If the moderator (graphite) crumbles into the channel it could jam it and prevent the control rods from entering the core and stop the nuclear reaction. However there are secondary (Nitrogen injection) and tertiary (Boron beads) modes of shutting down the reactor.

"In October 2014 a new kind of crack in the graphite moderator bricks was found at the Hunterston B reactor. This keyway root crack has been previously theorized but not observed. The existence of this type of crack does not immediately affect the safety of a reactor – however if the number of cracks exceed a threshold the reactor would be decommissioned, as the cracks cannot be repaired."

https://en.m.wikipedia.org/wiki/Advanced_Gas-cooled_Reactor

> "However there are secondary (Nitrogen injection) and tertiary (Boron beads) modes of shutting down the reactor."

Hunterston B has also been fitted with "super-articulated" control rods that are designed to be insertable even if the graphite channels are destabilised/distorted.

https://www.bbc.co.uk/news/uk-england-somerset-37789654

> "however if the number of cracks exceed a threshold the reactor would be decommissioned, as the cracks cannot be repaired"

This is the issue currently. The cracking in reactor 3 has progressed more rapidly than modelled, and has exceeded the threshold of 350 cracks. EDF are arguing that the reactor can be safely operated with more cracks, and want the threshold increased to 700.

Safety limits are very conservative and are established through experiments. If the reactor can still be shut down safely and will for a few more years then it can run.

As an example, the initial safety assessment may assume that a) 5mm is the limit for crack width, and b) by the time cracks are 1mm wide, they will be 2mm wide a month later, and, because of that growth rate, pick 2mm as the “take out of commission now” point (nb: numbers 100% made up; for illustration only)

If, in real life, it turns out that it takes a year for a crack to grow from 1mm to 2mm, the model used can be improved. Result _could_ be that the “take out of commission now” point gets moved to 3mm.

They also may remove some stuff with 2mm cracks and test it for strength, and learn more about how the stuff behaves after being irradiated for decades. That, too, can change (in either direction) the life expectancy of the reactor.

Perhaps, but surely you realize that there is currently a lot of pressure (and therefore bias) for keeping the plant operational: Jobs, energy supply and the operator's profits. And the thing is, those biases affect both the regulator (the government), the local population (who probably want to keep their jobs) and the operator (EDF Energy).

So at this point in time I would be skeptical of new experiments and model updates performed by the operator and approved by the regulator. IMHO: Don't approve the model change, do some lab experiments / examine more empirical evidence over time and then possibly change the model for a _future_ reactor.

I do, but there also is a lot of pressure to close nuclear reactors because the perceived danger of nuclear power is a lot higher than its real danger.

Also, would you be in favor of _not_ changing the model for current reactors if updated models predict a shorter life time? If you think the model should be updated in those cases, that introduces bias. It also could mean reactor owners would stop making measurements, and that, in turn, would mean we would learn less about making safer reactors.

If such conditions exist, they can be determined while the reactor is shut down. Otherwise that's recklessness. The original safety margins are there for good reason. Setting them aside without thoroughly testing new limits with justifications that include reasons why the original limits were wrong is reckless. "Nothing bad happened" is not a good reason.

Safety limits are very conservative on nuclear power for fairly well established reasons though. If your worst case scenario makes the two largest metropolitan areas in a country uninhabitable for a while, for the current power equivalent of around 6 months new build in offshore wind (Hunterston is 1 GW and we are currently building out around 2 GW a year in offshore wind), then perhaps the caution is worth it.

The strategic industrial behavior is to accept conservative safety limits initially and then apply for exceptions as needed to keep the site running. You'll see the same thing from nuclear power to various chemical plants to NASA.

Hanlon shows that people rate 'jobs' above all sorts of variables, including economic growth and shuttering zombie businesses which would make everyone richer.

Particularly bizarre in a country like the United Kingdom which is operating at record levels of employment.

The UK is operating at entirely fake levels of employment.

The statistics are based on a Labour Force Survey which counts anyone who works for one hour a week - or more - as employed. And also anyone who labels themselves as self-employed, irrespective of hours worked.

This is explicitly official policy, and not exaggeration or hyperbole.

https://www.ons.gov.uk/employmentandlabourmarket/peopleinwor...

This ^^^^. Thank you.

So, in the UK, if you're on unemployment benefit (UB) you're permitted to work up to 15 hours a week. But that wage is deducted from your UB. That work counts towards "employment" as far as the current UK stats are concerned. It's a lie and the UK government should own up to this....but statistics, damn statistics make them look good depending on how you cut things.

They are employed, though. They're just under employed.

These definitions aren't just made up to make the UK look good, they're international definitions of employment.

> which is operating at record levels of employment.

Depends on your classification of "employment". For the last 30 years the UK government DWP (Department for Work and Pensions) has being classifying all sorts of things as "employment" including folks who work on zero hour contracts, which aren't full employment. Governments get to classify all sorts of things as "record levels" of employment, meanwhile these "employed families" are still having to rely on foodbanks to feed themselves.

Is it bizarre? It seems really intuitive that a country where jobs are prioritized above most else has an incredibly high employment rate.

It might seem inventive, but in this scope/size intuition is often wrong. Same with all large or long-term things in general; humans are notoriously bad at those.

Take this specific case; stay the reactor's systems fail (all of them including the quadruple redundancies), those jobs and energy generators are kind-of irrelevant if you are dead...

I'm sure the reactor enables a lot of jobs far enough away that they'd be safe if it explodes or whatever

In that case, why not make many exploding reactors, jobs for everyone! /s

You do see what the point is, right?

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.

> 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.

> 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.

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...

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.

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...

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).

> 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.

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

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.

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.

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?

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.

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?

I don't think that was the reason. The reason they waived the safety rule is that a further year of modelling and tests determined that the plant was still safe to run. The motivation for keeping it running was jobs and energy supply. But the extra modelling allows it to continue.

> inside information

This is my biggest problem with the nuclear industry. To begin with much of the design and operations of nuclear power stations were classed as state secrets, today they're classed as commercial secrets. Until the full operational characteristics of these generating stations are properly revealed we just can't trust nuclear power.

>why is it that when a safety limit has been established and later on reached, that (seemingly) uninformed people start walling by screaming 'but the jobs' and 'but the power'. None of those should be more important than a reactor problem you'd think.

And if it depended on those people the reactor would keep running until it exploded. It's almost as if democracy was stupid and ineffective.

I don't think democracy is the problem. The current and broken system in many nations is. I'd rather not have a dictatorship and I don't see Anarchy being particularly effective either. What is your solution?

Edit: I understand the feeling of helplessness, but saying that democracy doesn't work is dangerous as this feeling of apathy spreads and people stop participating in the system and we lose many of the more reasonable voters and then less savory candidates get elected and make things even worse and more apathy spreads and so forth. It becomes a vicious cycle and self-fulfilling prophecy. I'm very guilty myself, but have vowed to start participating more in elections and local politics.