Renewable energy is defined as an energy source/fuel type that can regenerate and can replenish itself indefinitely(i.e. can be used for a very long period of time without being depleted) and also does not cause pollution E.g. Sun Wind Geothermal energy etc.
In most of the cases the energy released in nuclear energy is due to the bombardment of heavy nucleus and protons (such as of uranium) which then splits into smaller nuclei this is known as nuclear fission.
Now,
Nuclear energy is termed as renewable source of energy because, the amount of uranium present on the Earth is enough to provide energy for billions of years.
Now, Nuclear energy is also termed as non renewable source of energy because, during the production of nuclear energy lots of nuclear waste is produced which is harmful to the environment and cause pollution, unlike sun wind etc. which violates the condition of renewable energy.
Hence, nuclear energy can be termed as both renewable and non renewable source of energy.