Theoretical Studies Of Fission Product Behavior In Nuclear Fuel

Thoria is a promising nuclear fuel material for advanced nuclear energy system.Compared with UO₂,the most commonly used fuel,thorium dioxide has better radiation resistance and greater chemical stability.Moreover,thorium fuel cycles generate little minor actinides and plutonium,which advantage the reduction of long-term radiotoxicity of nuclear waste and nuclear non proliferation.Recently,thorim-based molten salt reactor?TMSR?has been studied in Shanghai Institute of Applied Physics?SINAP?,Chinese Academy of Sciences.As the development of the generation IV reactors,which emphasize safety,sustainability and economy,the study of ThO₂ for improving nuclear fuel performance becomes very attractive and necessary.A considerable amount of fission products are produced in nuclear fuels during operation.For instance,krypton and xenon are typical fission products in nuclear fuels.These gases tends to aggregate and form bubbles,which gives rise to fuel swelling and affect mechanical properties of nuclear fuels.Moerover,two main fission product precipitates may exist in the fuel.One is metallic white inclusions,which are composed of Ru,Mo,Rh,etc.The other is so-called of gray phases,such as ABO₃?A=Ba,Sr,and Cs,and B=Zr,U,Pu,and Mo and rare earth elements?.The compositions of these phases vary with the complex condition of the nuclear fuel,such as the oxygen/metal ratio and the temperature gradients of the fuel.The existence of these fission products will affect the thermodynamics properties of nuclear fuels.Therefore,to ensure the safe operation of the reactor,it is very important to understand the behavior of fission products in nuclear fuel.We have studied the fission product behavior in nuclear fuel by first-principles method.Our researches include three parts:the stability of noble gases in thorium dioxide,the aggregation behavior of helium in thorium dioxide and the stability and aggregation of fission products in thorium dioxide.The stability of noble gases?He,Ne,Ar,Kr and Xe?in thorium dioxide is studied by means of density functional theory.The following insertion sites of ThO₂ are considered for our computations:the interstitial sites,the thorium vacancies,the oxygen-thorium di-vacancy and three types of Schottky defects.Our results show that there is an approximately linear relation between the energies and the atomic radii.As the size of the noble gas atom increases,the noble gas atoms energetically prefer to diffuse in large vacancy defects rather than into interstitial positions.Specially,it is found that the most stable insertion site of xenon is the thorium vacancy,which corresponds to its chemical bonding with O atoms.Moreover,the binding energy of Kr or Xe interstitial in a Schottky defect is larger than the formation energy of a Schottky defect,suggesting the Schottky defects are thermodynamically favorable in the presence of these noble gas atoms.The aggregation behavior of helium atoms in thorium dioxide has been investigated by first-principles calculations.The results show that He atoms tend to form a cluster around an octahedral interstitial site.As the concentration of He atoms in ThO₂ increases,the strain induced by the He atoms increases and the octahedral interstitial site is not large enough to accommodate a large cluster,such as a He hexamer.We considered three different Schottky defect?SD?configurations?SD₁,SD₂ and SD₃?.When He atoms are located in the SD sites,the strain induced by He atoms is released.Thus the incorporation and binding energies decrease.The He trimer is the most stable cluster in SD₁.Large He clusters,such as a He hexamer,are also stable in the SDs.The stability and clustering behavior of fission products?Zr,Mo,Ru,and Rh?in thorium dioxide have been investigated by density functional theory.The thorium vacancy is the most energetically favorable trap site for all of these fission products.Zr and Mo can exist as oxide precipitates whereas Rh and Ru tend to form metallic clusters in ThO₂.Moreover,Ru and Rh dimers are the most stable clusters in the Schottky defect in ThO₂,suggesting that metal clusters in ThO₂ may be formed of these dimers trapped in Schottky defects.