These questions are inspired by a conversation I just had with a friend of mine. Asking him to further elaborate on some of these things probably would not bring any more clarity. Some of what he said just pinged my BS meter, but I don’t really understand enough about these issues. I hope some of the experts here can help me out.
My friend says that President Jimmy Carter lobbied for and got passed a law requiring American nuclear power plants to remove their fuel rods before they become “weapons grade”. Unfortunately, this causes much of the fuel to be wasted and creates an unnecessary waste stream. If power plants could use the fuel until it is truly spent, the fuel would last much longer, creating a smaller and less hazardous waste. Electricity would be insanely cheap. Carter’s concern was that the weapon-capable fuel could be used as a weapon, potentially causing a problem. Do American power plants remove otherwise useful fuel due to regulations? Did Carter advocate for these regulations?
My friend says that Canada “got it right”. He says that they take water from Lake Ontario, extract heavy water from it, and use it in the nuclear power plants (I don’t know for what). When they’re done with it, they dump the waste, water, back into Lake Ontario. The United States doesn’t have this large body of water to use (I don’t know why the other Great Lakes are unusable). Essentially, Canadian nuclear power is cheap and pollution-free. Huh? What could he have been talking about? What is this Lake Ontario miracle?
Thanks for your illumination of some of these things.
I believe Jimmy Carter banned reprocessing of fuel. The reasoning behind it was to prevent proliferation of plutonium. If we reprocessed fuel it would last longer and produce less waste.
Canadian reactors (CANDU) use heavy water as a moderator. This allows them to use unenriched uranium as a fuel. Using unenriched fuel is cheaper but using heavy water costs more, I’m not sure if its cheaper overall. It’s not cheap and pollution free.
CANDU reactors, as mentioned above, use heavy water (water containing the deuterium isotope of hydrogen) as a neutron moderator. They use ‘natural’ uranium fuel, which is to say, ‘unenriched’. However, they still generate radioactive waste. Here’s a link to Canada’s Nuclear Waste Management Organization.
In the CANDU reactor the heavy water moderator sits in a large tank (‘calandria’) surrounding tubes which contain the uranium fuel. This heavy water is heated by the nuclear reaction, and passes through pipes where it in turn heats up a second loop of ordinary water. This hot, ordinary water passes through the turbines to generate electricity. It is then discharged from the plant. This water retains some residual heat, but very little contamination. For the Darlington and Pickering nuclear stations, this water is vented into Lake Ontario; several other reactors exist in Canada which are not located on that lake.
Heavy water is indeed extracted from natural water sources, possibly including Lake Ontario. The heavy molecules are separated from the rest at great expense. It stays in the reactor, however, and is not discharged. It’s so valuable that Atomic Energy of Canada Limited has a stock which they can loan to people carrying out experiments that need it, such as the neutrino detector at the Sudbury Neutrino Observatory, which contained a third of a billion dollars’ worth of the stuff.
In the CANDU reactor the heavy water moderator sits in a large tank (‘calandria’) surrounding tubes which contain the uranium fuel. This heavy water is heated by the nuclear reaction, and passes through pipes where it in turn heats up a second loop of ordinary water. This hot, ordinary water passes through the turbines to generate electricity. It is then discharged from the plant. This water retains some residual heat, but very little contamination. For the Darlington and Pickering nuclear stations, this water is vented into Lake Ontario; several other reactors exist in Canada which are not located on that lake.
QUOTE]
This does not sound right. Just heating water and running the hot water through a turbine would not yeild much power.
I think you are confusing condencer cooling water with water in the steam system.
My understanding is the High pressure water in the primary loop is heated in the reactor. This primary hot water heats the water in the secondary loop, and turns it into steam. The steam is expanded in the turbines, turning them and producing power. The exhaust from the turbine enters the condencer and passes over the condencer tubes. The exhaust steam is condenced into water by lake water, sea water, or tower water is pumped through the condencer tubes. The water leaving the condencers returns to the lake, sea, or cooling tower. The only change in the condencer water is it had increased in temperature.
You are confusing the steam power cycle with the cooling cycle. The steam (not hot water) which spins the turbine goes through a condenser, where it is cooled in a closed system by cooling water. The condensed steam can now be sent back to the primary loop via a circulating/feed pump. The cooling water either goes to a cooling tower, or can be exhausted to a water source when done. This is the same principle used by pretty much all nuclear reactors.
Well, obviously the water boils, and of course it’s a steam turbine. That’s essential to the power station architecture, but the phase change isn’t important to showing the mass flows into and out of the plant.
I was just tying to show that the heavy water isn’t discharged as waste; rather, the only water discharge is from a separate system that may accept waste heat but doesn’t pass through the reactor. I left out a step or two in my simplification.
Ok, Just the engineer in me hates to see what I see as miss information about a process. And not everyone here understands a steam cycle and I was concerned that there could be confusion. Sorry
Others have mentioned that the heavy water (D2O)isn’t put back into the environment. This is not only to avoid radioactive contamination (D20 isn’t radioactive itself, but since it is in the primary loop of the reactor it can pick up radioactive particles and some of the deuterium captures neutrons to become tritium, which is radioactive) but becuase D2O is actually rather expensive.
I’m also under the impression that, due to large stockpiles, we don’t need to produce any more heavy water in Canada, and haven’t for quite a few years.
A general steam plant answer (Naval power plants, that is - see Una for a civilian power plant answer)…
A typical ship’s steam plant has two turbines, side-by-side: a high pressure turbine and a low pressure turbine. Each one is tuned for a different pressure range, and uses different blade shapes and such.
A typical oil-fired boiler with superheater will generate 1200psig; hence, the high pressure turbine will operate with an inlet pressure somewhere thereabouts.
Nuclear power plants operate at different pressures, but the concepts are similar.
There are throttle valves before the HP turbine inlet, so there will be some pressure drop there.
The outlet of the HP turbine is at a much lower pressure (I have forgotten that pressure) and may be run through a moisture separator and then it goes to the inlet of the LP turbine.
The shafts of the two turbines feed a set of reduction gears that then drive the main shaft.
The exhaust of the LP turbine goes to the condenser, which typically operates at a vacuum, maintained by the condensation action of the steam combined with “air ejectors” that suck any air out.
Here’s a pagethat gives a lot of info about one of these steam plants.
It depends on whether you’re looking at a non-reheat, reheat, or double-reheat cycle, and what type of power plant. Nuclear turbines generally run at pressures which are a bit lower than coal plants, and these can be down in the relatively low under-1000 psi range. Boiling water reactors try for around 1000 psi with saturated conditions, maybe hitting 950-975 psi at the HP turbine inlet. Final exhaust pressure at the condenser depends upon the cooling water temperature (and numerous other factors) but typically is about 2-6 inches of mercury absolute.
Actually, now that I think about it, I’m not certain there are any reheat-cycle nuclear plants, but I wouldn’t be surprised if there were.
D20 probably, but it’s not really consumed in operations like fuel is (there are always unintended losses).
Hmm, according to wiki Heavy water - Wikipedia “Atomic Energy of Canada Limited (AECL) is currently researching other more efficient and environmentally benign processes for creating heavy water. This is essential for the future of the CANDU reactors since heavy water represents about 20% of the capital cost of each reactor.” I hadn’t realised it was that expensive.
A ships main high pressure turbine inlet pressures will depend on design and age. From 440 PSI and 750 degrees to 1200 psi and I believe they were 1000 degrees. When on the open sea and not manuvering a merchant ships main throttle is wide open and the speed controled by opening or closing nozzles.
The exhaust pressure from the HP turbine to the LP will depend on the load on the main engine. The exhaust from the LP turbine will be in the range of 29 inches of vacumm. The temperature will be in the 80 degree range.
A stationary p[lant can be from maybe 150 PSI to 3205 PSI and from staturated temperature or superheated to 1000 degrees. Same exhaust temp and pressures.
Others have addressed other misconceptions, so I’ll limit my response to this. First of all, the advantage of heavy water (HWR) CANDU-type reactors is that one can use natural uranium (~0.7% [sup]235[/sup]U) or slightly enriched uranium (0.8-2% [sup]235[/sup]U), as opposed to light water reactors that have to use low enriched uranium (3-5% [sup]235[/sup]U) to achieve criticality.
Second, weapons grade material is about 80% [sup]235[/sup]U or better; the Navy strategic weapons (W88 warhead) use a supergrade that is (IMSM) ~96% [sup]235[/sup]U. This is probably about as pure a grade as extant technology allows, although there are a variety of systems that can make the production of HEU less energy intensive than the gas centrifuge method previously used. It is possible to make weapons with lower grade materials (~30%) but the yield is substantially less and fizzle (premature low yield detonation) is far more likely. Also, most fuel-grade uranium, and especially used fuel, will contain [sup]236[/sup]U, which is a total neutron hog that will poison any fast fission reaction.
Plutonium is even worse; while plutonium can be chemically separated from uranium in once-through fuel elements, [sup]239[/sup]Pu cannot be readily extracted from [sup]240[/sup]Pu and [sup]241[/sup]Pu, which will make any weapon so composed unstable to the point of being unusable. For weapons grade plutonium, the [sup]239[/sup]Pu is actually produced by a breeder cycle and the fuel is frequently cycled for extraction of the desirable plutonium before undesired isotopic species are generated.
In essence, if you have the technology and access to process uranium and plutonium into weapon-grade material, you don’t need to steal it from someone else. As far as waste, while the nuclear industry is no banner child for cost effectiveness, if they could make more use of fuel elements with the existing unmodified cycle and reactor they certainly would. The bulk of the waste stream doesn’t actually come from fuel elements, which can be vitrified and stored indefinitely with a reasonable level of safety, but waste products produced during the fuel processing cycle.