The great advantages of FS-MSRs with regard to operational safety form a very important but complex topic, which remains to elucidated in detail. As a starting point, and in honor of Oak Ridge National Laboratory (ORNL), we here provide some concise excerpts on this subject from ORNL’s recent paper, “Fast Spectrum Molten Salt Reactor Options.” [1]

First, let’s have a look at some of ORNL’s introductory notes on the FS-MSR:

“The unique characteristic of MSRs is the use of a liquid fuel rather than the solid fuels used in more conventional designs. Halide salts have been demonstrated to provide a high degree of solubility of actinides in concentrations sufficient to maintain a critical system. The use of liquid fuel enables many design options and fuel cycle opportunities that are not possible with solid fuel. Liquid-fueled reactors eliminate fuel or target fabrication, which presents technical challenges when using actinide and/or TRU fuel and can result in the need for capital-intensive facilities. In the MSR, each batch of fuel that is fed into the reactor is blended into the existing fuel inventory; consequently, the addition of fuel has a limited impact on the isotopic composition of the fuel so that no need exists to control the variability of the isotopic concentration of the feed fuel. The fuel feed can be in solid or liquid form.

“An FS-MSR fuel cycle consists of a fast-spectrum molten salt core, a heat removal and power conversion system, and a salt processing and cleanup system. The reactor can be designed to have a range of heavy metal conversion ratios so that it serves waste management functions (with a low conversion ratio) or fuel cycle sustainability functions (with a high conversion ratio, unity or greater). For the waste management function, the system would be configured with a front-end processing system for used fuel, like the ones used in LWRs; whereas for the sustainability mission, after an initial charge of fissile material, the feed material could consist of natural or depleted uranium or thorium. For a waste management reactor, the front-end processing system could be located onsite or the used LWR fuel could be processed at a central facility supporting several reactors. For the on-site reprocessing option, once the used LWR fuel is brought into salt form and the excess uranium is removed, the processing steps are identical to those for the FS-MSR’s used fuel salt. Hence much of the infrastructure can do double duty, removing fission products from both used LWR fuel and FS-MSR fuel salt.

“The safety aspects of FS-MSRs are also innovative. FS-MSRs have a negative salt void coefficient (expanded fuel is pushed out of the core) and negative thermal reactivity feedback that avoids a set of major design constraints in solid-fuel fast reactors. A passive core drain system activated by a melt plug enables draining the radioactive inventory into geometrically subcritical drain tanks that are passively thermally coupled to the environment. FS-MSRs have a low operating pressure even at high temperatures. The fuel/coolant is transparent, allowing visual inspection, and methods of maintenance for the system have been conceptually developed based on the MSRE experience. The high-temperature operation of the reactor is compatible with process heat applications and can be coupled to high-efficiency power conversion systems for electricity production.”

And now, the promised excerpts regarding safety:

“FS-MSRs have the potential for incorporating excellent passive safety characteristics. They have a negative salt void coefficient (expanded fuel is pushed out of the core) and a negative thermal reactivity feedback that avoids a set of major design constraints in solid-fuel fast reactors. Thus, an FS-MSR can provide a high power density while maintaining passive safety. The liquid state of the core also enables a passive, thermally triggered (melt plug) core draining into geometrically subcritical tanks that are passively thermally coupled to the environment. FS-MSRs have a low operating pressure even at high temperatures, and FS-MSR salts are chemically inert, thermodynamically lacking the energetic reactions with environmental materials seen in other reactor types (e.g., hot zirconium or sodium with water).”

“FS-MSRs can provide a high degree of passive nuclear safety while enabling fissile resource extension, maintaining high power output, and achieving high power density. This set of characteristics compares favorably with all other proposed reactor classes. The high degree of negative thermal reactivity feedback due to the large fuel salt coefficient of thermal expansion combined with the negative void reactivity feedback is a unique reactor characteristic. Also, the ability to passively drain the core into geometrically subcritical decay tanks that provide for passive decay heat removal (likely via heat pipes to the surrounding soil) provides a highly robust severe-accident response that compares favorably with the capabilities of solid-fuel reactors.”

“FS-MSRs have the potential for excellent passive safety characteristics. FS-MSRs have a negative salt void coefficient (expanded fuel is pushed out of the core) and a negative thermal reactivity feedback that avoids a set of major design constraints in solid-fuel fast reactors. A passive core drain system activated by a melt plug enables draining the radioactive inventory into geometrically subcritical drain tanks that are passively thermally coupled to the environment. FS-MSRs have a low operating pressure even at high temperatures; and FS-MSR salts are chemically inert, thermodynamically lacking the energetic reactions with environmental materials seen in other reactor types (hot zirconium and sodium with water). FS-MSRs do involve more intensive manipulation of highly radioactive materials than other reactor classes and thus small spills and contamination accidents appear to be more likely with this reactor class.”

REFERENCES

[1] D.E. Holcomb et al, “Fast Spectrum Molten Salt Reactor Options,” OAK RIDGE NATIONAL LABORATORY, July 2011, http://info.ornl.gov/sites/publications/files/Pub29596.pdf

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