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Comment by module0000 | Hacker News Reader

If graphite cracked/crumbled to prevent the control rods from being re-inserted, that's some scary stuff. The article ending with a request for the safety limit for number-of-cracks to be increased was really surprising for me. The whole "risk VS reward" test comes to mind... the reward is power for 1.8 million homes, but the risk is total catastrophic destruction of the environment for (tens of) thousands of years. The odds may look good, but geez...just a huge gamble to take with other peoples homes/property/lives.

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> but the risk is total catastrophic destruction of the environment for (tens of) thousands of years.

This is ranging on hyperbole. Even in the case of a meltdown the result wouldn't be another Chernobyl. The latter was an unshielded reactor. Literally the soviets didn't bother putting a concrete condom on their reactors, because that would be too expensive. We've already experienced meltdowns in properly built reactors: https://en.wikipedia.org/wiki/Three_Mile_Island_accident

Like Chernobyl, AGR reactors do not have secondary containment: the idea was that as they are gas cooled (no water to phase change) and have enormous cores (so much more time to deal with heat build up), they are safe enough to not need it.

I don't know where you're getting that info. This one says that AGR reactors have 5-7 meter thick reinforced concrete walls: https://inis.iaea.org/collection/NCLCollectionStore/_Public/...

This isn't a separate containment structure surrounding the reactor, but part of the reactor itself. From your own link, page 42, section 5.8 Containment systems:

> Thus, there seems to be a barrier less in AGR compared with LWR reactors. In AGR's the pressure vessel and containment is one unit, but the vessel contains the total primary circuit.The main reason for this difference is the single phase CO2 coolant used in AGR compared with the H2O coolant in LWR. CO2 cannot undergo suddenly phase change as a result of an unexpected rise in temperature or pressure, i. c. it cannot flash as water. It means that there can be no sudden discontinuity of cooling under fault conditions, and changes in flows, temperatures and pressures progress rather slow.

The Three Mile island accident mostly teaches us that we got lucky. Here‘s a quote from that Wikipedia article:

On the third day following the accident, a hydrogen bubble was discovered in the dome of the pressure vessel, and became the focus of concern. A hydrogen explosion might not only breach the pressure vessel, but, depending on its magnitude, might compromise the integrity of the containment vessel leading to large-scale release of radioactive material.

Sure, secondary containment might fail. But it buys a second chance and requires penetration of meters thick reinforced concrete on top of a failure of the pressure vessel. With such secondary containment it's likely that Chernobyl would have only resulted in the deaths of operators working near the reactor and water supply, and drastically less contamination of the surrounding areas.

And research papers dispute the risk posed by said bubble: https://www.sciencedirect.com/science/article/pii/0146572483...

> The Three Mile island accident mostly teaches us that we got lucky.

Why is it considered luck? For it to be luck wouldn't we have to show that there was some way in which the explosion breaching the containment was possible and should've occurred, but due to chance didn't. (Seems like it could've been modeled but I couldn't find anything on that.)

> From 30 March through 1 April operators removed this hydrogen gas "bubble" by periodically opening the vent valve on the reactor cooling system pressuriser. For a time, regulatory (NRC) officials believed the hydrogen bubble could explode, though such an explosion was never possible since there was not enough oxygen in the system. [1]

> Our main conclusions were the following: (1) Most of the initial hydrogen in the bubble was produced by the reaction of the Zircalloy cladding with the super-heated water. (2) During the first 16 hr after shutdown, when boiling of the primary coolant water took place, in the worst case stoichiometric amounts of hydrogen and oxygen could have been produced by radiolysis, leading to a maximum amount of oxygen in the bubble, of 0.7% of the hydrogen, which is well below the explosion limit. (3) After this 16 hr period, when boiling had totally ceased, no further oxygen could have been produced by radiolysis of the primary cooling water. On the contrary, oxygen was recombined with hydrogen due to radiolysis at such a rate that the oxygen in the water was completely removed in less than five minutes. The subsequent rate of removal of oxygen from the bubble by dissolution and radiolysis depended essentially on the rate of dissolution. [2]

[1] http://www.world-nuclear.org/information-library/safety-and-...

[2] https://www.sciencedirect.com/science/article/pii/0146572483...

That's the result of the post-mortem analysis.

The fact that the engineers were afraid it was going to blow up shows that the entire scenario had not been forseen. Hence, luck.

Well if we set the question like this then fossil fuels are about the same - power for everyone for the small chance to kill from thousands to billions.

In the case of fossil fuel, this is not an incremental risk per plant. Each fossil fuel plant gives electricity to a similar number of people, but can't individually render a subcontinent uninhabitable.

The WHO will tell you that air pollution from fossil fuels kills millions of people each year (multiple times more than even the most outrageous estimates of the combined death tolls of all nuclear accidents in history - even if you add Hiroshima and Nagasaki to the list). You also don’t have to look very hard to find information about how air pollution could lead to large portions of the planet being uninhabitable.

Again, this is aggregate air pollution, not caused by one individual fossil fuel power plant. My whole point was this distinction. Obviously fossil fuels in aggregate cause lots of death and damage.

How fitting in this case is the airing of the HBO Miniseries Chernobyl?

The timing sometimes is just so perfect

You're probably hearing about this because Chernobyl aired. It wouldn't be newsworthy otherwise.

There is some series on nuclear disasters going on all the time. PBS just did a big one on Fukishima last year and Command and Control has been put out again recently. Maybe it's a slight surge, but people have always been interested in these subjects. If you haven't seen either I would highly recommend. Command and Control is particularly frightening as many don't know about it.

Chernobyl is particularly relevant though due to the similarity of the potential failure modes (control rod thermal failure preventing full insertion leading to meltdown).

I've read about the cracks around a year ago.

https://www.bbc.com/news/uk-scotland-glasgow-west-43988385

https://www.theguardian.com/environment/2018/may/06/cracks-n...

> total catastrophic destruction of the environment for (tens of) thousands of years

More like 300 years (10 Cs137 and Sr99 half lives) but otherwise you're right.

Plutonium, a byproduct of all nuclear reactors and part of their waste stream, is dangerous for 240,000 years.

How much plutonium is likely to be released in a nuclear incident? I thought Three Mile Island and Chernobyl mostly released radioactive isotopes of iodine and a bunch of noble gasses, with cesium, plutonium, and strontium being present in smaller quantities and generally settling closer to the reactor.

Where did you get that number?

Seems like a typo. Half-life of Pu-239 is ~24,000 years, and of Pu-242 ~374,000 years.

I'm guessing Pu-239 was intended as 1) an extra "0" digit is the more likely typo, and 2) Pu-239 is (AFAIU) the principal Plutonium byproduct of typical Uranium fission reactors.

Half life means that radioactivity is reduced by half, not eliminated. A single particle of plutonium is enough to cause cancer and kill a person. This is why nuclear fuel rod storage (the waste products) must be guarded for hundreds of thousands of years.

Half life is inversely proportional to radioactivity. Hundreds of thousands of years = no worries, you only live up to 1e-3 that amount.

Not all reactors; fast neutron burner reactors output less Pu than they consume.

Those exist only on paper. I'm speaking of actual production reactors which create dangerous amounts of plutonium-laden waste that needs to be cooled, monitored, and protected from human intervention for hundreds of thousands of years -- a problem that we still have no way to solve.

Plutonium release is far less being less mobile and it doesn't have the biological issues strontium does.

Well, it depends, if you're at 10 times the safe level, you will need far more than the half life.

Most likely between 900 and 1200 years.

10 half lives = QTY/1024. Not one half life. At Chernobyl one half life has already passed.

Why do you assume the failure mode here is catastrophic?

Because we already know what happens when you can't insert the rods and stop the reaction.

How do we know? Are you referring to the Borax experiments or the tests on EBR-II? How are they relevant to an AGR?

No idea why you're getting downvoted...

It's not like modern nuclear reactors can only fail catastrophically, much less a predictable failure case that engineers are actively monitoring to ensure the safe operation of said reactor.

What's terrifying are the failure cases that the engineers aren't aware of -- the unknown unknowns -- which are probably significantly fewer than the cases they are aware of and are monitoring.

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