As provided by Beyond Nuclear's legal counsel, Terry Lodge of Toledo, Ohio:
The lawsuit -- a petition for review pending in the U.S.  Circuit Court of Appeals for the District of Columbia Circuit, involves a  challenge to the adequacy of the NRC's analysis and disclosure of the  HALEU project. Centrus Corporation, created by the US Congress after the  bankruptcy of the uranium enrichment firm USEC at the Piketon, Ohio  site (PORTS, formerly called the  Portsmouth Gaseous Diffusion Site),  received a $115M contract from U.S. DOE in 2020 to demonstrate the  manufacture of high-assay low-enriched uranium, or HALEU, which NRC and  DOE maintain is the nuclear fuel of the future, to be used in several  different designs of advanced nuclear reactors, aka small modular  reactors (SMRs).
HALEU poses problems. Some of  the planned new generation of reactors require intensively enriched  uranium-235 in order to fission adequately to produce sufficient heat to  produce steam to turn an electricity generator. While the current  generation of reactors requires a uranium fuel with a concentration of  2-3% U-235, SMRs will be designed to require a 19.75% concentration. The  NRC license granted to Centrus to install centrifuges to enrich uranium  fuel to that level allows fluctuations as high as a 25% concentration.  Any nuclear fuel enriched at 20% or more is classified as  "highly-enriched uranium", or HEU, and technically can be used as the  explosive core material in a thermonuclear bomb. That means that HALEU  could be desirable to thieves or weapons traffickers. The additional  effort needed to enrich 20% HALEU to 90% HEU is comparatively little.
   
As a comparison, Iran's controversial nuclear fuel enrichment program  has been enriching fuel to about 5% U-235, yet Iran is accused of  maintaining a bomb-making process and has to submit to international  inspectors. But the U.S. government intends to enrich to roughly 20% or  more concentrations without requiring sophisticated inventorying of the  volume of fuel created to prevent theft, nor special security measures.
Beyond Nuclear and the Ohio Nuclear-Free Network claim that the  environmental assessment written by the NRC neither mentions the weapons  proliferation potential of diverted HALEU and so does not evaluate in  any way what the prospects are, nor is the new fuel examined from the  standpoint of being classified as HEU, which should subject to to  international treaty obligations requiring greater security and  accountability safeguards. If the court finds in BN/ONFN's favor, it  could require the NRC to prepare and publish an environmental impact  statement covering these objections in detail and the public would be  informed of the potential for weapons trafficking/proliferation of  weapons material, as well as the additional environmental impacts caused  to Americans living near or working in the expanded U.S. uranium mining  and extraction that would be caused by a new generation of nuclear  reactors.
BN/ONFN files its brief on March 14,  NRC responds in mid April, BN/ONFN files a reply brief at end of April,  and some months later there will be oral argument and a decision.