Which nuclear accident ware worse Chernobyl or Fukushima

[Nibb31]

Yes I do not understand the exclusion zone, during the accident it was an good idea as things could go wrong, but afterwards?

On the other hand areas around Chernobyl are still radioactive, this is because of all the reactor fragments around.

Radioactivity doesn't disappear overnight. Some particles have a half life of a few days, others take years to disintegrate. Those particles fall to the ground and are absorbed by the soil, the water, the plants, the animals. The particles in the water evaporate with the water and end up airborne again. Absorption doesn't mean that the radioactivity disappears, it means that the particles are still there, but in the plants or living organisms.

You don't want to live near a puddle of radioactive water or near a woods that has radioactive trees. Dead leaves on the ground, mud, crud in roof gutters... these are all places where radioactivity can concentrate. This is why there are exclusion zones. It's pretty easy to remove a layer of soil if you do it in the early stages, but when there are hundreds of square kilometres to clean up, it can be a long process and the longer you take, the more those particles get assimilated by living organisms.

People in Tokyo have analysed air filters from cars or a/c units and found high rates of radioactive particles in them. If it's in air filters, it's also in peoples lungs. Once these particles gets into your lungs or your intestins, you are going to live with a small source of gamma radiation in your body for the rest of your life.

Edited May 11, 2013 by Nibb31

[TooManyErrors]

That would be expensive. Don't forget that nuclear power is about producing cheap energy. If you add so many safety features that it becomes more expensive than other power production methods, there would be no point.

Then why not just place the reactors in old mines that have no use anymore? Cable can't be that expensive.

[Rune]

Then why not just place the reactors in old mines that have no use anymore? Cable can't be that expensive.

Just to cite one inconvenient, water. Reactors are primarily placed where there is an ample source of water nearby to cool their cores and run the steam turbines that actually generate their electricity. The white smoke they output? Water vapor. So the places they are built today are almost invariably on the coast or next to big rivers. Not many of those places lack natural environments, precisely... but you work with what you have to, and so you build containment systems that work just as good as any cave, or better. Like in Fukushima, or better actually since that's a very old reactor.

Rune. You may think of the odd underground river. And I counter, that's odd indeed.

[Brotoro]

That would be expensive. Don't forget that nuclear power is about producing cheap energy. If you add so many safety features that it becomes more expensive than other power production methods, there would be no point.

This is the main problem with nuclear power. Operators will tend to skimp on safety because of the economic pressure. It happens in just about every country: nuclear plants are run by subcontractors who employ temporary workers who are poorly qualified. Regulatory bodies have a hard time forcing the operators to comply with safety rules, and operators have a hard time running the plants economically because so often the cost of (safe) nuclear power production was underestimated.

This is not the case in my experience. I know two guys who are reactor operators, and we have discussed the training they go through. One of them was telling me about the simulation they had be put through that day: Total power outage...backup power systems out...no emergency water feed. I laughed and asked how likely THAT was to ever occur. But he said they trained for it. And a month later, a monster tsunami slaps Fukushima. These guys are well-trained and dedicated. They tell me that the industry had gotten a bit sloppy back in the 1970s, but Three Mile Island was a real wake-up call, and they are much better prepared and knowledgeable now.

[Galacticruler]

Any nuclear accident just results in a huge public outcry about how dangerous nuclear power is and all that, then everyone goes into a knee-jerk reaction mode of stopping various projects in development, shutting down existing reactors, protesting against spacecraft that use RTGs (happened for Cassini and Curiosity), and so on. It's really a shame people get that way, nuclear is really the one thing that could provide us with clean energy for hundreds of years, with just some more developments to it (better waste storage/disposal, for example)

I'm confident that if we started launching our wastes into deep space on solar escape trajectories then we would not have issues with it on earth.

Not for hundreds of centuries anyway.

[Fractal_UK]

I'm confident that if we started launching our wastes into deep space on solar escape trajectories then we would not have issues with it on earth.

Not for hundreds of centuries anyway.

But how likely are rocket accidents? Rocket accidents are common stuff compared to their nuclear counterparts and a single rocket exploding during launch could scatter seriously dangerous stuff all over the planet. If anything went wrong, it would far more catastrophic than anything that could conceivably happen to it just by leaving it where it is.

Also, for much of the most high grade waste, "waste" is a bit of a misnomer anyway. Conventional nuclear reactors only consume about 0.1% of the fuel, with breeder reactors we could potentially power human civilisation for a number of centuries with the radioactive "waste" we have produced from our power plants. This process would have the added benefit of making the stuff vastly less dangerous in the process.

[falofonos]

The fukushima workers may have been well trained but the training and equipment they received was not up to the task.

At one stage some engineers were standing in 3 or 4 inches of water, no boots were used because nobody had thought anyone would ever have to carry out the safety procedures standing in highly radioactive water.

I propose a new testing regime for any reactor : The reactor process flow diagram is modeled as brightly coloured blocks with lines between them then a variable number of children under the age of 5 are let loose on it with hammers and dribble. The engineers then work through what would occur as a result of such damage to a real scale reactor.

The same is done with a scale model of the reactor.

Nuclear energy isn't cheap when total cost of ownership is taken into account. "nobody can live there or near there for a couple of million years and to be honest we're not sure how much material will escape over the next ten to fifty" seems a high cost to me but it's usually externalized to tax payers and the future.

[Dunbaratu]

I'm confident that if we started launching our wastes into deep space on solar escape trajectories then we would not have issues with it on earth.

Not for hundreds of centuries anyway.

The success rate of rocket launches is nowhere NEAR good enough to risk carrying radioactive waste by rocket. Once it's in orbit, then yes it's a great way to keep it from contaminating anything and in space you won't even notice the radiation compared to what the sun already puts out. In fact there's not much reason to bother sending it to the sun. Just get it away from earth and that's good enough. The problem is what happens during those first few minutes of flight trying to get it up out of Earth's gravity well. A success rate as high as 99% would still be too risky, and in reality it's not anywhere near even that good.

Here's a report just from a portion of 2012. Note: 78 successes and 6 failures. You don't want one of those 6 failures to be one that showers fallout from radioactive waste down onto the planet.

=========================================================    
Vehicle            Overall           By Orbit Type
                   Launches      Earth-Orbit Earth-Escape
                  (Failures)      LEO   >LEO  Deep Space
=========================================================
CZ                   19(0)       10(0)   9(0)    -
R-7                  14(0)       12(0)   2(0)    -
Proton               11(2)         -    11(2)    -
Ariane 5              7(0)        1(0)   6(0)    -
Atlas 5               6(0)        2(0)   4(0)    -
Delta 4               4(0)        1(0)   3(0)    -
Zenit                 3(0)         -     3(0)    -
PSLV                  2(0)        2(0)    -      -
Falcon 9              2(1)        2(1)    -      -
Unha                  2(1)        2(1)    -      -
H-2B                  1(0)        1(0)    -      -
H-2A                  1(0)        1(0)    -      -
Rokot/Briz KM         1(0)        1(0)    -      -
Vega                  1(0)        1(0)    -      -
Safir                 3(2)        3(2)    -      -
Pegasus XL            1(0)        1(0)    -      -
---------------------------------------------------------
Total                78(6)       40(4)  38(2)    - 

[a] Assumes that two unsuccessful, unreported Safir launch 
attempts occurred.

(Source: http://www.spacelaunchreport.com/log2012.html )

[magnemoe]

Note the falcon 9 fail was an single engine out, primary mission was completed, an secondary payload ended up in an too low orbit.

[Nuke]

we need to stop using archaic pressurized water reactors and replace the existing ones with current generation reactors. lftr looks especially nice because it can burn up all "spent" fuel from all the current reactors so you dont have to bury it and forget about it, and its waste only needs to be stored for 300 years instead of tens of thousands of years. you also do not need to keep them near water, so you can stick em underground or in the desert and away from fault lines and tsunami zones and other locations which are prone to natural disasters. this is also the kind of reactor which would probibly be compact enough to use for space applications provided you can design a zero g version (and at least one molten salt reactor was installed and tested in an aircraft, and this was in the 50s).

[Nibb31]

We produce 20000 tons of nuclear waste every year. If you wanted to lob that into solar orbit, you would need hundreds of SLS launches per year, or a thousand Falcon 9 Heavy launches (3 launches per day). Even if you could reduce the cost through mass production and reusability, it would still cost billions.

And even with an unrealistically optimistic failure rate of 1%, you would still have at least 10 major disasters every year.

[Galacticruler]

Good points.

Breeder reactors sound like the better way to go.

And we could store that "extra spent" fuel inside the middle of the Earth's crust, as we already do.

[OdinYggd]

Just to cite one inconvenient, water. Reactors are primarily placed where there is an ample source of water nearby to cool their cores and run the steam turbines that actually generate their electricity. The white smoke they output? Water vapor. So the places they are built today are almost invariably on the coast or next to big rivers. Not many of those places lack natural environments, precisely... but you work with what you have to, and so you build containment systems that work just as good as any cave, or better. Like in Fukushima, or better actually since that's a very old reactor.

Rune. You may think of the odd underground river. And I counter, that's odd indeed.

Most mines have to be regularly pumped out because groundwater is constantly seeping in and flooding the tunnel.

A reactor could easily utilize that groundwater as part of its coolant feed, reducing how much has to be sent down the hole from other sources, and it wouldn't take a whole lot to sink a number of wells around the reactor's mine to tap into additional aquifers.

Fukushima was indeed an old reactor. Most of the reactors in the world are- built in the 1960s and 70s with intention to be replaced after 30 years service. Between Three Mile Island and Chernobyl, public outcry was so intense that all new reactor construction was halted until recently, forcing many reactors to remain in service well beyond their design lifetime and leading to a greatly increased risk of accidents.

[DerekL1963]

Most mines have to be regularly pumped out because groundwater is constantly seeping in and flooding the tunnel.

If the mine was viable at all that's generally a (relatively speaking) slow seepage - not the Niagara Falls quantities that a reactor needs for cooling.

[architeuthis]

Most mines have to be regularly pumped out because groundwater is constantly seeping in and flooding the tunnel.

A reactor could easily utilize that groundwater as part of its coolant feed, reducing how much has to be sent down the hole from other sources, and it wouldn't take a whole lot to sink a number of wells around the reactor's mine to tap into additional aquifers.

Fukushima was indeed an old reactor. Most of the reactors in the world are- built in the 1960s and 70s with intention to be replaced after 30 years service. Between Three Mile Island and Chernobyl, public outcry was so intense that all new reactor construction was halted until recently, forcing many reactors to remain in service well beyond their design lifetime and leading to a greatly increased risk of accidents.

This seems an ironic result of the anti-nuclear movement doesn't it?

[Fractal_UK]

This seems an ironic result of the anti-nuclear movement doesn't it?

There are many ironies about the anti-nuclear movement. It's a continual source of amusement to me that the green movement tends to be, by and large, such an anti-nuclear force. Yeah, ignoring the single source of virtually carbon-free base load generation technique that we have is simply a great idea... It amuses me further that any kind of subsidy for wind or wave power is supporting the development of low carbon technologies while subsidies for nuclear power are apparently "market manipulation." The level of hypocrisy is astounding.

[Rune]

Most mines have to be regularly pumped out because groundwater is constantly seeping in and flooding the tunnel.

A reactor could easily utilize that groundwater as part of its coolant feed, reducing how much has to be sent down the hole from other sources, and it wouldn't take a whole lot to sink a number of wells around the reactor's mine to tap into additional aquifers.

Fukushima was indeed an old reactor. Most of the reactors in the world are- built in the 1960s and 70s with intention to be replaced after 30 years service. Between Three Mile Island and Chernobyl, public outcry was so intense that all new reactor construction was halted until recently, forcing many reactors to remain in service well beyond their design lifetime and leading to a greatly increased risk of accidents.

But then you wouldn't be safe from contaminating important stuff, right? That water ends up on the food chain somehow. But yeah, minor point. It's mostly cost in the end.

Anyhow, the point of the OP was which accident was the worst, and I think we can safely say: "the one where there was a core meltdown and no containment building".

Though on other order of things, I'm fiercely pro-nuclear, at least for baseload and until something better comes along. It kills the least amount of people when handled correctly.

Rune. Obvious answer is obvious.

[OdinYggd]

Pretty much. Although Fukushima is indeed leaking contaminated water, almost all of the core material remains inside the containment- it simply did what three mile island did, melting out of it's assembly to collect in a solid mass on the bottom of the reactor vessel.

Most of the contamination that is escaping is short-lived byproducts like tritium, caesium, and iodine. These will be naturally cleaned up to safe levels after a few years, and other than inside the reactor building the radiation level is for the most part just abnormally high- not seriously dangerous.

Chernobyl remains dangerously reactive to this day, the reactor hall only allows 15 minutes exposure for instance while the area around it is restricted to only a few hours. With radioactive dust and bits of core materials having been blown all over the place, not only was there the initial mess to clean up but the equipment used to do it also became highly contaminated and had to be abandoned as well as nuclear waste.

Three Mile Island on the other hand has been completely cleaned up. Today, Reactor Unit 2 is nothing more than an empty building with only traces of radiation left, and the bulk of its hardware removed to be dismantled or sold to other plants as spare parts. Mind you TMI unit 1 is still under power, the steam from it is visible in Harrisburg Pennsylvania.

Here's some pictures of Three Mile Island. There's a local road that goes near it that was close enough to get photographs, staying on public roads well outside the safety fence. According to the Nuclear Regulatory Commission website, the reactor was operating at full power when these were taken.

Unit 1 cooling towers

Unit 2 cooling towers, the empty unit 2 reactor building on the right of this pic

Reactor hall, Unit 2 on the left with the functional Unit 1 on the right. It looks like they were adding on to the building for Unit 1, probably in light of Fukushima.

Plant entrance

But really, other than the prominent steam clouds visible from the highway near Harrisburg, you almost wouldn't even know it's there. And when I first saw the steam clouds I thought it was just a factory until the people I was visiting told me that I was looking at Three Mile Island. These pictures were taken a few days after that when I looked up its location on google and saw a local road on the riverbank, close enough to get a good look without being in any real danger.

They regularly hold disaster drills in the area just in case anything happens.

As for the idea of putting a reactor in the bottom of a mineshaft, it has to do with the depth of the shaft and what rock formations are down there. The Comstock mines of the late 1800s and early 1900s had to remove tens of thousands of gallons of water a day from the mines to prevent flooding, and were producing such large volumes of silver that it was economical to do so, using some of the largest steam engines ever built to do it.

Edited May 13, 2013 by OdinYggd

[Nibb31]

Though on other order of things, I'm fiercely pro-nuclear, at least for baseload and until something better comes along. It kills the least amount of people when handled correctly.

Anyone who is fiercely pro or anti anything is going to get stuck in their dogma. Ideally, you will want a mix of power sources, so as to not rely on a single source. If your country is 100% nuclear and the price of uranium goes up, then you're screwed. If everyone went 100% uranium, this would certainly happen and the World reserves of uranium might not actually be able to meet the demand.

Dogma is what made France switch to almost 80% nuclear power (nowadays it's closer to 60% I believe, the rest of it is mainly hydro and a tiny proportion of wind and solar). France has 19 nuclear plants and 58 reactors. Many of its nuclear power plants are reaching their end of life and the bill for dismantling them is approximately $1 billion for each plant. This cost was never factored into the total ownership cost of nuclear power policies. Now, add the price of rising costs of Uranium, the dubious supply routes for uranium, the political compromises of bribing and supporting dictatorships in Africa, and the storage costs for the spent fuel... Nuclear power would never had been worth it.

Nuclear energy has advantages and drawbacks. The advantages are that it's reliable, continuous, and generally clean energy. The drawbacks are that it's extremely expensive, makes us rely on uranium, and the spent fuel is accumulating and we don't really know what to do with it. And when a freak accident does happen, the cleanup cost is tremendous. The cleanup cost for Fukushima is estimated at $250 billion, which pretty much negates any cost advantage that nuclear power might have had versus other power sources.

Renewable energy sources are cheaper, but they have other drawbacks. The main one is that they are unreliable because they are cyclic (hydro is seasonal, solar doesn't work at night, and wind... er... depends on wind). These cycles don't necessarily match the power demand cycles (which are also day/night and seasonal), so you have to maintain an auxiliary source for when they are not producing. The problem is that you have to maintain enough auxiliary power sources to cover the maximum demand when the renewables are at a minimum power output. This means that you have a power production capacity that exceeds your demand, which is a waste of money.

Because nuclear is expensive and can't really be powered down, it's not great as an auxiliary power source either. Gas or oil are better suited for auxiliary power, but they are dirty, and you rely on oil or gas.

So these are complicated issues and fiercely "pro" and "anti" stances are paralyzing the debate by making most information sources untrustworthy and biased.

Other energy sources have their own advantages and drawbacks. We should be using renewable energy wherever we can and only resort to nuclear power for peak

[Fractal_UK]

Dogma is what made France switch to almost 80% nuclear power (nowadays it's closer to 60% I believe, the rest of it is mainly hydro and a tiny proportion of wind and solar). France has 19 nuclear plants and 58 reactors.

French nuclear capacity stands at about 75% of their total power output, it hasn't changed significantly from the 80% figure. They are looking into the possibility of bringing that figure down to around the 50% region but that is a long term plan to be introduced over the next 20 to 30 years.

Many of its nuclear power plants are reaching their end of life and the bill for dismantling them is approximately $1 billion for each plant. This cost was never factored into the total ownership cost of nuclear power policies. Now, add the price of rising costs of Uranium, the dubious supply routes for uranium, the political compromises of bribing and supporting dictatorships in Africa, and the storage costs for the spent fuel... Nuclear power would never had been worth it.

If that is the case, it is purely a result of poor budgeting. Every modern nuclear reactor has priced into the total cost the cost of decomissioning the reactor at the end of its life. It's also worth noting that France has one of the lowest electricity prices throughout all of Europe which means that over the 30 or 40 year lifetime of a nuclear plant, the low cost of electricity has easily paid for the cost of decomissioning it at the end.

Also, I don't know where you're getting your information about fuel costs but uranium prices are rising marginally from being down in the dumps, Uranium is still ridiculously cheap at the moment and availability of supply is not really an issue. The Uranium price is so low that many mining companies are sitting on large reserves on moderately economically recoverable supplies but have no pressure to actually exploit those supplies until the demand and prices increase somewhat beyond their current levels.

Additionally, unlike sources of power like gas, the fuel makes up a tiny proportion of total lifetime costs of a nuclear powerplant. Fuel costs could in theory rise by 10x above current levels and it wouldn't make nuclear power a particularly uneconomical choice.

I don't know why you bring up supporting and bribing dictatorships in Africa either, the largest Uranium mine in the world is the McArthur River mine in Canada which accounts for over 14% of the world's total Uranium production. By far the two largest exporters of Uranium in the world are Australia and Canada. Hardly the most unstable countries and nowhere near Africa... That's before considering the fact that fuel recycling means that many countries wouldn't even need to buy much new Uranium at all.

Nuclear energy has advantages and drawbacks. The advantages are that it's reliable, continuous, and generally clean energy. The drawbacks are that it's extremely expensive, makes us rely on uranium, and the spent fuel is accumulating and we don't really know what to do with it. And when a freak accident does happen, the cleanup cost is tremendous. The cleanup cost for Fukushima is estimated at $250 billion, which pretty much negates any cost advantage that nuclear power might have had versus other power sources.

I've heard a number of estimates for what the cleanup cost might be in the future but let's what it actually turns it to be. Regardless, whatever it is, some of the the cost is unneccessary. Disposing of wide swathes of topsoil in regions as radioactive as Cornwall is both expensive and absurd.

Because nuclear is expensive and can't really be powered down, it's not great as an auxiliary power source either. Gas or oil are better suited for auxiliary power, but they are dirty, and you rely on oil or gas.

Actually, this is a common misconception. Power output from a nuclear plant can be cycled relatively easy now. The modern designs are capable of altering their power output substantially over only a matter of minutes.

So these are complicated issues and fiercely "pro" and "anti" stances are paralyzing the debate by making most information sources untrustworthy and biased.

There is nothing wrong with being fiercly pro or anti-something provided you have looked into the facts in detail. It only becomes a problem if you become dogmatically attached to something when it provably has better alternatives. An example might be promoting current nuclear power plant designs as an alternative to economical fusion power - chances are, if we had economical fusion, such a fission reactor would have very few notable benefits.

[Fuzzy Dunlop]

Obviously nuclear reactors can be powered up and down, think about nuclear submarines. They can accelerate from ultra low power to 50 mph in a matter of seconds. The reason this dosent happen in power plants is because the reactors are very expensive to build, but very cheap to run - so if you've invested in one it's sensible to run it at 100% 24/7.

Also there is no real issue with Uranium prices, raw Uranium ore counts for 3-5% of the life time cost of a nuclear plant. So if the price of ore went up by 10x then the price of electricity would go up by 30-50%. Annoying but by no means a disaster.

And Uranium packs a ubeliveable amount of energy in a very small volume. So if you were really paranoid you could stockpile enough Uranium to run a country for a decade in your back garden, making you completely immune to supply issues or price rises.

[Fractal_UK]

Also, the 10x Uranium price is actually quite interesting because at 10x the current price then even current methods of sea water extraction (something that has been done solely as an academic excercise to date, so far as I am aware) actually becomes cost effective. The implications of that are that 10x is actually the largest imaginable increase in Uranium price, assuming that for some reason, all non-sea water sources are magically unavailable for some reason and that commercial mechanisms for extracting sea water Uranium aren't more cost effective than they are at present.

Both of these worst case scenarios are so incredibly unlikely that we can effectively conclude two things about nuclear power: 1) Cost of fuel will never make nuclear power uneconomical and 2) Nuclear fuel is so incredibly abundant that we might as well call it renewable. I mean, I'm all in favour of thinking long term but I don't think humanity needs to be worrying about what we're going to do for power in a few million years.

[OdinYggd]

Yes, nuclear reactors do have a variable power output. Just they're extremely slow on the draw- nuclear submarines get around this by using a battery bank to capture surge loads while waiting for the reactor output to change.

However, they become unstable at low output percentages as demonstrated by Chernobyl, and the thermal mass of the system makes them take a very long time for the output power to respond to changes in reactivity. Plus there's the whole decay heat problem, the controls end up having significant lag on order of hours between what the operators commanded and what the reactor's actual output is.

Typically they are indeed used for base load, operating at their maximum permitted output at all times when not under maintenance.

Edited May 13, 2013 by OdinYggd

[Fuzzy Dunlop]

Yes, nuclear reactors do have a variable power output. Just they're extremely slow on the draw- nuclear submarines get around this by using a battery bank to capture surge loads while waiting for the reactor output to change.

However, they become unstable at low output percentages as demonstrated by Chernobyl, and the thermal mass of the system makes them take a very long time for the output power to respond to changes in reactivity. Plus there's the whole decay heat problem, the controls end up having significant lag on order of hours between what the operators commanded and what the reactor's actual output is.

Typically they are indeed used for base load, operating at their maximum permitted output at all times when not under maintenance.

Depends on design, most reactors were never designed to be throttled (because there's no economic sense in doing that). On the other hand some can be, Chicargo once got almost all it's power from nuclear plants - and so needed to reduce power in times of low demand. The boiling water reactors they used could go from 50% to 100% (electric output) in less than 5 minutes; that's much better than any coal plant could manage. The French also throttle their pressurised water reactors.

In contrast the RMBK design used in Chernobyl wasn't built for rapid throttling and it certainly wasn't designed to perform an emergency shutdown and then restart to full ouput within a matter of hours - which is what they were doing when it exploded.

[DerekL1963]

Yes, nuclear reactors do have a variable power output. Just they're extremely slow on the draw- nuclear submarines get around this by using a battery bank to capture surge loads while waiting for the reactor output to change.

Nuclear reactors can be quick on the draw or slow on the draw, it's all a matter of how they're designed... and submarine reactors are VERY quick on the draw. They have to be, because there are situations where you want to change your throttle settings rapidly, such as needing to go from all ahead full to all astern full to avoid hitting something, or when maneuvering in a confined space. The battery is an emergency backup and is used mostly while we claw our way up to the roof so we can raise the snorkel mast and start the emergency diesel generator set. If we're somewhere where we *can't* do that... well, that's why submariners are a special breed, because we'll have to fight like hell to rectify whatever caused the loss of electrical power before we die.

(Ah, I should introduce myself - in a former life, I was FTB2/SS Lyons, USN Submarine Service 1981-1991.)

Commercial power reactors can all be throttled, because otherwise they couldn't be started up... but their throttle response is slow by design, because it increases safety. They generally aren't throttled though, because stable operating conditions are the safest. The also generally run at full power because that makes the most economic sense given the cost of fuel.

However, they become unstable at low output percentages as demonstrated by Chernobyl

Not even close. The RBMK reactor used at Chernobyl was unstable at low power, but that's not what caused the explosion. The explosion was caused by the low power instability combined with a flaw in the control rod design that actually briefly *reduced* moderation as they were driven home. So a spike in power combined with a reduction in moderation, which lead to a further spike, equals pieces of reactor scattered across the landscape.

the thermal mass of the system makes them take a very long time for the output power to respond to changes in reactivity. Plus there's the whole decay heat problem, the controls end up having significant lag on order of hours between what the operators commanded and what the reactor's actual output is

Um, no. There was not a lag of "hours" between control input and reactor response - such a reactor would be impossible to operate. (And hideously unsafe even by Soviet standards.)