Вот здесь собрано почти все что известно об этих мифических чемоданчиках:
http://cns.miis.edu/pubs/week/020923.htm
and indeed:
...Short life span between scheduled maintenance. According to the chief of the 12th GUMO, Igor Valynkin, small munitions required replacement of components every several months (other sources mentioned six months).[20] Valynkin's statement is the most direct corroboration of the allegations about the existence of portable nuclear devices.
...
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.