Nuclear batteries for long-duration energy systems under 3R sustainability principles

Nuclear batteries and radioisotope power systems (RPS) occupy a distinct position within the broader landscape of long-duration energy technologies because their power evolution is governed primarily by radioactive decay rather than by electrochemical cycling. This review examines these systems from a system-level and time-explicit perspective, with particular emphasis on terminology and scope, isotope selection criteria, energy-conversion pathways, radiation-tolerant materials, reliability, and sustainability assessment. To avoid conceptual ambiguity, we distinguish direct-conversion nuclear batteries such as betavoltaic and alphavoltaic devices from the broader class of radioisotope power systems, which also includes thermal systems such as radioisotope thermoelectric generators (RTGs), radioisotope thermophotovoltaic systems (RTPV), and Stirling-based concepts. We argue that the defining advantage of these technologies is not high instantaneous power, but predictable and maintenance-independent operation over years to decades in environments where intervention is costly, risky, or impossible. From a sustainability perspective, the reduce, reuse, recycle (3 R) principle must be reformulated in temporal rather than purely material terms. Reduce as replacement avoidance over the mission horizon; Reuse is functional repurposing under predictable power downgrading; and Recycle as a combination of selective balance of system recovery and long-term radiological stewardship, rather than full circular recovery of the radioactive core. A service-based functional unit and break-even logic are proposed to support more rigorous comparison with alternative long-duration power solutions, including solar plus storage systems, primary battery replacement strategies, and low-power energy harvesting architectures. Overall, nuclear batteries are positioned not as competitors to mainstream electrochemical storage, but as enabling technologies for niche applications defined by inaccessibility, long service horizons, and maintenance intolerance. The central novelty of the review is the integration of time-explicit assessment, a temporal 3 R interpretation, and service-based break-even logic into a single evaluation framework.