Molten Salt Reactors

The expansion of the world's population to 10 billion people all striving for a better quality of life is expected to double the world's energy consumption by the year 2050. The increasing demand of energy cannot be addressed by today's energy production mix without devastating impact on the environment. Therefore, a renaissance of nuclear energy, which is the most prominent among the non-greenhouse emitting energy sources, seems at hand.

Today, 440 nuclear power plants produce about 17% of the world's electricity; a share that is steadily increasing despite temporary stagnation in the US and Europe. Within the next two decades, new nuclear power plants, like the Advanced Light Water Reactor and the High Temperature Reactor, will have to be built as a replacement of the current plants and to cover the increased energy needs.

These reactors will continue to produce plutonium and minor actinides. This means a strong need for reactors that can 'burn' these nuclides, while on the other hand the requirement of sustainability calls for high-conversion reactors or reactors that even breed new fissile material. The Molten Salt Reactor (MSR) can meet both requirements, and seems therefore the most promising and challenging concept for the future. Apart from the advantages of a flexible fuel cycle, the coolant temperature (850 °C) is much higher than of today's reactors leading to a high thermal efficiency and the application of the MSR for hydrogen production via thermo-chemical processes.

A MSR produces fission power in a circulating molten salt fuel mixture at ambient pressure. Only when the salt enters the graphite core, the neutrons released in a fission event can moderate to thermal energies and initiate new fission. Because the fluoride salt is used to remove the heat from the core and to circulate the fuel, a fraction of the salt stream can be diverged to extract the fission products and to add fresh fuel. This means that no refuel outages are needed, which reduces the operation costs considerably.

Due to the online removal of fission products fewer neutrons are lost by parasitic neutron capture. This enables the MSR to operate either as a burner reactor using the excess neutrons to transmute non-fissile actinides to fissile ones or as a breeder reactor converting abundantly available thorium to fissile U-233. The use of the Th-U fuel cycle is of particular interest to the MSR, because this reactor is the only one in which the Pa-233 can be stored in a hold-up tank to let it decay to U-233. To eliminate the proliferation risk of the highly fissile U-233, several innovative options could be considered.

Nuclear reactors can be controlled by the grace of delayed neutrons emitted by some fission products (precursor atoms) up to some tens of seconds after the fission event. This means the effective lifetime of neutrons is a thousand times larger than that of prompt neutrons. Due to the circulating fuel in a MSR, a fraction of these delayed neutrons is produced outside the core, which effectively reduces the margin to prompt criticality (the point at which a rapid power transient might occur). This poses no problem as long as the power feedback coefficient is sufficiently negative. In a properly designed MSR, this can easily be assured by the prompt effect of fuel expansion when the temperature rises. Knowledge about the density and the temperature distribution of the molten salt fuel are of utmost importance to predict accurately the dynamics behavior of the reactor during normal operation and during transients.

The coupled calculation of all physics aspects of a MSR is a challenging task never undertaken before. Applying present-day computing power, one should be able to calculate simultaneously the spatial distribution of the neutron flux, the power, the salt density, the precursor atom density, and the temperature during steady state reactor operation and during transients.

In conclusion: all nuclear reactors in operation today use solid fuel. The new nuclear reactor proposed here uses molten salt fuel with the potential of safe, clean and sustainable energy production. The application of molten salt fuel ensures:

• Safety (prompt negative power feedback / no fuel meltdown accidents).
• Efficiency (very efficient heat removal at high coolant outlet temperature).
• Flexibility (burning actinides or breeding new fuel / electricity or hydrogen production).
• Economy (online refueling without outages / high fuel burnup).

More information can be found in the PhD project description.

Results of the research can be found in the PhD thesis.

For more information, please contact  j.l.kloosterman (at) tudelft.nl