Radiation Effects on Hollandite Ceramics developed for Radioactive Cesium Immobilization

摘要

Progress on separating the long-lived fission products from the high level radioactive liquid waste (HLW) has led to the development of specific host matrices, notably for the immobilization of cesium. Hollandite (nominally BaAl_2Ti_6O_(16)), one of the main phases constituting Synroc, receives renewed interest as specific Cs-host waste-form. The radioactive cesium isotopes consist of short-lived ~(134)Cs and ~(137)Cs of high activities and ~(135)Cs with long lifetime, all decaying according to Cs~+→Ba~(2+)+e~-(β) + γ. Therefore, Cs-host forms must be both heat and (β,γ)-radiation resistant. The purpose of this study is to estimate the stability of single phase hollandite under external β and γ radiation, simulating the decay of Cs. A hollandite ceramic of simple composition (Ba_(1.16)Al_(2.32)Ti_(5.68)O_(16)) was essentially irradiated by 1 and 2.5 MeV electrons with different fluences to simulate the β particles emitted by cesium. The generation of point defects was then followed by Electron Paramagnetic Resonance (EPR). All these electron irradiations generated defects of the same nature (oxygen centers and Ti~(3+) ions) but in different proportions varying with electron energy and fluence. The annealing of irradiated samples lead to the disappearance of the latter defects but gave rise to two other types of defects (aggregates of light elements and titanyl ions). It is necessary to heat at relatively high temperature (T=800°C) to recover an EPR spectrum similar to mat of the pristine material. The stability of hollandite phase under radioactive cesium irradiation during the waste storage is discussed.