It is highly improbable that any systems (even nuclear launch systems) could be still powered after a full year with out someone alive to maintain them. If all human operators would vanish from a nuclear power plant it is safe to assume that something bad would happen very soon.
Purportedly a version of this film begins with the submarine Nereid visiting Japan, allowing Yoshizumi a last look at his homeland. The trip does not fit in with the sequence this scene fits into, in which the sub is on a desperate race against time from Antarctica to Washington, DC to deactivate a doomsday device. It is improbable that the sub would detour to the other side of the world for nostalgic reasons. However, the extended (Japanese) version begins with a flash-forward to Dr. Yoshizumi walking down to Patagonia and his last trip in the Nereid takes the direct route from Palmer Station to Washington, DC.
The ARS (a device that launches many nuclear missiles in retaliation to a strike) is activated by one man pushing a few buttons. Devices that launch nuclear missiles are normally (and believably) portrayed as requiring launch codes, special keys and the authorization of at least two people.
The chamber where the ARS is located is reached by Yoshizumi and Carter simply blowing two doors open with plastique and climbing down an elevator shaft. A chamber containing a doomsday device probably contains more elaborate security features, such as blast doors.
Upon entering the bunker, Yoshizumi watches in horror on several monitors as nuclear rockets launch all over the world. The footage on many of these monitors feature hand-held camera work. Due to the virus sweeping the world many months earlier, there would be no one alive at that point to be operating these cameras.
The goof item below may give away important plot points.
When Yoshizumi visits the ruins of a church, an internal monologue depicts him "talking" to a corpse. The corpse "asks" Yoshizumi if his girlfriend's child is his. Yoshizumi responds "No, but I love him very much." The child in question is a girl.