8.8 – Thermal windows and material choices

The  problems above demonstrates that the MSR has a relatively narrow thermal window. Its filtering plant will not work if the temperature of the fluid drops much below a certain threshold and the danger of fuel solidification raises the risk of the reactor being damaged. With UF4 the solidification temperature is 1,036 °C and its vapourisation temperature is 1,417 °C at atmospheric pressure. We can move this temperature “window” by altering the working pressure (the original MSRE operated at a work pressure of around 0.3-0.4 bar and as I recall operated within a thermal window of 700-560 °C) but this comes at the disadvantage of increased working stress (the pressure outside is trying to squeeze the reactor walls inwards, although the effect is generally minor) and lower thermal efficiency (more on that later) or by increasing the salt concentrations (lowers power density, may increase corrosive effects, thought that depends on a number of other factors). With TF4 our “window” is 1110 – 1,680 °C, but again we can potentially move this by lowering the pressure (or raising it if we want to go the other way…not that we do!). A low vapour pressure also creates a few potential problems in terms of keeping the reactor sealed (air is more likely to leak in if the pressure inside is less than atmospheric…possibly starting a fire!) and maintaining a good flow rate from our pumps.

Either way we are likely to be operating a MSR well outside of the operating temperatures of any stainless steel alloy and likely forced to rely on using Nickel Alloys such as Hastalloy instead. This will of course be expensive. For anyone thinking, given the temperatures posted above (a recap the operating temperature window for Hastalloy is 1050 – 1200 °C), that I’m sailing a little close to the wind here (as in have a forgotten about thermal creep). I would remind you that assuming we are not actually power cycle these reactors (that would be silly!) and that by maintaining a low pressure cycle (and again lowering pressure below atmospheric lowers the working temperatures, again it was 700-560 °C for the MSR) that should drop us to within the window of probability that we can get by with.

That said the issue of Inter-granular cracking would need to be investigated, as it could prove to be a show stopper for Hastalloy usage. Also the MSRE suffered, as noted, a very worrying thermal creep failure of part of its drain valve (mentioned on pg 39 of this report), probably due to excessive thermal cycling (again this is why power cycling the reactor would be a bad idea!). There were also problems with Grain-boundary embrittlement, and a decrease in creep ductility. Again I would include the caveat that these issues would also need to be addressed in detail before one could proceed with Hastalloy use. Many of the attempts at Fast Breeder Reactor development in the early days were held up by a failure to get to grips with material science issues from day one. Consequently if the MSR researchers neglect to answer these questions above they could find history repeating with a series of costly and time consuming delays.

With the LFTR however, weighting up all the evidence, I doubt you could operate one made out of Hastalloy, contrary to everything said on the internet. Bare in mind I’m thinking in terms of a good lengthy service life with a sensible factor of safety, not a flimsy test reactor in a lab (with a 100 mile exclusion zone!). While a LFTR/MSR might be less vulnerable to thermal creep than other reactors (thought that depends on a host of specific design issues), it certainly isn’t immune. Also, as a minimum standard of safety it would need to be easily able to withstand the vaporisation temperature of TF4 at atmospheric pressure. Failing to do so would undermine our entire concept of passive safety, and like the LWR we’d have to start adding this and that to the reactor and soon it will be the size of an aircraft carrier! Our LFUR (might!) just about limbo under the bar on this point (if we can address those material science and corrosion issues detailed earlier), but I seriously doubt the LFTR could do it. At the very least a good deal of material science would need to be done to prove the issue either way.

Thus the pressure vessel of any LFTR would likely have to be formed out of either an alloy of Tantalum or Niobium, or possibly a Nickel alloy with a coating of either of the latter two. Both of these options would “complicate” the design, though not nearly as much as the other alternative, using Ceramics! (very expensive and difficult to form, especially given how critical getting an air tight seal is given the graphite core). For both the LFUR and LFTR key internal components will in any event likely have to be made out of Refractory metals, particularly certain high temperature parts of any CPP, given talk of operating temperatures in the range of 1600 °C (again I direct the reader to  chapter 3 on materials where this is discussed in more detail). The bulk of the rest of the power plant however would run at much lower temperatures (and neutron fluxes) and thus could indeed be made out of Nickel alloy (such as Hastalloy), but again only if this inter-granular cracking issue gets tackled successfully. As mentioned in the section 8.5 as regards the graphite core, a double blanket core (see here) would have even tighter material requirements due to the increased level of neutron bombardment and the thermal gradients imposed on the core by such a split pressure vessel arrangement.

So my instinct from a materials science point of view would be to drop the LFTR idea altogether and focus instead on a LFUR. While this isn’t able to use the Thorium cycle, the point was raised earlier about how the Thorium cycle might not be all it’s cracked up to be. Its going to be a lot easier to build a LFUR than a LFTR, cheaper (relatively speaking) and likely safer too. Of course it does come at the disadvantage of a slightly awkward acronym! but overall that would be my focus of attention, at least initially.

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