Before a new nuclear reactor design can be constructed and operated, its safety must bedemonstrated using models that are validated with integral effects test (IET) data. However, because scaled integral effects tests are electrically heated, they do not exhibit nuclear reactor feedback phenomena. To replicate the nuclear transient response in electrically heated IETs, we require simulated neutronics feedback (SNF) controllers. Such SNF controllers can then be used to provide SNF capabilities for IET facilities such as the Compact Integral Effects Test (CIET) at the University of California, Berkeley (UC Berkeley). However, developing SNF controllers for IET facilities is non-trivial. To expedite development, we present the use of Digital Twins as testbeds for iterative SNF controller development. In particular, we use a Digital Twin of the Heater within CIET as a testbed for SNF Controller Development. This Digital Twin with SNF Capabilty is run as an OPC-UA server and client written almost entirely in Rust using Free and Open Source (FOSS) code. We then validate the Digital Twin with experimental data in literature. We also verify the transfer function simulation and Proportional, Integral and Derivative (PID) controllers written in Rust using Scilab. Moreover, we demonstrate use of data driven surrogate models (transfer functions) to construct SNF controllers in contrast to using the traditional Point Reactor Kinetics Equations (PRKE) models with the hope that they can account for the effect of spatially dependent neutron flux on reactor feedback. To construct the first surrogate models in this work, we use transient data from a representative arbitrary Fluoride Salt Cooled High Temperature Reactor (FHR) model constructed using OpenMC and GeN-Foam. Using the Digital Twin as a testbed, two design iterations of the SNF controller were developed using the data driven surrogate model. Compared to the potential development time taken in using physical experiments, using the digital twin testbed for SNF controller development resulted in a significant time saving. We hope that the approaches used in this dissertation can expedite testing and reduce expenditure for licensing novel Gen IV nuclear reactor designs.