Has this been detected in Japan or anywhere else recently?
We have not heard about any detections, and we probably won't, for a few reasons:
(1) The fission yield of I-129 is much smaller than the yield for I-131.
(2) I-129's half-life is 15 million years, as you point out. The decay rate of an isotope is inversely proportional to its half-life. So the ratio of I-131's decay rate to I-129's decay rate is (8 days)/(15 million years) = 1.5E-9.
Let's assume for argument that the fission yields of I-129 and I-131 are equal, and that the plant has operated for 40 years at a constant rate. The ratio of I-129 to I-131 would be roughly ~(40 years)/(8 days)=1.3E3 due to the 8 day half-life of I-131.
So then the I-129 activity should be smaller than the I-131 activity by a factor of at least one million (1.3E3 * 1.5E-9 ~ 1E-6). We are measuring ~10 Bq/L of rainwater for I-131, so for I-129 we would expect ~1E-5 Bq/L, which is much lower than our minimum detectable level of ~0.1 Bq/L. The same would go for any measurement elsewhere in the world -- if I-131 is detected, I-129 would be several orders of magnitude weaker, due to its much longer half-life.
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