"This has been the method of choice at the Department of Energy's Savannah River Site (SRS) production reactors. However, there are several shortcomings with this method of production. First, the production efficiency is quite limited, i.e., to approximately 13% efficiency. This is seen from the fact that the Pu-238 produced in the target after only 2.12 day's half-life decay, itself becomes a target for production of higher isotopes of plutonium, thus reducing the Pu-238 purity by producing Pu-239 and Pu-240. Second, this process produce a hazardous Pu-236 by-product. As noted on the decay chain, above, there is a η→2η or γ→η reaction that results in the production of Uranium-236 (U-236) and Pu-236. These reactions increase with exposure to fast neutron flux. Pu-236 decays to U-232, which has a hazardous gamma-ray energy emitting daughter product. Even a few parts per million U-232 increase the radiation exposure hazard to personnel dramatically. Lastly, Np-237 must be chemically purified before target fabrication. This is seen from the fact that Np-237 decays to Protactinium-233 (Pa-233), which in turn has a strong gamma-ray emission with its beta decay to U-233 (half-life 27 days). Therefore, the Np-237 was stored in solution at SRS and chemically processed immediately before fabricating targets. Solution storage of Np-237 may not be practical at an alternate production site."
Source: http://www.google.com/patents/US6896716
The extent to which this should worry our interstellar concerns is huge. The voyager 1 went several billion kilometers away from the Sun. You know it's crazy, it's so far away that it takes 15-20 hours for the signal to reach us.
The voyager 1 is probably going to go out in like 2020, but that's because of the Plutonium- 238.
There's 2 sites in the USA that can produce it.
Hanford in Washington State and Savannah River Site in South Carolina.
The Russians have Mayak, and thank goodness they kept making the stuff.
Hopefully we can pump some steroidal funding into High Flux Isotope Reactors.
I've been watching this issue since 2006. In 2009, someone finally spoke up about it and published some papers on it.
http://www.nap.edu/openbook.php?record_id=12653
Any NRE's or Particle Accelerator Designers on HN?