Hydrazine Radiolysis by Gamma-Ray in the N2H4–Cu+–HNO3 System

Radiolysis of chemical agents occurs during the decontamination of nuclear power plants. The γ-ray irradiation tests of the N2H4–Cu+–HNO3 solution, a decontamination agent, were performed to investigate the effect of Cu+ ion and HNO3 on N2H4 decomposition using a Co-60 high-dose irradiator. After the irradiation, the residues of N2H4 decomposition were analyzed by Ultraviolet-visible (UV) spectroscopy. NH4+ ions generated from N2H4 radiolysis were analyzed by ion chromatography. Based on the results, the decomposition mechanism of N2H4 in the N2H4–Cu+–HNO3 solution under γ-ray irradiation condition was derived. Cu+ ions form Cu+N2H4 complexes with N2H4, and then N2H4 is decomposed into intermediates. H+ ions and H● radicals generated from the reaction between H+ ion and eaq− increased the N2H4 decomposition reaction. NO3− ions promoted the N2H4 decomposition by providing additional reaction paths: (1) the reaction between NO3− ions and N2H4●+, and (2) the reaction between NO● radical, which is the radiolysis product of NO3− ion, and N2H5+. Finally, the radiolytic decomposition mechanism of N2H4 obtained in the N2H4–Cu+–HNO3 was schematically suggested.


Introduction
Hydrazine (N 2 H 4 ) is commercially used to produce plastics, medicines, and textile dyes and to reduce the corrosion of a boiler in a thermal power plant [1,2]. In the nuclear field, N 2 H 4 is added to the primary feed water to maintain the hydrogen concentration and to remove dissolved oxygen [3]. In addition, N 2 H 4 can be applied as a chemical decontamination solution to remove radioactive nuclides in an oxide layer of a primary system in the nuclear power plant [4]. The decontamination solution containing N 2 H 4 is developed to reduce the damage of the base metal and the secondary radioactive wastes compared with the decontamination using organic acid [5,6]. They are composed of N 2 H 4 and inorganic acids such as HNO 3 and H 2 SO 4 [5][6][7]. Furthermore, the metal ions can be added into the decontamination solution containing N 2 H 4 for improving the decontamination performance [8]. For this reason, a N 2 H 4 -Cu + -HNO 3 decontamination solution was suggested as the effective chemical decontamination solution [7]. The decontamination solution, however, can be decomposed under the high radiation field [9]. The radiolysis of the decontamination solution occurs by radionuclides in the primary system, such as Co-60 and Co-58, during the application. The decomposition of major compositions of the decontamination solution affects the decontamination performance. Therefore, it is necessary to analyze the radiolysis of N 2 H 4 , which is the major component of the N 2 H 4 -Cu + -HNO 3 decontamination solution, during irradiation.
In this regard, a number of research studies concerning decomposition of N 2 H 4 solution have been carried out. It is known that the decomposition of N 2 H 4 under irradiation conditions can occur through a reaction with radiolysis products of water. Various radicals and products, such as e aq − , • OH, and H 3 O + , are generated by the radiolysis of water [10]. The radiolysis products of water as represented in Equation (1) [11].
The decomposition reaction mechanism of N 2 H 4 in the aqueous solution by γ-ray irradiation can be found in the study of Buxton et al. [12]. They reported that radicals such as N 2 H 4 •+ , NH 2 • and N 2 H 3 • , generated from N 2 H 4 , are decomposed into N 2 and NH 3 . It is also shown that the NH 4 + ion was produced by a reaction between H • radical and N 2 H 5 + . Garaix et al. studied the decay mechanism of the NO 3 • radical generated by the radiolysis of NO 3 − ions in the N 2 H 4 solution using an electron beam [13]. They concluded that N 2 H 4 exists mainly as N 2 H 5 + or N 2 H 6 2+ in the acidic solution, both of which cause rapid consumption of NO 3 • radicals. Motooka et al. also reported that the deoxygenation reaction with radiolysis of N 2 H 4 can be suppressed by salt in the water in the γ-radiation field [14]. In this way, the molecular structure of N 2 H 4 after radiolysis and the decomposition mechanism of the N 2 H 4 depend on the composition of the solution. However, there are few research studies about the N 2 H 4 decomposition reaction in N 2 H 4 -Cu + -HNO 3 solution. Therefore, it is necessary to study the decomposition reaction mechanism of the N 2 H 4 in N 2 H 4 -Cu + -HNO 3 solution during γ-ray irradiation to simulate the decontamination condition.
In this study, we evaluated the effects of Cu + ions and HNO 3 on N 2 H 4 decomposition under the γ-radiation field. The study was performed by analyzing the concentration of remaining N 2 H 4 and the concentration of the products in N 2 H 4 -Cu + -HNO 3 solution after the irradiation. The decomposition mechanism of N 2 H 4 in the solution containing Cu + ions and HNO 3 was also suggested.

Radiolysis of Hydrazine in Acidic Solution
Hydrazine generally exists in the form of N 2 H 5 + by its reaction with H + ions in an acidic solution, as given in Equation (2) [13,15,16] In an acidic solution, the same as the condition of this study, e aq − , H • , OH • , and H 2 O 2 are generated as products after water radiolysis. It is possible that e aq − reacts with H + ions in the acidic solution and generates H • as in the following Equation (4) [18]. The reactions between the water radiolysis products and the chemical species of N 2 H 4 lead to the decomposition of N 2 H 4.
The principal decomposition reactions and rate constants of the chemical species of N 2 H 4 in the irradiation condition are listed in Equations (5)- (23). As shown in Equation (5), N 2 H 6 2+ reacts with OH • and produces N 2, the end product of N 2 H 4 decomposition at pH 1 [19].
In addition, N 2 H 5 + is the main species form of N 2 H 4 in the acidic solution. N 2 H 5 + reacts with the radiolysis products of water such as e aq − , H • , or OH • as shown in Equations (6)-(9) [12,20]. NH 4 + ion, one of the end products of N 2 H 4 decomposition, is produced by the reaction between N 2 H 5 + and H • , as indicated in Equation (7). The intermediates of N 2 H 5 + decomposition, N 2 H 4 , NH 2 • , and N 2 H 4 •+ , are generated by the reactions in Equations (6), (8) and (9). These intermediates cause the consecutive decomposition reactions of N 2 H 4 . In particular, N 2 H 4 can also be hydrolyzed into N 2 H 5 + and N 2 H 6 2+ as shown in Equations (2) and (3). The consecutive decomposition reactions of N 2 H 4 with OH • , N 2 H 4 •+ , and H 2 O 2 are listed in Equations (10)- (12) [12,20,21]. N 4 H 8 + is formed by the reaction between N 2 H 4 and N 2 H 4 •+ , as indicated in Equation (10). • N 2 H 3 is generated by the reaction between N 2 H 4 and OH • , as shown in Equation (11). The intermediates, N 4 H 8 + and • N 2 H 3 , participate in the other consecutive decomposition reactions of N 2 H 4 . However, N 2 is produced as the end product by the reaction between N 2 H 4 and H 2 O 2 (Equation (12)). NH 2 • generated by the reaction in Equation (7) causes the reactions with N 2 H 5 + or N 2 H 4 , as represented in Equations (13) and (14) [12]. N 2 H 4 •+ , • N 2 H 3 , and NH 3 are formed after the reactions shown in Equations (13) and (14). Among these products, N 2 H 4 •+ and • N 2 H 3 cause the consecutive reactions because they are the reactive intermediates. N 2 H 4 •+ produced by Equations (8), (9) and (13) participate in the consecutive reactions represented in Equations (15) and (16) [12]. As shown in Equation (15), N 4 H 9 •2+ is generated after the reaction of N 2 H 4 •+ with N 2 H 5 + . As indicated in Equation (16), N 4 H 9

•2+
reacts with N 2 H 4 •+ , and the reaction products are N 4 H 8 2+ and N 2 H 5 + . The N 4 H 8 2+ is directly decomposed into N 2 and NH 3 in the ratio of 1 to 2. On the other hand, N 2 H 5 + is repeatedly decomposed into other forms, as represented in Equations (6)-(9), (13) and (15).
• N 2 H 3 generated by Equations (11) and (14) is decomposed into various forms, as listed in Equations (17)-(22) [12]. The main end products of • N 2 H 3 decomposition are N 2 and NH 3 , as represented in Equations (19) and (20). The main intermediates, N 2 H 4 and N 2 H 2 , are also generated from the decomposition reaction of • N 2 H 3 , as shown in Equations (17)- (22). N 2 H 4 is hydrolyzed into N 2 H 5 + and N 2 H 6 2+ in the acidic solution or causes consecutive decomposition reactions. N 2 H 2 reacts with H • , and • N 2 H 3 is produced as shown in Equation (23).
As mentioned above, it is expected that various intermediates are generated during the decomposition of the chemical species of N 2 H 4 . Therefore, the intermediates can affect the reaction with Cu + ions or NO 3 − ions in the N 2 H 4 -Cu + -HNO 3 system.

Change of Copper Species during Irradiation
Copper ions in the solution would cause the decomposition of N 2 H 4 during the irradiation. The redox reactions mainly occur between copper ions and radiolysis products of water such as e aq − , H • , and OH • , as listed in Equations (24)-(28) [22][23][24][25]. The equations show that copper ions coexist in the forms of Cu 0 , Cu + ions and Cu 2+ ions regardless of initial chemical species. In addition, Fenton reaction occurs in an acidic condition, as represented in Equation (29) [26,27]. The above reactions can affect the decomposition of N 2 H 4 in the N 2 H 4 -Cu + -HNO 3 system.

Radiolysis of Nitrate Ion
The principal reactions of NO 3 − ions during the irradiation are listed in Equations (30)- (38) [13,[28][29][30][31][32][33]. The reactions can be classified by direct and indirect decomposition reactions. As shown in Equation (30), the NO 3 − ion is directly changed into NO 3 • and electron due to the γ-ray irradiation [13,28]. The NO 3 − ion is also changed into NO 3 2− ion or NO 2 • through the reactions with e aq − or H • , as can be seen in Equations (31) and (32) [29,30]. The NO 3 2−• reduces into NO 2 • in the water, as represented in Equation (33) [29,31]. During irradiation, NO 2 • reacts with the radiolysis products of water such as e aq − , H • , and OH • , and H + , NO 2 − ion, and NO 3 − ions are produced as listed in Equations (34)-(36) [32]. On the other hand, NO 2 • reacts with water and generates NO 2 − and NO 3 − ions, as shown in Equation (37) [13,33]. As represented in Equation (38), NO 2 − ions generated from the reaction in Equations (34), (35) and (37)  • and NO 3 − ions are regenerated by the reaction indicated in Equation (38). As mentioned above, NO 3 − ions and their radicals generated from the radiolysis of NO 3 − can also participate in the decomposition reaction of N 2 H 4 in the N 2 H 4 -Cu + -HNO 3 system.

Effect of Copper Ions on Hydrazine Decomposition
In order to investigate the effect of copper ions on the N 2 H 4 decomposition, γ-ray was irradiated to the N 2 H 4 -Cu + -HNO 3 solution and N 2 H 4 -HNO 3 solution at pH 3. The absorbed dose was varied from 0 to 20 kGy, and the [N 2 H 4 ] 0 in the solutions was equal to 50 × [N2H4]0 in the solutions was equal to 50 × 10 −3 mol dm −3 . The pH of the solution was adjusted to 3using HNO 3 . Figure 1 shows the change in the concentration of N 2 H 4 as a result of the γ-irradiation. The decomposed portion of N 2 H 4 increased with the increase in the absorbed dose regardless of the presence of the Cu + ions. This result indicates that the amount of radiolysis products of water participating in the N 2 H 4 decomposition was enhanced when the absorbed dose was increased. At the same absorbed dose, the decomposed portion of N 2 H 4 was higher when the copper ions existed than that when the copper ions were absent, as indicated in Figure 1. In particular, 12.48 × 10 −3 mol dm −3 of N 2 H 4 in the solution containing Cu + ions was decomposed after the 20 kGy of γ-irradiation. When the Cu + ions were absent in the solution, 9.05 × 10 −3 mol dm −3 of N 2 H 4 was decomposed. Moreover, the G-values for the N 2 H 4 decomposition were calculated for 20 kGy of absorbed dose and listed in Table 1. G(-N 2 H 4 ), for the solution containing Cu + ions, was higher than that of the solution not containing Cu + ions.  There are several explanations for the effect of copper ions on the decomposition of N 2 H 4 : (1) a catalyzed reaction of H 2 O 2 occurs [34], (2) copper ions lower the energy barrier of N-H bonds cleavage in the gas phase [35], and (3) the formation of Cu + N 2 H 4 occurs [36,37]. The experimental condition of Zhong and Lim is similar to that of this study [36,37]. They suggested that Cu + N 2 H 4 complex acts as a scavenger and it increases the decomposition reaction of N 2 H 4 . The predicted mechanism is given in Equations (39) (19) and (20). The Cu + ion regenerated from the reactions shown in Equations (40) and (41) repeatedly formed the Cu + N 2 H 4 complex and caused the decomposition reaction of N 2 H 4 again. Therefore, these two reactions offered new decomposition reaction paths of N 2 H 4 where the Cu + ion acted as the catalyst.
The electrochemical characterization, under the same conditions as this experiment, was also performed by Yang et al. [38]. Figure 2 shows the cyclic voltammograms using an ITO (Indium-Tin Oxide) electrode in solutions of 3 mM N 2 H 4 , 0.3 mM Cu(NO 3 ) 2 , 0.1 M NaNO 3 , and 3 mM N 2 H 4 + 0.3 mM Cu(NO 3 ) 2 . All the test solutions were adjusted to pH 3 using HNO 3 . The oxidation peak of the N 2 H 4 near 0.3 V increased significantly with the addition of Cu(NO 3 ) 2 . The peak is quite different from that of N 2 H 4 alone or Cu(NO 3 ) 2 alone. This result implies that Cu + ions make coordination compounds with N 2 H 4 , as listed in reaction Equation (39). Therefore, it is inferred that Cu + ions affect the N 2 H 4 decomposition by formation of the Cu + N 2 H 4 complex in the N 2 H 4 -Cu + -HNO 3 system.    Firstly, the above results can be explained by the effect of the H + ion on N 2 H 4 decomposition. As mentioned above, the reaction between the H + ion and e aq − caused the generation of H • , as represented in Equation (4). The increase in H • promoted the reaction between H • and the intermediates of N 2 H 4 decomposition, such as N 2 H 5 + , N 2 H 3 • , and N 2 H 2 , listed in Equations (7), (8), (18) and (23). When the reactions shown in Equations (7), (8) and (23) occurred, NH 4 + ions or the intermediates such as • NH 2 , N 2 H 4 •+ , and • N 2 H 3 were produced. As represented in Equation (18), N 2 H 4 is recovered through the reaction between • N 2 H 3 and H • . This N 2 H 4 could be decomposed after hydrolysis into N 2 H 5 + or be decomposed directly through the reactions listed in Equations (10)- (12). On the other hand, it was possible to form the Cu + N 2 H 4 complex with copper ions and cause a decomposition reaction of N 2 H 4 using Equations (40) and (41). In particular, N 2 H 4 and • N 2 H 3 generated from the reactions shown in Equations (18) and (23) have a high reaction rate, which are 7.0 × 10 9 M −1 s −1 and 3.0 × 10 9 M −1 s −1 , among the reactions concerned H • . The N 2 H 4 and • N 2 H 3 are mostly decomposed into N 2 and NH 3 , as mentioned above.

Effect of HNO 3 on Hydrazine Decomposition
Secondly, the increase in the decomposition reaction of N 2 H 4 with the lowering of the pH of the N 2 H 4 -Cu-HNO 3 solution could also be explained by the effect of the NO 3 − ion. When the NO 3 − ion reacts with NH 4 •+ , which is the chemical species of N 2 H 4 , N 2 H 2 and • NO 2 are produced due to the reaction, as represented in Equation (42) [39]. As listed in Equation (23), the N 2 H 2 reacts with H • , and • N 2 H 3 is generated. As mentioned above, • N 2 H 3 normally decomposes into N 2 and NH 3 , leading to N 2 H 4 decomposition. • NO 2 participates in the reaction with radiolysis products of water, and finally NO 3 − is formed by the reaction shown in Equations (34)- (38). On the other hand, NO 3 • generated during the radiolysis of NO 3 − ions also affects N 2 H 4 decomposition. When NO 3 • reacts with N 2 H 5 + , N 2 H 4 •+ and HNO 3 are produced, as shown in Equation (43) [13,39]. N 2 H 4 •+ is consecutively decomposed not only by the reaction listed in Equations (15) and (16) but also by the reaction shown in Equation (42). NO 3 − ions recovered from the reaction shown in Equation (43) participate in the decomposition reaction of the chemical species of N 2 H 4 . Therefore, the increase in HNO 3 in the N 2 H 4 -Cu + solution accelerated the decomposition of N 2 H 4 by increasing the occurrence of reaction concerned with H • and adding new decomposition reaction paths, including that of the NO 3 − ion.
Moreover, the decomposed portion of N 2 H 4 increased rapidly at pH 1 compared to at pH 3 and 5, when the absorbed doses increased at the rates shown in Figure 3. This was caused by H • and NO 3 • being generated by irradiation. Since the amount of HNO 3 added at pH 1 was larger than at pH 3 and 5, the amount of H • and NO 3 • produced was larger at pH 1 than at pH 3 and 5. The increase in H • and NO 3 • promoted the decomposition of N 2 H 4 through the reactions, as mentioned above.
In order to investigate the anionic effect, the remaining concentration of N 2 H 4 in the NO 3 − ion system was compared with that of the SO 4 2− ion system at pH 3. Quantities of 10, 20, and 40 kGy of the absorbed doses were irradiated to each solution. The initial concentration of N 2 H 4 in each solution was 50 × 10 −3 mol dm −3 . As shown in Figure 4, the decomposed concentration of N 2 H 4 increased when the absorbed dose increased regardless of the type of acid added. At the same absorbed dose, three times higher concentrations of N 2 H 4 in the NO 3 − ion system were decomposed as compared to the SO 4 2− ion system. As indicated in Table 3, the G-value for the decomposition of N 2 H 4 at 40 kGy was 3.98 × 10 −7 mol J −1 when the acid added in the solution was HNO 3 . G(-N 2 H 4 ) was 1.25 × 10 −7 mol J −1 when the acid injected in the solution was H 2 SO 4 . From these results, we found that the NO 3 − ions facilitated the decomposition of N 2 H 4 in the solution.

Decomposition Mechanism of Hydrazine in N 2 H 4 -Cu + -HNO 3 System
The decomposition reactions of the N 2 H 4 in N 2 H 4 -Cu + -HNO 3 system are schematically shown in Figure 5. N 2 H 4 in the acidic solution is hydrolyzed and coexists as N 2 H 5 + or N 2 H 6 2+ . These species are decomposed into intermediates such as N 2 H 4 •+ , • N 2 H 3 , and NH 2 • under irradiation conditions. N 2 H 4 can decompose into NH 4 + ion, N 2 , or NH 3 . However, it was verified that the end products were mainly formed with N 2 or NH 3. In addition, N 2 H 4 decomposition was promoted by the influence of the Cu + ion, H + ion, and NO 3 − ion in the N 2 H 4 -Cu + -HNO 3 system, as explained in Sections 3.1 and 3.2. As represented in green line in Figure 5, Cu + ions form the Cu + N 2 H 4 complex with N 2 H 4 . The Cu + N 2 H 4 complex participates in the reactions, as shown in Equations (40) and (41). The complex decomposes into • N 2 H 3 and further decomposes into the end products through the consecutive reactions, as listed in Equations (17)- (22). The recovered Cu + ion from the complex repeatedly forms an N 2 H 4 complex that acts as a catalyst to accelerate the decomposition of N 2 H 4 . H • was produced through the reaction between the H + ion and e aq − . Therefore, the decomposition reaction of N 2 H 4 by H • was promoted as the concen-tration of the H + ion increased. As shown in the orange line in Figure 5, NO 3 − ions or NO 3 • radicals accelerate the N 2 H 4 decomposition by providing the additional reaction paths to change N 2 H 5 + into N 2 H 4 •+ . They also cause N 2 H 4 •+ to decompose into N 2 H 2. NO 3 − ion and NO 3 • were regenerated by the radiolysis of NO 2 • and NO 3 • , as shown in Equations (34)- (38), and they participated in the N 2 H 4 radiolysis reaction again. Consequently, N 2 H 4 decomposition was promoted in the N 2 H 4 -Cu + -HNO 3 system through the mechanism shown in Figure 5. In order to verify the effect of Cu + ions on the decomposition mechanism of N 2 H 4 in the N 2 H 4 -HNO 3 solution, the fraction of N 2 H 4 and end products were analyzed after irradiation with 20 kGy of the absorbed dose. The initial concentrations of N 2 H 4 in the solution were 50 × 10 −3 mol dm −3 , and the pH of the solution was adjusted to 3. The results were compared with and without Cu + ions in the solution, as represented in Figure 6. Through the above reactions, it was verified that N 2 H 4 in the N 2 H 4 -Cu + -HNO 3 solution was finally decomposed into N 2 , NH 3 , and NH 4 + ion under an irradiation condition by the reactions with radiolysis products of water or consecutive decomposition reactions. For this reason, the fraction of N 2 and NH 3 in the solution after γ-ray irradiation was calculated by subtracting the amount of remaining chemical species of N 2 H 4 and NH 4 + ions produced after irradiation from the initial amount of N 2 H 4 . As shown in Figure 6, N 2 H 4 decomposed into N 2 , NH 3 , and NH 4 + ion. It is well known that most of NH 3 reacts with H + ions in the acidic solution and exists in the form of NH 4 + [40]. Therefore, it was considered that most of the remaining gas phase end product was composed of N 2 after irradiation in this study. It was judged that the NH 3 was converted into NH 4 + ion after the irradiation. When the Cu + ion is present in the N 2 H 4 -HNO 3 solution, N 2 H 3 • is generated, as indicated in Figure 5 and Equation (41). N 2 H 3 • participated in the reaction, generating N 2 or NH 3 , as shown in Equations (19) and (20). Therefore, it was confirmed that the fraction of N 2 and NH 4 + , the form of NH 3 in the acidic condition, increased when the Cu + ions were present in the N 2 H 4 -HNO 3 solution, as represented in Figure 6.
To confirm the effect of HNO 3 on the decomposition mechanism of N 2 H 4 in the N 2 H 4 -Cu + solution, the fraction of remaining N 2 H 4 and end products in each sample after 40 kGy of absorbed dose irradiation at pH 1, 3, and 5 was analyzed. The initial concentrations of N 2 H 4 in the solutions were 50 × 10 −3 mol dm −3 . The result is represented in Figure 7. As mentioned above, NH 3 is converted into NH 4 + ion in the solution because of the acidic condition. As shown in Figure 7, NH 4 + ions were not generated after 40 kGy of absorbed dose irradiation at pH 1. Therefore, it was considered that most of N 2 H 4 was decomposed into N 2 . The obtained result at pH 1 can be explained as follows. At pH 1, N 2 H 4 exists in the form of N 2 H 6 2+ as a result of hydrolysis, as shown in Equation (3). The N 2 H 6 2+ ion generated N 2 through the decomposition reaction, as indicated in Equation (5).
In addition, the amount of end products of N 2 H 4 decomposition were decreased with increasing pH. This was the case because the large amount of H • produced by the reaction between H + ions and e aq − affected the N 2 H 4 decomposition, as shown in Figure 5. For this reason, the decrease in the concentration of end products of N 2 H 4 decomposition following an increase in the pH was caused by a decrease in the N 2 H 4 decomposition reaction. However, the concentration of NH 4 + ions generated after irradiation increased with increasing pH. This was the case because the N 2 H 4 exists as a form of the N 2 H 5 + ion rather than the N 2 H 6 2+ ion as the pH increases. As indicated in Equations (6)-(23), the end products of the reactions with a high reaction rate, among the consecutive reactions of N 2 H 5 + ion decomposition in which H • participated, were mainly N 2 and NH 3 . The NH 3 reacted with the H + ion in an acidic condition and existed in the form of NH 4 + ions, as mentioned above. NO 3 − ions were also related to the generation of N 2 and NH 3 . The reaction between the N 2 H 5 + ion and NO 3 • shown in Equation (43) (23). The end reaction products involving • N 2 H 3 are also N 2 and NH 3 , as represented in Equations (17)- (22). The NH 3 generated by the reaction between N 2 H 5 + and NO 3 • also existed in the form of NH 4 + ion in the acidic condition. Therefore, it is concluded that HNO 3 can affect the decomposition of N 2 H 4 through the mechanisms listed in Equations (5)- (23) and Equations (42) and (43) by investigating the end product of the expected decomposition paths.

Chemicals and Sample Preparation
Hydrazine monohydrate (Junsei, Tokyo, Japan, 98.0%), nitric acid (EMSure, Darmstadt, Germany, 65.0%) and copper (I) chloride (SIGMA-ALDRICH, St. Louis, MO, USA, 97.0%) were used to prepare N 2 H 4 -Cu + -HNO 3 solution in this study. The conditions of each sample is listed in Table 4. All the solutions contain 50.0 mM of N 2 H 4 . In order to investigate the effects of Cu + ions on N 2 H 4 decomposition, the solutions were prepared according to the presence of 0.5 mM of copper ions. Each sample was adjusted to pH 3 by adding nitric acid. To analyze the effects of HNO 3 on N 2 H 4 decomposition, each solution was adjusted to pH 1, 3, and 5 by adding 144.7 mM, 50.8 mM, and 49.9 mM of nitric acid, respectively.
All the sample solutions included 0.5 mM of copper ions. The 30 mL of sample solutions were stored in the 50 mL vials. After storing the solution in the vial, the nitrogen purging was conducted for 10 min. during the γ-ray irradiation.

γ-rradiation
A high-dose γ-ray irradiator (Co-60 source) at the Korea Atomic Energy Research Institute was used for irradiation on the solutions. Quantities of 0, 5, 10, 20, and 40 kGy of absorbed doses were given to each sample to compare the dose effects on the decomposition of N 2 H 4 . All irradiation experiments were carried out with a dose rate of 10 kGy/hr at room temperature.

Analysis
The concentration of chemical species of N 2 H 4 in the solutions was measured using a UV Spectrometer (DR 5000, Hach Co., Ames, IA, USA). The p-dimethylaminobenzaldehyde method was applied to detect the chemical species of N 2 H 4 .

Conclusions
The radiolysis of N 2 H 4 in the N 2 H 4 -Cu + -HNO 3 solution during γ-ray irradiation was verified through the irradiation experiment and the analysis of a chemical species of N 2 H 4 concentration. When copper ions were present in the N 2 H 4 -HNO 3 solution, the N 2 H 4 decomposition, via the decomposition of the Cu + N 2 H 4 , complex was promoted by the catalytic reaction of Cu + ions. HNO 3 also accelerated the N 2 H 4 decomposition in the N 2 H 4 -Cu + -HNO 3 system through the influence of the H + ion and NO 3 − ion. This is because H • produced by the reaction between H + ion and e aq − participated in the N 2 H 4 decomposition reaction. Owing to the H + ion effect, the N 2 H 4 decomposition during irradiation was raised when the pH was decreased. NO 3 − ion and NO 3 • led to an increase in the N 2 H 4 decomposition through the reaction with N 2 H 4 •+ or the reaction with N 2 H 5 + . These additional paths, due to the existence of the Cu + and NO 3 − ions, improved the N 2 H 4 decomposition under irradiation condition. These findings can be applied, in accordance with the characteristics of radiolysis, to define the conditions of N 2 H 4 concentration during chemical decontamination processes.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.