The Nuclear Power Thread

[nikkkom]

@Astronuc in particular, but anyone is welcome. A question: I am keen on nukes, but am no nuclear engineer, and I had for long been under the impression that nuclear plants could be scrammed, typically by dumping them into enough water.

No. SCRAM is done by inserting all control and safety rods at once.

There are neither operational plans nor water allocated for flooding the containment. I found it surprising too.

All the other items I could have regarded with equanimity, but that combination struck me (a nuke freak, please note!) as simply crazy. To have ANY dependence on ANYTHING not fail-soft I would regard as totally nuts. So it would mean maintaining a reservoir of what... a million tonnes of water uselessly (pick a figure)? Under conditions that require no power to supply? And that would have cost how many millions of dollars extra? As compared to what the present rectification is costing and the direct harm that is accruing? (Never mind the political harm!)

I posted somewhat similar posts numerous times last few years.
Apparently our "nuclear people" managed to convince themselves that extended power outages won't happen. More amazingly, many STILL insist that planning for that is not necessary.

Fukushima is not the only place where fail-safe scramming is not available?

Well, scramming does seem to work. But yes, planning for reliable cooling of scrammed reactors in emergency such as extended power outage does not seem to be a priority even now. Neither filters on emergency vent lines.

I am no longer a supporter of nuclear power.

 

[Jon Richfield]

As we understand the information from TEPCO, the operating reactors at Fukushima (Units 1, 2 and 3) did scram on signals related to the earthquake. The nuclear reaction did shutdown. BWR control rods are hydraulically operated, and all rods, as far as we know, were inserted as designed.

However, ...

Thanks Astronuc, that was helpful.

 

[Astronuc]

I should add that the tsunami took out the fuel supply for the emergency diesel generators, so they quit. The tsunami also flooded the basements and shorted the electrical bus and flooded the emergency diesel generators, which are the major long term supply of electricity to the site when the transmission lines go down.

With electricity pumps and valves are inoperable, so even though the core is scrammed, the plant still needs the cooling system to function. Without power, the cooling system failed.

 

[Jon Richfield]

No. SCRAM is done by inserting all control and safety rods at once.

I am not sure whether you are referring (correctly) to Fukushima in particular, or (incorrectly) in general to nuclear reactors. The nature of a SCRAM varies with the reactor type. I had perhaps misused the term by extending it to cover flooding the reactor. However it seems that we agree on the need for a reactor design to include fail safe removal of waste heat, whether from isotopic decay, Wigner energy or any other post-SCRAM sources, whether by flooding or other realistic means.

I assume that we also agree that any design that could rationally be expected to permit failure of such safety features in a commercial or industrial installation would be unconditionally unacceptable. And I speak as (unlike you, it seems) a continuing nuclear power enthusiast.

Apparently our "nuclear people" managed to convince themselves that extended power outages won't happen. More amazingly, many STILL insist that planning for that is not necessary.

Here I share your amazement and no doubt your dismay. Accepting your remark at face value (I was not aware of such statements or views) I cannot accept that any engineer with such attitudes understands either engineering responsibilities or the statistics of disaster cost-benefits. And as you imply, to deny the responsibility to install filters on emergency vent lines strikes me as flatly irrational, even bearing in mind that they cannot filter out everything.

I am no longer a supporter of nuclear power.

There however, we diverge. The fact that nuclear power demands greater responsibility than many politicians and engineers display does not persuade me that the only or even the only rational alternative is to give up on such an important option for our most important resource, namely power.

 

[jim hardy]

Edit - oops i see other posts appeared whilst i was typing...

Without heat removal from the core, the fuel temperature continues to rise.

@ Jon --- That's the crux of it right there.

It has been long known that loss of all power for that generation of plants would lead to , well, just what happened.
For that reason they have redundant pumps, pipes, and heat exchangers, and redundant batteries, redundant diesel generators located in the basement where they're safe from earthquakes, redundant instruments and so on and so on.

Current designs do incorporate things like gravity fed heat removal and standardized connections for simple things like portable pumps and generators.How different might Fukushima turned out had some electrician found a few gasoline motor driven welding machines (ones that weren't full of seawater) and hooked them to the batteries?

Observe the two newer units there (5&6 ?), the ones with diesels higher up the hill, fared okay.
That those diesels were put way up there... hmmmm

Sorry for the ramble.

Glad to hear you aren't Anti-Nuke.
Thank you.

old jim

 

[Jon Richfield]

Edit - oops i see other posts appeared whilst i was typing...

@ Jon --- That's the crux of it right there.
...
Sorry for the ramble.

Glad to hear you aren't Anti-Nuke.
Thank you.

old jim

Hi Jim, no problem with the ramble. Interesting stuff. Here in South Africa we are helplessly watching developments and wondering whether our government is going to buy a job lot of VVER/PWRs. From Russia. I am left trying to imagine a worse prospect than combining Russian diplomatic free enterprise and control of their own installations, with our local politicians opportunism and Nuke nous. AFAIK most of them don't know kVA from kava and don't care as long as someone promises that their take-home pay will expand. Nukes I can follow with the help of some of the folks round here, but some things are beyond my conception, never mind comprehension.

 

[nikkkom]

Then why shouldn't we store it (above ground, I reckon, but conveniently in available chambers if someone insists) in a place that presents no marine or volcanic threats and that is reasonably weather-proof? Many desert nature reserves would be perfect. Most unapproachable nuclear hazards or processing or power plants are already de facto reserves. I live near a couple. They are great! And not unique either; ask naturalists that have inspected the evacuated regions round Chernobyl.
It would be important to catalog what is where, against the time that people begin to wonder where to get some of it out again for new use or re-use.
The storage medium should be in a form that could be extracted efficiently, but not rapidly, nor without special equipment, nor in particular inconspicuously. It should be indifferent to earthquake, fire or flood, and in particular to corrosion. Something like hundred-tonne cargo containers with special multiple walls and requiring special tools and much time to breach, and producing long-range distress signals if moved without approval, should do nicely. They even might be powered by waste heat from their own contents. Compared to alternatives such as vitrification (not to mention lifting into space) such storage would be dirt cheap and it all could be used up long before all the important half-lives had expired.
So who needs space launching or deep burial or indeed, long term storage at all? A few centuries should be plenty. Anything likely to survive after that, we could cheerfully burn down to inactivity in a high neutron flux unit.

This looks like inventing a bicycle to me. Why bother, when French have it nailed already?

Store spent fuel for 5-100 years (short periods let you recover more Pu; longer periods are more economic).
Then reprocess it. Save reprocessed uranium for use when prices of Uranium go up. Use recovered Pu to manufacture MOX fuel.
Vitrify fission products and transuranics.

This is being done today, every day, in La Hague facility in France. French will even build you a copy for something like $5bn if you ask them.

The only thing French don't do (yet), is geological burial of vitrified waste.

 

[Jon Richfield]

This looks like inventing a bicycle to me. Why bother, when French have it nailed already?

Store spent fuel for 5-100 years (short periods let you recover more Pu; longer periods are more economic).
Then reprocess it. Save reprocessed uranium for use when prices of Uranium go up. Use recovered Pu to manufacture MOX fuel.
Vitrify fission products and transuranics.

This is being done today, every day, in La Hague facility in France. French will even build you a copy for something like $5bn if you ask them.

The only thing French don't do (yet), is geological burial of vitrified waste.

I am glad that someone has had enough sense to steal my idea, even if it was a Frenchman ( :D ) The only thing that puzzles me is why they should want to vitrify anything such as transuranics.

My elaborations about containers etc are just to anticipate possible objections to deal with terrorism etc.
And incidentally, presumably like the French, I reckon that to reprocess fuel before you have a use for the products is crazy. It just lands you with a bigger storage bill for process waste plus an increased load of impurities in your Pu from decay products.

Hooray for French logic and practicality, say I!

 

[jim hardy]

I am glad that someone has had enough sense to steal my idea, even if it was a Frenchman ( :D ) The only thing that puzzles me is why they should want to vitrify anything such as transuranics.

Once upon a vacation i was chatting with a fellow at the motel pool while our kids were swimming.
He was on his way to the resort a block from where I lived in Key Largo.
Small world , he worked for a fuel reprocessing plant in England.
I mentioned i worked at the nuke plant near where he was going and would love to send him home with twenty years worth of spent fuel.
He said he'd love to have it.

US is delinquent in handling the stuff.

You have a Westinghouse-like PWR if I'm not mistaken. (Koeberg site?) I've communicated with a nice lady engineer responsible for its control rod drives through the user's group.

Small world indeed.

 

[nikkkom]

I am glad that someone has had enough sense to steal my idea, even if it was a Frenchman ( :D ) The only thing that puzzles me is why they should want to vitrify anything such as transuranics

Cost. Separating transuranics from fission products requires further processing steps.

Each additional step costs a lot when you work with very, very radioactive materials: it must be done in airtight building, all gaseous emissions and liquid effluents require elaborate filtering, all vessels and piping need to be very durable (since repairs are extremely costly, if practical at all), and all of this stuff requires heavy shielding.

 

[nikkkom]

My elaborations about containers etc are just to anticipate possible objections to deal with terrorism etc.

The final product at La Hague, a steel canister with vitrified waste, emits 1.5 million rem/h of gamma on contact. I dare any terrorists to try stealing THAT.

 

[Jon Richfield]

Pardon my cackling Jim; concerning your "US is delinquent in handling the stuff ", I was just reflecting on how miserably without perspective the views of outsiders are concerning almost any speciality. Imagine the reactions of anti-nuke activists who knew of such conversations...
The US is not alone in its delinquency of course; virtuously anti-nuke activists are instant experts with unbounded faith in their knowledge and understanding. Eg did you know that one of the differences between non-breeder and breeder reactors is that the latter permit a reaction time of at most two MICROseconds to prevent a nuclear explosion? No? Nor did I. We have a lot to learn. Unlike my passionate source.
Not that nuclear engineering is alone in this of course; get to trading war stories with other engineers, say process or chemical engineers trying to deal with safety inspectors who don't understand simple chemistry but quote standards that mention iron as a fire hazard, and accordingly want to see the precautions against fire in stacks of sheet steel. Or want to see the safety practices for dealing with cylinders of compressed N2, uncomprehendingly walking past cylinders of compressed H2 on the way there...
I also remember how miserably ignorant many specialists are about fields quite close to their own; I have met practising engineers who patently misunderstood the concept of nuclear shielding, being under the impression that it was like mechanical shielding: put up a metal plate and either the bullet gets through a thin sheet, or it fails to get through a thick sheet. That sort of thing. And of course I am not immune -- that is why I hang out in places like this where there are folks to help.
Besides, not being an engineer, I safely can claim to know everything. :D

You are correct about Koeberg. I live almost in sight of it (about 60 km away). It has been chugging away since 1984 with comparatively minor incidents, not counting a non-nuclear sabotage incident in the eighties if I remember correctly. However, lately there have been some worrying incidents that to me suggest incompetence and smugness; I am increasingly inclined to share some of nikkkom's concerns. :(

Regrettably I don't know any of their staff. I generally like female engineers in any field -- they tend to be girls with sense and competence.

But I am not sure whether the VVVR approach is any better. And if SA were to commit to Russian built and Russian plants in this country, our politicians would have sold us down the river to dependence on a power that could hold the country to ransom for price and obedience for the foreseeable future,and all for the cash in their own pockets. And IMO, that was the GOOD news.

 

[nikkkom]

Eg did you know that one of the differences between non-breeder and breeder reactors is that the latter permit a reaction time of at most two MICROseconds to prevent a nuclear explosion? No? Nor did I.

(1) You are confusing breeders with fast neutron reactors. Most breeders are fast reactors, but some are not.

(2) Fast reactors won't explode as a nuke even if they suffer power excursion. At worst, they can get their core melt. First, they are designed that way, and second, you can't get a sizable nuclear explosion by merely pushing uranium or plutonium chunks together, no matter how big are they - you need to make them collide or compress at significant velocities, hundreds of meter per second - otherwise, the energy release is "small" and the material "only" melts, doesn't reach millions of degrees.

 

[jim hardy]

We have a lot to learn.

Eric Hoffer said, succinctly:

"I am no longer young enough to know everything."

I took only one course in Reactor Physics
and it did no go at all into fast reactors.
I know just enough to appreciate the abysmal depth of my ignorance.

Anything worth doing should be done well. Particularly anything involving large amounts of energy.
A Windows crash deserves an ounce of prevention, a reactor excursion warrants tons.
The problems with nuclear are societal not scientific ones. Complacency, hubris, ambition, fear and the like.

And that's my opinion.

Do current design Russian reactors still have a large positive moderator void coefficient?
Chernobyl's was +$4(4X enough for prompt criticality). I am glad the one in Cuba never got finished for i don't think something like that should be built in the first place let alone turned over to civilians.

old jim

 

[Astronuc]

Do current design Russian reactors still have a large positive moderator void coefficient?

The VVER is the current commercial design being supplied by Russia, with the VVER-1000 and derivatives being the current models. The VVER-1000 is more like western PWRs, except is uses a triangular/hexagonal lattice. It uses control rods that drop in from the top, unlike the VVER-440 which used control assemblies.

The RBMK is a different animal and that design is more or less obsolete.

 

[Teen4Ideas]

I just understand how there is nuclear waste, I've been taught that radioactive materials are radioactive because they are inherently UNSTABLE, unstable material is needed for a fission reaction, so why is there waste? Shouldn't what leaves the plant be stable, non radioactive material? I agree with theory process on the above comment, waste could be used as another fuel source right?

 

[Jon Richfield]

I just understand how there is nuclear waste, I've been taught that radioactive materials are radioactive because they are inherently UNSTABLE, unstable material is needed for a fission reaction, so why is there waste? Shouldn't what leaves the plant be stable, non radioactive material? I agree with theory process on the above comment, waste could be used as another fuel source right?

As far as it goes you are correct, but the difficulty is that you need to read up a lot on the nature of the different types of instability involved. By way of analogy, think of nitroglycerine and of coal burning in air. Both release energy by oxidation and reduction of fuels, in other words, by exploiting instability of oxidants and reductants in contact. Both produce largely similar reaction products. And yet, coal would make a lousy material for blasting in mines, and nitroglycerine a lousy material for warming your stove.
OK?
Now, the nuclear instabilities in question are, as you realize, the source of energy, whether for bombs, power stations, or poisoning dissident Russians. Roughly they might be classed into two types:

• nuclear decay, which is what you get from unstable isotopes, and which proceeds in terms of half-life, each nucleus undergoing (usually) minor alterations at a random time while minding its own business and ignoring its neighbours and not doing anything on the lines of a chain reaction. Think radium, strontium 90, tritium and so on.
  
• nuclear burning, to which most radioactivity is only tangentially relevant, in which the nuclei undergo massive changes, either fission (splitting if you like) or fusion (joining, if you like) in which not only a lot of energy is given off, but particles that shoot off and split or fuse neighbouring nuclei that otherwise might have sat minding their own business. If this tickling up of your neighbour is effective enough, say every splitting/fusing atom causing two more to react, then we get a what we call positive feedback -- the faster it blows, the faster it grows. This is sometimes called a chain reaction and for serious power production, an effective, efficient chain reaction is crucial, either uncontrolled in a bomb, or carefully dripped out as in a power station.

Now, all the nuclear fuels in a fission reactor happen to be radioactive, but that is simply because only very, very heavy nuclei are any good for splitting, and all nuclei more massive than lead (even bismuth it seems nowadays) are at least slightly radioactive anyway. Most of their fission products are incidentally radioactive, usually intensely so, meaning that they don't last long, but the point is not that they are active, which has nothing to do with chain reactions. What does matter is that they are too light to undergo any useful fission and therefore support no chain reaction. In fact they tend to interfere with the main chain reaction and we speak of them as poisons, that we have to get rid of before we can use the remaining fuel.

At the light element end, where we can fuse hydrogen, lithium and similar nuclei,the process differs in detail, but the principle remains the same. Only the lightest nuclei are useful in fusion and the products ("ash") of the burn are either useless or a nuisance.

So I hope that helps you understand that it is not the instability of the "ash" that matters, but the fact that it is not suited for "burning" because it cannot support a chain reaction. It does in fact give off heat and particles in quantities that continuously grow smaller (half-life) till we get an inert product, but the heat, though valuable in space vehicles etc, is too little to be valuable for bulk use in power stations.

 

[rootone]

Spent fuel rods still contain a lot of unburned Uranium, and that is in fact recycled into fresh fuel rods.
The remaining 'ash' as has been pointed out, while it is radioactive, it isn't sufficiently radioactive to contribute to maintaining a self perpetuating fission reaction, and if anything interferes with the Uranium chain reaction.
Some of the waste by products can be separated and are useful in medical treatments and other applications, others can be processed in special reactors and transformed into stuff that can be useful.
Much of the waste has no potential use that we presently know of though, so the only thing that can be done is to lock it up in a safe storage facility where it doesn't pose a hazard.

 

[Jon Richfield]

Just want to add that when rootone says

"Spent fuel rods still contain a lot of unburned Uranium, and that is in fact recycled into fresh fuel rods"

it is important to understand that "spent" in this sense never meant "burnt up", but "containing too much 'poison' to be profitable to go on using like this".
The reprocessing then does nothing to the fuel uranium or plutonium or whatever elements we were burning, or that got produced in the fission process, it just removes the "ash", the fission products that would absorb the neutrons that otherwise would keep the chain reaction going.

 

[mheslep]

Spent fuel rods still contain a lot of unburned Uranium, and that is in fact recycled into fresh fuel rods.

Can be recycled, not necessarily is.

 

[rootone]

Can be recycled, not necessarily is.

Sure, some nuclear power operators do this and others don't.
Personally I can't think of why it isn't done by all as a matter of routine.
I would have thought that the cost of recycling the usable Uranium in used fuel rods would be less than producing new rods from raw Uranium ore,
but I guess the decision not to recycle in some cases must have something to do with economics ultimately.

 

[jim hardy]

Both release energy by oxidation and reduction of fuels, in other words, by exploiting instability of oxidants and reductants in contact. Both produce largely similar reaction products. And yet, coal would make a lousy material for blasting in mines, and nitroglycerine a lousy material for warming your stove.

Jon Richfield - i like that analogy, in fact i use a similar one for my non-science friends.

""
A campfire burns cellulose. So does a stick of dynamite. But they're put together differently.

Had the bomb evolved out of the power reactor instead of the other way 'round, people would say:
'What a dreadful thing to do with perfectly good Uranium, scattering it allover the countryside like that. You can make electricity with of that stuff. '
""

 

[jim hardy]

Teen4ideas

are you a student ?
you might enjoy poking around this page
http://www.nndc.bnl.gov/chart/reZoom.jsp?newZoom=7

it's sort of like the periodic chart you studied in chemistry but it shows all the isotopes .
Vertical axis is number of protons, which define what chemcal element it is.
Horizontal axis is number of neutrons, which determines which isotope of any element you are looking at.
The black line up the middle is all the stable isotopes, the rest are unstable .
How to use it:
example
in lower left click on 1H (Hydrogen), then on right side panel click "zoom 1"
you should see black squares for 1H and 2H, plain hydrogen and deuterium as in heavy water.
both are stable, and the numbers tell you natural hydrogen is is 99.9885% 1H and 0.0115% 2H.

Small wonder heavy water was so scarce in WW2...

3H is tritium with 12.32 year halflife
4H and 5H i never noticed before. I think they immediately emit neutrons thereby turning into tritium.

You've heard of Carbon 14, check that one out...have fun exploring

old jim

 

[Jon Richfield]

Had the bomb evolved out of the power reactor instead of the other way 'round, people would say:
'What a dreadful thing to do with perfectly good Uranium, scattering it allover the countryside like that. You can make electricity with of that stuff. '
""

Nice line Jim!

 

[Jon Richfield]

Personally I can't think of why it isn't done by all as a matter of routine.
I would have thought that the cost of recycling the usable Uranium in used fuel rods would be less than producing new rods from raw Uranium ore,
but I guess the decision not to recycle in some cases must have something to do with economics ultimately.

Right, plus politics. The nuke-killer nuts want to vitrify everything, good bad and potential, and bury it halfway down the mantle.
But there also are practical considerations. There is more fissile fuel in the spent fuel than we usually have capacity for using at anyone time and it contains a lot of isotopes, including "poisons" that accumulate in even highly purified reprocessed fuel, so once it is cleaned it is best used ASAP, if you don't want to do it all over again before you can use it.
Another thing is that while it is still full of highly radioactive "ash" it is practically unhandleable and hard to hijack for terrorist purposes.
So it is sensible to retain the dirty material as safely as practical while it cools down and decays into comparative safety until there is a need for it. Then you can recycle the fuel for use while it is still fresh and clean and crusty, and process the rest of the material into compactly storeable form, partly for safety till it decays, partly in case a use for it arises, and partly because if there are any particularly undesirable isotopes in it, they can be exposed to convenient sources of neutron radiation to accelerate their final decay.
Incidentally, except where they are used purely as sources of decay heat, the value of nuclear fuels is independent of how radioactive they are. They really are valued according to how fissionable or fusible they might be. In most contexts their radioactivity is a nuisance except when the decay products (usually neutrons, occasionally alpha particles) are needed to support fission or some types of fusion.

 

[Astronuc]

Sure, some nuclear power operators do this and others don't.
Personally I can't think of why it isn't done by all as a matter of routine.
I would have thought that the cost of recycling the usable Uranium in used fuel rods would be less than producing new rods from raw Uranium ore,
but I guess the decision not to recycle in some cases must have something to do with economics ultimately.

The nuclear power operators themselves do not perform the recycling/reprocessing, but rather spent/used fuel is sent to a reprocessor. In reprocessing, the fuel is basically dissolved, the fission products are separated and chemically formed into a stable configuration (immobilized), and the unused U and Pu is reformed into usuable fuel. All that is done remotely due to the radiation levels, which are many time greater than the ore. Remote handling greatly increases the cost of the fuel, and the disposition of the inventory of radionuclides (fission products) as opposed to mine tailings is a significant cost.

Right now, uranium ore is inexpensive, and the cost of fuel from virgin UO₂ is much less than recycled U and (U,Pu) MOX. The structural materials, i.e., the Zr-alloy or SS-alloy cladding and supporting structure are the same.

 

[nikkkom]

Sure, some nuclear power operators do this and others don't.
Personally I can't think of why it isn't done by all as a matter of routine.
I would have thought that the cost of recycling the usable Uranium in used fuel rods would be less than producing new rods from raw Uranium ore,
but I guess the decision not to recycle in some cases must have something to do with economics ultimately.

Reprocessed Uranium costs more than natural one, and it has slightly worse characteristics. It contains some U isotopes which don't occur in natural U, and these isotopes aren't very good at fission. IOW: 5% enriched reprocessed U is approximately equivalent to 4.5% enriched natural U. Since enrichment for civilian power reactors is capped at 5%, you can't "simply" use 5.5% enriched reprocessed U. It's not allowed. Thus, fuel fabricated from reprocessed U would be somewhat less efficient that one from natural U.

 

[Astronuc]

Reprocessed Uranium costs more than natural one, and it has slightly worse characteristics. It contains some U isotopes which don't occur in natural U, and these isotopes aren't very good at fission. IOW: 5% enriched reprocessed U is approximately equivalent to 4.5% enriched natural U. Since enrichment for civilian power reactors is capped at 5%, you can't "simply" use 5.5% enriched reprocessed U. It's not allowed. Thus, fuel fabricated from reprocessed U would be somewhat less efficient that one from natural U.

That's true for the most part. However, in the case of MOX (U,PU) fuel, the Pu-content is necessarily greater than 5% (more like 6 to 8%), but equivalent to 5% U-235, in order to compensate for the parasitic absorption of isotopes like Pu-240.

Research reactors, e.g., BR-3, could (can) and did use higher enrichments, 8 to 10%, for special fuel programs. Some research reactors used up to 20%, which is down from higher enrichments in the past.

Obviously, research reactors have different purposes, e.g., research or isotope production, than commercial reactors, which are generally used for base-load electrical supply. Some commercial reactors may be used to produce certain isotopes like Co-60.

 

[rootone]

So as things stand with current reprocessing technology, there just isn't a cost effective way to get most of the U235 out of used fuel rods without it having an admixture of unwanted isotopes.
I guess it might be feasible to do it in principle, but on economic grounds it's considered that this process takes an unreasonable amount of time and energy to do it.
There ARE however facilities in a fair number of countries where they do actually do this, and they wouldn't bother if the economic argument against was fully conclusive, what would be the point?

 

[nikkkom]

So as things stand with current reprocessing technology, there just isn't a cost effective way to get most of the U235 out of used fuel rods without it having an admixture of unwanted isotopes.
I guess it might be feasible to do it in principle, but on economic grounds it's considered that this process takes an unreasonable amount of time and energy to do it.
There ARE however facilities in a fair number of countries where they do actually do this, and they wouldn't bother if the economic argument against was fully conclusive, what would be the point?

The point is (1) to reduce the amount of waste and (2) natural U is not a particularly abundant resource. It can become much more expensive in the future.