If I ingest a grain of sand size piece of the Chernobyl Reactor No.4 core

The short answer is, we created it. The half-life of Uranium 235 is 700 million years, so there's plenty of it around. When you bring a bunch of U235 together and cause it to undergo fission, it creates a bunch of the shorter-lived stuff as a decay product. Especially when a reactor melts down; you get a lot more of the stuff you didn't want, in a much more uncontrolled manner.

I've watched this interesting (but long) talk by Brian Sheron, retired director of Nuclear Regulatory Research:

https://www.youtube.com/watch?v=ryI4TTaA7qM

It gives me a better understanding of the risk of nuclear power, particularly for current US reactors, and what our regulatory agencies actually focus on: really it's on preventing direct radiation induced deaths, and not so much on property damage. So: It's not so much that accidents directly kill people, instead they kill the land. The idea is that loss of cooling incidents are contained for a significant amount of time- at least 8 hours. I'm dubious, but this is thought to be enough time to evacuate people from the land that will eventually become contaminated. Only when people move back do people die, and then only from increased cancer risk (Brian says this becomes an EPA problem). So now the land is lost, because who would move back? [of course this focuses only on deaths from radiation, and not for example, deaths caused by stress to elderly people forcibly relocated].

I did not remember when people were evacuated after the Fukushima accident, but it was pretty quick, here is a timeline:

https://www.oecd-nea.org/news/2011/NEWS-04.html

There is another question I'm still trying to answer. If the final heat sink is lost (someone blows up a dam), can the reactor be shut down without incident, assuming no blackout? This would require that the decay heat is spread across a large enough surface area. I'm not sure if the containment building provides such an area (a 1000 MW reactor generates ~70 MW decay heat after shutdown). It reminds me that this is another area that NRC does not focus on: "terrorist attacks are a military problem".

Edit: well I answered my own question from wikipedia entry on containment building: "While the containment plays a critical role in the most severe nuclear reactor accidents, it is only designed to contain or condense steam in the short term (for large break accidents) and long term heat removal still must be provided by other systems." So if the heat sink is a man-made lake held by dam, it's a big risk (of course dam loss would cause direct loss of life anyway). I was wondering about this because my inlaws live near Duke Energy's Oconee Nuclear Station, on man made Lake Keowee https://en.wikipedia.org/wiki/Lake_Keowee

Season 2 of 24 I think.

The "ten minutes next to Chernobyl reactor core at the time of meltdown" is 50 Sv, not 50mSv.

3.6R was measured by the small dosimeters on site (and also happens to be their maximum read value). The bigger dosimeters didn't work or also maxed out and the person in charge didn't believe it.

The show mentions a 1500R value later on but outside the reactor building.

>>> That speck of spent fuel is still radioactive enough after 30 years to push most nuclear industry workers over their annual permissable dosage. Chernobyl itself is still phenomenally radioactive - but is far less so than it was immediately after the accident.

I was refelcting on this, not the original article and not on CS-137.