Salaries for software engineers at a US national labs like LANL don't deviate far from the $100k range [1]. For many projects, a security clearance and on-site presence are required. Add to that the requirement to train and use Fortran (old and new), the job proposition is only looking good to (1) people who love the problem domain (e.g., nuclear stockpile stewardship), (2) people who are specialized scientists who moonlight as programmers, or (3) people who simply like government lab environments (stability, science, ...).

[1] A SWE4 (BS + 12 YoE) has a pay scale of $109k - $182k and requires a Q clearance (top secret for nuclear): https://lanl.jobs/search/jobdetails/senior-software-applicat...