Is nuclear fission safe?

Post Reply New Topic

Hey, if it can essentially "eat" our existing nuclear waste and give us ANY electricity at all out of the process, it's worth trying! Build one super-good one out in the Nevada desert or something...
Even if it were nothing more than a waste consumer, it's worth doing, just to eliminate that huge negative issue.


And I think I really stumbled on something, that we can blame corporate greed for preventing the mega-development of solar tech... They want to SELL power!

One modern proposal is for a molten salt reactors, some with thorium.

In a molten salt reactor, the salt would be mixed in with the fuel. If it gets too hot, the salt expands which spreads out the fuel and consequently reduces the reactions. If it overheats more, a stopper will melt and let the fuel flow into a holding tank where the fuel would spread out and stop the reaction entirely.

From http://www.extremetech.com/extreme/150551-the-500mw-molten-salt-nuclear-reactor-safe-half-the-price-of-light-water-and-shipped-to-order:

From http://www.telegraph.co.uk/finance/comment/ambroseevans_pritchard/8393984/Safe-nuclear-does-exist-and-China-is-leading-the-way-with-thorium.html:

Precisely why thorium is ideal for generating heat to boil water to run turbines to produce electric current. And there is plenty of it. Enough for thousands, perhaps tens of thousands of years.

Eventually we will have to develop technology to tap the heat beneath the earth's crust. There is enough there for billions of years.

ruveyn

It would seem that the French have it right with nuclear power, as their reactors are of a standardized design, which means your aren't having to design things over and over again.

Just thought I'd chip in. I'm part of a laser plasma team that works really closely with Prof Cywynski on this topic and it's one of my pet interests - I'm running simulations based on shock neutrons at the moment.

So although this topic is miles down the board, if anyone wants to know anything about thorium reactors, ask

Back in 1960 they told us we would have controlled nuclear fusion in 30 years. In 1990 they told us we would have controlled nuclear fusion in 30 years. In 2013 they told us we would have controlled nuclear fusion in 30 years. I am sure in the year 2100 they will be saying we will have controlled nuclear fusion in 30 years.

ruveyn

Just one question comes to me immediately: since there is no chain reaction, how much energy does it take to drive the reaction and how much energy is released in the fission?

I always thought that the chain reaction was very important to get enough fission activity.

In plasma methods (like the Tokomag) the trick is diverting some of the output energy to maintaining the plasma. This has yet to be done for significant intervals of time. If the plasma cools down after being fused, then the method is simply not practical.

Controlled fusion has been achieved many times, but no one knows how to sustain it.

Fission is a different story. U-235 is just on the edge of instability. A U-235 atom wants to split and it doesn't take much of a nudge to do it. One slow neutron and it happens. Hydrogen does not want to fuse. The nucleus consists of a proton and pushing two hydrogen atoms together is fighting coulomb repulsion force. It takes a high temperature to bang two hydrogen atoms together hard enough so the strong forces can bring about fusion. In the Sun the grip of gravity makes the hydrogen atoms fall together so fast that they fuse into helium. In man-made devices no such force has yet been found to produce a sustained fusion processes.

ruveyn



ruveyn

My question was about the use of the laser to induce fission in a thorium reactor where there is no chain reaction as in conventional fission reactors.

From the article I linked earlier:

Speaking of fusion, I've stood within four or five feet of a small tokamak while it was being used to create a fusion reaction. It was quite noisy. After the first one, I kept a greater distance from the others during the demonstrations. That was in 1971. Do they still use tokamaks?

Lasers are capable of getting a small amount of hydrogen to fuse. The problem is deriving enough energy to keep the laser fusing more hydrogen. So far no one has been able to do this.

As I said, nuclear fusion has been accomplished by hot plasma and by lasers but the problem of sustaining the reaction is still unsolved.

ruveyn

I thought the problem was in trying to contain the plasma so that they could maintain fusion.

I readily admit that I haven't paid much attention to fusion research in the past thirty years, so maybe they solved that part.

By the way, I asked one solid state physicist in the late 1970s about nuclear fusion. He responded that there were much more interesting problems that actually had a chance of being solved in our lifetime. He didn't think that controlled fusion is going to be viable for a very long time. I'm starting to think that he was correct. In 1971 they were saying it was thirty years away. Forty two years later, it's still thirty years away.

Tokomags have contained plasma. But the plasma cools down after fusion so more energy is needed to sustain it. Magnetic confinement cannot be maintained with current technology.

ruvein

When I wrote about containing the plasma, I was talking about the magnetic confinement.

By the way, when I first saw your use "tokomag", I assumed you meant tokamat. Since you keep using "tokomag", are you talking about something different than a tokamat?

I know this is pretty far down the thread, but I'll throw in my opinions. I'll also let you know some of my background so you might believe that I know what I am talking about. I was in the navy and worked on submarine nuclear reactors. I slept for months at a time within 15 feet of the reactor compartment. I have a degree in radiation sciences, and I currently work as a health physicist in the Radiation Safety Department. I currently maintain and continually improve all radiation safety aspect of our radioactive materials (including nuclear medicine, electron microscopy, and the research department).

First off, radiation is everywhere. It is in the ground that you walk on, the air that you breathe, the food that you eat, and coming from space to hit you on the top of your head. Carbon dating is based on radioactive the radioactive decay of carbon-14, a naturally occurring radioactive element that is incorporated into your body the entire time you are alive. I'm sure you've heard of radon gas. It is produced by the radioactive decay of naturally occurring radium, which ends up in building materials. You also get radiation from x-rays, fluoroscopic procedures, nuclear medicine studies, and CT scans. These procedures do not make you radioactive, no matter what some people may tell you.

There is a huge difference between radiation and radioactive material. Radioactive materials produce radiation. Think of the difference like this (not entirely accurate from a physics standpoint, but a pretty decent metaphor): radioactive material/contamination is like a giant steaming dog turd. It can get spread around and contaminate a large area. Radiation is like the stink that the giant steaming dog turd gives off.

A substantial percentage of low-level nuclear waste is actually less radioactive than a cinder block basement. Much of the waste that is labeled as such only has that designation because of regulations regarding its production, not the radioactivity itself. The problem with the accumulating nuclear waste is that no one wants it disposed of in their state, as well as the fact that environmentalists seem to have trouble with the idea of disposing of it at all. There are many viable solutions to its disposal, but too many politicians and lobbyists are too busy arguing about it to actually do anything.

Baking our radioactive waste into high density glass bricks (a very stable form which is unlikely to degrade significantly enough to cause contamination) and dropping them miles below the ocean surface is a viable solution. Water is an excellent shielding material. It attenuates the energy of the radiation, which is then given off as heat; it does not become radioactive. This is how nuclear power is generated: massive amounts of heat are generated in the primary coolant. This superheated water is in a wholly self contained system. The primary coolant is then circulated through steam generators, where the heat is transferred to a secondary water system, producing high pressure steam which in turn drives turbines which then generate electricity.

As has already been stated, thorium reactors would be a much more ideal reactor plant design. High level radioactive waste would be excellent to use for breeding thorium usable in a power plant, helping to reduce the current build up of waste. It will not likely happen any time soon for a whole host of reasons, but cost is the big one. It costs a crapload of money to build a nuclear reactor. You have to pay the costs of the design team, the specialized construction, the massive regulatory costs, environmental studies, paying off the government in the area you want to build it (as well as surrounding areas), you have to have a significant amount of cash set aside against the possibility of something going wrong, and then you have to hire and train highly educated and skilled workers before you can even start it up. It is just not as profitable as frakking, coal mining, oil drilling, etc. It is also very heavily regulated, which creates an added layer of pain-in-the-ass crap that companies do not like to deal with.

Nuclear power plant incidents:
Chernobyl had major design flaws on many levels. In large part, it was a by-product of the cold war. Chernobyl was designed to produce power, but also to rapidly breed weapons grade plutonium. Its design was also inherently flawed in that its primary cooling medium had a positive coefficient for reactivity, meaning that as the temperature went up, it increased the rate of fission, which in turn increased the temperature (basically creating a self-feeding loop of craziness). Water, on the other hand, has a negative coefficient for reactivity.
TMI was a case of gross negligence, incompetence, violations of federal regulations, an apathetic work environment, lack of training and basic knowledge, lacks safety inspections, and ridiculously poor management. The regulations enacted after this incident (in the US, I can't say anything for other countries) made it nearly impossible for a reactor to meltdown. There are now so many safeguards and interlocks, plus backup interlocks, automated safety interlocks, and automated shutdowns that it would be extremely difficult to do on purpose, even by highly trained professionals.
Fukushima was a combination of several construction flaws that should have been caught in inspections prior to beginning operations. Dumping seawater on it only made the problem worse by several orders of magnitude. If they had used freshwater it would not have been nearly as bad. Purified freshwater would have been even better. Better still: an emergency system that would have filled the reactor compartment with boronated water would have neutralized the situation.

Fusion as a sustainable source of energy is a pipe dream. Without the massive gravity that makes the sun work, it will always take more energy to produce fusion than is generated by it. Perhaps in a hundred years we will figure out some new laws of physics that will alter that. Most current research into fusion has nothing to do with creating a viable energy source anyway. I put fusion up there with hydrogen as a fuel source. Yes, hydrogen is clean burning, but the amount of energy it takes to produce is much greater than the energy it gives off; not to mention the inherent combustible dangers of hydrogen itself.

Nuclear power plants that are built to exact specifications and safety requirements and run by competent employees are only minimally dangerous.
Radiation and radioactivity can be dangerous, very dangerous, but it doesn't have to be.

If you have any questions about nuclear power, fission, radiation, its interactions with matter, or radiobiology just send me a PM.

All nuclear reactions are safe as long as you show the components the respect they demand... the same principle applies with any energy source let it be a campfire or a PBR. Fission by its nature releases high amounts of radiation, however controlled fission reactions *pretty much all used in power plants and labs* reactors and fuel are well shielded to prevent radioisotopes and EM radiation from being released into the surrounding environment. IF the source of the uncertainty is from the usage of Fission in nuclear weapons, I can whole-heartedly assure you the U235 and Pu241 cores used to make fission components for nuclear and thermonuclear (hydrogen) weapons are specifically designed to have a very high purity of the desired isotope to result in a supercritical mass, which makes the runaway reaction and hence the nuclear fireball. Reactor grade isotopes will not runaway like weapons grade will, unless the catastrophic failure of an experimental "breeder" reactor were to take place... which I have only read of a single such facility in the US.

Hi! Sorry, I was too busy doing work to answer earlier. I'll reply now

Yes, that's more or less the running in-joke of the fusion community. It is always 25 years away. The truth is when we started looking into nuclear fusion in the 1960s we really knew nothing about plasma at all. We had some basic understanding through Maxwells equations and gas laws but plasmas as a medium are not a simple construct.

However we are getting closer and very quickly as computational power allows us to model new experimental parameters far quicker than we used to. NIF is at about 70% of ignition (meaning the hot spot in the centre of the fusion capsule is producing 70% of the energy being fed into the lasers. ITER will break even on its first real test run (that's not the problem with MCF) and will allow engineers to test and produce materials able to withstand the neutron bombardment resultant from tokamak operation.

At the risk of a cliche.. 25 years is about right based on what we know now.



Okay. What happens in accelerator driven thorium reactors is a beam of protons is directed onto a spallation target in the centre of a rod of thorium. Thorium itself is not fissile but thorium upon interaction with neutrons produces a fissile uranium isotope, uranium 233 which decays through a different decay series than conventional u255 and u238. How do we get the neutrons? The impact of the proton beam on the spallation target produces a flux of neutrons in all directions from the spallation target, which then irradiates the thorium. This is a -very- efficient way of producing a high flux of neutrons and we see the same approach in ISIS and other such neutron spallation facilities already in use.

This results in two things.

1. A decay series which does not involve long lived actinides. This results in a 10 to 100 fold reduction in long lived radioactive waste.
2. A reactor which is completely incapable of going critical. The rate of reaction is regulated by the flux of the proton beam. The power driving the proton beam (probably on the order of 50-60MW will produce 500-600MW from the reactor itself with an increasing amount with the larger the proton beam becomes.

So why haven't we done this already?

1. We don't have accelerators capable of producing a high enough proton beam current.
2. Any such accelerator capable of doing it would be exorbantly expensive with present methodologies.

Thus we need to come up with a new field geometry or a new method of acceleration. Two possible approaches present themselves, FFAGs (fixed field alternating gradient accelerators) or laser plasma wakefield accelerators which will have a lower beam current but produce intense pulsed bursts of protons in a relatively short space.



ITER in France is a Tokamak design. It is incredibly noisy but that noise is mostly vacuum pumps and generators used to drive the electromagnets.