Chemical Decontamination Of Nuclear Stainless Steel And Research For Waste Water Adsorption

The oxide layer produced in the primary coolant system of a PWR nuclear power plant reactor will adsorb radioactive elements to generate extra-nuclear radiation.In order to solve this problem,it is necessary to run chemical decontamination process.According to the composition of the oxide layer,four kinds of simulated oxide powders with different chromium contents were prepared and then using the Oxidation-Reduction two-step method to dissolve the prepared simulated oxides.The optimal chemical decontamination formula was found by a series of experiments,which is applied to the actual pipeline and found that the radioactivity dropped to the same with the surrounding environment.In the meantime,it is necessary to treat the Cr（Ⅵ）waste water that does not meet the emission standards during the oxidation treatment of the simulated oxide.In this paper,a new type of organobentonite was prepared for the adsorption of hexavalent chromium,and the optimum conditions for the adsorption of Cr（Ⅵ）on organically modified bentonite were investigated.The results of the tests were as follows:（1）Fe_3-xCr_xO₄（x=0.3,0.45,0.6,1）were prepared by coprecipitation method using FeSO₄·7H₂O,Cr₂（SO₄）₃·6H₂O and KOH as raw materials.It is found by the characterization that the obtained product has good crystallinity and a grain size of0.3-1μm,forming a counter-spinel structure,and chromium is into the lattice no precipitation to replace part of the location of ferric iron.（2）The optimum chemical decontamination formulation for the simulated oxides dissolution experiment is NP-NTA-Vc-OX formulation:2mmol/L potassium permanganate oxidation pretreatment the simulated oxides at 80℃and pH=2;0.12%of mass fraction（NTA+Vc+OX）;m（NTA）:m（Vc）:m（OX）=7:18:3 reagent for the simulated oxide reduction complexation step.The use of low concentrations of oxalic acid will not produce intergranular corrosion of stainless steel,and chemical decontamination effect can be doubled.（3）The NP-NTA-Vc-OX formulation can dissolve the oxides of Fe_3-xCr_xO₄ x<0.5 up to70%-98.31%.With the increase of chromium content,Fe_3-x Cr_xO₄ will have low dissolution efficiency.When x=1,the dissolution efficiency is only 11.18%.The Oxidation-Reduction two-step cycle can be repeated to increase the dissolution effect.As the redox cycle is repeated several times,it will produce non-compliance with the discharge of Cr（Ⅵ）waste water.（4）The NP-NTA-Vc-OX formulation was applied in the actual primary coolant system of a PWR nuclear power plant reactor.It can be found that after chemical decontamination,the radioactivity dropped to the same level as the surrounding environment.（5）Organobentonite was prepared from sodium bentonite and cationic surfactant1-n-dodecyl-3-methyl-imidazolium.Through FTIR and XRD characterization it is found:1-n-dodecyl-3-methyl-imidazole cationic organobentonite compared to the sodium bentonite skeleton structure did not change,and cationic surfactants enter into the aluminosilicate lamellae,increasing interplanar spacing d₀₀₁ of organic modified bentonite from 1.22nm to 1.51nm.（6）The 1-n-dodecyl-3-methyl-imidazolium cation organobentonite was used to deal the hexavalent chromium generated by the oxidation treatment of the simulated oxide,and the optimum adsorption conditions are as follows:50 mL of a 50 mg/L K₂Cr₂O₇ solution,organobentonite dosage 0.5g,pH=5-7,adsorption time t=40min,the removal rate of hexavalent chromium up to 98.51%.The adsorption of Cr（Ⅵ）by organically modified bentonite agreed well to the Langmuir isothermal adsorption model,which illustrated the adsorption process is a single molecule layer and uniform on the surface.And the adsorption kinetics data fit pseudo-second-order kinetics model well.