Saudi Arabia’s construction of a research reactor is stoking concerns about whether Riyadh’s plans for nuclear power plants include a hidden military element.

Saudi Arabia is nearing completion of a nuclear research reactor at King Abdulaziz City for Science and Technology (Kacst) in Riyadh.

The plant is being built by Argentina’s state-owned Invap, under a contract signed with Kacst in 2014. The 30kW rated unit will produce neutron beams for research, training and...