|>>>>>>>>......but the proton beam that is produced at the start is also going to be used in experiments such as neutralising radioactive waste by transmuting the radioactive elements into stable ones.<<<<<<<<|
I think this part of DPAD could have really big implications ,but I don't see much about it on your site (unless being an NTL user has screwed my mind) ,is it very likely?
Any more info about this part of the project?
|That project is sort of running parallel to the Neutrino Factory - it shares the very beginning part of the accelerator technology but not the muons end.|
There aren't actually that many sites about it - I did a Google search a few months ago for it (Accelerator Driven Transmutation Systems) and got a load of sites that were abandoned around 2001. Perhaps someone pulled the funding.
|Any news on the transmuting front?|
Outside of the primary goal of pure research this is the more potentially important part of the project ,especially as some goverments are looking to Nuclear fission power stations again.
(though I'd rather they spent money on nuclear fusion research etc).
|absolutely nothing wrong with fission.|
Its safe, a lot cleaner overall, and if said transmutation system becomes possible, it'll be very good
Tell you what, in 5-10 years, after I've made my millions, hows about i start a research group.
|Making fission work well, dealing with the waste properly (that is, properly-managed storage and then either transmutation or firing it into the Sun on rockets) is very important as a stage before fusion comes online. I don't think we can have sufficient power just with wind turbines, solar etc. (although those should be provided as a backup). The UK has recently reopened the idea of building new fission plants after a period since 2002 when they weren't recommended and a longer period before that when we just did nothing. Perhaps eventually that will mean transmutation research gets funded again.|
lol ,you must be kidding me? ,what about the fact the fission waste has a 1/2 life measured in centuries to millenia?
Its all very well burying it(its not all recycleable right?) but what if the site gets disturbed by geological activity?,something we can't be sure over millenia!
Nuclear fusion will be the answer when they've sorted out the materials problem for the plasma chamber (can't remember its proper name).
They'll produce far less waste & its radioactive waste 1/2 life is measured in decades not centuries.Something we can safely manage.
As for transmutation ,its not happening atm it seems ,so until then its a mute point.
Another issue with fission is its possibilty of a chain reaction if things go wrong ,granted its rare but it CAN happen.
In fusion reactors it is impossible to have a chain reaction ,if something goes wrong it snuffs out the reaction.
Having said all that ,I agree that we maybe forced to build new fission reactors as we can't relie on fossile fuels indefinatly & it seems that fusion power is 10-20yrs+ away.
Anyway ,way off topic
So I take it the transmutation side of this is dead atm then?
And when transmutation is picked back up (by me and perfected, there's all that lovely waste to work on.
You've been watching 24 ahven't you?
With proper design, runaways (chain reactions are always happening, thas what fission reactors do, no chain reaction, no heat generation) are near impossible to hapen. Yes, with certain reactor types, such as those in Chernobyl, it can happen, with modern edsigns, it can't. In an old type, removing the water or sodium coolant casues a heat build up, unless the carbon moderator rods are inserted. Heat bild up causes what coolant left to flash steam, and then the materials melt.All the time they're fissioning, and generating heat.
Modern reactors use water itself as the moderator, not just as the coolant. Remove the water, and the chain stops, the heat stops building, and the system goes cold. then control rods are inserted to keep the system from chaining, even if its refilled with water.
Au contrair. If it breaches its containment vessel, it'd say its rather devestating. Maybe not so 'long term' devestating, but never the less...
|All I've read suggests the modern generation of fission reactors have been made a lot safer, to the point where comparison with Chernobyl is rather like comparing a modern diesel train to an old traction engine! Another thing not often mentioned is the "fast breeder" option where waste plutonium (I think it is) is re-used to make more fuel, which apparently gets like 20-50 times as much energy out of the same stuff.|
|I have worked with fission reactors for almost 30 years now. The waste that everyone seems so worried about is not as big a problem as described. The spent fuel is definitely HIGHLY radioactive when removed from the reactor. It glows a brilliant blue. But - the way that modern commercial PWR and BWR fuel is designed makes it easy to handle. The fuel itself is in a ceramic pellet and is very stable. The pellets are stacked inside a long tube made of Zircalloy. The Zircalloy tube is welded shut at both ends to keep water out and radioactivity in. Zircalloy is very corrosion resistant, even with the high heat generated by PWR fission reactors. So, you remove the spent fuel assemblies from the reactor vessel, keep it 20 to 30 feet underwater to minimize radiation exposure (remember, it still has the "blue glow"), and you put it into a Spent Fuel Pool. These are basically stainless steel swimming pools, seismically qualified, and they have a capacity to store at least 10 years worth of spent fuel. Some newer plants (like mine - Comanche Peak) can store over 40 years of spent fuel. |
Assimilator, you mention the half-life. The decay curve is very, very steep for the first five years. The highly radioactive elements have very short half-lives. Admittedly, there are some elements in the fuel with long half-lives, but they are relatively weak radiation emitters. Fuel assemblies stop glowing within months and after a few years stop producing much heat from their radioactive decay process.
This behavior (reduced heat and radioactivity) allows power plant owners to remove old assemblies (> 10 years) and put them into "dry storage" casks if they are pressed for space. These casks are still maintained on-site and are extremely heavy and welded shut (terrorism concerns, perhaps?). While many utilities would like to move these assemblies to a long term storage facility like Yucca Mountain in Nevada, most utilities are currently resigned to keeping all their fuel on site for the lifetime of the plant and beyond.
|Why not send em to Sellafield for reprocessing?|
|I believe it is still illegal in the US to reprocess spent fuel. There is a LOT of Plutonium 239 in spent fuel. That's how breeder reactors work, by turning U-238 into Pu-239 by neutron bombardment. Jimmy Carter (President back before Ronald Reagan) banned it all to reduce weapon proliferation. The UK and France routinely re-process spent fuel.|
|yeah, sellafields in the UK, Cumbria in fact. Reproceses fuel from all over the place. They do MOX pellets for Japan, and even take in waste flights from Canada.|
Was some concern back in 02, i seem to recall, a boat carrying MOX pellets that were 'faulty' came back to get new fuel, only armourment apart from sidearms was a pair of 20mm cannons.
for others - http://news.bbc.co.uk/1/hi/uk/647981.stm
|How many Sv/hr is the waste at when it's glowing blue? I think it's safe to say I've never seen anything anywhere near that radioactive here (although I guess accelerator targets might glow after being bombarded for a while - I'll have to ask).|
|I'd love to know how you found it had the blue glow. Safe to say, you can't exactly look out a window at it.|
|K'Tech (and Stephen), the blue glow is clearly visible by standing at the side of the spent fuel pool. The fuel assemblies are 7 meters below the surface and therefore give no radiation dose to those above.|
As far as the radiation dose rates (you mention Sieverts/hr while we use RAD/hour), I'll have to ask a Radiation Technician. They would know.
I'll poke around in some old pictures tomorrow and perhaps attach one showing a glowing fuel assembly. Quite pretty if it weren't so dangerous at that time in its life.
|1 Sv = 100 REM, and REMs are basically RADs except you get more of them when you have hadronic (neutron & alpha) radiation, providing a multiplication factor of up to 5-20 for those sources.|
In our accelerator here - a moderately radioactive machine - we have a cordon at the "20uSv/hr" line, about 2 metres from the ring and magnets themselves. So that's 2 milliREMs/hr, probably VERY small compared to what you get in fission work, but our lab is concerned primarily with chronic exposure so they don't want people working for days on end within that region.
Parts of the ring where beam is actually lost (hits something, intentionally or not!) are worse and have concrete blocks stacked in front of them. Injection is the one of these. The highest rate I've seen reported on the ring was 10mSv/hr (1 REM/hr) for a magnet near injection that had had a lot of beam hitting it.
As it happens, the target is always much more radioactive than the accelerator. I don't have the figures for ours except to say that it's tungsten and has up to 150kW of fast protons hitting it for long periods of time, so it could be comparable to reactor fuel by the end of that.
Our neutrino factory tantalum rod (actually a toroid) will have to cope with many times that and so most of the surrounding area and early decay channel you see in Muon1 will be highly radioactive.
It's interesting how good an absorber water is... The light seems to be Cherenkov radiation, so I suppose theoretically if the fuel were in air instead of water there wouldn't be a glow (unless it was so radioactive you ionised the air).
|As far as I know, air doesn't display Cherenkov blue glow, just water.|
I asked an RP tech for a radiation dose value. He said they didn't know exactly, just that their highest range meter pegs high if the probe is close to freshly removed fuel. Those meters measure in RAD/hr and the high scale is 1000x and the meter scale is from 0 to 100, so it is at least 100,000Rad/hr on contact. That sounds kind of high so I hope I got it right.
And it isn't that water is such a great shielding medium, just that is so inexpensive when compared to lead, concrete or steel. It is a great shield for neutrons but just so so for gammas.
|Yes, very high - lethal dose in 10 seconds. I hope the meter probe was remotely operated!|
|Actually you do get cherenkov light in air, it's the basis of some types of cosmic ray /gamma ray telescopes for example Cangaroo http://www.physics.adelaide.edu.au/astrophysics/cangaroo.html which I played with while I was at Uni|
|Wow this thread's grown nicely! (wish I knew about it via email subs!) ,interesting stuff.|
>>>>Au contrair. If it breaches its containment vessel, it'd say its rather devestating. Maybe not so 'long term' devestating, but never the less...<<<<<<
True it would be devastating to the immediate area ,but it wouldn't spread pollution for miles ,as well as it being 'short term' radiation problem.And the reaction would just snuff out on containment breech.......... lol sounds like Star Trek stuff
(& yes I meant to say a runaway chain reaction)
Btw ,never seen 24
Think of me if you get rich on transmutation
(think I know you from somewhere )
What's the critera for that?
>>>>there are some elements in the fuel with long half-lives, but they are relatively weak radiation emitters.<<<<<
How weak ? ,don't give me a figure it won't mean much to me, ,what sort of damage would it do to living things at xxxx distances for example?
Btw ,somewhat rusty on what I was taught about radiation ,if an item (say a rock) was placed next to a radioactive material & then removed from the area is it still radioactive?
Or would it only be radioactive when exposed to the source?
>>>>most utilities are currently resigned to keeping all their fuel on site for the lifetime of the plant and beyond.<<<<<
And there in lies the possible problem.
What sort of shielding does that waste need?
|Not all radiation is 'harmfull'|
you say "what sort of damage would it do to living things at xxxx distances for example?" but you're exposed to radiation every day of your life. CArbon dating is a prime example. We all have levels of Carbon-14 in us, which decay over time, radioactively. They measure the amount you have, against the amount you should have had at the time of death, and then work out the difference in half-lifes. half life of C14 is something like 1400 years, iirc.
Second, therse 3 main types of radiation. Alpha, beta and gamma. Alpha is a helium nucleus. Paper will stop it, as will smoke, which is why its used as the detector in a smoke alarm. Beta is an electron. Mildly penetrating, does very little damage again, because of what it is. Gamma however, is the biggie. Thats the one that needs the heavy lead shielding, and the concrete bunkers.
"if an item (say a rock) was placed next to a radioactive material & then removed from the area is it still radioactive?"
WEll, radioactivity is the breaking down of an element to another, and the emillion of radiation as a result. as such, its not 'contagious', you place a piece of paper next to a gamma source, and then remove it, the paper won't be a source of gamma, unless its been chemically broken down by the radiation (very unlikely). If however, it was a material that did respond to radioactive bombardment, then yes, it'd become radioactive, as its own elemental atoms broke down. Generally, though, materials are contaminated, my having them touch, they pick up small bits of material, which are radioactive. Its the transfered material that is radioactive, not the base material.
|lol ,finally got around to reading this thread again |
More interesting info again ,though I was aware that thier is a constant background radiation & that this is relatively harmless (though some research points to slighter higher cancer rates in areas with higher natural background radiation).
I was also aware of the 3 types of radiation & thier relative dangers (or lack of) ,though I'd forgotten that 2 of them were parts of an atom (so to speak).
IIRC gamma radiation is at the extreme top end of the EM spectrum ,& thus is pure energy rather that particles (sound right?)
Anyhow what I was trying to get at was how strong the radiation is from the ' ....elements in the fuel with long half-lives, but they are relatively weak radiation emitters' that Jon mentioned.
This is the waste that could be a long term problem.
On your last paragraph ,got ya ,makes sense
I just did a google search on 'Accelerator Driven Transmutation Systems' & the top listed site , http://www.ne.anl.gov/research/afc/index.html ,shows a 'last modified' date of July 06 ,does this mean its back on or is their research for something else?
I haven't had time to read it yet ,& I might not understand it anyway! (looks like the research could be back on)
|Yes, that does appear to be an accelerator-driven transmutation study.|
The DOE Advanced Fuel Cycle Initiative (AFCI) was launched in fiscal year 2003 as an outgrowth of the Advanced Accelerator Applications (AAA) Program,
I think it so happens that George Bush is a supporter of nuclear power, so has actually increased investment in this sort of thing via the DoE. Having said that, I've heard that the "4th generation" nuclear reactor programme was once widely believed to be a way to postpone thinking about whether to initiate new nuclear build in the US.
Also since I last posted to this thread, Tony Blair in the UK has given approval to a new energy policy in which there is a 30% nuclear component to the power mix along with increased renewables (~20% up from the current 4% or whatever).
|At the moment, I think it's fair to say the prospect of climate collapse is a lot worse than having stacks of plutonium etc. sitting around.|
|Mmmmm, activity (non-radioactive)|
K'Tetch, let me continue your discussion about the types of radiation. You said, "Not all radiation is harmful" and I would like to modify that to "Not all radiation causes harm." I make that distinction because all types of radiation CAN cause harm (alpha, beta, gamma and neutron).
While alpha particles are stopped by paper or smoke (paper, smoke and your epidermis are all dead material), if they are moving very fast, they can cause relatively large amounts of tissue damage because of their high mass (4 units) and charge (+2). If a radioactive, alpha emitting material is inhaled or swallowed, the alpha particles won't be stopped by your dead epidermal layer, they'll be stopped by living cells in your lungs or intestinal tract. An alpha particle is very likely to kill any living cell it impacts because of the energy (read that as "damage" it will give to the cell. In the nuclear biz, we go to great lengths to keep alpha emitters out of our nose, mouth or even cuts in our skin.
Beta particles are lower mass (a measly electron) but high charge (-1), so it will always interact with anything it hits. Betas don't penetrate deeply because of their charge, but they can cause cell damage if they hit a living nucleus or DNA strand. Energy deposited is also heat and too much heat will kill things, too. So, Betas aren't too bad but we don't ignore them, either.
Gammas are tricky. They come in a super wide range of energy levels. Low energy gammas will not cause much cell damage unless they interact with something vital like a DNA strand. Gammas have no appreciable mass and no charge, so how do they cause damage? They do it with "collateral damage." If a gamma actually impacts something with mass like a proton or neutron, it will impart some energy to object and then continue on its way as a lower energy gamma. The proton may now leave the nucleus of its atom, leaving it ionized (-1) plus the proton has a charge of +1. Ions cause cell damage as they seek to become non-ionized. This is called compton scattering. A gamma can also cause "pair production" as it interacts with matter. An electron and positron can be generated by a collision. We're talking matter and anti-matter here. The two opposing particles will now begin to circle each other, finally spiralling in and annihilating each other in a flash of pure energy. Bad news for the living cell containing the flash. There are other interactions, but less common. A gamma is pretty much a gamma in some ways - they travel in straight lines at light speed, but their energy levels can be in KeV (kilo electron volts) to MeV (mega or million) and higher. Cobalt 60 is a common radioactive material of concern at nuclear plants and when it decays, it gives off a moderate energy beta particle and a 2.5 MeV gamma. It takes a lot of lead to reliably stop a 2.5 MeV gamma. (more than 10 inches if you want to be sure)
Then there is neutron radiation. This isn't common. You won't find it around your home. Operating nuclear reactors give off lots of them but, when shut down, give off virtually none. Lead doesn't phase them much. No charge, so they don't interact with matter unless there is a direct collision. If a neutron hits a lead atom, it is like a golf ball bouncing off of a steel plate. Virtually no energy is imparted and the neutron just keeps going, though perhaps in a different direction. But, if a neutron hits a Hydrogen or Helium atom, something closer to its own weight, it will give up a large chunk of energy, as much as half, before moving on. Water is full of hydrogen. Lots of it. That makes water a good shield for neutrons, much better than lead. Then again, our bodies are mostly water. We stop neutrons pretty well. If you stop too many of them, you will get sick or die from the heat damage to your cells. Neutrons are bad news. Stay away.
Anyway, I'm tired of typing for now. Feel free to ask specific questions or to point out inaccuracies. I don't teach this stuff any more so I'm rusty.
|I'm a little surprised by the "smiley" that popped up above. I didn't intend that to be humorous, but punctuation can be touchy here.|
|Radiation exposure itself can be dangerous (and usually is if it is intense enough!), but there seems to be widespread confusion between that and storage of radioactive material. The point in the latter case is that the material *is* surrounded by 10 inches of lead etc., so while there is activity inside, outside it is generally harmless (though it still requires care to make sure the casing doesn't leak or split). The care required leads me to think the stuff is best stored in *supervised* underground storage areas, not "bury and forget". But if done well it's not a hazard to society or anything like that and isn't nearly as large scale as landfill in terms of waste volume.|
|Interesting info again Jon |
I understand that radioactive waste properly stored is not a hazard to people locally ,but I think the problem we haven't talked about here (though it's been ages since Ive read the whole thread through, so I may of forgotten) is how long does it need to be stored? ,& if its over many centuries can we be almost certain that any area will be safe from geological activity and look after properly?
(I'll have to reread the whole thread at some point )
|"I understand that radioactive waste properly stored is not a hazard to people locally..."? This link may be of some interest: http://www.pnl.gov/news/release.asp?id=211|
|See my definition of "properly" Bury and forget for 1400 years is not "proper" storage.|
|They just need to build that "Space Elevator" ( http://en.wikipedia.org/wiki/Space_elevator ) , and then just slowly start lobbing this stuff toward the Sun.|
|i dont know if that is an good idea =)|
|Wouldn't cause any harm to the sun ,but might cause problems for any passing spaceships!|
|Funnily enough, an e-mail about accelerator-driven reactors appeared in my inbox via a circuitous route this afternoon. I spent the afternoon wandering around talking to people making sure the appropriate experts knew the UK was contemplating a research study on such a thing, as they'd certainly not sent it to us all at RAL directly.|
For transmutation of waste in particular, I've heard views that becuase of the need to recirculate many intermediate radioactive isotopes back into the system and separate them from the stable nuclides, the studies generally suggest more waste is produced than consumed by such a thing! However economies of scale may operate, so if a facility tried to process waste for the whole world, it may end up reducing it significantly.
|So are we (the UK) going to do transmuation experiments then?|
JonB are you still reading this thread? ,you missed some questions of mine from my post on 2006-02-01 & 2007-06-08.
|The answer to that is "probably not", it would require a lot of funding at a time when the government is running very low on funds, and may be politically unpopular - just look at the amount of problems people have with conventional nuclear reprocessing sites like Sellafield - I'm sure they'd _really_ love having a highly experimental reprocessor built that contains just about every radioisotope in the book. The accelerator-driven reactor may be a bit more popular, but I'm wondering if conventional reactors aren't safe enough already, what's the point of driving it with an expensive accelerator system just to get a safety improvement that may be entirely notional.|
What may be possible is that the UK would get involved in design studies for such things, and then maybe in the years 2020 and far beyond could build one (though I would think Japan and France would be more likely candidates for building the first of such a machine). Japan already are commissioning a small system with a test reactor on the end.
|Assimilator - hadn't forgotten so much as got too busy over Christmas with my synchronized light display. |
Danger to passing spaceships and the space elevator. I suppose that adding heavy elements to the sun, which is mostly lighter elements, might have some impact on things like sunspots and solar flares. While the sun is huge, we thought the Earth was too large to ever notice things like burning coal and oil, etc... I'd say, let the spaceships watch out for themselves. If visiting aliens can't handle that, then how did they cross galactic space, anyway?
As far as storage for millenia is concerned, my reasoning is influenced by a novel called "Evolution" by Stephen Baxter. Very interesting future for our descendents. I think that by the time our best efforts to safely and securely store nuclear waste begin to fail (let's say 2000 years) that our science and technology will have progressed to the point that there is no problem, or our worldwide civilization will have collapsed to the point where nobody will care. 2000 years is a lot of scientific development.
|lol @ light display .|
I didn't mention about spaceships & elevators, I assume that's for someone else.
Re long term waste storage, could we even guarentee safe storage for 500 yrs let alone 2000yrs?, true if we are still about in 2 millenia then I'm sure we would have the technology then to neutralise it & probably in 500yrs too.
But what if containment was damaged in 100-200 yrs from an unexpected earthquake etc? could we be sure to have the technology then? What if our civilisation partially collapses where we are still spread around the world but technology has been severly restricted/damaged say by running out of oil & coal with no real replacement?, or severe worldwide damage from a large volcanic erruption or an asteroid impact? Could we deal with it then even if we had the knowledge?
I think the last thing a critically 'damaged' civilisation would want to deal with is leaking radioactive waste, I know it's a lot of 'ifs' but that's the trouble where we are dealing with waste which at the least could be a problem for several hundred years, or alot more if technological advancement is halted or reversed by a worldwide catasprophe.
How long can they guarentee a storage site will be safe for? (excluding the incredibley unlikely event of a direct asteroid hit ).
Als if it does 'leak' into say the water table, would it contaminate drinking water in a wide area? (oh & even if we didn't exist we don't wanta ruin parts of the planet for the remaining life ).
|Generally, I'd say designing a storage facility for 200 years is a good plan and the idea is in the year 2200 you'd have another review of current best practice and hopefully transfer the waste into a more efficient one. You'd also keep monitoring the waste throughout the 200 year period.|
If a radioactive waste repository leaked, it's difficult to see a realistic situation where it would cause widespread damage outside its local area. Chernobyl certainly made a mess, but all the UK got was a plume of moderate radiation for a few days; even most of the Ukraine is fine.
|Balarus is the country that got most of the damage as Chernobyl is almost on the border and winds were going north west at the time.|
|Leakage into water table is not a credible scenario for Yucca Mountain. The waste stored there will be predominately (perhaps exclusively) dry material. The spent fuel assemblies from nuclear power plants stay under water at the plants for several years, during which time they have substantial decay of short lived radionuclides. That means they are still giving off plenty of heat and the water keeps them cool. After 5 to 10 years, they don't produce enough heat to damage themselves even without water, so the current practice is to place them in "dry storage" casks. This frees up space in the "pool" for spent fuel after each refueling outage (12 to 18 month cycle, usually). When/If Yucca Mountain opens for business, the dry casks will be shipped there. I'm not sure whether they will remove the fuel assemblies from the cask for storage, but they will still be completely dry. No water table problems. |
Other countries are doing this already, but the US is finally looking at processing the spent fuel assemblies to remove usable fissionable material (Uranium 235 and Plutonium 239) and blending it in with freshly enriched U235. That should extend fuel supplies for a few more centuries until Fusion power finally gets the breakthrough it needs. The side effect of using blended fuel is it gets rid of Plutonium, which isn't naturally occurring anyway.
|Seems like a good plan on the face of it, storing nuclear waste relatively high up in the base(?) of a mountain should keep it a long way from the water table.|
I assume they are pretty solid & resistant to earthquakes too .
Are the only storage sites considered for nuclear waste in the world that high up & stable?
How radioactive are spent fuel rod materials that have been re-processed?
Btw re modern reactors mentioned earlier, how does the water enable a controlled chain reaction to happen?
Trying to summarise here:-
In modern reactors (how modern btw?) if their's a severe coolant (water) loss then the nuclear reaction stops & it cannot runaway? Is that 100%?
Storing radioactive waste can be safely stored IF its regularly monitered (for how long?) & not buried & forgotten, stored away from the water table,& is siesmically stable.
Transmutation is still in developement & maybe possible in the near to medium term?
Stephen you said >>>>Japan already are commissioning a small system with a test reactor on the end.<<<<
Is that a reactor for transmutation experiments?
|"...how does the water enable a controlled chain reaction to happen?"|
The two hugely prevalent type commercial plants today are Pressurized Water Reactors and Boiling Water Reactors. They both use plain water as coolant and as part of the control mechanism in the nuclear reaction. In a PWR, the type I'm most familiar with, the reactor's fuel is surrounded by pressurized water of approximately 2000psig (2000 psi, = 13.8 MPa, = 138 bar). At that pressure, water doesn't boil until the temperature is very high (640F or 335C). Using this, the reactor can heat the water and then pump it out to a "steam generator" that is just a boiler. The steam given off by that boiler powers a normal steam turbine. So, in a PWR, the reactor coolant does not boil. Your question, though, is about the nuclear chain reaction. To explain that, I must tell you about neutrons. They are the key to both creating and maintaining the chain reaction. Uranium fuel, more specifically the isotope Uranium 235, will fission if it absorbs a neutron. In fact, it will fission almost instantly after absorbing a neutron. Too many neutrons at once and you will have uncontrolled fission, otherwise known as a bomb. If you can control the quantity of neutrons, you control the power production rate and can make heat and power. Water is a good neutron control media because it is mostly hydrogen (2/3 hydrogen, in fact). Now, let's get more technical. When a Uranium 235 (U235) atom fissions, it breaks into 2 or more smaller elements, a fair amount of heat and also releases an average of 2.38 neutrons per fission. If every one of those 2.38 neutrons then caused another fission, then we could never control a U235 reactor. In fact, it is actually hard to get ANY of those released neutrons to be absorbed because they are too fast to be absorbed. Their kinetic energy is just too high and they cannot be captured by the nuclear of a U235 atom to cause another fission. Water is very good at slowing down neutrons because of the hydrogen. Think about this - the only difference in atomic weight between a lone neutron and a hydrogen atom (a single proton and electron) is the electron. When a neutron hits the proton of hydrogen, the neutron slows down by half and the proton speeds up. After a few more collisions, the neutron slows down enough that it is referred to as a "thermal" neutron because its Kinetic energy is the same as the water it is bouncing around in. Once it becomes "thermal" it is readily absorbed by a U235 nucleus and then fission occurs. The design of a PWR core matches the quantity of water, the quantity of fuel and balances that with various safety measures like safety shutdown control rods to easily control the chain reaction so that for "every fission event, just one of the 2.38 neutrons survives to cause another fission." When that One to One ratio exists, the reactor is referred to as "critical." I mention that word because "critical" really means normal and safe even though movies would have you believe it is a bad thing.
"Severe coolant water loss" would definately stop the reactor because the neutrons won't thermalize. That would be the least of your worries, though, because a total loss of coolant would mean that the fuel wasn't being cooled and it would soon get hot enough to become damaged. That is why the most modern reactor protection systems have huge gravity fed tanks of water above the reactor that will dump into the reactor to keep it full.
|A bit late , but thanks for the explanation Jon |
Re your post 2008-01-07, is that a reactor for transmutation experiments?
|I've heard more on the transmutation in the last year. Apparently the worst parts of the nuclear waste (the transuranics) can be "burnt off" in a relatively simple accelerator-driven-reactor (without the need for isotope separation). That would still leave radioactivity but its half-life would be of the order 100 years, not 100 thousand.|
That makes the accelerator-driven-reactor(ADS) for "transmutation" a very similar thing to the one for energy generation.
|Sounds good, thx for the info|