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Radiochemistry in Nuclear Power Reactors 6. RADIATION CHEMISTRY IN REACTOR COOLANT 6.1 INTRODUCTION The radiation energy generated in the reactor core and absorbed in the coolant is mainly attributed to fast neutrons and gamma rays. Absorption of energy in the coolant results in water radiolysis, which occurs in both BWR and PWR. However, because of hydrogen gas over-pressure in the PWR system, there is no net water decomposition nor oxygen gas production in the PWR system (Section 6.4). The addition of hydrogen in the BWR coolant has become an important technique to reduce the dissolved oxygen level in the coolant and minimize the susceptibility of intergranular stress corrosion cracking (IGSCC) of stainless steel in the BWR primary system. Because of its reducing nature in the PWR coolant, practically all radioactive and non-radioactive impurities in the coolant are in the reduced chemical forms. On the other hand, the BWR coolant under normal operating conditions is under oxidizing condition and the chemistry of the BWR coolant system is rather complicated due to many oxidizing species (both radicals and stable species). An attempt will be made to describe this complicated system in a limited scope. 6.2 WATER RADIOLYSIS IN BWR COOLANT Boiling water reactors use high purity water as the neutron moderator and primary coolant in the production of steam. In an operating BWR, most of the radiolysis occurs in the high flux core region. Under normal operating conditions, the core contains an average steam void of ~30% and the core radial average void fraction increases from 0 at core inlet to ~70% at the top of the core. A brief overview of radiation chemistry in the BWR coolant has been reported by The radiation energy generated in the reactor core and absorbed in the coolant is mainly attributed to fast neutrons and gamma rays; the contributions from thermal neutron and beta particles are relatively small. The core average total neutron dose rate is estimated at 1.5×109 R/hr and the total gamma dose rate is estimated at 3.1×108 R/hr for a 50 W/cm3 power density standard plant. The total radiation dose rate in the core region
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Radiochemistry in Nuclear Power Reactors is nearly proportional to the core power density. As a result of water radiolysis, liquid-vapor phase equilibrium and recirculation, the reactor recirculation water contains oxygen and hydrogen peroxide in the concentration range from 100 to 300 ppb, and about 10 ppb (less than stoichiometric ratio of 8 to 1) of dissolved hydrogen. It is known that the dose rates decrease as the distance from the core increases, and the fast neutron fluxes decrease faster than the gamma fluxes. At out-of-core regions, the total dose rate could be ≈0.1–1% of the core dose rate. The production of radiolytic species is not important in the peripheral regions, but the levels of radiation are still high enough to initiate the “recombination” of dissolved O2 and excess H2 in the coolant (see Section 6.4). The overall simplified general expression for the radiation-induced water decomposition can be written as: (6–1) which is not chemically balanced. Although the detailed mechanism of water radiolysis is complex, a simple scheme has been well established to explain the observed effects.(2,3) The primary process is to produce H and OH radicals, (6–2) Many of these radicals react with each other in regions of high local concentration to form molecular products, H2 and H2O2, or reform water according to the reactions: (6–3) (6–4) (6–5)
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Radiochemistry in Nuclear Power Reactors The molecular products H2 and H2O2 may be destroyed and water is reformed by the following chain reaction: (6–6) (6–7) Production of molecular O2 is brought about by the following reactions: (6–8) (6–9) (6–10) (6–11) The O2 is destroyed mainly by the reaction: (6–12) Within seconds of irradiation, steady-state concentrations of molecular and radical species can be reached in a static system which contains pure water. The examples of radiolysis model calculations(21) for the production of H2, O2, and H2O2 in air-free and air-saturated water at ambient temperature are shown in Figure 6–1. In air-free water, all three radiolytic products reach steady state concentrations at ~200 Gy of total dose (Figures 6–1, A and B). In the presence of excess oxygen in water, both H2 and H2O2 increase and they may continue to increase until the oxygen is consumed to an equilibrium level (Figure 6–1, C). The chemical yield of the decomposition product is denoted by the symbol G. The G value is defined as
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Radiochemistry in Nuclear Power Reactors Figure 6–1. Radiolytic Products in Water Radiolysis with 1 Gy/s Dose Rate (Reproduced with Permission, Radiochim. Acta, Ref. 21) A. In air-free pure water, doses up to 100 Gy B. In air-free pure water, doses up to 1000 Gy C. In air-saturated water
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Radiochemistry in Nuclear Power Reactors The symbols GH, , , etc., refer to primary radical and molecular yields, while G(H), G(H2), G(H2O2), etc., refer to the observed yield under experimental conditions. When a species is decomposed by radiation, the negative yield is indicated by a minus sign, i.e., . The chemical yield (G value) for a species in the radiolysis of water may vary as a function of several factors in an irradiation system. Variations of radical and molecular yields with water pH, temperature, LET*, and does rate have been well documented.(4) The variation of chemical yields with temperature is significant at reactor operating conditions, and the LET is probably the most important factor when one calculates the chemical yields from gamma rays (low LET) and fast neutron (high LET) in a reactor. The yields of some major species at ambient and higher temperatures are compared in Table 6–1.(5) Table 6–1 G-VALUES OF PRIMARY RADIOLYTIC SPECIES IN WATER (Ref. 5) Temperaure Radiation e(aq)− H+ H H2 OH HO2 H2O2 25°C Gamma 2.70 2.70 0.61 0.43 2.86 0.03 0.61 Neutron 0.93 0.93 0.50 0.88 1.09 0.04 0.99 280°C Gamma 3.76 3.76 0.7 0.8 5.5 — 0.28 (Tentative)** Neutron 1.4 1.4 0.75 1.32 1.64 0.06 1.49 * The rate at which energy is lost (locally absorbed) per unit of length traveled by an ionizing particle, −dE/dx, is called the linear energy transfer (LET). ** Best estimated values from current literatures.
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Radiochemistry in Nuclear Power Reactors 6.3 RADIOLYTIC GAS PRODUCTION IN BWRS In an operating BWR, most of the radiolysis occurs in the high flux core region where boiling takes place. The net average production rate of radiolytic gases (containing approximately stoichiometric mixtures of H2 and O2) in BWRs has been determined to be 0.041 SCFM/MWt (19.35 cc/s/MWt) (Figure 6–2). The majority of the data shown in Figure 6–2 were determined by measuring the radiolytic gas (H2+O2) content in a gas sample vial and the offgas flow rate. As shown in Figure 6–2, considerable variation in the measured radiolytic gas production rate can be seen. The variation may be partly attributed to analytical error and the difficulty in accurately calibrating the gas flow rate in the off-gas line. However, the data measured in steam samples appear to be more consistent with the average production rate. It should be noted that the actual production rate in the BWR core may slightly vary due to differences in design and/or operating characteristics (e.g., core power density, steam void fraction, operating pressure, coolant flow rate, and impurity levels in the coolant). For the average offgas production rate of 0.041 SCFM/MWt in BWRs, the “apparent” H2 and O2 yields are G(H2)BWR=0–0056 and G(O2)BWR=0.0028, respectively. These apparent yields are produced by a mixture of neutron and gamma radiation in the core region. It is entirely possible that most of the radiolytic gases are produced in the boiling region and quickly partition between the steam phase and the liquid phase. The steady-state concentrations of dissolved gases in non-boiling water are expected to be relatively low. In the presence of excess H2 in water, the production of O2 may be effectively suppressed in non-boiling water. 6.4 SUPPRESSION OF WATER RADIOLYSIS BY H2 ADDITION In the presence of excess H2 in water, the water decomposition and production of O2 can be suppressed through a chain reaction which rapidly reduces the concentration of OH and H2O2, in the reactions.
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Radiochemistry in Nuclear Power Reactors Figure 6–2. Radiolytic Gas Production Rates In BWRs (Ref. 6)
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Radiochemistry in Nuclear Power Reactors (6–13) (6–14) These two species are normally the precursors of O2 in the reactions (6–15) (6–16) When H2 is in excess, the O2 concentration is reduced by the fast reaction (6–17) and the decrease in H2O2 concentration occurs finally because the overall rate of reactions which destroy H2O2 is faster than that of reactions which produce H2O2 including its radiolytic formation. For the recombination of H2 and O2, a balanced set of reactions can be written as: An example of a radiolysis model calculation for the suppression of dissolved O2 by surplus H2 in water at ambient temperature is shown in Figure 6–3. A set of reactions and rate constants used in water radiolysis simulation is given in Table 6.2.
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Radiochemistry in Nuclear Power Reactors Table 6–2 REACTION AND RATE CONSTANTS USED IN WATER RADIOLYSIS SIMULATION (Ref. 8) Chemical Reactions Rate Constant at 25°C (1 mole−1 s−1) Activation Energy K cal mole−1 eaq−+H2O→H+OH− 1.6×101 3.0 eaq−+H+→H 2.4×1010 3.0 eaq−+OH→OH→OH− 2.0×1010 3.0 eaq−+H2O2→OH+OH− 1.3×1010 3.0 H+H→H2 1.0×1010 3.0 eaq−+HO2→HO2− 2.0×1010 3.0 eaq−+O2→O2− 1.9×1010 3.0 2eaq−→2OH−+H2 5.0×109 3.0 2OH→H2O2 4.5×109 3.0 OH+HO2→H2O+O2 1.2×1010 3.0 OH+O2−→OH−+O2 1.2×1010 3.0 OH−+H→eaq−+H2O 2.0×107 4.5 eaq−+H+H2O→OH−+H2 2.5×1010 3.0 eaq−+HO2−+H2O→OH+2OH− 3.5×109 3.0 H++OH−→H2O 1.44×1011 3.0 H2O→H++OH− 2.6×10−5 3.0 H+OH→H2O 2.0×1010 3.0 OH+H2→H+H2O 4.0×107 4.6 OH+H2O2→H2O+HO2 2.25×107 3.45 H+H2O2→OH+H2O 9.0×107 4.5 H+O2→HO2 1.9×1010 3.0 HO2→O2−+H+ 8.0×105 3.0 O2−+H+→HO2 5.0×1010 3.0 2HO2→H2O2+O2 2.7×106 4.5 2O2−+2H2O→H2O2+O2+2OH− 1.7×107 4.5 H+HO2→H2O2 2.0×1010 3.0 H+O2−→HO2− 2.0×1010 3.0 eaq−+O2−→HO2−+OH− 1.3×108 4.5 OH−+H2O2→HO2−+H2O 1.8×108 4.5 2H2O2→2H2O+O2 0.3/H2O2 —
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Radiochemistry in Nuclear Power Reactors 6.5 THE ROLE OF IMPURITIES Some ionic impurities, especially halide and metallic ions, are known as “radical scavengers” which have profound effects on water radiolysis as well as radiation induced chemical reactions. The examples of reactions involving Br− and Cu++ ions are illustrated by: and Figure 6–3. Depletion of O2 in Water by Irradiation in the Presence of Surplus H2. Dose Rate at 1 Gy/s (Reproduced with Permission, Radiochim. Acta, Ref. 21)
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Radiochemistry in Nuclear Power Reactors The scavengers compete with H2 and H2O2 in reactions (6–6) and (6–7) for OH and H radicals. The reaction rate constants for some reactions involving Cu+/Cu++ and halide ions can be found in the literature.(7) As shown in Table 6–3, the reaction rates for the Cu+/Cu++ ions are comparable to most of the free radical reactions involved in radiolysis (Table 6–2). Thus, if the impurity levels are high enough (e.g., ≥10 ppb), the scavenging effect would significantly interrupt the radiolytic chain reactions. Table 6–3 EXAMPLES OF IMPURITY REACTION RATE CONSTANTS(7) Reaction k(L/mol/s) CL− +OH →Cl+OH− 106 Br− +OH →Br+OH− 109 I− +OH →I+OH− 1.5×1010 Cu+2 +H →Cu++H+ 9.8×108 Cu+2 →Cu++(H2O) 4.0×1010 Cu+ +H2O2 →Cu+2+OH+OH− 2.3×109 6.6 HYDROGEN WATER CHEMISTRY IN BWR COOLANT The first full-scale hydrogen water chemistry (HWC) test in the U.S. was performed at Dresden-2 in 1982.(9) Subsequently, similar tests have been carried out in several reactors. A typical example of the H2 and O2 concentrations in steam as a function of H2 concentration in reactor water is shown in Figure 6–4. It can be shown (Section 6-4) that the excess H2 provides the initial H radicals for a chain reaction and there is no net consumption of H2 in the process. That is, for each H2 molecule that is added to the coolant and consumed to remove 1/2 O2, one H2 molecule is liberated by water radiolysis. The steam phase is dominated by the H2 added in the feedwater. The initial decrease of the H2 content in the steam may be due to some direct recombination of H2 and O2 in the core region when the O2 concentration is sufficiently high.
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Radiochemistry in Nuclear Power Reactors Figure 6–4. Hydrogen and Oxygen Concentrations in Steam as a Function of Hydrogen Concentration in Reactor Water
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Radiochemistry in Nuclear Power Reactors The dissolved O2 concentration in the recirculation system also responds very quickly to hydrogen addition to the reactor water. When sufficient H2 is added, the O2 concentration decreases with increasing H2 concentration according to the following equation: Experimental data obtained at three reactors are shown in Figure 6–5. It has been hypothesized that a radiation induced water decomposition-recombination equilibrium. is established in the downcomer region when sufficient H2 is added to the reactor water. The equilibrium constant Keq is a strong function of the radiation field in the downcomer region.(22) Measurements have not detected H2O2 in any of the plant measurements of the recirculation system chemistry. Ullberg and Rooth(10) have suggested that H2O2 may be present in the large diameter pipe, but decomposes heterogenously on high temperature, small diameter sample piping surfaces, to be measured as oxygen. Recently, a laboratory study(11) has shown that indeed H2O2 decomposes quickly in a stainless steel tubing at higher temperatures. However, the same study also shows there is no evidence of reactions between H2 and O2 or H2O2 in high-purity water. Thus, it is certain that in high purity water the dissolved H2 measured in a BWR coolant sample line is representative of the concentration in the coolant from which the sample is taken.
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Radiochemistry in Nuclear Power Reactors Figure 6–5. Recirculation Water Oxygen Concentration as a Function of Recirculation Water Hydrogen Concentration. (Ref. 22)
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Radiochemistry in Nuclear Power Reactors Similarly, the measured dissolved O2 should be the combined concentrations of H2O2 and O2 in the coolant. Nevertheless, with impurities in water, the same study indicates that copper ions at ~30 ppb would catalyze the reaction between H2 and H2O2 to some extent at higher temperatures. 6.7 CHEMICAL EFFECTS OF RADIATION IN BWR COOLANT Radiolytic oxidation of impurities including corrosion products (CrO4=) fission products (IO3−, TcO4=, NpO2+), and water activation products (NO3−) in the primary coolant under normal water chemistry conditions are well known,(12) but the actual reaction mechanisms may not be easily understood. The radiation effects on nitrogen species have been extensively investigated.(13–16) When dealing with N-13 and N-16, which are activated from the oxygen atoms in water (Section 5.4), the reactions of “hot atoms” also have to be considered. In an earlier study, Schlieffer and Adlogg(17) reported that the chemical forms and distribution of N-16 produced in pure water were: Chemical Form: N2 NO NO2− NO3− NH4+ NH2OH Distribution (%)*: 1 9 25 10 30 16 This result suggests that when a newly produced N-16 atom is broken away from an H2O molecule, the thermalized nitrogen atom (N*) may react equally with various radicals, ions and molecules in the immediate surrounding area. Under a high radiation field, such as in the reactor core, the nitrogen atoms are expected to react with radiolytic species from water radiolysis. The initial reactions may include: * The data do not total 100% as reported in Ref. (17).
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Radiochemistry in Nuclear Power Reactors and the following subsequent reactions may occur until stable compounds are formed:(13) It may be expected that under oxidizing conditions, more and will be formed. Even if ammonia is present in reactor water at low concentrations, ammonia will be converted to nitric acid in the reactor core under conditions of unsuppressed radiolysis of water,(14,15,16) according to the following equations: On the other hand, when H2 is added to suppress water radiolysis, more NH3 is expected to form as most of the oxidizing species are eliminated. Although N2 is thermodynamically stable, its formation from two nitrogen atoms at very low concentrations in reactor coolant is very unlikely. The results of measurements in reactor coolant and steam in several reactors have been described in Section 5.4.
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Radiochemistry in Nuclear Power Reactors The radiation chemistry of iodine is as complex as nitrogen chemistry. Iodine can exist in several stable and unstable chemical forms in aqueous solutions, particularly at very low concentrations similar to those normally observed in reactor coolant. The effect of gamma-ray radiation on the I-131 activity in aqueous solutions was investigated in a laboratory study at ambient temperature.(18) The I-131 activity in the iodide (I−) form was found easily oxidized to iodate as a result of high intensity (>105 R/h) gamma-ray irradiation. The chemical yield of was found to vary with water pH, dose rate and concentration. Thermodynamically the primary oxidizing species, OH and HO2 (oxidation potentials are 2.8 and 1.35 V, respectively)(19) are capable of oxidizing all iodine species to and, in the core region, the concentration of OH is expected to be higher than iodine species in the coolant. Thus, the mechanisms may consist of successive oxidation of iodine by OH, with I, IO, HIO, IO2, and HIO2 as possible intermediate species. The reactions may be represented by: A number of studies on the chemical behavior of radioiodine in the BWR system have been reported.(20) The results of those studies are summarized in Section 8.3. 6.8 REFERENCES (1) C.C.Lin “An Overview of Radiation Chemistry in Reactor Coolants”, Proc. 2nd. Int. Symp. Environmental Degradation of Materials in Nuclear Power Systems—Water Reactors.” Monterey, California, p. 160, (September 1988). (2) C.J.Hochanadel, J. Phys. Chem., 56., 587 (1952). (3) A.O.Allen, et al., J. Phys. Chem., 56., 575 (1952). (4) See for example, I.G.Draganic and Z.D.Draganic “The Radiation Chemistry of Water” Academic Press, New York (1971). (5) W.G.Burns and P.B.Moore, “Radiation Enhancement of Zircaloy Corrosion in Boilng Water System. A Study of Simulated Radiation Chemical Kinetics”, Water Chemistry of Nuclear Reactor System 1, BNES, paper 33, 229 (1977); private communication (1991).
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Radiochemistry in Nuclear Power Reactors (6) C.C.Lin, Nucl. Sci. Eng., 99, 340 (1988). (7) National Bureau of Standards, NSRDS-NBS-43 (1973) and 43 Supplement (1975), ibid-46 (1973), ibid-51 (1975), ibid-59 (1977). (8) C.P.Ruiz and C.C.Lin, “Modeling Hydrogen Water Chemistry for BWR Applications”, EPRI NP-6386 (June 1989). (9) R.J.Law, et al., “Suppression of Radiolytic Oxygen Produced in a BWR by Feedwater Hydrogen Addition,” Water Chemistry of Nuclear Reactor System 3, BNBS Vol. 2, 23 (1983). (10) M.Ullberg and T.Rooth, “Hydrogen Peroxide in BWRs,” Water Chemistry of Nuclear Reactor System 4, BNES, Vol. 2, 67 (1986). (11) C.C.Lin and F.R.Smith, “Decomposition of Hydrogen Peroxide at Elevated Temperatures,” EPRI NP-6733 (March 1990); Int. J. Chem. Kinet. Vol. 23, 971 (1991). (12) C.C.Lin and H.R.Helmholz, “Radiochemical studies of the BWR at Monticello Nuclear Generation Plant,” NEDE-12586 (June 1975) (GE Internal Report). (13) M.T.Dmitriev, Zh. Prik. Khim., 36, 1123 (1963). (14) J.E.LeSurf, G.M.Allison, Nucl. Tech., 29., 160 (1976). (15) L.Hammar, et al., Water Chemistry Research at the Halden Boiling Water Reactor, Proc. Int. Conf. Peaceful Uses of Nuclear Energy, 3rd: Geneva, 1964, Vol. 9, 408 (1965). (16) F.W.Fessenden, et al., J. Phys. Chem., 82, 1875 (1978). (17) P.J.J.Schlieffer and J.P.Adlogg, Radiochem, Acta, 3 145 (1964). (18) C.C.Lin, J. Inorg. Nucl. Chem., 42, 1101 (1980). (19) E.J.Henley and E.R.Johnson, “The Chemistry and Physics of High Energy Reactions”, University Press, Washington, D.C. (1969). (20) C.C.Lin, J. Inorg. Nucl. Chem., 42, 1093 (1980). (21) E.Bjergbakke, Z.D.Draganic, K.Sehested, and I.G.Draganic, Radiochimica Acta, 48, 65 (1989). (22) C.C.Lin “Prediction of Electrochemical Potentials in BWR Primary System”. Vol. 1: Evaluation of Water Chemistry and ECP Measurements under HWC”, EPRI TR-102766, Vol. 1 (August 1993).
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