Quantum key distribution (QKD), one of the latest cryptographic techniques, founded on the laws of quantum mechanics rather than mathematical complexity, promises for the first time unconditional secure remote communications. Integrating this technology into the next generation nuclear systems - designed for universal data collection and real-time sharing as well as cutting-edge instrumentation and increased dependency on digital technologies - could provide significant benefits enabling secure, unattended, and autonomous operation in remote areas, e.g., microreactors and fission batteries. However, any practical implementation on a critical reactor system must meet strict requirements on latency, control system compatibility, stability, and performance under operational transients. Here, we report the complete end-to-end demonstration of a phase-encoding decoy-state BB84 protocol QKD system under prototypic conditions on Purdue's fully digital nuclear reactor, PUR-1. The system was installed in PUR-1 successfully executing real-time encryption and decryption of 2,000 signals over optic fiber distances up to 82 km using OTP-based encryption and up to 140 km with AES-based encryption. For a core of 68 signals, OTP-secure communication was achieved for up to 135 km. The QKD system maintained a stable secret key rate of 320 kbps and a quantum bit error of 3.8% at 54 km. Our results demonstrate that OTP-based encryption introduces minimal latency while the more key-efficient AES and ASCON encryption schemes can significantly increase the number of signals encrypted without latency penalties. Additionally, implementation of a dynamic key pool ensures several hours of secure key availability during potential system downtimes. This work shows the potential of quantum-based secure remote communications for future digitally driven nuclear reactor technologies.