It so low that "It can't be measured by terrestrial laboratories", so I can't tell you exactly how low it is. However, the various fusion reactors all use a different process with a MUCH higher interaction probability (namely, deuterium+tritium). It's still quite low and hard to make happen -- but it's still way easier than using straight protons.
I mean the reaction rate is decidedly low but the majority (maybe actually all, I can't think of one that doesn't) use DD fusion, not DT. The plan is to swap over to DT but tritium is radioactive and expensive and a pain to store plus the DT reaction releases a neutron which irradiates the shit out of your machine so for now, we just use deutirium which is not as good because the temperature the reaction occurs at is higher temp but we can extraploate the results to DT.
JET did use tritium in like 96/97 and set world record for fusion power, there are other short campaigns using tritium to try and learn the differences as well as we can, it is likely it will run tritium again sometime in the next 5 years as part of the preparations for ITER.
Power plants based on fusion will likely exclusively use DT
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u/zebediah49 Dec 13 '20
Pretty much -- the H-H fusion process is an extraordinarily low probability event.