Nuclear Decisions 3 – Spent Fuel

Can nuclear reactor waste and spent fuel  from nuclear power plants be rendered harmless to the Earth environment?

The first post in this sequence, Decisions-1, brought up the two top-most safety issues that are associated with nuclear reactors,  (1) loss of coolant for the active reactor core, and (2) safe storage of spent fuel rods and other nuclear waste.

Loss of coolant:   Decisions-2 addressed this – can the nuclear industry cope with a loss of coolant accident (LOCA) without suffering a fuel melt-down, or the threat of one?

There is no technical reason why fuel melt should remain a reactor issue –  an intrinsically safe reactor certainly could be the choice for power generation.  The true issue is human psychology… pride, hubris and willingness to learn from mistakes, to move into new concepts generated by other people.   The result was a qualified yes, we could build safe reactors, but… would we?

Spent fuel disposal:   Nuclear waste is a true danger to everyone on Earth.  I would like to think that there are there techniques that can do for spent fuel what helium gas cooled reactor can do for fuel meltdown –  turn it into a moot point.  I just do not know of any that are tried and true, waiting to be picked up off the shelf.

SpentRodPool_img

Water sequestering of spent fuel in the UK

Right now, all waste products from most reactors are stored in pools of water right at the site of the reactor.  No dangerous underground mines, no dangerous transport, no dangerous burning (or its equivalent) .  Sounds good, right?

If the Fukushima tragedy shows anything, it shows that these waste storage pools are massive threats that sit passively on reactor sites around the world, waiting quietly.

What to do with spent fuel is the central issue for reactor use.  We examine a lot of options and the Post is long.

Index       Click on any of the underlined discussion sections to jump there.

The Waste Problem Nuclear waste is dangerous.
Short History of Nuclear Waste How did we get into this mess?
Long Term Solutions 1 Pool cooling,
2 Dry cask storage,
3 Sequestration,
4 Reprocessing
Summary Put the conclusions together

For a background on nuclear fuel, fuel rods, assemblies, and radioactive waste generation, jump to appendix About Nuclear Fuel .

The Waste Problem

FukushimaH2_img

Burning H2 at Fukushima disaster

The hydrogen gas from the burning Fukushima reactors was surprising:  H2 is not used anywhere, so where did it come from?  The answer is that the fuel rod shells are made of zirconium, which, at elevated temperatures, reacts with water to evolve hydrogen.  Scenario: the reactor walls cracked, the storage pools cracked, the cooling pumps failed to run, the rods were exposed to atmosphere and heated; then they generated hydrogen, which burned.    H2 is not explosive outside a critical region of concentration in air,  but it can generate a fire storm that would spread radioactive materials across the countryside.  This is not idle scare  talk, Fukushima burned.  Fuel rods almost certainly melted.

Used fuel was originally considered to be an easy fix.  It has become the central issue.  Can we would find someplace to safely sequester the waste and live happily ever after?  Does such a place exist?

Short history 

MapToYuccaMt_img

Yucca Mt nuclear respository is near the Nevada nuclear weapon test site

The U.S. National Academy of Science proposed in 1957 that deep underground storage should be the way to sequester the nuclear waste that was being generated by power reactors and weapons manufacture.  What was needed was somewhere that would never have an earthquake threat and be far from water.  After 25 years of technical and political  discussion, President Reagan selected 3 sites for final analysis.

The de-facto favorite was the site at Yucca Mountains, a large ancient earthquake ridge. Yucca  is near Nellis air force base and adjacent to the Nevada Nuclear Weapons Test Site, 90 miles north west of Las Vegas, NV.  After another 20 years, President Bush (II) cut off the analysis.  Nuclear waste disposal at Yucca Mt. was authorized in 2002.  This was to be the location for permanent sequestering.

This has been a highly politicized process.  Evidence that the site was located over the Bow Ridge earthquake fault surfaced in 1995, and one of the tunnel drills demonstrated this in 2007.   In 2009, President Obama killed the entire project.

This reset plans to about 1957.  As of now, virtually all wastes are stored in pools at the place of original generation. The Fukushima disaster shows that this is not a really great idea.

YuccaMtWasteStorage
Proposed nuclear waste underground disposal at the Yucca Mountain facility
YuccaDgm_img

Diagram of Yucca Mt facility

The waste would arrive by rail [1] and deliver reinforced storage containers to the receiving site [2].  Each container would be transported down the main tunnel [3] and into side storage tunnels [4].     As many bloggers have pointed out, something like $10 G  (U.S. billion) had been sunk into the program.

The Yucca Mt Repository is a lovely way to continue to sink money for a long time to come; there is much yet to be done.  Currently, [2] has been built (not out-fitted);  [3] dug; and [4] is TBD.   A major consideration:  The radiation levels would be too high for human entry and all tasks must be fully automated by equipment that must function for at least 10 or 20 half lives of 239Pu (10 or 20 times 24,100 years).   Or, at least the remote monitoring equipment must do so.  Or at least all this stuff must last for as long as our current society stands.  (Those guys in the afterwards can take care of themselves…   Hey! Life is hard all over.)   Back to top 

What are Long Term Solutions?

CoolingPool_img

One of many cooling pools

The U.S. currently has about 100,000 tons of radioactive waste and growing.  We must do something with it.

One kg is 10% larger than 2 lbs; the metric ton is 10% larger than the 2000 lb English ton. For our purposes 10% is insignificant so we will use ton as a generic size.
1  Pool Cooling    This is what is mostly being done, everywhere. Fukushima was an old installation and there were over 1 million spent fuel rods in storage there (see Appendix).  This is calm and quite safe while the water is present, the power is on, and the pool is actively cooled.

All reactors at Fukushima Daiichi lost cooling power and the diesel generator for all the plants flooded.  Apparently all six reactors used a common cooling backup and all were left in trouble when the single system failed.  The rods became uncovered, the zirconium rod shells heated, contacted water, evolved copious hydrogen;  fuel melted.  Why were all systems dependent on the same emergency backup?  It was the cheaper way.

Pool storage is a terrible idea for safe storage over the many years needed to convert radioactive waste to safe but very poisonous trash.  Review Appendix > Spent Fuel > Long Lived Wastes for details on these chemicals.    Back to top 

DryCaskV_img

Dry cask for long term storage

2 Dry Cask Storage At this moment, the best that can be proposed is to

(A) store the spent assemblies in a chilled pool for a couple years (until the assemblies can be removed from water and not melt from their own intense heat).

(B) transfer assemblies  to a cask that has significant neutron shielding in its walls.

(C) interdict forever.

Dry casks in the horizontal orientation would be used at Yucca Mountain, if it were ever completed.

DryCaskTanks_img

Foreground: 2 dry cask tanks at reactor site

Currently, dry cask storage is in use at a few reactor sites in the U.S. and around world.  The technique has proven safe enough if humans are kept a football field  away.

If the currently common UOX fuel is used, the assemblies could be moved to casks within in a couple years.  If MOX is used, as in Fukushima Daiichi Unit 3, a longer time is needed .  It is MOX fuel and bomb wastes that force the true safety timeline for the Yucca Mountain facility to be a on the order of a million years. (UOX, MOX – see Appendix.)

Although dry cask storage is the best sequestration technology currently available, power company executives are reluctant to spring for a dry cask “farm” to store waste products.  I think the fear is that this would solidify resistance to find a better, cheaper (for the Owners) sequestration solution.  Some commentators say it is because there are no regulations that force use of dry cask storage.

Dry cask storage is hugely better than active pool cooling, but not good if the idea is to keep the material out of human hands for the next million years.  We need a truly permanent solution.    Back to top 

3 Sequestration Strategies   These are isolation strategies and would appear to be the simplest.  The basic issue is to find a location that once delivered, the spent fuel will be safe forever.  Many sequestration strategies have been proposed, we will examine 3 variants:  Outer Space, Deep Ocean Deposition, and Underground storage.

Outer Space
The Delta-IV-H and Atlas-V-H boosters from United Launch Associates are both rated at about 23 tons to low Earth orbit; the SpaceX  Falcon-H is rated at 53 tons.  ULA and SpaceX both publish paths to reach Saturn-V’s capability of 120 t to LEO (the  Saturn-V last flew in 1973).   If we assume 100 t to LEO,  1000 launches would be needed shift 100,000 T of waste from Earth to the ISS.  ULA charges $5,000/kg,  SpaceX targets $1,000/kg ($1M/t).  We need $100 – 500 billion to lift all wastes into the ISS orbit.  And this is just first stop in the journey outwards.

  • Earth orbit  –    We cannot leave the waste near the ISS in LEO because drag from the residual atmosphere would cause all of it to reenter and either disperse in the atmosphere or bombard the landscape as high lethality meteorites.  We are obligated to move the waste on outward.  The LEO first stop, though, would minimize the ΔV required to reach the final destination (and thus the overall cost).
  • Lunar burial –  We could put the waste on the moon, under the assumption that we would never want to operate mining ventures there.  The energy balance is so bad that the cost of lunar burial would far outweigh the gain from the power generation, itself.
  • Solar elimination  –  I keep hearing about shooting waste to the sun.  Hitting sun only happens once we have zeroed the solar orbit velocity out here at Earth’s orbit, a huge fuel expenditure.  Cost is same argument as that opposing lunar burial.
  • Launch failure  –  This is the final, action-stopping argument.  What if lunar or solar prove viable?  I was at Cape Canaveral when one of the high reliability work-horse Delta boosters blew up during launch ascent and made a beautiful carnation high in the sky.  Picture yourself watching one of the monster heavy lifters launch, when you see the lovely bloom as 100 t of nuclear waste vaporize over your head.  How do you feel about being under this blossom?  Now, guess how people in Orlando would react to suggested launches of waste products.

Deep Oceans
This is an attractive option, actually.  Put the casks into the subduction trenches in the ocean and the very earth would transport the waste deep into the mantle, not to reappear for millions of years.  Yet, here there be problems … or at least issues.

  • Waiting time –  Surface subduction  occurs very slowly, and the casks must remain stable in the ocean water for many, many years to come.
  • Safe descent –  How can you assure safe deposition 5 miles down?  An accident as you lower the casks would disperse the radioactive sludge throughout the seas of the planet.  Might make you reconsider swordfish for dinner?  This issue is similar to rocket ascent  into space.
  • Long term stability – These or any of the other deep sea solutions suffer from survivability issues.  Deep ocean beds are mostly very cold, and feature high pressures, with strange pH and dissolved gas concentrations.  How will the storage containers withstand any of the ocean floor earthquakes?  After all, the Fukushima disaster was due to a magnitude 9 quake.    I am not sure that we can detect fault zones as easily as on land, and I would want detailed analysis before we commit to this kind of “forever” storage.  Bore-hole storage has been suggested (a kind of underwater Yucca mountain), but … we can almost drill oil in the Gulf of Mexico, what difficulties would appear when drilling a wide vertical tunnel 3 miles or more under the surface?

Deep Underground
This is the classification for the Yucca Mountain Repository (YMR).

Earthquake_map

M>2.5 for 20 yr period

How can anyone forecast earthquake patterns 1000 years from now?  This cannot be done reliably for next year, even. YMR is over an earthquake fault, as this map indicates.

Nevada ranks third in the nation for quake activity. It is difficult (impossible) to guarantee that YMR will be earthquake free for the next million years, and yet this is what the Repository is all about.  Some geologists are quoted as saying that it is a low risk zone;  how far into the future are they looking?

Would any care to indemnify survivors of a large quake over the next 1,000,000 years?  YMR was suggested because of its supposedly placid existence.   The timelines required for sequestration are so huge that any such a statement is self-invalidating.

The current president took major hits on his decision in 2009, mostly from the right wing paranoids. You can still hear them doing heavy breathing about this.  There are  websites open for business that say this move was to keep waste horribly dangerous so that the left wing environmentalists can have an issue to tantrum against.  Easterbrook (at the site just linked) actually indicates that any source of safe clean power is a terrible threat to these (hated) liberal nuts.

Actually, Obama should be given international awards for his astoundingly far sighted thinking.  This is as opposed to that of the president who started YMR construction while he simultaneously opened war on two separate fronts against religious fanatics who hated each other, and declared an unsuccessful anti-ballistic missile prototype “operational.”  Far right extremists and their comments embarrass any true conservative, who wants to regain our manufacturing power, keep active technology development alive, and build a strong common future for all American citizens.

Sequestration by any technique for a million years or so is a bad idea.  Mostly because no one can guarantee the safety of the stored waste during the cool-down cycle.  The issue is dominated by the number of zeros in the half lives of the elements in the nuclear trash.  (Jump down to the Appendix > Spent Fuel > Long Lived Wastes for a very short selection from the many isotopes generated during the fission process.)   Back to top

4  Waste Usage   This is probably the least popular solution to the nuclear waste problem.  We will discuss 2 paths to use up as much radioactive waste as possible: chemical reprocessing and nuclear transmutation.  For a favorable review see World Nuclear Association.  For an unfavorable review, see Union Of Concerned Scientists.

A. Reprocessing
This is a chemical processing of nuclear waste to extract uranium and plutonium.  The most widely used process is called the PUREX method (plutonium uranium extraction).   Recall that spent fuel is only 3% used up before fission products have poisoned its ability to generate power. The object is to extract the fissionable parts and reconstitute new fuel pellets. With reprocessing, we can use nearly 30% of the fuel in a pellet, without, only 3%.

The spent fuel is dissolved into nitric acid and the final result is about 97% uranium (called RepU for reprocessed uranium), 3% waste, and  1% reactor grade plutonium (RGP is 240Pu and unsuited for bombs, whereas weapons grade WGP is 239Pu).  The RGP can be mixed with RepU to form “mixed oxide” reactor fuel, MOX and made into pellets.  All else is vitrified… combined with borosilicate glass (the original composition of Pyrex®) and made into blocks.  Vitrified waste handles high temperature well and can then be sequestered as desired.

Final comment on a bit of physics that explains how atomic weapons can be made from spent fuel.  If the fuel rod is use only for a short time in the reactor,  238U turns into 239Pu and WGP can be extracted. If it is left for an extended period, the WGP will have been converted substantially to RGP (240Pu).

The US suspected diversion of plutonium or highly enriched uranium to Israel in the mid 1960s from the NUMEC facility in Apollo, PA. (This smuggling has been contested to this day.)  Ten years later, India had reprocessed plutonium for atomic bombs.

To block potential diversion, President Ford issued a directive in 1976 that suspended US commercial reprocessing.  In 1977, President Carter issued an outright ban;  in 1981, President Reagan lifted the ban but did not fund developing any reprocessing plant.  Major reprocessing activity is currently performed in the UK, France, Russia, and the other nuclear bomb countries.

B. Transmutation
Transmutation of waste is sometimes called burning it.  The idea is to naturally accelerate the decay process and turn all the long lived actinides (atoms such as uranium and plutonium) into short lived daughter atoms that can become as radioactive as potash or concrete in a few hundred years.  (Compare with  the million years for some of the actinides.) This is not a reuse strategy as in A but a path to eliminate the material, itself.

Waste is transmuted into safer material by placing it in an intense neutron environment.  The waste atoms are net absorbers of the neutrons, not generators, so this is not a power generation process.  There are too many poisons to cause an explosion, so this is not explosive,  as in atomic bombs. The neutrons cause induced fission and the heavy atoms will break apart into tamer daughter atoms.  There are three processes that we know will work.   Fast neutron reactors, accelerator facilities, and fusion neutron generators.

Fast Reactor – So named because it has no moderator and uses high energy (fast) neutrons.  Waste is placed about the periphery and immersed  in the neutron bath. The converted waste can be used to fuel satellite reactors and generate a net 60% more power from the fuel.  This is a big value added item to the burn-up process.  The down side is that there is a high inventory of plutonium and human greed at any of these facilities.  Though a fast breeder reactor is a huge amplifier for power, it could easily become a conduit for diversion to outside parties.

Accelerator Transmutation of Wastes– ATW would use a proton accelerator to make a lot of neutrons by slamming the proton beam into metal.  Nuclear waste would be arranged about the target as a blanket and absorb most of the neutrons generated.  With enough accelerator shots, the material would be converted from actinide atoms  into much more benign daughter atoms.  I believe an ATW test is being conducted at the Argonne National Laboratory, near Chicago.

Fusion reactor blankets – The generation of copious high energy neutrons by fusion machines is called the “wet-wood-burner” fusion concept.  This is proposed as a device to generate lots of neutrons but little if no energy.  A  WWB is not a power generator and could have been done anytime in the past 30 years with any of the large machines doing plasma fusion research.  You would use deuterium and tritium as gaseous fuel, heat to the stellar temperatures by current techniques and process the spent fission reactor fuel loaded as a blanket about the fusion core.  Although this may sound similar to the fast breeder concept, no long lived actinide fuel would be used.  Tritium (12 year half life) and locally activated materials are the only radioactive byproducts.

WWB units have been proposed since the early 1970’s when I was a graduate student.  If I had to guess, I would say China or Japan will be the first nation to build a WWB machine to transmute fission reactor waste.    Back to top 

Summary

Radioactive waste production is the hard, bitter core of fission power generation.  Currently we are leaving spent rods in water pools that need actively powered cooling.  The published disasters show this is not sustainable.  We have considered a number of sequestering schemes.   The problem with each is that interdiction must hold for hundreds-of-thousands to tens-of-millions of years.    There is no way we can assure that life on earth could not be eradicated by events at the Repository currently labeled “improbable.”  After all,  improbable events do occur, just not very often.

To misquote Sherlock Holmes:  After you have eliminated all reasonable choices, what is left, no matter how bizarre, must be the solution.  Transmutation is what is left after sequestration is eliminated.  For assurance against nuclear diversion, we must rule out the fast breeder reactors that feed satellite reactors.  This leaves accelerator  beam transmutation, or fusion driven wet-wood burners.  Concrete suggestion:   I suggest using the JET tokamak in the UK (once it has completed its final physics experiments) to run wet-wood-burner tests to  demonstrate conversion efficiency.

click for LastTechAge on fission technology

Bottom line: The  solution  is to eradicate the waste now, during this current era:  Until this becomes our accepted policy, we must not build new reactors.  End of sentence, end of paragraph.

This was to be the final post in the Nuclear Decision chain.  However, there will be at least one more to frame the long discussion into a coherent result.     Back to top 

– – – – –
Update: 2013 Nov 10:   Good infographic from Union of Concerned Scientists here.  Advocates moving all waste storage to dry cask  containment for safer short-term storage.  Absolutely.  But:  be careful with percentages of safety increase using casks.  All engineering estimates are linear guesses.  Real accidents have been confluence of multiple events –  true for reactors,  spacecraft, aircraft, naval craft, etc.  

……………………………….

Charles J. Armentrout, Ann Arbor
2011 May 8
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Appendix – About Nuclear Fuel

UO2Pellet_img

Uranium Dioxide fuel pellets

Reactor fuel pellets   are made of uranium dioxide (UO2) and heat sintered into ceramic cylinders.  These pellets are called UOX for uranium oxide,  and handle high temperatures well.  Fissile uranium-235 (235U) is about 3.5% of the total uranium in the UO2.   235U is about 0.7% concentration in natural uranium.  The fission operation converts the abundant natural heavier uranium-238 into plutonium-239, which is quite fissile.

An alternate pellet used is a mix of natural uranium and reactor grade plutonium, 240Pu;  this is called mixed oxide fuel, or MOX.  We will return to MOX later.

FuelRod_dgm

Diagram of fuel rods

Fuel Rods  are hollow tubes filled with fuel pellets. The rods have zirconium alloy walls which resists oxidation as well as stainless steel but is resistant to swelling in the reactor neutron flow, this swelling was discoverd during early reactor operations, led to early incidents and were a really big “oops” for the industry.

Refer to the diagram (from the Nuclear Regulatory Commission).

For numbers, we will use data from a 2010 Japanese presentation on nuclear waste issues at the two Fukushima stations (Daiichi and Daini).

Fuel assemblies   are made up of 63 rods per bundle (Fukushima) and it is these that form the fuel elements in the reactor core.

FuelAssy_Dgm

Diagram of a fuel bundle

Like all else associated with a reactor, this is not an industry standard number.  Other places have assemblies with  other rod counts

After they no longer can generate sufficient neutrons to maintain the required activity, they are removed, said to be spent, and treated as dangerous as waste.

In early March 2o11, Fukushima had 7 pools, six attached to the 10 reactors  (holding 3450 assemblies each and one huge common pool with 6291 assemblies.Some of these have been stored for 20 years or more.

FuelBundleChange_img

Fuel bundle being removed from core

Count:  6×3450 + 6291 is 26,991 bundles.  At 63 rods per bundle, there are in excess of 1.7 M spent fuel rods in storage.

Fukushima is a huge installation, most sites are much smaller. Nevertheless, every fuel rod ever delivered to your friendly neighborhood  reactor will still be on site, waiting.

Spent Fuel   is what remains in the fuel rod after it no longer is able to generate enough power to keep in a working reactor. Fission activity generates isotopes of elements from zinc through the actinides (atomic numbers 90-103).   Some of these are reactor poisons, elements such as xenon, samarium, certain uranium isotopes, many others, that absorb neutrons and stop the fission activity.

Long lived wastes
Depending on the actual fuel used in your local reactor, fission products can have huge half lives.   Here is a small selection:    129I (157,000 yr),  236U (23,000,000 yr),  237Np (2,100,000 yr),  we must not leave out plutonium 239Pu (24,100 yr).  These products are due to to neutron absorption or isotope decay;  they are what we must protect ourselves from.  A duration of five half lives reduces the amount of radiation 32 (=25).  An engineering rule of thumb  is that we should keep these products sequestered for 10 to 20 half lives – for a reduction in radiation intensity by thousand to a million times.

ControlRodsBundles_img

Control rods and fuel bundles moving in the core

Remaining usable fuel
After about 3% of the fissionable fuel has been used, the poison products will have nearly stopped fission reactions;  the fuel assembly must be removed.  To repeat:  spent fuel is 97% good fuel but with enough fission products  to keep  the fuel pellets from producing power.

Back-story of the waste issue
In the US, spent fuel assemblies are never touched again.  They remain hot and dangerous for a very long time and require secure handling/storage for even longer.   The American decision was to reduce the chance of diversion into terrorist hands.  Diversion was not an issue to European and  other countries. These people try to make use of all the fuel, for better or for worse.

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About LastTechAge

I am a physicist with years of work in fusion labs, industry labs, and teaching (physics and math). I have watched the tech scene, watched societal trends and am alarmed. My interest is to help us all improve or maintain that which we worked so hard to achieve.
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