1. “One of the primary technological rainbows that might lead to this pot of gold is to use reactors that are cooled by liquid metal, with the common choices being limited to sodium, lead, or a eutectic mixture of sodium and potassium called NaK…”

    It’s a nit, but would one not rather prefer “a eutectic mixture of lead and bismuth called LBE?”

    LBE is liquid between 124 C and a toasty 1670. Although hot molten lead has corrosive properties that introduce challenging opportunities in reactor vessel and piping design, not everyone believes them insurmountable, and Westinghouse has proposed a consortium to pursue lead fast reactor commercialization.

    NaK, on the other hand, is truly fascinating stuff; with high surface tension and a eutectic mixture liquid between -12.6 and 785 C.

    That low melting point is attractive for research reactors, but unlike pure sodium (mp 98 C), NaK in contact with oxygen can form a potentially explosive super-oxide. IAEC has a standard for NaK handling that recognizes the hazard. Perhaps I am mistaken but to my knowledge NaK is not proposed for use in commercial power reactors for this reason.

    Pure sodium is a different matter: exposed to air it spontaneously burns with a slow smoky flame, and as we all know from high school chemistry, reacts a bit more vigorously with water. Neither of which are an issue within a reactor vessel or primary heat exchanger, and unlike lead, liquid sodium metal is quite benign in this environment.

    1. LBE seems like it would be attractive for creating high temperature reactors with something like a supercritical CO2 turbine? I think one limitation of sodium reactors, is that the lower boiling point of sodium means you can’t run them at the high temperatures necessary for high efficiency cycles, is that right?

      1. Vaclav Dostal’s thesis, IIRC, mentioned that CO2 begins to crack to CO and coke at around 550°C, so ultra-high temperatures wouldn’t help for an sCO2 system.  Industrial process heat and thermochemical applications are a different matter.

        sCO2 can push 50% if memory serves, and given how cheap and clean nuclear heat is I don’t think it’s worth spending much money just to get into CCGT territory.

        1. EP – OK, thanks, didn’t know that was a limit. So, putting aside sCO2, what about other high temp cycles like helium – can’t those get up to like 60% efficient?

          1. what about other high temp cycles like helium – can’t those get up to like 60% efficient?

            (I guess I’m the resident thermodynamicist, so…) There are issues of expense with helium, for a number of reasons.

            The two biggies are things most people barely grasp.  One is that it’s a very low-density fluid with an extremely high speed of sound.  When you expand it through a nozzle, it comes out FAST.  You also need very high blade speeds to compress it.  All your rotating machinery winds up going twice as fast or more than it would have to with steam, and close to 3x as fast as with air.

            The other is a thermodynamic factor called ratio of specific heats, often abbreviated as k or γ.  Everyone’s familiar with Boyle’s Law, Pv = RT, but until you take thermo nobody tells you the “ideal gas law” for adiabatic processes:  Pv^γ = constant.

            (γ-1)/γ equals R, the gas constant.  γ for diatomic gases like nitrogen and oxygen at normal temperatures is 1.4 (7/5).  But for monatomic gases, it’s a whopping 1.667 (5/3).  This means that a given pressure ratio in compression or expansion results in a MUCH greater change of temperature for gases like helium than gases like air.  It also means the specific heat is a lot smaller, so you need to move more gas to handle the same amount of heat.

            What this means practically is that a reasonable pressure ratio in a helium-cycle system requires a compression stage, an intercooler, another compression stage, another intercooler… and finally the last compression stage before adding heat.  The expansion is similar:  expand, reheat, expand again, reheat again… finally expand and dump to the heat sink.  All those separate stages and heat exchangers cost money.

            The beauty of the supercritical CO2 cycle is that, right around the critical point, γ falls to a very low value.  This means that the specific heat goes through the roof… and the temperature rise during compression is very small.  This allows large pressure ratios without intercooling.  That slashes a lot of cost out of the system and also slashes compressor back-work, which is where your major efficiency gains come from.

            I hope that made sense.

            1. @E-P

              Understood every word. You’ve done a good job of explaining why AAE shifted from He to N2 in the very early stages of design. While the supercritical CO2 cycle might be even better someday in the near future, it wasn’t – and still isn’t – a common, off-the-shelf option today.

              AAE’s primary insight was that it would be wonderful to find a way to combine the abundant, low cost, zero emission, energy dense fuel used in nuclear heated steam propulsion plants with the low capital cost, simple to replace, manufactured in factory setting, and well proven air-breathing Brayton cycle gas turbines used in responsive ship propulsion systems and in a growing number of land-based applications. (We were making our design decisions in 1993-94.

              The path we chose was to use compressors and turbines designed to breathe air and combustion products and determine if they would be suitable for moving N2 gas – which is the 79% majority component of atmospheric air. They are.

              We discovered that N2 has been used as a nuclear reactor coolant once before. It worked fine, but that experimental system used improperly an matched compressor/turbine and produced about 60% of design power. It was in 1960-62, the early days of gas turbines; even the aircraft versions were only about 15 years past their earliest demonstration units. Designers were still learning.

              There is an objection to N2 – it has a small neutron abosorption cross-section so it is a minor “poison” in a power reactor and it gets activated with a moderate production of C-14. That is a solvable issue, one whose cost is expected to be quite minor compared to the difference in cost between a steam turbine power plant and a gas turbine power plant of the same output.

              The AAE “wall chart” will look very different from those of conventional nuclear plants. Those have an almost invisible little reactor compared to the rest of the plant’s structures, systems and components. The AAE power plant will have a relatively large reactor, a relatively large heat sink heat exchangers and a small compressor that may or may not be physically connected to a small turbine.

              The highest pressure in that reactor will be the output pressure of the compressor, whose input will be at or close to atmospheric pressure. That makes the large reactor reasonably affordable because never is exposed to a substantial pressure. There is no mechanism for sudden pressure increases to anything higher than the shutoff pressure of the compressor.

          2. I goofed on one thing.  The difference between the constant-pressure and constant-volume specific heats, Cp and Cv, is the gas constant R.

            Cp – Cv = R

            γ is dimensionless.

  2. RE: “They will not thank us for putting valuable material in places where it is difficult to retrieve.”
    In fact, most national DGR projects (Deep Geological Repositories) for used nuclear fuel, are mandated to design it in such a way as to make the fuel retrievable, should future generations decide to make use of it.
    Even during the site selection process, long before construction, agencies like Canada’s NWMO periodically re-assess the likelihood that the fuel will in fact be re-used in the relative near-term.
    (See 2015 update here: https://www.nwmo.ca/~/media/Site/Reports/2015/10/22/11/49/2763_watching_brief_on_reprocessing_-_update_2015.ashx?la=en )

    For the foreseeable future, it does not appear that advanced fuel cycles will supplant the need for DGRs, anywhere on earth.

    That being the case, government support of DGR projects (outside the US) is based on the notion of responsible stewardship of used fuel — meaning not leaving future generations with the burden of having to deal with it, should they forego the option of used fuel recycling (but leaving them the option to do so, by making DGR material retrievable).

    Of course, it is understood that the ease & cost of retrieval gradually decreases with time.
    Ergo, there is an incentive of sorts, to make a retrieval decision sooner rather than later.

    In any case, if the intent of recycling used fuel is to “burn up” all the heavy metals in it — chiefly U238 — then we need to look at how long that might take, because the equivalent stored energy is enormous.
    For instance if, under a particular Gen-IV deployment scenario, it takes several thousand years to use it all up, then a DGR may be preferable to surface storage anyway, because surface structures typically last only about a century, and would therefore require many replacements and associated used fuel transfers.
    Partial used fuel retrieval from a DGR may therefore be a possibility.

    For example, if we compare LWR used fuel,

    LWR fuel burnup: 45,000 MW-d/tonne
    Total fuel burnup: 900,000 MW-d/tonne

    Hence a 20-fold difference in total energy output.

    So, if a 100,000 tonne used fuel inventory was the result of operating a 100% fleet of LWRs for 50 years, then a fleet that includes 25% FNRs (Fast Neutron breeder Reactors) will take several thousand years to use it all up (as one example).

    Lastly, for purposes of managing radwaste and designing long-term storage facilities that are compliant with regulatory release limits, the isotopes of concern are long-lived things like Tc99 (211ky), Se79 (295 ky), Cl36 (301 ky), Cs125 (2.3 My) and I129 (15.7 My).
    Separating them out, as part of used fuel recycling, still leaves you with the same problem of long-term storage, which is to be compliant with regulatory release limits.

    For long term storage, over periods of a 1,000 years or more, reliance on surface structures is much more difficult to justify to a regulatory authority, than reliance on geology deep below surface, in specially selected sites (where the rock is known to have remained stable over millions of years, and has a known “hydraulic conductivity” that allows scientific prediction of radionuclide transport).
    The only way to change that situation, is to change the regulatory release limits.
    It’s debatable how likely such a policy change may or may not be, or how far it might go.
    But storage facility design cannot be based on anyone’s murky crystal ball.

    1. @Jaro

      “Retrievable” isn’t a sufficient criteria. It should be cheap and easy to retrieve.

      We should not be vain enough about our current state of knowledge and understanding to attempt to make too many choices for future generations. We should also not put ourselves into the role of the indulgent parents who expect their children to be able to make good choices without teaching them as much as possible and providing them with the tools they need to learn even more than we have learned.

      Anyone who has read Atomic Insights with any kind of regularity should know that I am adamantly opposed to the existing release limits and their lack of scientific basis.

      Finally, what kind of discount rate are you assuming when you say that it is more expensive to maintain or rebuild surface facilities than to spend tens of billions MORE in the near term to build deep underground repositories? As a former financial analyst, I’d love to see the assumptions claiming that it is better to spend a whole lot of money now than to defer expenditures into the distant future.

      1. Interesting report, in NEI Magazine, on an international conference on used fuel management….

        “ There is both the perception and reality that the amount of radioactive waste and spent fuel held in non-final storage increasing. “This may lead to consideration of storage as a long-term option, which is hardly justifiable with regard to inter-generational equity,” the summary observes.”

        “the Policy & Strategy session also raised issues around retrieval of wastes from a proposed disposal facility.
        From the discussions, the session summary reports, it became apparent “there is apparently not a common understanding of what retrievability really means.”


        1. @Jaro

          It’s time to put a new spin on “intergenerational equity.” We are not leaving them a burden, but an inheritance. Shutting down nuclear in favor of natural gas invariably leads current generations to use up a depleting resource that would be valuable for future generations. That’s selfish of current generations, don’t you think?

  3. In 1992, the FFTF was ordered to be placed in standby by President Clinton and Hazel O’Leary, his first Secretary of Energy. On Jan 19, 2001, the last day of the Clinton Administration, …

    That’s quite a trick that Clinton pulled off considering that the first day of the Clinton Administration wasn’t until Jan 20, 1993. 😉

  4. “One of the many reasons that the U.S. has not reached any long term agreement resulting in a successful and sustained program of final disposal for used fuel is that a significant group of nuclear-knowledgable scientists and engineers are professionally offended by the idea of permanently placing a vast source of potential energy into a location where it is as inaccessible as possible.”

    Yeah, with “friends” like this, nuclear doesn’t need enemies. I’ve become more and more convinced that, in addition to external adversaries like the anti-nukes and the fossil industry, a significant fraction of nuclear’s struggles come from people working inside it, who have put their various specific agendas ahead of nuclear’s overall well being. Examples include hyping nuclear “problems” to justify research funding, and to make margins on large unnecessary expenses. This is another example.

    The cost of uranium ore is a negligible fraction of nuclear power’s overall cost, and technologies like seawater extraction make limitless supplies available at an additional cost of at most a fraction of a cent/kW-hr. And even if Yucca is needlessly expensive, it only cost ~0.1-0.2 cents/kW-hr. Thus, this is all a solution in search of a problem.

    The bottom line is that any closed fuel cycle (i.e., combination of reprocessing and some advanced reactor design) will be more expensive than just using raw uranium ore. The only conceivable benefit is a reduced number of repositories, but even that is largely a red herring (experts have told me that Yucca could hold an almost indefinite amount). Even if one accounts for a potentially less expensive repository, it is not clear at all that the overall fuel cycle would be less expensive, given the costs of reprocessing, etc. And again, any differences between these options are only on the order of a fraction of a cent/kW-hr.

    Yet, over this unimportant issue, these people (within the nuclear community) are willing to delay final resolution of the nuclear waste “problem” for several more decades, and to let the public continue to (erroneously) believe that it is a problem that we have no technical solution for.

    To very roughly paraphrase Yoda:

    Public belief that nuclear waste is unique in that we have no technically viable way to dispose of it leads to fear and lack of support of nuclear energy.

    Fear and lack of support lead to excessive regulations and to unfair policies (such as lack of credit for not polluting).

    Excessive regulations, as well as unfair policies, lead to high costs and lack of economic competitiveness.

    High costs and lack of competitiveness will result in the death of nuclear power….

    How about this. Let’s put all the waste in Yucca, and if some wonderful fuel cycle and advanced reactor combination actually comes into existence (on large scale), then we can pull the waste out and recycle it. As Jaro said, the waste will be retrievable for centuries. Any added costs of this approach will be negligible in the grand scheme of things (on the order of a fraction of a cent/kW-hr).

    The risks and costs of transportation are negligible. And to minimize that, we could put a reprocessing facility next to the repository. Nevadan’s have always liked the idea of such a facility far more than the idea of the “dump”. (Far more jobs after all.) More generally, the costs of retrieval as well as the “needless” cost of an overly expensive repository, are negligible in the grand scheme of things (a fraction of a cent/kW-hr). Far lower than the costs that are indirectly related to leaving the waste “problem” unresolved, as a result of unfair policy and excessive regulation, IMO.

    At a minimum, we should finish the Yucca licensing process, as the public has already paid for all that science. Perhaps I’m being too hopeful, but an NRC license may at least reduce the public belief that there is no *technical* solution for the nuclear waste problem.

  5. The US NRC ensures that anyone that subscribes to it cannot build any new reactors.
    India has two and Iran one new Russian VVER producing power. Russia also has a new fast reactor working to produce power.
    Mainland Eurasia from Russia to China and India is the centre of nuclear building.

  6. I drive by FFTF everyday on my way to work. I also work with some individuals who were once employed there. Restore it!,

  7. Russia is building a brand-new facility. Invited anyone to participate, even at the design stage, if they made a somewhat greater commitment. Why not? Unfortunately we know the answer. The current dangerous and dishonest hysteria about Russia, makes that sadly impossible.

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