The N2 type weapons were a non-nuclear ordinance developed in the first year of the twnety-first century. The very first N2 device to ever be employed was used to destroy Old Tokyo (what we know as Tokyo today). After its destruction, Old Tokyo was sealed up by the government (something they had planned to do with the NERV HQ facality as well in EoE - this screen capture shows the blast created when the JSSDF dropped one on Tokyo-3 during the movie). As previously mentioned, it's not a nulcear, so radiation isn't a problem (though residual heat is). It's possible that the N2 is modeled on an outgrowth of current weapons technologies (incendiaries, etc) or may even be some sort of fouth or fifth generation radiological weapon that doesn't leave behind residual radiation (as opposed to atom bombs and hydrogen bombs) but still has destructive power (as opposed to the neutron bomb, which kills organisms, but leaves structures intact).

For more information on N2, here's a post fromt he EVA ML, made by Avery Davies (edited for length - original):

The N2 or NN Mine is an explosive that was developed shortly after the Second Impact. The first mention of it's use is in the destruction of old Tokyo (or Tokyo-1).

While many of us here on the ML have discussed this before, there is some information I obtained that sheds light on the N2 acronym. In Neon Genesis Evangelion Iron Maiden Story Book by GAINAX and SimpleText, there is a keyword on the N2 Mine. N2 or NN stands for Non Nuclear (or No Nuclear as it's listed in the book).

Some facts about the N2 mine.

1) N2 Mines tend to come in the same shape (big cansiter) and but have different yields. Example, the one used in the 1st episode runs around 40 to 50 Kiloton range. (enough to dish out 3psi at 800m, just enough force to knock over a house or tip over a car). But, the one used in EoE, which cracks open the Geofront is a much larger yield (possibly going into the Megatons (I ended up downloading a declassified DOS program used by the DOE from http://www.enviroweb.org/issues/nuketesting/hew/ Let's just say I can't accurately model the N2 based off the effects shown in EoE.)

2) Many people believe it maybe some form of clean nuke or just a larger conventional explosive like a FAE

3) In the original script of the TV series, the bomb is referred to the P-Bomb (Photon-Bomb, perhaps? Particle-Bomb?)

Contents

Mentioned a couple of times in the series, Matsushiro is the area in which the new capital of Japan (Tokyo-2), the UN-2 headquarters, and a NERV facility (with MAGI SCS-2 and EVA testing faciilities) are all located. What's interesting about the area is what already exists there in present day. The following is an excerpt from a post to the EVA ML by Bochan_bird (original):

Of all the places in Japan, why did the Japanese government pick Matsushiro (Nagano Prefecture) for the site of the new Capitol?

Well, aside than the fact that the rise in sea level caused by 2nd Impact happened to submerge most major Japanese cities (which are located along the coasts), there is also the underground complex in Matsushiro which dates back to WWII.

This unfinished tunnel complex is hewn out of solid bedrock and consists of two portions: a 'functional' area consisting of a tunnel grid several hundreds of meters in each direction, and separate Imperial quarters complete with shrine and emergency escape tunnel.

The complex was constructed during the late WWII years as Japan came within range of bombing raids, and was intended as an emergency bomb shelter (it would be difficult to destroy even with a direct nuclear blast) capable of supporting government functions should Tokyo become uninhabitable.

Contents