This is an impressive feat of analytical chemistry. Note the extremely low radioactivity of plutonium contained in their samples, which peaked at just over 7 millibecquerels per gram:

Total 239Pu + 240Pu activities in BMC19 core reach their maximum of 7.14 mBq/g at 48.5 cm bsf and then sharply drop to ca. 2.09 mBq/g.

For comparison, a single banana contains approximately 2000 times more radioactivity just from its naturally occurring potassium content:

https://en.wikipedia.org/wiki/Banana_equivalent_dose#Source_...