8.7 – Why power cycling a LFTR would be an even worse idea!

Some LFTR supporters seem to be of the opinion that LFTR’s can be used for more than just baseload power, with them being used to meet the needs of variable electrical loads  (which is currently performed by fossil fuel plants and hydroelectric systems). I think this one got going over confusion regarding the MSR’s aforementioned negative thermal co-efficient, which some mistakenly assumed implies a variable load supporting capability. In this video here the speaker seems to imply this (but he specifically didn’t say it). It is worth noting the differences between the MSRE (a small test reactor) and a much physically large production reactor, with inevitably a slower rate of reaction.

Firstly, it is worth noting that existing nuclear stations are capable of some level of power cycling anyway, just not much! The truth is that the LFTR is as constrained in it power output capabilities as other reactors, possibly more constrained in fact. You will recall what I said in the materials section about thermal creep. You will also recall me stating the need to avoid thermal cycles (i.e regular increases and decreases in temperature). Power cycling a LFTR would necessitate such cycles, worsening our already narrow materials choice and requiring a much more heavily constructed reactor. Indeed the MSR suffered a number of problems likely related to (or made worse by) the excessive thermal cycling of its core (which went through nowhere near the sort of paces it would be if the LFTR fans proposal was adopted). Notably the aformentioned distortion of of graphite core elements under irradiation (discussed here and here) and the failure of its freeze valve due to “thermal fatigue” (pg 39 of this report).

A typical load daily load cycle graph for the United states in summer weathe

Furthermore there are a host of other good practical reasons why we would wish to avoid any power cycling. That CPP, which requires the input MS/fuel mixture at a series of given temperatures, won’t work terribly well (i.e less efficiently) if the temperature falls much below a certain threshold (i.e. we’ll be “pinching” more heat from the heat exchanger or having to install supplementary heating elements to bring up the temperature). Indeed the chemical plant might even be damaged by such an event (requiring a reactor trip on safety grounds).

Also a MSR with dissolved Thorium (or Uranium) present has the problem that as it cools back down towards its solidification temperature, the Thorium (or Uranium) below a certain threshold of temperature and pressure will begin to solidify with small flakes of solidfied fuel forming within the mixture. This can cause all sorts of problems, with fuel channels, valves and pumps being potentially clogged. Several meltdowns of liquid metal cooled reactors have resulted from such clogging incidents. The usual cop out for the MSR (dump the fuel & salt) won’t work if channels are blocked by solidified fuel. Also corrosion, abrasion and expansion related damage becomes a danger. Most pumps are designed to either handle a gas or liquid. They tend to be intolerant of a mixture of both ( as would be the case if we pushed the temperature too high, towards the vapourisation temperature). Passing a mixture containing partially solidified fuel through them would be the equivalent of taking a sandblaster to the pumps. While the above happening occasionally under timid operating conditions, as it inevitably will during reactor shutdown or start up scenarios, is nothing to worry about. However, using a MSR to perform short aggressively power cycling on a daily basis will very quickly invalidate your warranty and endanger safety. So no, you can’t use a MSR for anything other than baseload power, or industrial heat provision.

Sodium Secretions within the Dounreay PFR demonstrate the hazards of channels being clogged by solidified fuel

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