The Study On Irradiation Behavior And Leaching Performance Of Apatite Glass Ceramics

The disposal of nuclear waste has greatly restricted the development of nuclear energy.The development of nuclear waste immobilization materials that can remain stable in a complex and extreme long-term immobilization environment has become the current research focus in the field of nuclear waste disposal.Glass ceramics,as a new generation of nuclear waste immobilization material with high potential,has received extensive attention.However,there are relatively few studies on the irradiation resistance of glass ceramics,and there is no report on the leaching performance of glass ceramics after irradiation.In this article,apatite glass-ceramics were studied in terms of irradiation resistance and chemical stability for immobilization of nuclear waste.The irradiation resistance of apatite glass ceramics was studied by Kr²⁺ and He⁺ ion irradiation,and the change of leaching rate of apatite glass ceramics before and after irradiation was tested by MCC-1 method.In this paper,the lanthanide element samarium was used as the simulated nuclide.Three kinds of fluorapatite glass ceramics with different samarium doping contents of 0 wt.%,3.53 wt.%and 6.93 wt.%were prepared by the traditional two-step melting sintering method（named GCO,GC1 and GC2 respectively）,and the heat treatment process was studied.When the primary melting temperature was 1350℃ for 2 h,and then 700℃ for 2 h sintering,the obtained sample has the highest density,and the crystalline phase of the sample was a single apatite phase.The crystalline phases were spherical with an average grain size about 95 nm,evenly distributed in the glass phases.The sample showed good thermal stability at 600℃.Three kinds of apatite glass ceramics were irradiated by 800 keV Kr²⁺ ions.The GIXRD results show that at different influence,the crystalline phase of the undoped GCO sample has the highest proportion of amorphous transformation,followed by GC1 with the smaller samarium doping,and the GC2 sample with the highest samarium doping has the lowest rate of amorphization in the crystalline phase.Under the irradiation influence of 1×1016 Kr²⁺/cm²,this value is 53.07%,49.20%and 48.72%,respectively,and the amorphous transformation has not completely occurred.On the one hand,the addition of samarium improves the anti-amorphous ability of the apatite crystalline phase in the sample;On the other hand,because the crystalline phase is about 95 nm nanocrystals,its higher interface density enhances the anti-amorphous ability of the sample,making it impossible to completely undergo full amorphization.At the same time,the results of XPS and ATR-FTIR showed that Kr²⁺ ion irradiation caused a change in the structure of the glass phase in glass-ceramics,and the[BO4]tetrahedral structural unit in the glass phase was transformed into a[BO3]triangular structural unit.The leaching performance of glass ceramics before and after Kr²⁺ ions irradiation was tested by MCC-1 method.After Kr²⁺ ions irradiation,the normalized leaching rate of B and Sm elements both increased.This was due to the conversion of[BO4]to[BO3]structural units in the glass phases due to Kr²⁺ ions irradiation.The triangular[BO3]was not as stable as the tetrahedral[BO4],the LRB of the sample increased;and the Kr²⁺ions irradiation destroyed the crystal structure of the original crystalline phases,which caused the amorphous transformation of the crystalline phases.The Sm atoms on the lattice site lost the constraints of the lattice,which led to the increase of LRsm.The apatite glass ceramics were irradiated with He⁺ ions of 50 keV with an irradiation influence of 2.6×1016 He⁺/cm² at 320℃.In the GC1 sample.Spherical helium bubbles with an average size of about 11.8 um were observed in the crystalline phase of the GC1 sample,but no helium bubbles were observed in the glass phase.The crystalline phase transformed into nanocrystals of smaller size（about 10-20 nm）,and no helium bubbles were observed of the GC2 sample.On the one hand,the concentration of vacancies is low in the ordered crystalline phase,and it is difficult for helium atoms to be annihilated by recombination with vacancies,thereby continuously forming helium bubbles.On the other hand,the addition of samarium enhances the irradiation resistance of the apatite crystalline phase,thereby reducing its critical amorphous temperature.Near the critical amorphous temperature,amorphization and recrystallization compete with each other to produce nanocrystals.The appearance of these nanocrystals further improves the radiation resistance of the sample,reduces the concentration of helium interstitial atoms,and makes it difficult to form helium bubbles.