Decontamination of a Contaminated RCP Shaft Using the SP-HyBRID Process

Abstract

It is very important to minimize the waste generation for decontamination of the reactor coolant system in a nuclear facility. As an alternative to commercial decontamination technologies, an inorganic acid chemical decontamination (SP-HyBRID) process can be effectively applied to the decontamination because it can significantly reduce the waste generation. In this study, the decontamination of a contaminated reactor coolant pump shaft from a nuclear facility was conducted using the SP-HyBRID process. First, equipment for a mock-up test of the decontamination was prepared. Detailed experimental conditions for the decontamination were determined through the mock-up test. Under the detailed conditions, the contaminated shaft was successfully decontaminated. The dose rate on the shaft surface was greatly reduced from 1400 to 0.9 μSv/h, and the decontamination factor showed a very high value (>1500).

1. Introduction

A corrosion oxide layer containing radioactive nuclides is deposited in the RCS (reactor coolant system) in a nuclear facility operated at a high temperature and a high pressure [1,2,3,4]. This corrosion oxide layer generates radiation build-up, and this can increase the risk of radiation exposure to workers [3,4,5,6]. It thus must be removed from the RCS. A chemical decontamination process can be effectively utilized to decontaminate the layer [6,7,8,9,10,11]. The HP CORD UV (Permanganic acid Chemical Oxidation Reduction Decontamination based on the acid permanganate Ultra-Violet light) process using an organic acid is widely used as a representative chemical decontamination process because it has a high decontamination factor (DF), and it has a problem that a considerable amount of radioactive ion exchange resin is generated as a secondary waste [9,10,11].

To resolve this problem, KAERI (Korea Atomic Energy Research Institute) has been developing the SP-HyBRID (Sulfuric acid Permanganate Hydrazine Based Reductive metal Ion Decontamination) process, a chemical decontamination process using inorganic chemicals, as diagrammed in Figure 1 [12,13]. H₂SO₄ and KMnO₄ are used as decontamination reagents in the oxidative process, and the chemical reagents used in the HyBRID process are N₂H₄, H₂SO₄, and CuSO₄. Cr-rich oxides are dissolved in the oxidative process, and Fe-rich oxides are dissolved in the reductive process [12,14]. In the SP-HyBRID process, the wastewater is purified by a precipitation method using Ba(OH)₂, and a filtration method. Finally, BaSO₄ waste containing metal hydroxides is generated, and the use of ion exchange resin is very minor [14,15]. For this reason, the SP-HyBRID process thus can significantly reduce the secondary waste generation when compared with the HP CORD UV process. However, it is necessary to demonstrate the performance of the SP-HyBRID process for application to nuclear facilities.

In this study, a decontamination test of a contaminated RCP (reactor coolant pump) shaft from a nuclear facility was conducted using the SP-HyBRID process. Based on the results of this test, the performance of the process was evaluated.

2. Experimental and Methods

2.1. Materials

The SS 304 specimens (50 $\times$ 50 $\times$ 2 mm) with a simulated corrosion oxide layer were prepared for a mock-up test of the decontamination test, as shown in Figure 2. The specimens were manufactured using an acidic solution in an autoclave (Ilshin autoclave, Daejeon, Republic of Korea, ISA-BC-0100-SS-20-315-SYS). The acidic solution was composed of 0.1 M EDTA-2Na (Junsei, 99.5%) and 0.1 M EDTA (Junsei, 99.5%). The autoclave was operated at 250 °C for 96 h. The simulated oxide layer was identified as magnetite (Fe₃O₄) by means of XRD analysis, as shown in Figure 3.

A contaminated RCP shaft was obtained from a nuclear power plant in Korea, and it is shown in Figure 4. This shaft is composed of a stainless steel material. The total length of the shaft is about 2.3 m. The area red-lined in Figure 4 is directly connected with the RCS as a contaminated area. The contact dose rate of spot A in Figure 4 was measured using a teletector radiation detector (Automess, Ladenburg, Germany, 6150ADT, energy range: 65 keV~1.3 MeV, dose rate range: 0.01 μSv/h~9.99 Sv/h), and it was about 1400 μSv/h as the average of three measurements.

In the decontamination test, H₂SO₄ (Showa, 97%), KMnO₄ (Daejung, 99.3%), N₂H₄·H₂O (Junsei, 98%), CuSO₄ (Junsei, 97.5%), and H₂O₂ (Daejung, 35%) were used as decontamination reagents, and their concentrations are provided in

Table 1. Concentrations of chemical reagents used in this study.

Cycle	Step	Chemical Reagent	Concentration [mM]
Iterated cycle (1st, 2nd)	SP oxidative process	H2SO4 KMnO4	0~3.25 6.33
	Removal process of Mn	N2H4	5~6.00
	HyBRID reductive process	H2SO4 N2H4 CuSO4	25~30.00 50.00 0~0.50
	Decontamination process of hydrazine (N2H4)	H2O2	125~150.00
3rd	SP oxidative process	H2SO4 KMnO4	— 6.33
	Decomposition process of MnO4−	N2H4	7.91
	HyBRID reductive process	H2SO4 N2H4 CuSO4	25~30.00 50.00 —
	Dissolution process of Mn oxides on the filter (filter cleaning)	—	—

.

2.2. Experimental

Equipment was prepared to conduct the decontamination test of the contaminated shaft, as shown in Figure 5 [13]. The equipment has a capacity of 30 L scale based on the SP-HyBRID decontamination solution, and it consists of a decontamination reactor, a circulation system, and a decontamination wastewater treatment system. The decontamination reactor (15 L based on the decontamination solution) was designed to accommodate the contaminated RCP shaft, and it contained a mockup of the RCP shaft head, as shown in Figure 5. The simulated specimens can be mounted on the mockup. The circulation system circulates a decontamination solution, and it consists of an injector, a buffer tank, a heater, a circulation pump, a chiller, a monitor, and a metal filter. The injector supplies a constant amount of decontamination reagents using a metering pump. The buffer tank accommodates a decontamination solution, and it has a capacity of 45 L. It also contains a condenser and a charcoal filter to minimize the evaporation of the decontamination solution and prevent the emissions of air pollutants. The heater heats a decontamination solution by an operation temperature. It is possible for the circulation pump to adjust the flow rates by 45 L/min using an inverter control system. The chiller reduces a decontamination solution with a high temperature (90~95 °C) to a temperature below 30 °C to measure the pH of the decontamination solution. The monitor observes the pH of the decontamination solution in real time. The metal filter collects insoluble particles generated from the conversion of MnO₄⁻ into an oxide form. Finally, the decontamination wastewater treatment system consists of a reactor, a circulation pump, and a filter. The reactor conducts precipitation and decomposition reactions in the decontamination wastewater, and it contains a metering pump to inject chemical reagents. The circulation pump is a diaphragm type, and it is used to mix the decontamination wastewater in the reactor and transfer it to the filter. The filter separates insoluble particles from the decontamination wastewater after the precipitation and decomposition reactions.

2.3. Decontamination Test

A mock-up test of the RCP shaft decontamination was conducted as follows. First, three simulated specimens were mounted on the RCP shaft mockup, and the decontamination reactor was sealed. Next, 30 L of deionized (DI) water was charged into the buffer tank and then circulated and heated in the decontamination reactor and the circulation system. After the temperature of the DI water reached 90 °C, chemical reagents used in the oxidative process were injected into the buffer tank, and the oxidative process was conducted for a constant time. N₂H₄ was then injected into the oxidative process solution for the Mn removal process while circulating the solution to the loop containing the metal filter. After finishing the Mn removal process, chemical reagents used in the reductive process were injected into the buffer tank, and the reductive process was conducted for a constant time. At this time, the metal filter was not incorporated in the circulation loop of the reductive process solution. Lastly, the decomposition of N₂H₄ in the reductive process solution was conducted using H₂O₂. These processes became one cycle. This cycle is an iterated cycle, as shown in Table 1, and it was conducted two times in the mock-up test. In the 3rd cycle shown in Table 1, the decomposition process of MnO₄⁻ was conducted after the oxidative process, and Mn oxides collected on the metal filter were dissolved using the reductive process solution after the 3rd cycle reductive process. The operation times for the mock-up test are presented in

Table 2. Operating time for the mock-up and decontamination test [h].

Operation Step	1st Cycle		2nd Cycle		3rd Cycle
	OP 1	RP 2	OP	RP	OP	RP
Mock-up test	4	6	3	5	2	4
Decontamination test	10	6	8	4	6	4

¹ OP: oxidative process, ² RP: reductive process.

.

The decontamination test of the contaminated RCP shaft was carried out following same procedures as the mock-up test, and the contaminated shaft head was loaded into the decontamination reactor instead of the RCP shaft mockup. The operation times for the decontamination test were scheduled based on the results of the mock-up test, and they are given in Table 2.

2.4. Analysis

The simulated oxide layer was characterized using an XRD (Bruker, Billerica, USA, D8 Advance A25, Conditions: Cu target, 40 kV, 40 mA, 0.2 °/s). The pH of samples was observed using a measuring device (Thermo Fisher Scientific, Waltham, USA, Orion STARA2115), and concentrations of N₂H₄ in a sample were analyzed using UV-VIS spectroscopy (Hach, Billerica, USA, DR5000). Concentrations of metal ions in samples from the mock-up test were measured using an ICP-OES (Analytikjena, PQ 9000). Concentrations of radioactive nuclides in samples from the decontamination test were analyzed using an HPGe detector (Ametek Ortec, Oak Ridge, USA, GEM-C7080P4). The samples were collected twice after the unit process in each cycle.

3. Results and Discussion

3.1. Decontamination of the Specimens Mounted on the Mockup of the RCP Shaft

Before conducting the decontamination of the contaminated RCP shaft, a mock-up test of the decontamination was carried out using the specimens mounted on the mockup of the RCP shaft shown in Figure 5. As shown in Figure 4, the surface of the red-lined area was the mainly contaminated area in the RCP shaft. The decontamination results of the specimens mounted on the mockup thus can be used to determine detailed operation conditions for the RCP shaft decontamination.

The pH of the process solution is an important indicator for the decontamination process [12,13,15]. Thus, it is necessary to monitor it in real time during the decontamination process. In the SP-HyBRID process, the oxidative process is effective at a process solution pH below 2.3, and the pH of the reductive process solution is proper in a range of 2.5–3.0 [12]. These pH conditions can be changed with dissolution of the corrosion oxide layer during the decontamination process. If the changed pH deviates from the pH conditions, it must be adjusted using H₂SO₄ or N₂H₄. Figure 6 shows the pH changes in each unit process solution during the mock-up test. It is found that the mock-up test was conducted in the proper pH conditions. The pH of the 2nd and 3rd oxidative process solutions showed lower values when compared to the pH of the 1st oxidative process solution. After the 1st and 2nd reductive process, the reductive reagent (N₂H₄) in the process solution was decomposed by injecting H₂O₂ (weak acid). For this reason, the pH of the 2nd and 3rd oxidative process solutions were decreased. These decreases of pH reduced the use of H₂SO₄. It is also believed that the decreased pH can contribute to improving the effect of the oxidative process as a pretreatment process.

The Mn removal and N₂H₄ decomposition in Figure 1 are processes to decompose or remove the decontamination reagents from the SP-HyBRID process solution. They must be conducted at a high efficiency for high performance of the SP-ByBRID process. The Mn removal is conducted by the conversion of MnO₄⁻ in the process solution after the oxidative process into oxide forms by reactions with N₂H₄ and the collection of the oxides in the metal filter [13]. In the mock-up test, the Mn removal was conducted at a high efficiency above 82.5%. The Mn removal efficiency was dependent on the injection rate of N₂H₄. Mn was effectively removed as the N₂H₄ injection rate was lowered because the reactions between MnO₄⁻ and N₂H₄ were conducted in the circulation system. Diluting N₂H₄ 10 times using DI water and injecting it at a rate of 1.5–2.0 mL/min was the most effective protocol. The N₂H₄ is decomposed by the hydroxyl radical generated from the Fenton reaction delineated Equation (1) [13]. This N₂H₄ decomposition was also dependent on the injection rate of H₂O₂. The proper injection rate of H₂O₂ was determined as 10 mL/min. The theoretical molar ratio of H₂O₂ over N₂H₄ is 2.0 for the decomposition of N₂H₄ using H₂O₂ [16]. However, the appropriate molar ratio was 2.75 for the N₂H₄ decomposition in the mock-up test. This N₂H₄ decomposition was conducted at a low pH condition where the decomposition is not effective. For this reason, the molar ratio was increased by 2.75. Excess H₂O₂ is decomposed into a hydroxyl radical (▪OH), as shown in Equation (1), and then converted into H₂O [17]. Thus, excess H₂O₂ was not a problem in the next process. In the mock-up test, the N₂H₄ remained at a level below 1 ppm.

Fe²⁺ + H₂O₂ + H⁺ = Fe³⁺ + ▪OH + H₂O

(1)

In general, metal oxides in the corrosion oxide layer are converted into ionic forms in a decontamination solution when the oxide layer is dissolved in the solution [18]. Thus, concentrations of metal ions in a decontamination solution can be used to understand the dissolution of the corrosion oxide layer during a decontamination process. Figure 7 shows concentrations of metal ions dissolved in each unit process solution during the mock-up test. As seen in Figure 7, as the number of the decontamination cycles increased, the concentrations of metal ions dissolved in each unit process solution tended to increase. In particular, the increase in the concentrations was significant in the second cycle despite the shorter operating time of the first cycle compared to the second cycle. This trend was clearly observed in the dissolution of the Cr oxide layer on the specimens. It was assumed that the Cr oxide layer is under the iron oxide layer although it was not characterized in the XRD-patterns in Figure 3 [3,4]. This means that the Cr oxide dissolution can be lowered during the first cycle because the iron oxide layer was inhibiting the solution contacting the Cr oxide layer, as shown in Figure 7. In the other hand, the Cr oxide dissolution was significantly improved in the second oxidative process. These results indicate that the simulated oxide layer on the specimen was converted into a form that is easier to dissolve in the first cycle when compared to the initial form. This means that the first cycle is an important pretreatment process for the second cycle, and the first oxidative process conducted at a pH below 2.3 can be an effective pretreatment process for the conversion. For this reason, the operating time of the first oxidative process in the decontamination test of the contaminated RCP shaft was scheduled as about 10 h. Figure 7 also shows that the concentrations of Cr and Fe in the process solution after the filter cleaning were increased when compared to those after the 3rd reductive process. In particular, the concentrations of Cr in the 1st and 2nd reductive process solutions decreased when compared to those in the 1st and 2nd oxidative process solutions, as shown in Figure 7. These results indicate that some of the Cr and Fe ions in the oxidative process solution were converted into an insoluble form and collected in the metal filter during the Mn removal process. This decrease in the metal concentration in the process solution can contribute to improving the dissolution performance of the corrosion oxide layer in the next process. On the other hand, the ionic form of Ni was maintained in the Mn removal process.

After the filter cleaning, the specimens mounted on the mockup of the RCP shaft were recovered, and they are shown in Figure 8. The simulated corrosion oxide layer was almost completely removed, as shown in Figure 8. It is thus thought that the detailed conditions of the SP-HyBRID process established by the mock-up test can be applied to decontamination of the contaminated RCP shaft.

3.2. Decontamination of the Contaminated RCP Shaft Head

Based on the mock-up test results, the decontamination test of the contaminated RCP shaft was conducted with monitoring of the changes in the pH of the unit process solution. The changes in the pH of the unit process solutions during the decontamination test showed similar trends to those in the mock-up test, as presented in Figure 6. The Mn removal was performed at a similar efficiency to that obtained in the mock-up test, and the N₂H₄ was decomposed and removed at a level of about 0.5 ppm. These results indicate that the decontamination test was performed well in accordance with the conditions derived from the mock-up test.

After each cycle, the dose rate of spot A in Figure 4 was measured to evaluate the decontamination performance using the teletector radiation detector, and the results are shown in Figure 9. As presented in Figure 9, most of the radioactive contaminants on the RCP shaft head were removed during the 1st cycle. The initial dose rate (1400 μSv/h) was reduced by 67 μSv/h after the 1st cycle. It is considered that the long operation time (10 h) of the 1st cycle oxidative process contributed to this significant reduction of the dose rate. The dose rate was further decreased by 0.8μSv/h after the 2nd cycle, and it was not changed during the 3rd cycle. This means that the decontamination of the RCP shaft head was completed after the 2nd cycle. The dose rates after the 2nd and 3rd cycles were almost at the same level as the ambient dose rate, and this indicates that the activities on the RCP shaft head were almost completely removed. As seen in Figure 10, photos of the RCP shaft head after each cycle showed similar trends to the decontamination results. The DF was calculated using Equation (2), and it showed a very high value (>1500) [11,19,20].

DF = Initial dose rate/Final dose rate

(2)

After completing the filter cleaning, the concentrations of radioactive nuclides in the decontamination wastewater were analyzed using HPGe detector, and the results are provided in

Table 3. Concentrations of radioactive nuclides in the wastewater (30 L) after the decontamination test of the RCP shaft head.

Nuclide	Mn-54	Co-57	Co-60	Zn-65
Concentration (Bq/mL)	2.56 × 10	5.37	8.16 × 102	4.21
Removal activity (Bq)	7.68 × 105	1.61 × 105	2.45 × 107	1.26 × 105
Activity fraction (%)	3.01	0.63	95.87	0.49
Removal amount (μg)	2.68 × 10−3	5.16 × 10−4	5.86 × 10−1	4.14 × 10−4
Mass fraction (%)	0.46	0.09	99.39	0.07
Specific activity (TBq/g)	2.87 × 102	3.12 × 102	4.19 × 10	3.05 × 102

. The removal activity was calculated by using the concentration of radioactive nuclide and volume of wastewater. From this result, the removal amount of radioactive nuclide was calculated from following Equation (3).

(Removal amount, [μg]) = (Removal activity, [Bq])/(Specific activity [TBq/g]) × 10⁶

(3)

The fractions of activity and mass were derived based on the removal activity and amount. As listed in Table 3, radionuclides such as Mn-54, Co-57, Co-60, and Zn-65 were dissolved from the RCP shaft head, and 2.55 × 10⁷ Bq of gamma-emitting activity was removed during the decontamination. The activity fraction of Co-60 was over 95%. Removal amounts of the radionuclides were calculated using their specific activities, as shown in Table 3, and a total of 0.6 μg was removed during the decontamination (Co-60 > 99%).

Through the above decontamination results, it was confirmed that corrosion oxides containing radionuclides such as Co-60 on the metal surface can be almost completely removed using the SP-HyBRID process. It is thus considered that the SP-HyBRID process as an alternative to commercial decontamination technologies can be effectively applied to the decontamination of RCS in a nuclear facility.

4. Conclusions

In this study, the decontamination of the RCP shaft head from a nuclear facility was conducted using the inorganic chemical decontamination process, SP-HyBRID. The contaminated RCP shaft head was perfectly decontaminated in two cycles, and the DF showed a very high value (>1500). It is thus thought that the SP-HyBRID process as an alternative to commercial decontamination technologies using organic chemicals can be effectively applied to the decontamination of RCS in nuclear facilities.