Here's what would happen if Washington, DC got nuked...

Gizomodo points us to a DHS and FEMA analysis (PDF) of the after-effects of a nuclear detonation in the heart of the District of Columbia. Everything from blast effects, fallout, evacuation procedures, casualties and first-responder preparations are described in great detail.

Unfortunately, our instincts can be our own worst enemy. The bright flash of detonation would be seen instantaneously throughout the region and may cause people to approach windows to see what is happening just as a blast wave breaks the window. For a 10-kT detonation, glass can be broken with enough force to cause injury out to 3 miles and can take more than 10 seconds to reach this range.

Another urge to overcome is the desire to flee the area (or worse, run into fallout areas to reunite with family members), which can place people outdoors in the first few minutes and hours when fallout exposures are the greatest. Those outside or in vehicles will have little protection from the penetrating radiation coming off fallout particles as they accumulate on roofs and the ground.

...Unless a given shelter location is considered unsafe due to fire or structural damage, the length of time individuals should remain sheltered depends on instructions from regional emergency management agencies. For those in good shelters, such as a large concrete, brick, or underground structure, optimal shelter times will likely be in terms of days. In the absence of specific guidance from authorities and adequate supplies of food and water, or for those who are in smaller 2- to 3-story structures or shallow basements, evacuation should be considered after 12 hours. Upon leaving shelter, the best course is to follow routes that take advantage of sheltered passages (subways, underground connectors, or through building lobbies) that lead away from damage and heavy fallout areas. Once clear of potential fallout areas, evacuees should seek a change of clothes (including shoes) and wipe or wash exposed skin surfaces...

...The Severe Damage Zone (SDZ) is the area that immediately surrounds a detonation site and extends to ~0.5 mile radius for a 10-kT explosion, as shown in Figure 10. In the SDZ, few, if any, above-ground buildings are expected to remain structurally sound or even standing, and few people would survive; however, some people protected within stable structures (e.g., subterranean parking garages or subway tunnels) at the time of the explosion could survive the initial blast...

...Very high radiation levels and other hazards are expected to persist in the SDZ making the zone gravely dangerous to survivors and responders. The SDZ should be considered a no-go zone during the early days following an explosion...

...Emergency response and access to the MDZ will be greatly affected by the substantial rubble as well as crashed or overturned vehicles that will completely block streets and require heavy equipment to clear. Broken water and utility lines are expected, and fires will be encountered. However, many casualties in the MDZ will survive and will benefit most (compared to casualties in other prompt effect damage zones) from urgent medical care (AMA, 2011). Responders approaching from the blast-area periphery should be cognizant that when they begin observing that most buildings are either severely damaged or have collapsed, they are entering the SDZ...


...In addition to prompt effects that radiate outward from a detonation site, a nuclear blast can produce nuclear fallout, which is generated when dust and debris excavated by the explosion are combined with radioactive fission products produced in the nuclear explosion and drawn upward by the heat produced...

...The cloud rapidly climbs through the atmosphere, potentially up to 5 miles (8 km) high for a 10-kT explosion, forming a mushroom cloud (under ideal weather conditions) from which highly radioactive particles drop back down to earth as the cloud cools...

...Hiroshima and Nagasaki did not experience substantive fallout because the detonations occurred well above ground at altitudes of 1,900 ft (579.12 m) and 1,500 ft (457.2 m), respectively. At such altitudes, fission products do not have the opportunity to mix with excavated earth...

Net net: this is why the nation's capital is reportedly ringed by extremely sensitive detectors for radioactivity.

And -- sorry, Paulbots -- this is also why we need to continue to invest in missile defense.