Part 5/8:
Historically, research into nuclear thermal propulsion is not a new endeavor; the US government invested heavily in nuclear technology in the 20th century. However, past designs that relied on highly enriched uranium are no longer feasible due to proliferation risks. To combat this, today's research focuses on high-assay low-enriched uranium (HALEU), which is safer and unsuitable for weaponization. Nonetheless, the reduced enrichment level poses challenges in terms of fuel requirements and reactor efficiency.