Nuclear Safety and Automatic Rod Extraction

" withdrawing the fuel rods during emergency and sending these rods to secure locations"

No. It just doesn't work that way.

Any speculation at this point is not productive.

Lyn

I fail to understand your response. I was not trying to speculate anything but was rather looking for an answer to why the rods from a nuclear plant cannot be removed when an emergency situation arises. It was not my intention to look at ways to correct the current problem in Japan but rather to find a way to make these plants ever safer in the future. Usbport has provided what appears to be a logical answer to quick removal of the rods and placed in a secured position. I understand SCRAM methods of shutting down nuclear reactors quickly but these systems do not remove the rods to secure locations. I have lived in the shadow of Pickering nuclear plant near Toronto and had always felt safe. New building of nuclear plants should be able to incorporate some idea or mechanism for removing the rods. If Japan had such technology maybe the problems we now see would not be a big deal.

Please if you have some constructive criticism of my question, explain it better than just; "No. It just doesn't work that way."

kevinm,

As i sit here, I'm shaking my head in disbelief.

I fail to understand how you and Usbport could have solved all the problems, just like that.

No offense, but this is an INCREDIBLY complex facility. These things take many years of development before any design begins in earnest. Permitting is even more brutal. Then building, transporting and installing will only take a few decades.

You can't just push a button and turn them off. Then come in the next day and slip in that spiffy new lead box. I can't even contemplate how ridiculous it is to consider such a folly.

Now that I have offended you, I'll go back and stand in the corner.

@lyn "...come in the next day and slip in that spiffy new lead box."

Ah, c'mon. It'd take at least a week to build it. Then you grab one end of the reactor, kevinm grabs the other end, and while you lift it up I shove the lead box in place. Easy squeezy.

I'd want some thick gloves.

The "Candu" reactor uses heavy water not only as a coolant but is also the moderator for fission. Lose the water and fission will stop. The Japanese used strait RO water with moderator rods. Lose the coolant and they will still stay hot, even with the moderator rods pulled out. They lost there back up system when the tsunami washed away the fuel tanks to run the back up generators to run the coolant pumps.

There is no comparison between Canada and Japan.

Understood and thanks. Candu reactors are still not without their own set of dangers. Candu reactors, unfortunately, create plutonium and was instrumental in allowing India and Pakistan to develop nuclear weapons. That is probably their biggest and most dangerous drawback. The international agreements were worthless to prevent this type of abuse.The second danger is that the Candu uses heavy water which also helps create tritium another radioactive byproduct and known carcinogen. It can get into water supplies through accidents. However, there is a lot of concern and work in developing tritium containment.

The chance of an accident are still much more remote as the fuel is protected by the heavy water moderator. Not totally immune from disaster but still very remote as you point out.

Give this a GA, you think you can look back at 80 YEARS of engineering and tell what they missed and you are so smart you got it. so sad, and there are boys taking more than they shoud to keep it in control, have some respect, WAIT A FEW DAYS

Sure. Just design reactors with a cavity in the center into which a conventional bomb could be dropped. In the event of emergency, the bomb is dropped in and detonated. This scatters the fuel rods like so many pick-up sticks, spreading them out into a non-critical geometry. Problem solved!

Doh...

--Homer

Whoever didn't like that, I am curious to know why not. To be sure, I was being somewhat facetious, but just what did you think was wrong?

Not a bad idea, actually. I think you'd want something simple and failsafe, like a method of simply dropping the fuel rods into a large lead containment device located below the reactor - perhaps way below, farther than I show in my sketch:

Ga to Usbport and Tornado (sarcasm humor is not off topic). Usbport, your concept seems very doable and simple. It would also be cost effective. I assume that the rods could be extracted form the emergency position once the emergeny has been given an "all clear" signal. If the lead vault were build along the lines of rod transport cask, they should be very robust. I still think there may be ways to retrofit existing nuclear plants with some sort of fail safe storage in times of emergency. The industry has a good record but a couple of accidents really point out a potential problem on mega-scale size and very dangerous. Japan's accident seems as dangerous as the one in Russia and we do not want one happening where the downwind path is heavily populated.

Great concept.

The designs of the Chernobyl and the Fukushima reactors are in concept and execution completely different. The Chernobyl design is inherently unstable. The Fukushima design (as most in the world) is inherently stable. "Stable" is generally regarded as not explosive. "Explosive" does not refer to relatively minor hydrogen combustion - even though it can and has destroyed the Fukushima support building. Consequences of massive thermal excess are dire, whether or not nuclear, against which almost all nuclear plants have many layers of protection. Unfortunately, Fukushima lost one important layer of protection when the Tsunami disabled the diesel powered generators that ran the cooling pumps. Seemingly rational generalizations such as those offered by "Kevinm" remind me of how dangerous can be the well-intentioned.

You forget a quite simple thing: A nuclear reaction doesn't stop automatically as a combustion motor when you close the fuel feed.

The reaction decays exponentially and continues generating heat. Why do you think the "spent fuel pools" exists? And why we need to keep it under continuous cooling. Just read the Japan news and look at the temperatures in that pools rising slowly everyday.

The "rods" as you say are inside a rector pressure vessel, made of steel and can be melted. Have you take into account of the melting temperature difference between steel and lead? Someone really think that a group of fuel elements able to melt a zircalloy tube and a thick walled steel vessel will be stopped by low melting point lead?

It's easy to criticize or as we Spanish say "to see the bullfighting behind the barrier"

Please be a bit more informed before posting nonsense ideas.

Kind regards

@Kwetz Next time you fire off a reply and accuse someone of 'nonsense' try reading all of the other comments first and being informed yourself. In post #19 (well before your post #34) I had explained that my diagram was deliberately simple to get the main point across and that the actual vessel would more likely be a high-temperature ceramic impregnated with lead beads, or whatever materials are best suited as the moderator for the given fuel.

Also, I was not addressing the issue of spent fuel rods, I was replying only to the original OP with an idea of a way to remove some of the fuel from an out-of-control reactor to slow down the reaction and maintain enough spacing between the fuel rods to bring the reaction under control.

There are many issues with what is going on in Japan, and the spent fuel is one of the issues; but it is a side issue to the original OP to which I replied. Perhaps there should be another thread regarding a safer method of storing spent fuel on site, but that wasn't the issue I addressed.

As I was instructed the reactor core is set up much like Usbport's diagram. During emergency shut down they are suppose to drop down. The problem is it tested regularly. These reactors run 24/7. To due a emergency shut down means being off line for a while it's just not like turning a switch off and on. It may take a day or two to get them back on line maybe longer because of procedures that need to be followed. Then you have the loss of power to the grid. Can the power be acquire else where while the test is done.

As I have followed and the information given out is the rods did drop out. The problem was the super heated water with no pumps to transfer heat away from the core. Primary water is under a lot of pressure to keep the fluid in a liquid state. If it heats up to much then it starts to vaporize forming gas pocket. Insulation properties of a gas cause it to retain more heat.

Maybe some of you have heard different. Let me know.

I think Usbport has intended his design as something a little different than a normal nuclear plant employs. The drop down of the fuel rods could be much farther below the plant than I think the Japanese plants. It is normal to suspend the rods and drop them into a neutron slowing material during shut down. I stand to be corrected, but these rods are still within access of a few feet below the plant. I would not be sure on the depth but 30 feet is likely close to a maximum depth. I envision Usbport's clever design to bottom out at 500 feet or so below the core. That would remove the likely-hood of rods being exposed and away from any damage to the core.

I like your point about testing but as I understand it, these plants use SCRAM methods of shutting down reactors as a routine. Correct me if I am wrong, but if the rods are not in the core there is nothing to generate the heat. It seems to this rank amateur that the problem would be abated very quickly if the rods were now 500 feet below the core. Maybe Lyn is correct and I am being very naive. At this point I feel frustrated because I think there remain safer designs. The world can ill afford not to design these nuclear plants with the ultimate of safety. All I want is someone to explain why I am naive. Does CR4 have any nuclear plant engineers?

Love the SCRAM link and it may hold the answer to our questions in some part.

There is an indication there that a reactor that has been in constant power operation for more than 100 hours will continue at around 7% power output even when the shutdown has happened. I suppose this means that the 500MW reactor would be popping out around 35MW and this is what has evaporated the coolant.

If the rods were dropped 500ft into the earth, then there would be a 35MW heat source with very poor thermal release and basically inaccessible for post failure recovery.

I really suppose that we have to trust the experts in this field. Containment vessels still seem to be intact, but of themselves are restricting access to the core to enable cooling.

A little like many other "engineering disasters" (Like Piper Alpha for instance) I will be very interested to see the conclusive investigations and reports that will happen in the cool period after the current panic period has expired. My image is that these reactors should have "gravity fed cooling" (like a hydro dam) with sufficent volume to quench a hot core. But even that concept has issues relating to where does the hot water go to? (released as steam?)

Interesting subject, this shutting down a nuclear plant. I had always thought it was quick but you are right about the residual decay heat. I googled decay heat and found this;

"at the moment of reactor shutdown, decay heat will be about 6.5% of the previous core power if the reactor has had a long and steady power history. About 1 hour after shutdown, the decay heat will be about 1.5% of the previous core power. After a day, the decay heat falls to 0.4%, and after a week it will be only 0.2%. The decay heat production rate will continue to slowly decrease over time; the decay curve depends upon the proportions of the various fission products in the core and upon their respective half-lives "

Essentially the core reactor will fall to about 0.4% of output after 24 hours. I am not sure how far the heat must decay to be considered secure for intervention. Having the rods away from the reactor core seems a good thing. If plants had some design to remove them and not store used rods, even temporarily, near the core. Heck, I think the plants could then withstand these natural occurrences very well. Even if the plant itself blew up. I am not sure if it could withstand a man made attack as we may well see in the future (particularly in some rising nuclear power countries). But removal of the fuel rods seems a possibility and that action will provide operators time to fix or squelch the problem.

I hope when the reports come out of the Japanese disaster, that all man will learn to handle these plants much better. I also hope they come to grips with the existing problem post haste.

@kevinm

Yes. My understanding (which may be a bit dated) is that in most reactors the fission material is installed semi-permanently in the reactor, and the speed of the reaction is controlled by cadmium (?) control rods that absorb neutrons to slow down the chain-reaction. The control rods drop-in, if there is a problem, to slow the reaction to a crawl.

My idea is to have the rods containing the fissionable material drop out into a deep lead 'safe' that stops the reaction completely. It wouldn't even have to be all of the material, just enough of the material near the central core to completely negate any chance of a chain reaction. The stop locations of the fuel rods would be spaced to minimize the inter-rod neutron flux too.

@mikespike

You said "Removing fuel rods and sinking them into lead containers would stop chain reaction, however, uranium in each rod would still "burn" for number of years, therefore dropping these rods into lead containers would cause lead to melt away and evaporate/explode".

I suppose your are referring to my post. If you read my post carefully you would see I anticipated your concern about keeping the fuel rods too close together. The lead would only 'melt away' or 'explode' (LOL) if the fuel rods were too close together, but by shielding them in a lead vessel with enough separation between them there would not be sufficient heat to melt the lead, let alone cause an explosion. Obviously, lead is used as shielding when making the fuel rods... otherwise how would the fuel rods even get made. (!?) [The containment vessel wouldn't even need to be solid lead; it could be a high-temperature ceramic impregnated with lead beads, for example.]

A little bit of common sense would show that my sketch was deliberately simplified; there would be ways to keep the removed rods cool, and re-insert them into the reactor once the danger had passed. The key point is that no current reactor (to my knowledge) uses any method to remove the fuel from the reactor in an emergency; so when the safety systems suffer multiple failures, the heat from the fuel becomes difficult to control and other means like using seawater are needed.

In my earlier post I'd mentioned cadmium (not graphite) as the control rod material. I was correct, though there are a number of other materials used depending on the type of reactor: http://en.wikipedia.org/wiki/Control_rod. Depending on the type of reactor, some of these materials could also be used in the emergency containment vessel.

Keep the rods in a graphite core? Graphite burns. That was one of the problems at Chernobyl.

How did this get rated a 'Good answer'?

It seems that there is a lot of misinformation going around.

1. Reactors shut down (Scrammed) at first on set of earthquake. AS THEY ARE DESIGNED TO DO.

2. While a scrammed reactor is not producing power, the fuel rods are still very hot and need to be cooled.

3. Tidal wave destroyed back up generators that were supposed to provide power to keep pumps running and cool reactor. THIS is a fault of the engineers to consider this highly unlikely situation.

4, since these are boiling water design, lack of enough cooling water will cause water remaining in core to boil and create pressure that needs to be vented. Unfortunately this caused the explosion that damaged the outer building and released some radiation.

5, Spent fuel assemblies are stored in big pools. These were damaged (Leaking) and fires occured, These spent fuel assemblies also are source of radiation

6. The problem now is to get water flowing into pools and reactors ASAP

This is a very serious problem. But so far damage has been remarkabley contained. Yes there is a lot to learn from this situation but is not time to panic.

Modern reactors do, in fact, have automated shut-down systems to drop the control rods into place in an emergency. This is referred to as "scramming" the reactor. Those systems worked exactly as designed in all 4 reactors at one plant and in 2 of the 3 operating reactors at the plant now experiencing the worst problems. In that plant, Reactors 1, 2 and 3 were operating and Reactors 4, 5 and 6 were already in cold shut-down for maintenance. Reactor 2 was slow to "scram", but eventually did.

The problem is that, although inserting the control rods stops the fission process, it doesn't cool the reactor pile down quickly. Therein lies the problem.

After the reactor is shut-down, it still takes a considerable amount of time for the thermal mass of the core to give up its heat. During that time, the coolant must be continually circulated just as it would with the reactor operating. The cooling system requires power to drive the pumps, and that was lost due to the earthquake and tsunami. With no power to pump in additional cooling water or to circulate what was already there, the coolant quickly turns to steam and begins to dissociate into hydrogen and oxygen. This steam, rich in hydrogen must be bled off to prevent the pressures in the reactor containment vessel exceeding limits and causing that vessel to burst. It was this hydrogen-rich steam being bled off that resulted in the explosions that were widely reported. Under the circumstances, such explosions are to be expected and the containment buildings are actually constructed so as to allow the force of explosion to be directed up and away from critical systems. The upper portion is essentially just a dust and rain cover made of sheet metal.

The fires that have been reported have involved spent fuel rods from Reactor 4 and stored in a water pit. Apparently, fires started in ancilliary equipment by the hydrogen explosion from Reactor 3 impacted that storage pit and resulted in the spent fuel rods there being exposed to air and their temperatures to rise considerably.

I'll be eagerly awaiting the chronology of this accident when it is ultimately released.

there is a fail safe device that shuts the reactor down during emergency

the problem is that cooling is still needed for some days after shut down, and this was the problem in Japan as the back up diesel generators were located below the reactors and were flooded by sea water when the tsunami hit the site

if the back up generators had not been damaged the problems Japan is having now would have been avoided

WHY didnt they have power lines form other nuke sites to supply power ie site to site links independent of the power out lines

so the back up genys would have only be used if the site to site power lines where damaged

also why didnt they have water resovoirs up the hill side so dependence on power to pump could be avoided using gravity is free

as for rod storage it would be safer to store them in a seperate building located next to the reactors but not in the same building.

all your eggs in one basket springs to mind

If you wish a nuclear reactor that is safer to shut down and has less risk then goggle wikipedia and check out the CANDU heavy water reactor. To shut it down in very simplistic terms you drain the heavy water (Deuterium) that stops the reaction since heavy water is the moderator and exchange it for ordinary light water to cool off any fuel in the system.. This system does not need massive amounts of cooling water to control the reaction. Again I say this is a very simplistic comment.

simple is good, simple is less likely to break down

Diesel engines on vehicles were very reliable at one time but the engineers soon cured that by adding lots of electronics now they are as relable as petrol engines

and almost impossible to fix at the roadside without complex computer equipment

I agree on both counts Diesel and Nuclear since I have worked in both industries. However everything is reliable until it stops functioning. That is where the fun begins.

http://www.nuclearcounterfeit.com/index.php?s=fires+in+japan+plant

taken from above

Figures provided by Tokyo Electric Power on Thursday show that most of the dangerous uranium at the power plant is actually in the spent fuel rods, not the reactor cores themselves. The electric utility said that a total of 11,195 spent fuel rod assemblies were stored at the site.

That is in addition to 400 to 600 fuel rod assemblies that had been in active service in each of the three troubled reactors. In other words, the vast majority of the fuel assemblies at the troubled reactors are in the storage pools, not the reactors.

Now those temporary pools are proving the power plant's Achilles heel, as the water in the pools either boils away or leaks out of their containments, and efforts to add more water have gone awry. While spent fuel rods generate significantly less heat than newer ones, there are strong indications that the fuel rods have begun to melt and release extremely high levels of radiation. Japanese authorities struggled Thursday to add more water to the storage pool at reactor No. 3.

Four helicopters dropped water, only to have it scattered by strong breezes. Water cannons mounted on police trucks - equipment designed to disperse rioters - were deployed in an effort to spray water on the pools. It is unclear if they managed to achieve that.

Nuclear engineers around the world have been expressing surprise this week that the storage pools have become such a problem. "I'm amazed that they couldn't keep the water in the pools," said Robert Albrecht, a longtime nuclear engineer who worked as a consultant to the Japanese nuclear reactor manufacturing industry in the 1980s and visited the Fukushima Daiichi reactor then.

Very high levels of radiation above the storage pools suggest that the water has drained in the 39-foot-deep pools to the point that the 13-foot-high fuel rod assemblies have been exposed to air for hours and are starting to melt, he said. Spent fuel rod assemblies emit less heat than fresh fuel rod assemblies inside reactor cores, but the spent assemblies still emit enough heat and radioactivity that they must still be kept covered with 26 feet of water that is circulated to prevent it from growing too warm.

Gregory Jaczko, the chairman of the United States Nuclear Regulatory Commission, made the startling assertion on Wednesday that there was little or no water left in the storage pool located on top of reactor No. 4, and expressed grave concern about the radioactivity that would be released as a result. The spent fuel rod assemblies there include 548 assemblies that were only removed from the reactor in November and December to prepare the reactor for maintenance, and may be emitting more heat than the older assemblies in other storage pools.

Even without recirculating water, it should take many days for the water in a storage pool to evaporate, nuclear engineers said. So the rapid evaporation and even boiling of water in the storage pools now is a mystery, raising the question of whether the pools may also be leaking.

Michael Friedlander, a former senior nuclear power plant operator who worked 13 years at three American reactors, said that storage pools typically have a liner of stainless steel that is three-eighths of an inch thick, and they rest on reinforced concrete bases. So even if the liner ruptures, "unless the concrete was torn apart, there's no place for the water to go," he said.

At each end of a pool are 16-foot-tall steel gates with rubber seals, used to swing fresh fuel rod assemblies into a reactor and to swing out and store the spent assemblies. The gates are designed to withstand earthquakes, Mr. Friedlander said, but could have sprung leaks given the power of last Friday's quake, now estimated to have had a magnitude of 9.0.

Even if water gushed out of the gates, there would still be about 10 feet of water left on top of the fuel rod assemblies.

When the water in a storage pool disappears, residual heat in the fuel rods' uranium left over from their time in a nuclear reactor continues to heat the rods' zirconium cladding. This causes the zirconium to oxidize, or rust, and even catch fire. This breaks the seal of the rods, and pressurized radioactive gases like iodine, which accumulated in the rods while they were in the reactor, suddenly spurt out, Mr. Albrecht said.

Each rod inside the assembly holds a vertical stack of cylindrical uranium oxide pellets. These pellets sometimes become fused together while in the reactor, in which case they may stay standing up even as the cladding burns off. If the pellets stay standing up, then even with the water and zirconium gone, nuclear fission will not take place, Mr. Albrecht said.

But Tokyo Electric said this week that there was a chance of "recriticality" in the storage ponds - that is to say, the uranium in the fuel rods could become critical in nuclear terms and resume the fission that previously took place inside the reactor, spewing out radioactive byproducts. Oh Great it gets worse

Ive just watched NHK where they were interviewing workers from the power station.

when the earthquake struck the lights went out and they had to grope the walls to get out as there were no emergency lights.

as for the radiation readings they get these are from a guy in a car who drives around the site at regular intervals, thats would explain why they keep saying the readings at the front gate are 8989 or whatever

Its unbeliviable that such a site has no emergency lights, and no remote readings

they have no idea whats happening inside the reactors as they cant get close enough to operate the equipment

also when they do connect power to the reactors i hope they stand back cos everythings soaked in salt water, should be intresting