Efficiency of nuclear power plants.

[Degenerate]

I don't fully understand the intricacies of a nuclear plant, but how efficient is it?
Lets just say the radioactive fule is aleady available. it is basically allowed to decay and in the process producs heat and other forms of energy. the heat is used to turn water into steam and turn turbine....

What happens to the radiation (not in the for of heat), like gamma rays, beta and alpha? do the engineers make this radiation hit lead or some object and produce heat?

My friend has an idea to use the radiation in the photoelectric effect?

I recetly read on a New Scientist magazine that people have developed fule in the form of small capsules (radioactive fuel enclosed in a ceremic ball). Does this capture the radiation?

Also. i hear that nuclear plants habe to have coolers. why is this? why can't they control the temperature perfectly so that they dont need to cool it down. it seems rather pointless if they are cooling what they set to heat up.

How close are we getting in developing feasible fusion? i am guessing not far.

I know my questions are out of place, but some discussion?

 

[Ynog]

The question is no more out of place than say What if the moon cease to be or what happened to the dinos.

I could explain how nuclear power works and why you need a cooler, but I found this website that does a
much better job explaining what you want to know than I could.
Nuclear Power

 

[stebesplace]

maby all the nukes destroyed the dino's

 

[dakels]

lol steve you crack me up. Maybe you're right tho. Its wasn't a huge asteroid but a dinosaur sized ICBM. The IceAge was actually a nuclear winter.

btw whats "The Best Picture" link about?

 

[PowerEngineer]

Nuclear power plants actually operate a lot like fossil fueled plants. Water is boiled by a heat source and the steam is run through a steam turbine that coverts some of the steam's energy in to rotational energy that turns an electric generator. Steam exiting the steam turbine is dumped into a condensor that extracts enough additional energy from the steam to condense it back into water. And the water is pumped back into the boiler... Nuclear plants use the fission reaction in the reactor as the heat source (this from the conversion of mass to energy; the radiation is not converted into heat); that's the only real difference. The "coolers" you're probably referring to are used to dump the heat extracted by the condensor. I know it seems wasteful, but (alas) there are unavoidable laws of thermodynamics that place limits on the amount of useful energy (i.e. electricity) one can extract from a steam "cycle" like this; the rest does have to be dissapated somehow. The efficiency of the steam cycle is related to the temperature of the steam. Nuclear plants operate with temperature restrictions on their cores that do not allow them to produce really hot steam, so their efficiencies are in the 35-40% range. Fossil fuel plants are often 5-10% higher. Some combined-cycle plants that generate electricity using gas turbines and then use the hot exhaust to produce steam for another generator can reach 60%.

 

[dakels]

Well there are 2 major things that control heat or energy released. The heat comes from the decay of the uranium fuel rods that causes the heat that creates the steam. The Uranium must be kept at a certain control/temperature range or else the fission reaction will get out of control and melt the core. The other heat control is more like what powerengineer stated. Transferring the heat for electrical usage.

The control of the nuclear core's fission reaction is done with control rods. These rods are primarily silver and a few other metals. These control rods are raised and lowered around the fuel rods. They absorb the free neutrons shooting around the uranium fuel rods. If you were to freely let the neutrons go, they will cause a chain reaction that will grow exponentially. Thats how a nuclear meltdown is caused. The more you surround the fuel rods the more you slow down the fission. If the control rods are lowered all the way, it will stop the fission reaction. The rods in normal function are balancing the tightrope between a controlled reaction to produce enough heat, and an outright nuclear meltdown. The reason why you won't see a Hiroshima style explosion is due to the type of fuel. The normal nuclear fuel rods are comprised of only around 2-4% Uranium 235. Weapons grade uranium contains about (IIRC) 80-90% 235 thus causing a much more dramatic and violent reaction. A nuclear bomb.

So the 2 heat controllers a control rod and the coolant, usually water (special type of water) are used. The control rods prevent nuclear meltdown and the water that is used to carry the heat away from the core for electricity.

As for the ceramic, well thats what the fuel pellets are made from nowadays. The pellets are a mix of ceramic, just like your dinner plate, and uranium powder. The pellets are then loaded into a metal rod. Those rods then go into the reactor. The ceramic is not really used to not block radiation, but to control and contain the uranium as it breaks down. Uranium breaks down and becomes other elements including gas. The ceramic holds the radioactive solids but is pourous enough to release the gasses. I believe this is all done for better containment and control over the nuclear reaction and its radioactive waste.

 

[KenGr]

To continue to make this more technical, it is important to understand that only a small portion of the heat in the reactor comes from "decay". Most comes from the fission reaction which is not decay. The Uranium (or Thorium or Plutonium) is made unstable by the addition of a free neutron and fissions. The two or three resulting elements atoms have a summed weight less than the original fuel atom. The difference (a la Einstein) is given off as energy, heat and light, which is our major heat source. Some of the product atoms are unstable isotopes of the elements. With half lives (average durations) of between fractions of a second and thousands of years they give off a neutron or an electron (with resulting gamma rays) and become a different isotope - stable or unstable.

All of the rays given off (alpha, beta, gamma) subsequently interact with matter (coolant water, structure, people, etc) and turn into heat. (There is a bit of light also, but that ends up being absorbed and becoming heat.) As far as the photoelectric effect, it does exist. The junk you see on TV with the blue "nuclear glow" actually has some truth to it. Some of the emitted energy ends up as "Bremsstrahlung" (OK, I had to look up the spelling, it's been a while.) These are x-rays (effectively very low energy gammas) given off as Beta rays slow down in matter. This is accompanied by a visable light emission. Research reactors are often "swimming pool" reactor types and the blue glow is quite lovely.

The control rods have various composition but the key element is often Boron. Boron sucks up neutrons like crazy. Commercial pressurized light water reactors always operate at power with the control rods completely withdrawn, to keep the fuel evenly burned and maximize fuel usage. The power level is controlled by adjusting the boric acid content of the coolant.

The fuel pellets are in long tubes made of a highly corrosion resistant alloy like Zircaloy. The tubes are first level of containment for the fuel and fission products. The pellets are Uranium Oxide in a ceramic matrix. Individual pellets are small cylinders about the size of a dime but a half inch thick or so. The ceramic fuel you saw on TV may have been for the Pebble Bed Reactor. This design is cooled by gas (Helium) and the fuel is in a spherical form about the size of a tennis ball. The "pebbles" are slowly removed and added to the "bed" which is just a pile of pebbles. This allows refueling without power reduction.

Also, this design, like several other new designs does not need control rods to shut down. Most reactors have negative reactivity coefficients (Chernobyl being a notable exception) meaning as they increase in temperature, the power level drops. The newer fuel designs have enough high temperature capability that the reaction shuts down and the temperature stops rising before the fuel can melt.

 

[Shalmanese]

What the public doesn't understand is that modern Nuclear power plant designs mean that, even if we wanted to, we couldn't cause a catostrophic meltdown. Meltdowns rely on a positive feedback loop, that is, the hotter it gets, the more active the source becomes. If you have a negative feedback loop, the hotter it gets, the less active it becomes. While stuff like the japan incident can still ahppen, certainly nothing like cheynobyl can ever happen with modern designs.

 

[KenGr]

Shalmanese, don't overdo it.

I'm a strong proponent of nuclear but current "modern" designs certainly can "melt down" if you bypass the safety systems. Although the negative temperature reactivity coefficient is effective, a rapid power increase transient can create enough decay heat to damage the fuel even though the reaction has gone subcritical. "Melt down" is not a well defined term. Although I don't think it's feasible to end up with a molten puddle at the bottom of the containment, a gross deformation of the fuel and rupture of the cladding is clearly possible and that would be messy.

The advanced designs would not melt but while our politicians think nuclear safety is important enough to scare the public, they don't seem to think it's important enough to support the construction of the next generation of nuclear plants.

 

[dpopiz]

Some of the numbers posted here are incorrect. Here are the officials:
traditional light-water/pressurized water reactor (LWR/PWR) -- about 32% thermal-electrical efficiency
gas-turbine reactors (GT-MHR, etc) -- about 49% thermal-electrical efficiency
all other high-output electricity generating technologies are much less efficient than even LWR/PWR

 

[everman]

I do recall hearing about fusion reactors being built and functional, unfortunately they require more energy to run than they produce.

 

[KenGr]

Light Water Reactors are in the 30 to 35% range. Gas reactors theoretically should be in the low 40's but we haven't built the newest generation yet. They could be in upper 40's. Conventional coal/oil fired power plants reach about 40%. Supercritical coal fired plants reach the upper 40's. Combined cycle gas plants (gas turbine, Heat Recovery Steam Generator and steam turbine) run in the upper 50's with manufacturers claiming they can get into the 60's.

 

[YingYang]

I was goin to tell you to join the US Navy but then I noticed you're from Australia. I'll skip the question about steam is produced since power engineer and dakels pretty much covered it. About your question about what happens to the radiation I think I can answer that. To reduce the amount of exposure to radiation you have to control the time, distance, and shielding from radiation. When you're inside the reactor compartment you minimize the amount of time you are in there. If you have to go in there you rotate people around so that they don't get exposed to too much alpha, beta, and gamma radiation. (You don't worry about being bombarded with neutrons since you don't go in the reactor while it's online). Distance is pretty much self explanatory. You put as much space between you and the reactor so you get exposed to less radiation. The shielding part is done by water, steel and lead. All material have some neutron coefficient factor. You have to engineer the shielding by combining enough of the lead, water and steel so that a safe level of radiation is coming out of the reactor compartment.
The Russian Navy prides itself on having the fastest subs in the world. I think you can draw your own conclusions on how they accomplished that.

 

[Degenerate]

So still, even outside the reactor, or even the power plant itself, there are still more radiation than natural ones?

 

[KenGr]

It's all relative as to where the radiation is greater than "natural radiation" because it varies so much. At the fence around a nuclear plant, (or for that matter, in the non-nuclear work areas inside the fence) even very sensitive instruments will not be able to detect radiation above background. Most workers in nuclear plants get only a very low level of additional radiation (say, equivalent to a few x-rays) and less than workers in other industries, like the crew of an airliner. A few workers in specific jobs get higher levels but these are still a fraction of the level known to cause illness and within the variation of background radiation around the world.

 

[Walleye]

Originally posted by: Degenerate
I recetly read on a New Scientist magazine that people have developed fule in the form of small capsules (radioactive fuel enclosed in a ceremic ball). Does this capture the radiation?

yeah, i want my flying cars. just kidding.

you could go for fusion, use lasers to implode glass balls filled w/ 1000 atmospheres of deutrium.



Oh, and just to say this, Nuclear power is the safest, most economical, most environmentally friendly power source available to us in this day and age. (that sounds so fricking cliche. argh)

 

[ant80]

Quote

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Originally posted by: Degenerate
I recetly read on a New Scientist magazine that people have developed fule in the form of small capsules (radioactive fuel enclosed in a ceremic ball). Does this capture the radiation?
Quote

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yeah, i want my flying cars. just kidding.

you could go for fusion, use lasers to implode glass balls filled w/ 1000 atmospheres of deutrium.



Oh, and just to say this, Nuclear power is the safest, most economical, most environmentally friendly power source available to us in this day and age. (that sounds so fricking cliche. argh)

-------------------------
Walleye World

Walleye's In Box

Edited: 12/01/2002 at 4:09 AM by Walleye

Hehe. Meet the unending quote.

 

[ant80]

Uh, got distracted. What I wanted to ask was this.

Nuclear power plants need more money to set up. I dont know how long they last, but I am guessing not long as fossil fuel plants. Besides you need periodic maintainance, as well as waste disposal.

Is it really possible to recoperate the cost and waste disposal in terms of the electricity generated? I have always doubted that.

 

[KenGr]

Nuclear plants have a larger capital cost than coal plants which in turn have higher capital costs than gas or oil plants.

Conversely, nuclear fuel is very cheap compared to coal which is cheap compared to gas or oil.

Solar and wind plants have even higher capital cost than nuclear but no fuel cost.

Nuclear plants have a design life of 40 years but can be extended to 60 years. This is only a regulatory (licensing) limit and it has been suggested that a 100 year life is possible before the cost of refurbishment becomes so great a new plant would be cheaper. A few plants which started in the 60's are being decommissioned but most have had the life extended.

Coal plant have a design life of 30 to 40 years but many operate 50 to 60 years or even more. The only reasons they are retired is usually because the basic design is not efficient enough to justify continued operation. Power plants can be "rebuilt" almost infinitely at costs which are not much greater than similar initial construction costs.

Due to skyrocketing interest rates and unexpected delays in construction in the 1970's and 1980's, many US nuclear plants had very high construction costs and, from a total viewpoint, are not as low cost as modern coal plants. On the other hand, the plants which were built without large cost overruns produce lower cost power than coal plants. Today, a new nuclear plant in the US would provide lower cost electricity in the Eastern and Southeastern portions of the US, similar costs in the Midwest, and higher cost in the West (excepting California, where it's still almost impossible to build any thing except quick fix gas plants). In other countries without indigenous low cost fossil fuels, nuclear wins hands down.

For those who may have questions, nuclear electricity costs include waste disposal and end of life decommissioning fees as part of the electricity rate.

 

[Walleye]

beyond that, nuclear plants, operating normally, dont pollute the environment, dont rely on fossil fuels that may be gone within 500 years, (i dont know how many years worth of uranium we can get but i'm sure it's in the hundreds of thousands, long enough to research cold fusion, and develop new power sources.) It's illogical to expect that a new magical power source will save us. the best hope we have for now, and for the immediate and relatively distant future is Nuclear Fission.

 

[Mark R]

Current estimates are around about 100 years supply of easily-obtainable cheap ($50/kg) uranium at current levels of consumption.

However, to say that we will run out after 100 years is highly misleading, because so little uranium is actually required as fuel - enormous price rises in the cost of uranium would have little impact on cost of the electricity generated. Compare this to coal, gas or oil fired generating stations, where fluctuations in price of the fuel, have significant repercussions in the cost of the electricity.

The use of nuclear fuel reprocessing can dramatically improve the usable reserves - if all nuclear fuel used was reprocessed into plutonium, the reserves of 100 years (above) could be extended to over 2000 years. Given the technical difficulties of reprocessing, together with enormous cost, many regard it as not economically viable at current uranium prices. Debatebly, it makes waste management easier, by concentrating the waste to less than 3% of its original mass.

By far the largest source of Uranium is seawater - at present, however, it is not an economically viable source. Cost of extraction is estimated to be about $1000/kg. At this point, however, the vast seawater reserves are essentially limitless, and are replenished faster by natural processes than we could ever use them.

Unfortunately, it's not all a rosy picture, the major nuclear power generator in Britain is in dire financial trouble - having needed an emergency government loan, or face bankruptcy. The problem is that of a rapid (an unpredicted) fall in electricity costs caused by the government's deregulation of the electricity generating industry. The result was a large excess of power produced by cheap gas plants, which has pushed wholesale prices below the costs of nuclear generation.

 

[PowerEngineer]

KenGr, I agree with virtually everything you've stated and would like to add another couple of points about nuclear power.

First, the cause of many construction delays was the accident at Three Mile Island which led to extensive changes in regualtions and correspondingly expensive redesign of plants under construction (as well as modifications to completed plants). These regulation changes also significantly increased the expense of operating nuclear plants.

Second, Three Mile Island also demonstarted the huge financial risks that electrical utilities were undertaking by deciding to go nuclear. High initial investment costs, rising operating costs, and uncertain operation under increasing stringent regulation can all lead to trouble getting cost recovery through state-approved customer rates. Utility execs weren't willing to "bet their company" on a game of nuclear chance; much safer financially to commit to other forms of generation. I suspect that nuclear power will not make a come back in this country unless the federal government is willing to build and operate these plants.

As far as spent fuel and decommisioning costs go, most utilities are allowed to collect some money from rate payers in anticipation of these costs. However, very few nuclear plants have actually been decommisioned yet, making it hard to know just what the costs might turn out to be (especially with changing regulatory requirements). Also, spent fuel costs do not include the government's cost of providing permamnent storage for the spent fuel.

 

[KenGr]

Decommissioning is looking pretty good. We have a lot of good data from dismantling government research facilities. Big Rock Point nuclear plant is Michigan is well along in decommissioning. Maine Yankee has done a lot of work. People have got comfortable enough with the costs to start asking what to do with the extra money when it doesn't cost as much as expected. There are a lot of options here but the only thing exotic is disposal of the radioactive bits, and that's an easily quantifiable plan.

The spent fuel is a separate Federally mandated surcharge. How adequate this is depends on how much you trust the government. Basically the Federal government has pledged to take and dispose of the fuel for the amount collected. It's a huge amount. The surcharge is 0.01 cents per Kilowatt hour generated by nuclear plants. The total collected for the fund is approaching 20 billion dollars at this time. The problem is that instead of solving the problem, the Department of Energy has been wasting a lot of the money doing study after study and the Congress figured out by blocking actual waste facility expenditures they could hold onto the money and use it to offset spending programs, making the deficit appear smaller in the current year. The whole thing does not speak well for our Congress. (Anyone surprised at that?)

 

[ReiAyanami]

homer simpson would know all this too, but only when they take the crayon embedded in his cranial cavity

 

[Snooper]

I see a couple of things that needs to happen before nuclear power will be trusted and cost effective:

1) We need to move to safer designs (Personally, I love the pebble ball concept)

2) We need to design a couple of different "power modules" of a standard design and spend a lot of time and a lot of money working the bugs out of the design, hardware, and software.

3) Standardize the training for all the reactor folks (maintenance, operations, etc)

4) Standardize the reactor layout down to the last instrument and valve.

Normally, I'm not much into the government doing anything. But these little gems are each custom designed and built like a giant Swiss watch. The people operating it have to figure out how to operate each and every one of them (yes, I know there is some standarization right now). With a fully standard design, major parts for the thing could be built in a factory and then shipped to the approved reactor site and assembled (kind of like a giant moble home manufacturer who makes nuclear reactors!). Anyone who is certified for the type would be able to run any of them nation wide. Also, it would be a WHOLE lot cheaper to fully design two or three reactor sizes and then use the size and number of modules you would need to get the power output you required. It would also be a whole lot easier to expand the reactor down the road if it was designed to be modular in the first place.