Study On The Treatment Of Typical Organic Liquid Waste Simulants From Nuclear Power Plants Maintenance By Supercritical Water Oxidation

With the vigorous development of China’s nuclear power industry,and actively responding to the national low-carbon strategic goal,it is inevitable that radioactive organic liquid waste（ROLW）will be produced during the operation,repair and maintenance of nuclear facilities.Among these ROLW,little attention has been paid to a certain amount of organic waste liquids produced from nuclear power plants（NPPs）maintenance every year,which mainly includes radioactive oil waste and spent organic solvents,which have complex components and great differences in physicochemical properties.Due to the lack of suitable treatment technology,the ROLW with flammable,explosive and chemical toxicity can only be temporarily stored,but long-term storage has potential safety hazards such as leakage and fire.Therefore,to solve the final outlet problem of ROLW from NPPs maintenance,an efficient and green supercritical water oxidation technology（SCWO）was introduced to conduct experimental research on its degradation.It makes up for the basic experimental blank of treating ROLW by SCWO,and further promotes the development of industrial application of SCWO technology.In the continuous supercritical equipment,VG 32 lubricating oil and DURSET oil cleaning agent were selected as simulants of oil waste and spent organic solvents from NPPs to explore their removal effect and degradation mechanism during SCWO,and a quasi-first order kinetic equation was preliminarily established.The main conclusions are as follows:（1）During VG 32 lubricating oil of SCWO,the COD removal efficiency（CRE）increased with the increase of reaction temperature,reaction time and excess oxygen coefficient.The influence of each factor was a process of first fast and then slow.And the effect of reaction temperature and reaction time on CRE would be more significant.When the initial conditions were the feed concentration 2 wt.%and pressure 24 MPa,the optimum process parameters for VG 32 lubricating oil of SCWO were:reaction temperature 550°C,reaction time 80 s and excess oxygen coefficient 250%,CRE was up to 96%.In addition,by gas phase product analysis,the C element in lubricating oil was almost converted into CO₂,with a very small amount of CO.（2）In the catalytic SCWO experiment of VG 32 lubricating oil,the addition of transition metal salt catalyst was beneficial to promote the oxidative degradation of organic matter in supercritical water（SCW）.The removal of organic matter will be more significant at low temperature.The order of catalytic effect of nitrate was:Cu²⁺>Fe³⁺>Zn²⁺>Co²⁺.Cu（NO₃）₂ had the best catalytic effect,and the CRE reached 99%,which was increased by 2～3 percentage points.With the same degradation effect,the reaction temperature was reduced by about 20℃,which helped to prolong the reactor life.（3）Based on the response surface methodology（RSM）,the process parameters of SCWO of DURSET were optimized.The test results showed that the influence of single factor on CRE was:reaction temperature>reaction time>excess oxygen coefficient.The interaction effect of two factors was:reaction temperature and reaction time>reaction temperature and excess oxygen coefficient>reaction time and excess oxygen coefficient.The analysis of variance fitted a quadratic polynomial prediction model（R²=98.12%）,and the optimal process parameters optimized by the model were:reaction temperature 515℃,reaction time 66 s,excess oxygen coefficient 211%.The verification experiment showed that the CRE of DURSET was 99.49±0.23%,and the COD concentration of liquid product was 84.32±38.03 mg·L^-1,which met the discharge standard of conventional industrial water pollutants.（4）The quasi-first order kinetics of VG 32 lubricating oil and DURSET during SCWO were studied using a power function model.The initial conditions of the feed concentration 2 wt.%,pressure 24 MPa and excess oxygen coefficient 300%remained unchanged.At the temperature of 420～580℃and reaction time of 30～90 s,the reaction activation energyE_α and the pre-exponential factor A of VG 32 lubricating oil were43.10 k J·mol^-1 and 8.86 s^-1;respectively.At the temperature of 430～550℃and reaction time of 25～85 s,theE_α and A of DURSET were 101.44 k J·mol^-1 and 3.11×10⁵ s^-1,respectively.（5）The oxidative degradation and reaction pathways of VG 32 lubricating oil and DURSET oil cleaning agent were studied by GC/MS and FT-IR.On the one hand,VG32 lubricating oil was mainly composed of straight-chain alkanes and cycloalkanes,and the intermediate products were mainly monocyclic aromatic hydrocarbons such as acetophenone,benzoic acid,benzaldehyde,etc.And also contained many straight-chain alkanes and their derivatives.In addition,it was also found that a small quantity of polycyclic compounds such as indanone,phthalide,biphenyl,naphthalene,fluorenone,anthrone,and anthraquinone remained in the liquid phase.On the other hand,the main components of DURSET are monocycloalkanes and bicycloalkanes.The intermediate products were mainly short-chain molecules such as diethyl ether,acetone,ethyl propionate,2-pentanone,and 2-hexanone.Additionally,benzenes or cyclic ketones were also detected,such as benzene,toluene,cyclohexanone,benzaldehyde,4-methylbenzaldehyde,naphthalene,salicylaldehyde,etc.It can be concluded that the·OH free radical played an important role in promoting the occurrence of side reactions such as coupling,pyrolysis,hydrolysis and isomerization in SCW.Finally,the conceivable degradation reaction pathways of VG 32 lubricating oil and DURSET in SCW were proposed.