Historically, neutrinos from nuclear reactors have played very important roles in understanding neutrino properties. Currently, thanks to many experiments based not only on reactors but also on solar and atmospheric neutrinos as well as accelerator based ones, we are in the precision era for measurements of various fundamental neutrino properties such as mixing angles and mass squared differences. In the following decade, we expect to have significantly improved measurements of these mixing parameters by upcoming new experiments like JUNO, DUNE and Hyper-Kamiokande. Therefore, we expect to have a better condition to perform more precise test of the standard three flavour neutrino paradigm and, in this context, to be able to test of unitarity in neutrino oscillation experiments. In this work, we focus mainly on the reactor neutrinos, paying particular attention to the electron neutrino sector. We first try to review the current situation of the constraints on the unitarity for the first row of the PMNS (Pontecorvo-Maki-Nakagawa-Sakata) neutrino mixing matrix. We shall emphasise the critical role of today’s less precision in absolute knowledge, such as the the neutrino flux uncertainty as a key objective to be able to probe possible Unitarity violation as a direct and unambiguous exploration for physics beyond the standard model (BSM). Then, we will discuss some possible strategies to further improve the current constraints mainly in the context of reactor neutrinos.