8.6 – Why air cooling a LFTR would be a very bad idea

Another misconception is that LFTR’s can be air-cooled (here and here) rather than being dependant on the water cooling process we utilise in most other power stations. I’m assuming this rumour got going as a result of the fact that the MSRE was air-cooled. While this is true, you could air cool any power station (indeed many small diesel fired units are typically air-cooled), it’s just there are a host of good reasons not to!

Firstly, fire safety, air is an oxidising substance. Fires start all the time at power stations (fossil fuel fired and nuclear ones), especially in the turbine halls and the last thing we want in an emergency is a load of big cooling fans blasting in air and literally fanning the flames! In this scenario we’d face the dilemma between stopping the fans and cutting of the source of cooling (forcing us to SCRAM the reactor to prevent a LOCA scenario) or risk the fire spreading out of control, possibly to the point where it compromises the reactor’s safety. This was of course very similar to the dilemma faced during the Windscale fire, which was air cooled (although in this case directly, rather than indirectly as we currently discussing). And on the subject of Windscale, you will recall what I said earlier about fires and that Graphite core, so we’d be opening a very serious potential safety loophole.

Cooling fans also aren’t terribly reliable, which is why the MSRE was down for several months due to a cooling fan failure. Air based cooling is also very weather dependant, indeed I note that the fans at the MSRE seems to have failed in the late spring, when they would have likely been struggling to cope with higher daytime temperatures.

Thirdly, it’s the matter of thermal efficiency. Air based cooling is not very efficient, largely because air has such a low heat capacity compared to water (1.15 against 4.2 J/kg K). A typical COP (Co-efficient of Performance) for fans would be of the order of 2 – 3.7, while you can get 5 – 7.5 with water based cooling. Assuming a COP of 3 (it would be more like 2.5 at the temperatures in question, but bear with me!) and assuming a 1,000 MWth LFTR with a thermal efficiency of 50% (to keep my numbers easy!) = 500 MWe. Our cooling fans, in order to dispose of that 500 MW’s of excess heat, would be consuming 166.67 MW of electricity, dropping our effective plant efficiency down to 33%, barely Rankine cycle levels! This is why we use water in most power stations for cooling.

Also this air based cooling argument strikes me as a bit of a red herring, LFTR fans essentially inventing reasons why their “precious” is better than anything else. With the exception of a few geothermal power stations in arid areas (or hydroelectric plants!), I’m unaware of any major power project that was derailed for lack of cooling water. Either you can use cooling towers (forced draught or natural convection types) and minimise water losses to an acceptable level or simply move the plant next to a ready water source and transmit the power to where it is needed. Many desert countries operate large thermal power stations from around the coasts and several such as Iran, UAE and Libya are even planning to build nuclear stations too. So I fail to see how “air based” cooling offers any real benefits.

One option for MSR’s is so called “dry cooling” using condensers (see here). This relies on the high thermal mass of a liquid working fluid to take away heat as its passed through the condenser matrix (important note, it does not rely on the evaporative cooling effect as the two previously mentioned cooling methods do). However, obviously it requires water on site (which has various design implications for our thermal plant) although the water usage levels are low. Such arrays can be bulky (compared to forced draft or cooling fans, though smaller than Hyperbolic cooling towers) , less energy efficient (though better than direct air cooling, i.e. fans) and thus will consume some part (maybe as much as 5-8%) of the power stations electrical load. But again, whether you utlise such a system or not really depends on the circumstances where it is built.

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