Assessing the risk of proliferation via fissile breeding in ARC-class fusion power plants

Construction of a nuclear weapon requires access to kilogram-scale quantities of fissile material, which can be bred from fertile material like U-238 and Th-232 via neutron capture. Future fusion power plants, with total neutron source rates in excess of 1e20 n/s, could breed weapons-relevant quantities of fissile material on short timescales, posing a breakout proliferation risk. The ARC-class fusion reactor design is characterized by demountable high temperature superconducting magnets, a FLiBe liquid immersion blanket, and a relatively small size (∼4 m major radius, ∼1 m minor radius). We use the open-source Monte Carlo neutronics code OpenMC to perform self-consistent time-dependent simulations of a representative ARC-class blanket to assess the feasibility of a fissile breeding breakout scenario. We find that a significant quantity of fissile material can be bred in less than six months of full power operation for initial fertile inventories ranging from 5 to 50 metric tons, representing a non-negligible proliferation risk. We further study the feasibility of this scenario by examining other consequences of fissile breeding such as reduced tritium breeding ratio, extra heat from fission and decay heat, isotopic purity of bred material, and self-protection time of irradiated blanket material. We also examine the impact of Li-6 enrichment on fissile breeding and find that it substantially reduces breeding rate, motivating its use as a proliferation resistance tool.核武器的研制需要获取千克级易裂变材料，此类材料可通过中子俘获过程，由铀-238（U-238）、钍-232（Th-232）等可增殖材料制得。未来聚变发电厂的总中子源强度可超过1×10²⁰中子每秒，能够在短时间内增殖出与武器相关的易裂变材料，构成突发核扩散风险。ARC级聚变反应堆设计的特点为：采用可拆卸式高温超导磁体、FLiBe（氟化锂铍）液态浸涂包层，且尺寸相对较小（等离子体大半径约4米，小半径约1米）。我们采用开源蒙特卡罗中子学程序OpenMC，对典型ARC级包层开展自洽时变模拟，以评估易裂变材料增殖突发场景的可行性。研究发现，当初始可增殖材料装载量为5至50公吨时，仅需不足6个月的满功率运行即可制得大量易裂变材料，这一结果代表着不可忽视的核扩散风险。我们进一步通过分析易裂变材料增殖带来的其他影响，对该场景的可行性展开研究，这些影响包括氚增殖比降低、裂变额外产热与衰变热、增殖材料的同位素纯度，以及辐照后包层材料的自防护时长。此外，我们还考察了锂-6（Li-6）富集度对易裂变材料增殖的影响，发现其可显著降低增殖速率，因此可将其作为核扩散防护手段加以利用。