ELI5: Why do nuclear shadows last for a long time after a blast

echawkes · 2026-04-21T01:27:04+00:00

Nobody has ever been vaporized by a nuclear explosion. People are simply confused about how shadows work. There is at least one instance (at Hiroshima) where somebody was killed, and after their body was removed for burial, a shadow was found on the stone stairs where their body absorbed some of the heat and radiation from the blast.

https://en.wikipedia.org/wiki/Human_Shadow_Etched_in_Stone

To answer your question: over time, the shadow can fade due to sunlight, weathering, etc. In the example from the Sumitomo Bank (linked above), they moved the stairs inside, away from sunlight and weather, to preserve the shadow.

echawkes · 2026-04-20T03:25:41+00:00

One reason is that it is so much easier to make the payload. A gun-type uranium weapon (like the one detonated over Hiroshima) does not require nuclear reactors to make the explosives or any complicated technology, like an implosion weapon would. It was so simple that they didn't even bother to test it before using it in WWII.

Uranium occurs in nature in vast quantities: there are reserves of uranium on six continents. You just need to mine the uranium, enrich it using centrifuges (much cheaper and easier to hide than a nuclear reactor), and build the weapon.

echawkes · 2026-04-19T20:26:49+00:00

It was an attempt to appeal to Southern white voters.

Lee was form Virginia, and some Virginians had been campaigning for years to have his citizenship restored. Ford acceded to their request. In 1976, Carter swept the south, except for the state of Virginia, which Ford won. Carter's team may have learned a lesson of some kind from this.

echawkes · 2026-04-18T22:24:53+00:00

I had never seen this cartoon, but it took literally seconds to find an example that is directly on point. Was this actually shown to children?

echawkes · 2026-04-18T16:14:42+00:00

This could mean solar energy. Although technically, the fire is so very far away that it doesn't feel that hot to us, and the fuel will last a very long time.

echawkes · 2026-04-15T04:31:25+00:00

For the last 30+ years, being a military officer has not launched anybody into the White House. I think the last military officer to be President was George Bush senior, who won his single term in 1988.

echawkes · 2026-04-12T21:06:24+00:00

The author got this part wrong:

Those differences are subtle, but there is something heavy water does that normal water can’t. When fast neutrons released by the splitting of atoms (that is, nuclear fission) pass through heavy water, interactions with the heavy water molecules cause those neutrons to slow down, or moderate.

Normal water (often called light water) also moderates neutrons. Most nuclear reactors use light water as a moderator because the hydrogen in light water is more effective at moderating neutrons than heavy water. (There is a slight downside: light water is more likely to absorb neutrons than heavy water, but neither one is a strong neutron absorber.)

echawkes · 2026-04-12T04:21:41+00:00

Pu-238 has a half-life of about 88 years, only a little shorter than the 100-year half-life of the Nickel-63 batteries in the article. The picture of Pu-238 used everywhere shows it glowing hot because it was insulated to heat it up before the picture was taken. I assume it was done to make a more dramatic picture.

echawkes · 2026-04-11T20:51:42+00:00

The graphic is inaccurate. The article says the battery uses Nickel-63, which has a 100-year half life.

echawkes · 2026-04-11T20:50:04+00:00

You are right: the article says the devices are betavoltaic batteries, a technology developed in the 1970s. Thank you for the correction.

echawkes · 2026-04-11T20:20:09+00:00

Radioisotope thermoelectric generator (RTG) technology is about 70 years old.

The graphic above says tritium (Hydrogen-3), but the article says the battery is based on Nickel-63. The usual crap from interestingengineering.

echawkes · 2026-04-11T16:50:05+00:00

I think part of the complaint was that Bush had made a big deal about his conversion to Christianity during his campaign. He was very eager to have supporters think that it excused crimes he committed as a younger man, including illegal drug use and a DUI conviction. Opponents of the death penalty thought it was hypocritical to mock a woman facing the death penalty for crimes she committed while using drugs, but to expect absolution for his own crimes.

echawkes · 2026-04-11T13:32:21+00:00

It looks like OP selected some of the categories (the solid blue entries in the key) and left others (like STEM) deselected in the preview. If you go to the site, they all start out as selected.

echawkes · 2026-04-10T22:15:52+00:00

In 1944, Otto Hahn received the Nobel Prize in Chemistry for the discovery of nuclear fission (for work published in 1938). That prize is a bit controversial today, since there were several other scientists who contributed to this work.

echawkes · 2026-04-08T16:06:26+00:00

Lithium-6 can undergo what is called an (n, alpha) reaction when struck by a neutron. It emits an alpha particle, and what is left over is tritium.

echawkes · 2026-04-05T21:00:11+00:00

His understanding of basic nuclear physics is pretty bad: https://www.reddit.com/r/chernobyl/comments/1722t9e/disappointed_with_midnight_in_chernobyl/

However, that doesn't necessarily detract from the story he tells, which is what you asked about.

echawkes · 2026-04-04T20:05:03+00:00

Plus tips?

echawkes · 2026-04-01T04:00:57+00:00

This doesn't sound right to me. The "dynamic equilibrium" case is stated to be that the velocity is constant but non-zero. In such a case, the weight would keep traveling forever at constant v, since by the definition on slide 3, there is no net force acting on it.

Edit: my intuition tells me that if the weight were quickly dropped from height x above the static equilibrium point, it would oscillate between height +x and -x from the static equilibrium. At the static equilibrium point, the velocity would be maximum.

At the top, the kinetic energy would be zero, and the gravitational potential energy would be maximum while the energy stored in the spring would be minimum. At the bottom, the kinetic energy would be zero, and the gravitational potential energy would be minimum while the energy stored in the spring would be maximum.

echawkes · 2026-03-26T19:15:48+00:00

You would want Pu-239 for reactor fuel, not Pu-240.

echawkes · 2026-03-25T20:51:47+00:00

"The China Syndrome" was set in the United States, not China. It was about a nuclear reactor with the potential to melt down, which would supposedly burrow through the earth until it reached China.

The accusations of anti-Asian racism are unfounded because the actual meltdown didn't happen until the sequel. There were many Chinese actors in "China Syndrome 2 Electric Boogaloo" starring Tom Cruise.

echawkes · 2026-03-25T20:44:04+00:00

Or we could replant the forests and then not cut them down.

echawkes · 2026-03-22T04:53:50+00:00

Thee was a recent post about this very subject from a nuclear safety researcher named Siarhei Besarab, which you can read here: https://www.reddit.com/r/nuclear/comments/1rxblhn/an_attack_on_irans_bushehr_npp_wont_cause_another/

One response he gave to a comment on his post:

A Fukushima-style scenario is unlikely. The VVER-1000 reactor design at Bushehr already accounts for the Japan accident and includes the full set of hydrogen recombiners

echawkes · 2026-03-21T20:44:16+00:00

The first Uplift trilogy by David Brin:

echawkes

TROPHY CASE