Nanofluids for enhanced economics and safety of nuclear reactors: An evaluation of the potential features, issues, and research gaps

Nanofluids are engineered colloidal suspensions of nanoparticles in water and exhibit a very significant enhancement (up to 200%) of the boiling critical heat flux (CHF) at modest nanoparticle concentrations (<= 0.1% by volume). Since CHF is the upper limit of nucleate boiling, such enhancement offers the potential for major performance improvement in many practical applications that use nucleate boiling as their prevalent heat transfer mode. The Massachusetts Institute of Technology is exploring the nuclear applications of nanofluids, specifically the following three: 1. main reactor coolant for pressurized water reactors (PWRs) 2. coolant for the emergency core cooling system (ECCS) of both PWRs and boiling water reactors 3. coolant for in-vessel retention of the molten core during severe accidents in high-power-density light water reactors. The main features and potential issues of these applications are discussed. The first application could enable significant power uprates in current and future PWRs, thus enhancing their economic performance. Specifically, the use of nanofluids with at least 32% higher CHF could enable a 20% power density uprate in current plants without changing the fuel assembly design and without reducing the margin to CHF The nanoparticles would not alter the neutronic performance of the system significantly. A RELAP5 analysis of the large-break loss-of-coolant accident in PWRs has shown that the use of a nanofluid in the ECCS accumulators and safety injection can increase the peak-cladding-temperature margins (in the nominal-power core) or maintain them in uprated cores if the nanofluid has a higher post-CHF heat transfer rate. The third application can increase the margin to vessel breach by 40% during severe accidents in high-power density systems such as Westing house AP1000 and the Korean APR1400. In summary, the use of nanofluids in nuclear systems seems promising; however, several significant gaps are evident, including, most notably, demonstration of the nanofluid thermal-hydraulic performance at prototypical reactor conditions and the compatibility of the nanofluid chemistry with the reactor materials. These gaps must be closed before any of the aforementioned applications can be implemented in a nuclear power plant.