Cleanup time constant, βc=1.2×10−4s−1 5.4 N-13 AND N-16 IN BWRS The N-16 activity is the primary source of radiation fields in the coolant and steam systems during power operation. In a BWR, the total nitrogen activity carried by the steam is only a few percent of the total nitrogen activity produced in the core under normal BWR water chemistry (NWC) conditions(3). However, during recent tests in BWRs with hydrogen addition in the coolant, the radiation fields in the steam-turbine

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Radiochemistry in Nuclear Power Reactors systems increased by a factor of ~5(1,4), mainly due to the increase of the N-16 activity in the steam phase (Figure 5–2). The gamma-ray spectrum of N-16 and other nuclides (0–19 and C-15) observed at an operating BWR is shown in Figure 5–3. The higher levels of radiation fields in the steam-turbine systems are the major concern in reactor operation under hydrogen water chemistry (HWC) conditions. The N-13 (t1/2=10 min) and N-16 (t1/2=7.1 s) are two radioactive nuclides produced from oxygen by the 16O (p,α)13N and 16O (n,p)16N reactions, respectively, in the reactor coolant. The recoil energies for the nitrogen nuclides from nuclear transformations are estimated at ~0.4 MeV, which is several orders of magnitude larger than required to break a chemical bond (~3 eV). Thus, these two activated nuclides are expected to behave similarly in chemical reactions after they become thermalized. Because of its short half-life, the chemical forms of N-16 have never been measured. However, the chemistry and steam transport behavior of N-13 have been extensively studied(3,5). As a result of radiation effects, the nitrogen activities can exist in many chemical forms in water and steam (Section 6.7). In some early reactor measurements under normal operating conditions, most of the N-13 and N-16 activities were found in the anion forms in the reactor coolant, presumably NO2− and NO3−, while most of the activities in the steam condensate were found in the cationic form, most likely NH4+. In recent radiochemical studies in four reactors during hydrogen chemistry tests(5), the cationic form of nitrogen activity has been identified as NH4+ by ion chromatographic separation, and the anion fraction was found to contain two major species, NO3− and NO2−, in both reactor coolant and steam condensate. The test results are summarized in Table 5–3, and the typical results are shown in Figures 5–4 and 5–5, in which the N-13 concentrations in both cation and anion fractions in the reactor recirculation water and the steam condensate are shown as a function of dissolved H2 in reactor recirculation water. The dissolved O2 is also shown in each figure for comparison.

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Radiochemistry in Nuclear Power Reactors Figure 5–2. Variation of Radiation Fields and N-16 Concentrations in Steam as a Function Concentration in Feedwater (Ref. 3)

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Radiochemistry in Nuclear Power Reactors Figure 5–3. Gamma-Ray Spectrum Observed at a High Pressure Turbine with a Shielded Collimator

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Radiochemistry in Nuclear Power Reactors Figure 5–4. Variation of N-13 Species in Reactor Water with H2 Concentration. (Ref. 1,5) Figure 5–5. Variation of N-13 Species in Steam Condensate with H2 Concentration in Reactor Water. (Ref. 1,5)

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Radiochemistry in Nuclear Power Reactors Table 5–3 SUMMARY OF N-13 CHEMICAL FORMS MEASURED DURING HWC TESTS (Ref. 5) (in % of chemical fraction)   NWC HWC* Reactor Cation Anion Cation Anion In Reactor Water:   Pilgrim 0.6 99.4 80.0 20.6 FitzPatrick 1.2 98.8 97.7 2.3 Hatch-1 1.8 98.2 59.2 40.8 NMP-1 1.2 98.8 81.0 19.0 NMP-1 (Incore) 4.6 95.4 56.4 43.6 In Steam Condensate:   Pilgrim 55.6 44.4 99.5 0.5 FitzPatrick 52.5 47.5 98.7 1.3 Hatch-1 58.7 41.3 100.0 0.0 NMP-1 65.7 34.3 100.0 0.0 *HWC conditions are: 130–140 ppb H2 and 1–2 ppb O2 in reactor recirculation water. System chemistry has been found to have a profound influence on the chemical forms and the distribution of nitrogen activities in the BWR primary coolant system. Apparently, the NO3− or anion fraction dominate the N-13 activity in reactor water under NWC condition. As the dissolved H2 increases and dissolved O2 decreases, the NO3− fraction decreases quickly and the cation fraction, or NH4+ increases significantly. Similarly, the anion fraction (NO2−) of N-13 in steam decreases and the cation fraction (NH4+) increases sharply and becomes dominant as the dissolved H2 in water increases. The average total N-13 production rate is experimentally determined to be ~25 μCi/s/MWt, which is in excellent agreement with the theoretically calculated value of 23 μCi/s/MWt(6). The production rate of 16N is rather difficult to determine; however, based on the measured 16N concentration in steam lines, the core exit N-16 concentrations have been estimated to be 54±15 μCi/g under normal water chemistry conditions and 240±65 μCi/g under hydrogen water chemistry conditions (Table 5–4). The activity presented in non-condensable chemical forms and released through the offgas system was only a few percent of the total production, and the release rate decreased by a factor of ~2 under HWC conditions, in spite of the increase in steam activity. Apparently, the chemical form of N-13 in the offgas was dominated by the NO and/or NO2 under NWC conditions.

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Radiochemistry in Nuclear Power Reactors Table 5–4. N-16 STEAM CONCENTRATION AT VARIOUS LOCATIONS IN THREE CLASSES OF BWRs (μCi/g Steam) (Ref. 1)   BWR/2 BWR/3 BWR/4 Location NMP-1 Pilgrim Dresden-2 Hatch-1 PB-3 Measurement Point   NWC 34 25 24 46 21 HWC 159 129 104 201 78 Vessel Nozzle   NWC 38 28 27 51 23 HWC 180 139 117 221 87 Core Exit   NWC 64 46 49 73 37 NWC Ave: 54±13   HWC 300 230 212 319 137 HWC Ave: 240±65   5.5 F-18 IN BWR F-18 (t1/2=110 min) is formed in the reactor coolant by the 18O(p,n)18F reaction. Its production rate has been theoretically estimated to be 0.67 μCi/s/MWt(6), which is in good agreement with the experimentally measured values (~0.6 μCi/s/MWt). The F-18 activity has been observed only in the fluoride (F−) form in either the reactor coolant or the BWR steam condensate, which is the most stable and only form of fluorine in water. The typical concentrations of F-18 in the BWR coolant and the steam condensate are 1.0 and 2.0 μCi/kg, respectively, independent of water pH and conductivity in reactor water. Frequently, the F-18 activity has been found to be the major source of radiation fields in the BWR hotwell. More than 90% of the F-18 activity produced in the coolant is transported through the steam system and condenses in the condensate system.(3) The mechanism responsible for the formation of the volatile form of fluorine in steam is not clear. A complex of H2O.F or its equivalent has been proposed to explain the appearance of F-18 in the steam phase.(3) In a water environment, the newly formed F-18 may tend to be associated with H2O molecule as a complex H2O.F when its recoil energy from the nuclear reaction is totally reduced by numerous collisions. H2O.F may

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Radiochemistry in Nuclear Power Reactors be transported with the steam, and the F atom is reduced to F− as the steam condenses in the water phase at lower temperatures. Since the F− concentration in reactor water is believed to be very low (≤1ppb), the volatility of F-18 as HF in steam transport may be very low. 5.6 REFERENCES (1) C.P.Ruiz, C.C.Lin and T.L.Wong “Control of N-16 in BWR Main Steam Lines Under Hydrogen Water Chemistry Conditions”, EPRI NP-6424-SD (July 1989). (2) Westinghouse Electric Corporation, “Source Term Data for Westinghouse Pressurized Water Reactors”, WCAP-8253 (May 1974). (3) C.C.Lin, “Chemical Behavior and Distribution of Volatile Radionuclides in a BWR System Water Forward-Pumped Heater Drains”, Water Chemistry of Nuclear Reactor Systems 3, Vol. 1, 103 (1983), BNES (London). (4) R.J.Law, et al, “Suppression of Radiolytic Oxygen Porduced in a BWR by Feedwater Hydrogen Addition; ibid, Vol. 2, 23 (1983). (5) C.C.Lin, J. Radioanal. Nucl. Chem., 130, 129 (1989). (6) M.S.Single and L.Ruby, Nucl. Tech., 17, 104 (1973).