Theoretical Study On Substoichiometric Plutonium Carbide And Nitride

Fast neutron reactor has the advantages of energy saving,environmental protection and safety in comparison with thermal neutron reactor.Compared with the well-stablished oxide fuels,the carbide fuels and nitride fuels of the fast neutron reactor have the advantages of good thermal conductivity,sodium coolant compatibility,and high neutron breeder rate,as a very promising nuclear reactor fuels.The stoichiometric behavior of plutonium monocarbidePuC1-x and plutonium mononitride PuN as the important components of the carbide fuels and nitride fuels should be widely studied.However,the privious experimental reports are relatively old,the early experimental conditions are limited,and some data still have uncertainties.Due to the toxicity and radioactivity of plutonium,carrying out plutonium-related experiments is extremely limited,and it is difficult to rely on a large number of repeatable experiments to obtain accurate material data.In addition,the privious theoretical studies have focused on the positive stoichiometric plutonium monocarbide PuC1-x and plutonium mononitride PuN,and the theoretical research on the substoichiometric behavior is relatively lagging.Therefore.this work carried out the theoretical studies on the substoichiometric behavior of plutonium monocarbide PuC1-x and plutonium mononitride PuN.We use the special quasi-random structure method to model the substoichiometric plutonium monocarbide PuC1-x.The substoichiometric ratio range includes the reported experimental range.The energy and volume of ground state of plutonium monocarbide PuC1-x with different substoichiometry were calculated by density functional theory.The ground state volume is converted to the corresponding lattice constant,which is in good agreement with the experiment.The formation enthalpy of plutonium monocarbide PuC1-x with different substoichiometry was calculated,and its thermodynamically stable composition range was obtained,that is,its substoichiometric ratio was PuC0.741-PuC0.923,which is also in good agreement with the previous experiment.Through Bader charge,density of state,and crystal orbital hamilton populations analysis,we analyzed the microscopic mechanism of carbon vacancy to its thermodynamic stability.The original Pu-5f electrons in the local state become more delocalized due to the existence of carbon vacancies.The Pu-5f electrons in the delocalized state participate in more bonding,and the position of bonding is in the opposite direction of the carbon vacancy,thereby it improves the thermodynamic stability and reduces the enthalpy of formation.In this work,the range of substoichiometry of plutonium monocarbide PuC1-x is found,and the microscopic mechanism of the carbon vacancy to its thermodynamic stability is given.Plutonium mononitride PuN with different stoichiometric ratios was also established by the special quasi-random structure,and the energy-volume curve was calculated by VASP.The energy and volume of ground state were obtained by fitting the equation of state.The formation enthalpy is calculated from the ground state energy of plutonium mononitride PuN with different stoichiometric ratios.From the formation enthalpy analysis,plutonium mononitride has no substoichiometric behavior,that is,it is unlikely to form nitrogen vacancies.Through the conversion of the ground state volume to the corresponding lattice constant,the calculated lattice constants match the experimental lattice constants well.Through COHP analysis and comparison with plutonium monocarbide PuC1-x,this work shows that nitrogen vacancies and carbon vacancies have similar enhancement effects,which have a positive effect on their thermodynamic stability.However,the nitrogen vacancy formation enthalpy of the plutonium mononitride PuN is speculated.When the plutonium mononitride PuN loses an N atom,the overall weak Pu-N bonds plus the enhanced Pu-N bonds may not be a good relaxation and stability.Therefore,the overall weak Pu-N bonds of plutonium mononitride PuN may be one of the reasons for its non-stoichiometric behavior.In summary,we studied the substoichiometric behavior of plutonium monocarbide PuC1-x and plutonium mononitride PuN by the special quasi-random structure combined with density functional theory,and gave a possible microscopic mechanism of substoichiometric behavior.It has laid a certain theoretical foundation for the development of fast neutron reactor fuel.