8.9 – The Brayton cycle and MSR reactors

Another misconception is that an MSR can operate on an open cycle with a Brayton cycle turbine. While true, it could be run this way, there are a host of practical reasons not to do it. Not least of them the fact that our turbine would have to be designed to withstand having a mixture of molten salt and fluorided fuel passed through it at very high temperatures. This would be tricky to say the least, likely requiring the use of those super expensive refractory metals, and while using such materials to make the odd turbine blade is one thing, an entire turbine casing is an entirely different matter. It would likely cost much more than the reactor itself!

Furthermore it won’t be terribly efficient, you will recall what was said earlier about our MSR having a very narrow thermal window. In most other situations (involving Gas turbines) we would get around this problem by using large pressure drops or a multi-stage turbine cycle to improve efficiency, but neither of these are options if running a MSR on an open cycle, at least so long as we want to run the reactor at a low vapour pressure. To be blunt if you managed to beat Otto cycle efficiencies (20-30%) with a MSR running on an open cycle (with a Brayton turbine), you’d be doing well! Also, as this would involve circulating radioactive material outside of the pressure vessel, the entire turbine set would need to be built under our concrete containment dome, further increasing costs.

It would make far more sense, from an energy efficiency, safety and economics point of view to simply split the cycle in two, as shown in my schematic previously (nicked off wikipedia), one half operating within the pressure vessel, passing through a heat exchanger (where we lose some energy) where heat is then transferred to the secondary loop and into the turbogenerator plant (or hydrogen plant), which runs on either steam or an inert gas.

Brayton cycle against Rankine cycle

I might also take the opportunity to correct another misconception I’ve seen several MSR bloggers make about the Brayton cycle. A number seem to be convinced that the key to the Brayton cycle is that its efficiency is dependant on Tmax = Constant, where as the Rankine is dependant on ΔT. That’s not entirely correct; a substantial temperature drop is still required in a Brayton cycle to achieve good efficiency. Think about it, if we keep ΔT at zero (i.e the fluid does not change temperature) then obviously the energy output from the process drops to zero and thus our efficiency is therefore zero! The “trick” with the recuperative multi-staged Brayton cycle (and we can’t do this with an open cycle) is to utilise large changes to ΔP (the pressure) and the gas laws  to produce large values of “Δs” the change in entropy. In essence what we’re trying to do is arrange such that the critical ΔT is the temperature difference between the exit temperature from our heat exchanger (to the turbine) and the temperature of the cooling water inlet (T3 and T1 respectively in the page here). With a Rankine cycle we are constrained to whatever temperature drop can be produced over the turbines, which is generally smaller than that achievable with a Brayton cycle.

So while its not as critical to get a nice large ΔT with a Brayton cycle as a Rankine, you still need to get some sort of a temperature drop. This is especially true for any MSR for as I’ve pointed out, its efficiency is ultimately going to be dependant on the temperature drop we can produce over the heat exchanger.

Again our narrow thermal window and low vapour pressure conspires against the MSR here. Given our limited ability to modify ΔT we must instead increase the mass flowrate through the heat exchanger to ensure good heat transfer. What this means in simple terms is having a pair of big high capacity pumps either side of it running at full speed. Unfortunately that low vapour pressure, means the volume of fluid we need to process from the reactor is quite high. And given what we’re pumping (molten salt at temperatures of 700 °C), this quickly leads us into needing very large and powerful pumps made out of exotic materials – which are of course expensive and energy hungry. That energy being charged against the power output of the reactor. Again, some sort a balance point will emerge between increasing pump power consumption (and capital costs) and optimum reactor efficiency. This also leads us to rule out MHD as proposed by some LFTR activists, either for pumping purposes or otherwise, as they are simply too energy hungry and expensive. Plus the radiation they’d be exposed to might well give them a short service life.

A set of large pumps also rules out the possibly of us using natural convection to maintain fluid flow (another misconception). Much as I am a fan of natural convection (did my PhD in it!) it just would not provide the necessary heat flows. This of course opens up a possible safety window in that our reactor will no longer be able to cool itself down via natural convection and thus becomes vulnerable to a LOCA scenario in certain situations (fairly rare situations mind, and even then were likely just talking about localised core damage rather than anything more serious).

All in all my suspicion is that our heat exchanger would struggle to produce thermal efficiencies any greater than 70-75%. Assume a good high efficiency Gas driven Brayton cycle the other end (55-60%) so that yields us an overall efficiency of 38% – 45%, oh! but we almost forgot about that chemical processing plant and its net energy inputs, say we deduct 5-10% of reactor power output to account for running that and the cooling plant, so overall between 29% – 40%, with a 35% overall efficiency being my best WEG. This is about comparable to current day power stations, but much less than what can be achieved with other nuclear reactor designs. While if we can get that CPP to work reasonably effectively, we could achieve a high fuel efficiency and lower levels of nuclear waste, they would not be not as much as the supporters say. And the danger is that if we were forced to ditch the CPP as uneconomic or not technically feasible, then we could end up with a MSR that burns more fuel and produces more waste, and yet still is less efficient that its principal competitors (namely those gas cooled reactors).