Liquid-fueled Molten Salt Reactors (MSRs) do not contain their fuel in assemblies. It is then not possible to perform traditional item counting and visual accountability of the salt fuel. These facilities are closer to bulk accounting facilities, such as reprocessing plants, and require inventory determinations based on measurements of the actinide content of salts. This can be problematic due to the difficulty of sampling and the destructive analysis of actinide-containing molten salts. Some problems arise from the unique combination of high temperature and high radiation environments present in molten salt fuels. Another challenge is the continuous change in the isotopic concentration of fuel salts due to burn-up, conversion, plating out, and online chemical processing. There is a potential for fuel stocks outside the reactor containment vessel in on-site salt processing. In terms of proliferation resistance of 233U-232Th fuel cycle, the nuclide 232U is an important nuclide in thorium fuel cycle from the standpoint of proliferation resistance, because its daughter Thallium (208Tl) is a strong gamma (2.6 MeV) emitter. The hard gamma ray is not only barrier from to nuclear material theft, but also an effective means of detecting lost fissile material. However, there is a theoretical weakness in obtaining pure 233U at the core of the initial two weeks with a concentration of 232Pu less than 1,000 ppm. Therefore, Pu separation process is one of the most sensitive parts in online reprocessing facility. The decision to use a fertile blanket should also be based on proliferation risk considerations in addition to operational parameters. MSRs can be designed without a separate fertile blanket, which should be considered. In the case of the MSFR, even if fertile blankets are used, the production of 232U is large enough to make difficult the utilization of blankets for proliferation purpose. For the liquid-fueled MSRs without fissile materials separations, many of the observations from the previous section apply, except salt processing is minimized. The reactors will still need some method of estimating total actinide content. These reactor designs reduce proliferation risk for the reactor by not separating any actinides during operation.

Long-lived Small Modular Reactors are being promoted as an innovative way of catering to emerging markets and isolated regions. They can be operated continuously for decades without requiring additional fuel. A novel configuration of long-lived reactor core employs a mixed neutron spectrum, providing an improvement in nonproliferation metrics and in safety characteristics. Starting with a base sodium reactor design, moderating material is inserted in outer core assemblies to modify the fast spectrum. The assemblies are shuffled once during core lifetime to ensure that every fuel rod is exposed to the thermalized spectrum. The Mixed Spectrum Reactor is able to maintain a core lifetime over two decades while ensuring the plutonium it breeds is below the weapon-grade limit at the fuel discharge. The main drawbacks of the design are higher front-end fuel cycle costs and a 58% increase in core volume, although it is alleviated to some extent by a 48% higher power output.