Investigation On Migration Mechanism Of Trace Nuclides In Pyrolysis And Gasification For Low-level Radioactive Solid Waste

The problem of efficient reduction of low radioactive solid organic waste generated during daily operation and maintenance of nuclear power plants has plagued the sustainable development of nuclear power plants.The high contents of volatile and fixed carbon in low-level radioactive wastes make pyrolysis and gasification technologies become a available way to get volume reduction.However,the mor core scientific problem is how to have trace nuclides retained in the residues in the thermal treatment.In this paper,the migration path of trace nuclides during the pyrolysis and gasification of low-level radioactive waste was evaluated based on isotope substitution technology and chemical equilibrium theory,and the migration of trace nuclides was restricted by regulating reaction conditions and in-situ chemical fixation reaction.Through micro/macro thermo-gravimetric experiments,the non-isothermal and isothermal carbon dioxide gasification characteristics of glove char and coverall char were examined.The effects of heating rates（5,10,20,30℃/min）and temperatures（600,700,800,900℃）on the gasification reaction and kinetic mechanism of the samples were addressed.The gasification mechanism function of glove char and coverall char could be described by Avrami-Erofeev model（n=3,n=4）,and the average activation energies were 180.0 k J/mol and 42.2 k J/mol,respectively.With lifting the temperature from 600℃ to 900℃,the gasification reactivity index of the two char samples increased by 85%and 73%,respectively.The random pore model could better describe the isothermal gasification of the two kinds of char samples.The activation energy of glove char ranged between 96.6 and 99.6 k J/mol,and the activation energy of coverall char was between 54.4-64.7 k J/mol.The migration and transformation of trace nuclides（cobalt,cesium and strontium）during pyrolysis and gasification of low-level radioactive waste（polyethylene,glove,coverall,paper towel,sawdust and mixture）and their chars were evaluated by isotope alternative technology combined with chemical equilibrium theory,which was based on two lab-scale fixed-bed reactors and chemical equilibrium calculation,respectively.The effects of reaction temperature,pressure,residence time,Cl content,atmosphere,fixing agent,nuclide type and content on the migration characteristics of micro-nuclides were estivated.Co-based products mainly existed in the form of Co₉S₈（s）between the pyrolysis temperatures of 470 and 550oC.The pyrolysis temperature for the mixture should be kept below 509oC to prevent the cesium migration.Sr-based products between 470oC and 1200oC existed mainly in solid forms.The gasification temperature should be kept below 1200oC to prevent cobalt and strontium migration.Negative micro-pressure operation could not impact greatly the migration of the trace nuclides.CsOH（g）in polyethylene and Cs Cl（g）in mixture decreased by about 90.5%and 90.6%with lifting the pressure from 0.1 MPa to 1.0 MPa,respectively,indicating that lifting the pressure could effectively resist the migration of the trace nuclides.Phosphorus and sulphur in low-level radioactive waste could resist the migration of Co.Chlorine in the samples could aggravate the migration of Cs.SiO₂ and TiO₂ in the samples could restrain the migration of Sr.The influence of atmosphere on the migration of trace nuclides is not significant.Only in the lower temperature range（below 675°C）,the mobility of trace nuclides is the lowest under oxygen atmosphere,followed by carbon dioxide and steam.The addition of ZrO₂ could inhibit the cesium migration in the glove and glove char.The addition of ZrO₂（Zr/Cs molar ratio 0.5-1.5）could promote the conversion of Cs₂ZrO₃（s）and Cs₂Zr₃O₇（s）with higher thermal stability,but the fixation effect of Zr on Cs was significantly affected by temperature and Cl content.Ash melting experiment was used to identify the melting characteristics of low-level radioactive waste and the influence of micro nuclides on the melting characteristics of residual residue during the pyrolysis and gasification of the low-level radioactive waste.The melting characteristic temperatures of the low radioactive waste were above 1300℃,meaning that the effect of high temperature melting of the low radioactive waste was not considered in the actual gasification treatment.The existence of the trace nuclides had not any obvious effect on the melting characteristic temperature of the low-level radioactive waste.At high temperatures,CoO would react with SiO₂ and Al₂O₃ in the low-level radioactive waste to form aluminosilicate eutectic..The research results in this paper can provide basic data for the industrial application of the pyrolysis and gasification disposal of low radioactive organic solid waste from the nuclear power plants,and attribute some reference for the exploration of the mechanism of the migration and immobilization of the trace nuclides in the radioactive waste treatment.