A short maintenance schedule is an intriguing feature of portable nuclear devices, which is particularly important from the counterterrorism perspective: If that information is correct, such devices would be useless or have limited utility after only a few years, begging the question of whether terrorists would envision the same purpose for such devices as the Soviet Union. The period between routine maintenance--only six months--might seem very short, but short maintenance periods appear to be a typical feature of all Soviet warheads.
Without detailed knowledge of the design of Soviet warheads, it would be impossible to know which components needed replacement at what time intervals. Two potential candidates are tritium and the neutron generator, which may use radioactive materials that decay over time. It seems possible, for example, that Soviet designers balanced on the threshold, using only just enough plutonium to achieve critical mass and relied on tritium to generate required yield. In that case, even modest degradation of tritium could have resulted in a significant drop of yield. Thus, it would be safe to assume that without proper maintenance, portable nuclear devices might still produce chain reaction, but yield would be minimal, and with time, possibly non-existent.
There is techonology to detect the neighborhood-level proximity of these bombs, even if they are lead shielded. I think it would be nice, prudent, if we made a few thousands of that technology and established a nationwide network.
How difficult would it be for al-Qaeda to get tritium into the country? Its radiation is only beta, so they could shield it from detection, right?