Introduction
Pressurized Water Reactors (PWRs) use light water (H2O) both as a coolant and as a moderator [1]. In the reactor core, water is circulated under extreme conditions such as high pressure, high temperature and heavy mixed neutron/-radiation fields results in chemical decomposition of water (radiolysis) [2]. By means of radiolysis of water, various species such as free radicals (e−aq, H•, •OH, and HO2•/O2•-) and molecular products (H2, H2O2 and O2 (as a secondary product)) are generated continuously. These species can lead to corrosion, cracking, and hydrogen pickup both in the core and in the associated piping components of the reactor [2, 5]. As this issue has been an important consideration in PWRs, thus optimal water chemistry control is expected to play important role to mitigate the corrosion caused by radiolysis of water.
Recently, chemical compound being added into primary systems of PWRs such as: 1) dissolved hydrogen to suppress the oxidizing radicals and molecular products formation, 2) boron in form of boric acid to control the neutron reactivity and to suppress the oxygen concentration that play role on corrosion process [6]. While Ishida et al. (2006) have developed a selected method to mitigate stress corrosion cracking in boiling water reactors (BWRs) named hydrazine and hydrogen co-injection (HHC) [7], which is injected through feed water system. Hypothetically speaking, hydrazine could suppress the oxygen formation as a major of corrosion initiator. In this work, we developed a calculation model to understand the effect of hydrazine addition toward the oxygen under PWR condition. Our great interest is to study whether this strategy would also be effectively applied in PWRs.
In this work, FACSIMILE computer code were used to investigate the effect of hydrazine to suppress the oxidator, O2, on primary cooling water radiolysis at temperatures between 25 and 350 °C under neutron irradiation. We considered that G-values, the number of species produced (or consumed) per unit of energy absorbed, and rate constant of all species involved that we used in this calculation would be acceptable up to 350 °C. How far is the effects of hydrazine addition toward the formation of oxygen were discussed in this work.
Methodology
The absorption of ionizing energy by the coolant results in coolant radiolysis which can generate continuously free radical species and molecular products (where free radical species react with each other). In this study, we assumed a continuous radiation in a homogeneous chemical stage batch system, as coolant radiolysis is categorized in this type. It has been described in detail previously [8] that radiation chemical simulation is an analysis of simultaneous chemical reactions and is in effect the integration of time-dependent multivariable simultaneous differential equations. Therefore, in this present work, FACSIMILE was used to solve differential equations.
The important key parameters to evaluate the chemical effects of ionizing radiation are temperature dependence of the radiation-chemical yields or G-values of the species (in this case for fast neutrons), and the rate constants for all of the reactions involving these species in pure water. These two parameters are needed as inputs for our code. In addition to that, high dose rate of neutrons 104 Gy/s, aeration and deaeration system, and the reaction set of a hydrazine system. Details information were discussed as follow.
G-value is given in the units of “molecule per 100 eV”; for conversion into SI units (mol/J), 1 molecule/100 eV ≈ 0.10364 µmol/J. In this work, we need G-values of reactive radical species, molecular product and water itself by fast neutron irradiation. G-value of fast neutrons were taken from Ref. [9] (Table 1), it is a combination of measurements and computer modeling; irradiations carried out using the YAYOI fast-neutron source reactor at the University of Tokyo, with an average energy for fast neutrons of ~0.8 MeV. G-values measured at 250 °C were adopted for temperature above 250 °C. In order to provide the reaction set and rate of reaction as one of important parameter for one to understand radiolysis process, we use the self-consistent radiolysis database that recently compiled by Elliot and Bartels [10] (the reactions set not shown in this work). This new database provides recommended values to use in high-temperature modeling of light water radiolysis over the temperature range between 20-350 °C. In addition to that, the reactions set of hydrazine is also necessary in order to know the effect of hydrazine addition. Table 1 provides a numbered list of reactions and corresponding rate constants used in this work. The mechanism reaction of hydrazine with free radicals and molecular products, also with O2 radicals is quite complex.
Table 1
Main chemical reactions of hydrazine and their corresponding rate constants (k) at 25 °C used in our FACSIMILE. For first-order reactions (indicated by the symbol Ŧ), the value of k is given in s−1.
In this work, the concentration of hydrazine added to the system is under a range (10−6 to 10−3 M) and using a well-accepted mechanism for radiolysis in the presence or absence of air. Dose rate is also one of the input data to perform radiolysis simulation. Dose rate of neutrons are assumed as high dose rate conditions, a typical condition in reactor core (104 Gy s−1). Once again, the purpose of this study is to understand the best estimation of hydrazine addition to suppress the O2 formation in primary coolant water system.
Result and Discussion
The effect of hydrazine on coolant radiolysis at temperature range between 25-300 °C, high neutron dose rate condition and in the presence or absence of air were carried out using computer code FACSIMILE. Our code has been validated with other calculation in pure and other solution. The validity of the simulation is checked from the aspect of mass balance and numbers of hydrogen and oxygen atoms in major products are approximately 2:1 [8]. For the sake of comparison, the time evolution of O2 concentration were calculated in pure water system (black lines) and by varying the concentration of hydrazine added to our system under a range, 10−6, 10−5, 10−4, and 10−3 M (magenta, blue, green and red lines, respectively).
Simulation at room temperature
Figure 1 shows the time evolution of [O2] as reproduced by our simulation at 25 °C in deaerated (a) and aerated (b) condition. For deaerated condition, the varying concentration of added hydrazine in the range between 10−6 – 10−4 M has no effect in reaching the steady state concentration of O2 which is similar as in the pure water system. However, a striking feature of our simulated results obtained at 10−3 M hydrazine addition, where it increases with time, was the large produce of O2. Our hypothesis for this result is that there are two reactions contribute to the formation of O2 (based on the importance) at longer time:
Reaction (2) is much slower with a rate constant about 3 order of magnitude lower than for reaction (1). In reverse the concentration of hydrazine about 10−3 M can suppress the formation of O2 significantly pass through a minimum point then reach the steady state concentration at very low level ~5×10−9 M, while the addition of hydrazine in the range between 10−6 – 10−4 M is confirmed in Figure 1(b) as the same as in pure water system.
The results for the concentration of O2 in aerated aqueous 10−3 M hydrazine solutions show that this long-lived molecular species are decreased compared to those in deaerated solutions. The reactions of oxygen consumption, reaction R13 and R14 are much faster than the reactions of oxygen formation such as reaction (1) and (2), thereby cannot protect the oxygen molecule from further reactions with these species in the homogeneous stage of radiolysis, or in other words, this phenomena leads to a decrease in the concentration of O2. Indeed, as can be seen in Figure 1, the steady state concentration of O2 in aerated hydrazine solutions are six magnitudes lower than in deaerated solutions.
Figure 1
Time evolution of [O2] (in Molar) of pure water (black lines) and aqueous hydrazine solutions at 25 °C, calculated using our FACSIMILE in deaerated (a) and aerated (b) condition at the assumption of dose rate 104 Gy s−1. and by varying the concentration of hydrazine added to our system under a range, 10−6, 10−5, 10−4, and 10−3 M (magenta, blue, green and red lines, respectively).
Simulation at 250 °C and 300 °C
Figure 2(a), (b) and Figure 3(a), (b) show time variations of the oxygen molecule as a function of time in pure water and hydrazine solution at deaerated and aerated condition respectively. Overall, in all case, steady state concentrations of O2 are lower than those in Figure 1 both in the presence or absence of air. In deaerated condition, Figure 2(a) and Figure 3(a), shows obviously how each concentration of hydrazine addition suppress the formation of O2. It is readily explained that the higher the concentration of hydrazine the lower the generation of O2. This explanation is described in combination of reaction R.. and R.. that convert the N2H4 and N2H3 into water, N2H2 and N2.
Figure 2:
Time evolution of [O2] (in Molar) of pure water (black lines) and aqueous hydrazine solutions at 250 °C, calculated using our FACSIMILE in deaerated (a) and aerated (b) condition at the assumption of dose rate 104 Gy s−1. and by varying the concentration of hydrazine added to our system under a range, 10−6, 10−5, 10−4, and 10−3 M (magenta, blue, green and red lines, respectively).
Figure 3
Time evolution of [O2] (in Molar) of pure water (black lines) and aqueous hydrazine solutions at 300 °C, calculated using our FACSIMILE in deaerated (a) and aerated (b) condition at the assumption of dose rate 104 Gy s−1. and by varying the concentration of hydrazine added to our system under a range, 10−6, 10−5, 10−4, and 10−3 M (magenta, blue, green and red lines, respectively).
In the other side, the highest concentration of hydrazine which is 10−3 M can suppress the formation of O2 until it becomes negligible << 10−20 M. Figure 2(b) and Figure 3(b) show that in absence of hydrazine addition give the same result with 10−6 and 10−5 M of hydrazine whereas a slightly decrease of O2 occurred due to the addition of 10−4 M hydrazine. For aerated conditions, similarly to the results at room temperature in Figure 1b, concentrations of O2, falls down significantly at aqueous solution of 10−3 M hydrazine concentration.
For the case of increasing 225 centigrade temperature and above, concentrations of the O2 reach steady state values more rapidly than those in room temperature. In other words, the higher the temperature the faster decreasing of O2 since the reactions for O2 consumption take faster proportionally to the temperature then O2 production as in room temperature.
Unfortunately, regarding the water chemistry in hydrazine aqueous systems, there is however a complete lack of information on the radiolysis of water by fast neutron irradiation as the experimental data is not available in the literature to the best of our knowledge therefore we are not able to validate our results especially at high temperature.
The simulation results for room temperature, for deaerated condition, the varying concentration of added hydrazine in the range between 10−6 – 10−4 M has no effect in reaching the steady state concentration of O2 and it is similar as in the pure water system and for 10−3 M hydrazine addition, a large produce of O2 were obtained. In reverse, for deaerated condition, the concentration of hydrazine about 10−3 M can suppress the formation of O2 significantly, while the addition of hydrazine in the range between 10−6 – 10−4 M is confirmed to be the same as in pure water system.
Overall, in all case, steady state concentrations of O2 in the results of simulations for 250 and 300 °C are lower than those in room temperature both in the presence or absence of air. In deaerated condition, hydrazine addition can suppress the formation of O2 proportionally to its concentration
Conclusion
In this work, FACSIMILE was used to calculate oxidator concentration, O2 on coolant water radiolysis by fast neutrons irradiation at temperatures between 25 and 300 °C. The simulations also conducted in pure water and hydrazine aqueous solution both in the presence and absence of air. The time evolution of [O2] were obtained by taking into consideration the assumption of high dose rate conditions of fast neutron, a typical condition in reactor core (104 Gy s−1).
The higher hydrazine concentrate, the lower O2. The highest concentration of hydrazine can suppress O2 form until it becomes negligible. In aerated condition, hydrazine absent addition give the same result with 10−6 and 10−5 M of hydrazine where a slightly decrease of O2 occurred due to the addition of 10−4 M hydrazine. However, experimental data are required to test more thoroughly our modeling calculations, and to specify the potential role of hydrazine to suppress of O2 form .