SUS304 is used as the main material for primary system facilities and piping of nuclear power plants due to its excellent corrosion resistance and heat resistance. As the coolant of the primary system of the nuclear power plant continuously circulates, SUS304 changes to a state containing radioactiv...
SUS304 is used as the main material for primary system facilities and piping of nuclear power plants due to its excellent corrosion resistance and heat resistance. As the coolant of the primary system of the nuclear power plant continuously circulates, SUS304 changes to a state containing radioactive elements and emits radiation. Therefore, decontamination must be performed inevitably.
In this study, electro-decontamination experiment was conducted with SUS304 using phosphoric acid as an electrolyte, and hydrazine and oxalic acid were added to the waste solution generated after electro-decontamination to obtain the reaction characteristics and removal rate of each metal ion in the waste solution. Finally, based on these reaction characteristics and the removal rate of metal ions, a study was conducted to confirm whether it is possible to repeatedly reuse as an electrolyte for electro-decontamination, and the following conclusions were obtained.
1) Experiments was conducted in which the simulated waste solution containing metal ions was reacted with an excess of oxalic acid. As a result, it was confirmed that the particles produced by the reaction of divalent metal cation(Fe2+, Ni2+, Co2+) with oxalate form metal oxalate dihydrate. However, the trivalent metal cation(Fe3+, Cr3+) and oxalate did not react.
2) SUS304 was decontaminated for 60 min at a voltage of 4 V to obtain a waste solution, and it was confirmed that the chemical composition of the main elements Fe, Cr, and Ni of the waste solution was the same as that of SUS304.
3) An excess of oxalic acid was added to the waste solution to proceed with the reaction. The removal rate of iron ions was 23.81 %, and the removal rate of nickel ions was 65.80 %, and it was confirmed that there was no reaction of Chromium ions. In conclusion, it can be estimated that only divalent metal ions are not generated during the electro-decontamination process, and metal ions with a high oxidation number are also generated.
4) After setting the temperature of the waste solution to RT, 60 ℃, 90 ℃, hydrazine was added to reduce metal ions having a high oxidation number in the waste solution. Then, oxalic acid was added to react with metal ions, and the removal rate of metal ions was obtained. When the reduction reaction of hydrazine occurred at a higher temperature, it was confirmed that the removal rate of metal ions was higher. However, chromium ions did not react even when hydrazine was added.
5) The circulation process consists of the following four steps:
(1) Electrolytic decontamination of SUS304 (4 V, 60 min)
(2) Hydrazine Reduction at 90 ℃
(3) Regeneration by oxalic acid at RT
(4) Separation of solution and particles
(The separated solution is reused as electrolyte during decontamination)
The hydrazine reduction and regeneration process using oxalic acid do not fundamentally remove chromium ion from phosphoric acid electrolyte, and there is a problem in that the chromium concentration in the waste solution continuously increases. Therefore, the decontamination amount of iron and nickel decreases due to the accumulation of chromium ions. In addition, the accumulation of chromium ions is estimated to be the main reason of reducing the efficiency of the regeneration reaction. In conclusion, the electrolyte in the circulation process can be reused repeatedly for a certain number of times, but there is a limit to its permanent use.
SUS304 is used as the main material for primary system facilities and piping of nuclear power plants due to its excellent corrosion resistance and heat resistance. As the coolant of the primary system of the nuclear power plant continuously circulates, SUS304 changes to a state containing radioactive elements and emits radiation. Therefore, decontamination must be performed inevitably.
In this study, electro-decontamination experiment was conducted with SUS304 using phosphoric acid as an electrolyte, and hydrazine and oxalic acid were added to the waste solution generated after electro-decontamination to obtain the reaction characteristics and removal rate of each metal ion in the waste solution. Finally, based on these reaction characteristics and the removal rate of metal ions, a study was conducted to confirm whether it is possible to repeatedly reuse as an electrolyte for electro-decontamination, and the following conclusions were obtained.
1) Experiments was conducted in which the simulated waste solution containing metal ions was reacted with an excess of oxalic acid. As a result, it was confirmed that the particles produced by the reaction of divalent metal cation(Fe2+, Ni2+, Co2+) with oxalate form metal oxalate dihydrate. However, the trivalent metal cation(Fe3+, Cr3+) and oxalate did not react.
2) SUS304 was decontaminated for 60 min at a voltage of 4 V to obtain a waste solution, and it was confirmed that the chemical composition of the main elements Fe, Cr, and Ni of the waste solution was the same as that of SUS304.
3) An excess of oxalic acid was added to the waste solution to proceed with the reaction. The removal rate of iron ions was 23.81 %, and the removal rate of nickel ions was 65.80 %, and it was confirmed that there was no reaction of Chromium ions. In conclusion, it can be estimated that only divalent metal ions are not generated during the electro-decontamination process, and metal ions with a high oxidation number are also generated.
4) After setting the temperature of the waste solution to RT, 60 ℃, 90 ℃, hydrazine was added to reduce metal ions having a high oxidation number in the waste solution. Then, oxalic acid was added to react with metal ions, and the removal rate of metal ions was obtained. When the reduction reaction of hydrazine occurred at a higher temperature, it was confirmed that the removal rate of metal ions was higher. However, chromium ions did not react even when hydrazine was added.
5) The circulation process consists of the following four steps:
(1) Electrolytic decontamination of SUS304 (4 V, 60 min)
(2) Hydrazine Reduction at 90 ℃
(3) Regeneration by oxalic acid at RT
(4) Separation of solution and particles
(The separated solution is reused as electrolyte during decontamination)
The hydrazine reduction and regeneration process using oxalic acid do not fundamentally remove chromium ion from phosphoric acid electrolyte, and there is a problem in that the chromium concentration in the waste solution continuously increases. Therefore, the decontamination amount of iron and nickel decreases due to the accumulation of chromium ions. In addition, the accumulation of chromium ions is estimated to be the main reason of reducing the efficiency of the regeneration reaction. In conclusion, the electrolyte in the circulation process can be reused repeatedly for a certain number of times, but there is a limit to its permanent use.
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