<p>Multiscale multiphysics simulation is a key technology for nuclear reactor system design and analysis. However, its application and development are limited by the complexity of cross-scale simulation coupling and long calculation times. To satisfy the real-time simulation requirements of modern reactor digital twins, this study establishes a digital twin of the reactor circuit using multiphysics and multiscale reduced-order methods. This digital twin is based on the plug-and-play approach, and all simulations of the components are replaced by independent 1D and 3D multiphysical reduced-order surrogate models. The complete system circuit can be composed of a combination of these surrogate models, which allows for the easy integration of new components and modification of existing components. A digital twin circuit is established for the test case. The reactor core is described using the 3D neutronics/thermal-hydraulics model, whereas the steam generator is described using the 3D CFD model. The other components, including the heat and cold pipes, are described using a 1D reduced-order model. The numerical results show that the digital twin can accurately predict the multiphysics and multiscale behavior of the reactor circuit. The maximum relative error of the tested circuit is not larger than 0.05%, and the simulation time can be reduced to less than 2 ms. The proposed plug-and-play digital twin can be used to develop a new real-time digital twin system that can support reactor system design and analysis.</p>