Aligning System Boundaries between Scopes 1–3 CO₂e Accounting and Life Cycle Assessment (LCA): A Comparative Case Study of Fossil-Fired and Nuclear Power Plants

Abstract

The objectives of this research are twofold: First, we quantify cradle-to-grid (CtG) life-cycle greenhouse-gas (GHG) intensities for three major electricity-generation technologies and map these results to the GHG Protocol Scopes 1–3. We do so by harmonizing system boundaries across technologies and disaggregating impacts into direct operational emissions (Scope 1), purchased-energy emissions (Scope 2), and upstream/downstream value-chain emissions (Scope 3). Second, we examine how organizational GHG accounting aligns with, and can diverge from, LCA boundary definitions by comparing nuclear (PWR and BWR) generation with coal-fired and natural-gas combined-cycle (NGCC) power plants, and by explicitly linking scope classifications to CtG and cradle-to-grave (CtGr) LCA boundaries (with CtG defined at the busbar/plant gate).

Results show that nuclear electricity has a very low life-cycle GHG intensity relative to dispatchable fossil alternatives. Unlike fossil generation, nuclear emissions occur predominantly in Scope 3, especially in the front end of the uranium fuel cycle (mining, conversion, enrichment, and fuel fabrication) and in embodied emissions from capital goods, rather than in Scope 1 emissions from plant operations. By contrast, coal and NGCC remain materially higher due to dominant Scope 1 combustion emissions alongside nontrivial Scope 3 fuel-cycle contributions. For NGCC, upstream methane leakage is a pivotal Scope 3 driver that can materially shift CtG outcomes compared with Scope 1 combustion-only reporting. With carbon capture and storage (CCS), residual fossil GHG intensity is governed by capture rate, efficiency (energy) penalties, and upstream fuel-cycle emissions. We also find that scope allocations can change when moving from a plant-owner (investor-owned) boundary to a utility holding-company boundary, particularly where generation is not operationally controlled and purchased electricity for resale becomes a major indirect category. Overall, the study provides a transparent mapping between LCA boundaries and Scopes 1–3 that reduces boundary mismatches in technology comparisons and sensitivity analyses (e.g., methane leakage and capital-goods emission burdens) and supports clearer, more credible emissions reporting for regulators, investors, and other stakeholders.