  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 hours around the Solar flare. No excess or deficit
exceeds the 2$\sigma$ global significance. We set a conservative lower limit on
the decay rate deviation over an 84-hour period around the two correlated Solar
flares in September 2017 to $0.044\%$ with 2$\sigma$ confidence. A fractional
change of $~0.1\%$ in the decay rate of $^{54}$Mn over a period of 84 hours was
claimed with 7$\sigma$ significance during multiple Solar flares in December
2006. We exclude such an effect at 4.7$\sigma$ significance.
