We have performed atomistic modeling to better understand nuclear fuel and materials. For the application of accident tolerant fuel, uranium nitride UN instability in air and water must be mitigated. To examine the initiation of corrosion, we studied dissociated water and oxygen on UN surfaces. Different concentrations and temperature were analyzed to probe the causes of formation of various products. The electronic structure was calculated to reveal its electronic behavior. In addition, High Temperature Irradiation Resistant Thermocouples (HTIR-TCs) are under development to monitor the temperature of research reactors in real time. The voltage produced with respect to temperature can be determined with the Seebeck coefficient of each HTIR alloy leg. However, changing structure at high temperature leads to drift in the accuracy of the TCs. To better understand the atomistic and electronic causes of drift, we calculated the Seebeck coefficient of HTIR alloys before, during, and after heating.