I'm not a YEC, but I can't figure this out:
Q1. Wouldn't an age of billions of years cause the earth's magma to run out of radioactive isotopes?
C1. One would think so. (Otherwise, a mechanism to replenish the radioactive isotopes would have to be found.) However, newly-formed rocks continue to contain radioactive isotopes.
Q2. What would happen if pure radioisotope samples were generated?
C2. There would be a gradually-increasing rate of decay. This would self-limit as the decay rate would make the sample less pure. The rate of accumulation of the isotope and the rate of its decay are factors that offset one another. If the rate of accumulation is nearly instantaneous, and the mass were large enough, there would be detonation. Otherwise, the sample would merely show an artificially-old age. Of course, as the purity goes down, assuming constant mass of the isotope, the rate of decay would decrease. However, if the sample were exposed to a second large radioactive sample of high purity, the original sample would show an artificially-old age.