We report on finding no correlation between the two strongest observed Solar flares in September 2017 and the decay rates of 60 Co, 44 Ti and 137 Cs sources, which are continuously measured by two independent NaI(Tl) detector setups. We test for variations in the number of observed counts with respect to the number of expected counts over multiple periods with timescales varying from 1 to 109 h around the Solar flare. No excess or deficit exceeds the 2 σ global significance. We set a conservative lower limit on the decay rate deviation over an 84 h period around the two correlated Solar flares in September 2017 to 0.062% with 2 σ confidence. A fractional change of ∼0.1% in the decay rate of 54 Mn over a period of 84 h was claimed with 7 σ significance during multiple Solar flares in December 2006. We exclude such an effect at 4.7 σ significance.