I was under the impression that (1) making a weapon from Pu gets considerably harder when it the Pu249 (from initial neutron+U238 reaction) gets significantly mixed with Pu240 (from Pu239+neutron later), (2) that mixing happens naturally if you leave the fuel in the reactor for very long, and (3) it’s not very practical to undo it (because isotopic separation is hard and doesn’t get any easier when you’re dealing with highly radioactive materials like Pu240). Is my impression incorrect, or are you arguing that Pu from long-burned fuel rods is an unacceptable proliferation risk despite these difficulties, or what?

Also, do you happen to have a number for how much Pu would be present in one of these reactors compared to a conventional reactor with comparable power output? You make it sound as though the Pu risk would be quantitatively worse in one of these reactors (unacceptable because “the EM2 core will contain large quantities of weapons-usable plutonium”, implicitly not true of the alternatives you approve of, which I gather must only contain small quantities), but it’s unclear whether you mean merely one order of magnitude larger quantities, or multiple orders of magnitudes larger quantities.

The EM2 introduces additional safety and practical engineering challenges beyond the conventional GCFR. The EM2 fuel is an unproven concept and is expected to vent (release) its radioactive fission products while the reactor is operating, which essentially eliminates the fuel as a barrier to radioactivity release and defeats the concept of defense-in-depth to radioactivity release required by the U.S. Nuclear Regulatory Commission. The EM2 proponents also claim the reactor core can last 30 years without requiring refueling. Proving a new nuclear fuel can last this long without significant levels of failure is practically impossible, especially from a nuclear regulatory licensing perspective. Furthermore, this type of fuel cycle represents a significant risk for proliferation of nuclear fissile material, since the EM2 core will contain large quantities of weapons-usable plutonium long before the end of its claimed 30 year fuel cycle. In terms of safety and proliferation risks, the EM2 is an unacceptable nuclear reactor concept, especially for commercial deployment in a post-Fukushima world.

Until recently, General Atomics was the industry champion of the world’s safest reactor concept, a modular, helium-cooled thermal neutron spectrum reactor, sometimes referred to as a Modular Helium Reactor (MHR). In contrast to the EM2, this reactor concept has a large quantity of material in the core that absorbs heat and prevents the reactor fuel from reaching meltdown temperatures, even if all of the coolant is permanently lost. Unfortunately, the senior management at General Atomics abandoned the MHR in favor of EM2, and has stuck with this strategy even in the aftermath of the Fukushima accident. The proponents of the EM2 concept have falsely claimed the EM2 has the same inherent safety characteristics as the MHR.

Japan has the high temperature engineering test reactor (HTTR), which is an operational, engineering-scale prototype of the MHR. It has been used to demonstrate the intrinsic safety characteristics of the MHR. Perhaps the events in Japan can lay the foundation for developing, demonstrating, and commercializing a next generation of nuclear power with inherent safety. International collaboration among the U.S., Japan, and other nations on the MHR would provide a relatively quick path for achieving this goal. More information on the HTTR is available at jaea.go.jp.