The Gen IV nuclear reactor program is a program that is trying to save humanity from its own stupidity, and is thus probably not a good bet.
All of the criteria raised by scientifically illiterate mystics against nuclear energy, sustainability, war, reliability, terrorism, accidents, economics and (most incredibly ignorant of all) so called "waste" are arbitrarily examined by people who don't know shit from shinola about nuclear energy specifically or any other form of energy in general, and given consideration only for nuclear energy, and not for any of its alternatives. As it happens, there is no such thing as nuclear "waste" a point I have made repeatedly in this space. As for the other criteria, it is trivial and easy to establish that nuclear energy is clearly superior to all of its alternatives on these scores, although nuclear energy is not perfect.
Nuclear energy need not be perfect to be vastly superior to all of its alternatives. It need only be vastly superior to all of its alternatives, which it is.
The papers from the primary scientific literature that I will discuss today are...
"Conceptual design of an indirect-cycle, supercritical-steam-cooled fast breeder reactor with negative coolant void reactivity characteristics" Annals of Nuclear Energy by T. Jevremovic, Y. Oka and S. Koshizuka, at the University of Toyko...
...and...
"Thermal and stability considerations for a supercritical water-cooled fast reactor with downward-flow channels during power-raising phase of plant startup" Nuclear Engineering and Design 239 (2009) 665–679
...and...
A Linear Stability Analysis of Supercritical Water Reactors, (II)
Coupled Neutronic Thermal-Hydraulic Stability, Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 41, No. 12, p. 1176–1186 (December 2004) (no link).
In several diaries recently, I have described what a fast reactor is.
In a diary yesterday, for instance, I wrote...
Implicit in recovering 100% of the energy [in uranium] are two things: 1) Multiple fuel recycles of the plutonium into which uranium is transmuted in all nuclear reactors and 2) having reactors that operate on the "fast" spectrum, in which neutrons have speeds much higher than average gas molecules at ordinary temperatures. (In a gas, the average speed (m/s) of a molecule is equal to the square root of the quantity 3kT/m where k is Boltzmann's constant, T is the temperature using the Kelvin scale, and m is the mass of an atom in kg.)
In general, the world supply of fast reactors are all liquid metal reactors, generally cooled by sodium metal or a eutectic mixture of sodium and potassium metals, although a few Russian liquid bismuth/lead reactors have operated (and I think, still do).
It is well understood among people who understand about what they are talking, that the conversion of all of the so called "nuclear waste" into useful energy and useful materials requires fast spectrum reactors.
The liquid metal type reactors are only one option. There are many others as only a small fraction of the possible types of nuclear reactors have actually been built, almost all of them for the purposes of merely generating electricity, which they do, by the way, with superior economic, environmental, safety, and reliability to any form of energy that operates at a 10 exajoule/year power level, and almost all of them that don't operate at this level.
Nuclear energy can, in theory, provide the primary energy for all human needs however, but other types of reactors besides light water reactors - the most common kind of reactor in the world - heavy water reactors and liquid metal reactors will be required.
The supercritical water reactor is actually another type of light water reactor, and it involves the use of supercritical water, water that has a temperature higher than 373C and a pressure of 22.1 MPa, which is about 218 times as high as atmospheric pressure at sea level.
The relatively high temperature and pressure - which are commonly accessed in modern power plants that run on the dangerous fossil fuel coal - allow for increased thermal efficiency of the power plants. Although Congress and the legislature of California have been trying to repeal it - egged on by members of Greenpeace - the second law of thermodynamics is still operable and there is a maximum amount of exergy - work - that one can obtain from a given system. The best way to maximize this exergy is to raise the temperature of a power plant to the highest achievable energy level.
The advantages of supercritical water reactors in the nuclear case are listed in the Nuclear Engineering and Design 239 (2009) 665–679 paper.
I quote:
(1) high enthalpy rise and low flow rate in the core, which is very effective for reducing the main pump power,
(2) once-through direct cycle, which makes the reactor system compact and simple,
(3) high main steam enthalpy, which makes the turbine system compact and the thermal efficiency high,
(4) utilization of the current well-developed technologies of fossilfired
power plants (FPPs) and light water reactors (LWRs),
(5) single-phase coolant without boiling transition or dryout phenomenon,
(6) being available for both thermal and fast neutron spectrum
cores with the same plant system.
Point number 6 is, I think, a biggie, but the reason is technical and I won't go into it too deeply.
Nevertheless, if you are scientifically literate about nuclear energy - and thus are not one of those people who oppose things based on one's own ignorance of it - you will recognize that in general, water, both normal and heavy is generally a moderator. Collisions between neutrons and light nuclei in water, chiefly hydrogen (or deuterium) but also oxygen, cause neutrons to transfer much of their momentum in inelastic collisions. This makes the water get hotter, but it has the effect of making neutrons - which may be thought of as a very exotic gas - slow down to temperatures typical of their surroundings. A neutron emerging from a nucleus may be thought of as having an average "temperature" of billions of degrees Kelvin.
However the moderation of neutrons by water is in part, a function of the density of water. Steam is not very dense, whereas liquid water is. Supercritical water however is neither liquid nor gaseous steam, it has properties of both as well as properties that are not found in either state of water. It's density is therefore intermediate. Thus the water need not "thermalize" the neutrons, reduce their "temperature" too close to that of the surrounding ordinary matter. The paper makes the point that this is a very valuable property for burning what are known as the "minor actinides," the elements neptunium, americium and curium that are recovered from used "nuclear fuel" to make energy. Making energy, of course, is superior to throwing them away, as expressed by the rather silly "waste mentality" on which the members of say, Greenpeace, like to fixate.
Another point raised in this paper involves the fact that the reactor actually has a subcritical component, the inlet temperature of the water entering the reactor is 280C, below the critical temperature, but the outlet temperature is 500C.
The Annals paper (Ann. Nucl. Energy, Vol. 20, No. 5, pp. 305-313, 1993) in which is an early paper on this concept, points to an important safety feature that this type of reactor has, a negative void coefficient. A negative void coefficient refers to what happens in a putative "loss of cooling accident," of the type that was observed at Three Mile Island - which according to dumb anti-nukes wiped out Harrisburg, PA - and Chernobyl - which also according to dumb anti-nukes wiped out Kiev, Ukraine.
In the former case, the reactor shut down when the water was lost and the nuclear fission reactions stopped. Although residual heat from the fission products caused part of the fuel and core to melt, the vast majority of the radioactivity was contained within the containment building. In spite of what you may have heard, Harrisburg is still occupied by human beings.
In the latter case, Chernobyl, there was a positive void coefficient - now understood internationally to be a "no-no" - so that when the water was lost from the reactor the fission reactions actually sped up. This caused the reactor to heat up, catch fire, and widely distribute the bulk of it radioactivity to the environment, answering for all time what the "worst case" of a nuclear accident is. (It had considerably less impact than the "best case" of dangerous fossil fuel waste and dangerous fossil fuel accidents routinely have.)
According to the Annals paper, the former property of having a positive void coefficient led to the abandonment of study of the supercritical water reactor in the United States.
Steam-cooled FBRs were studied previously in both Germany (Engelmann et al., 1970) and the U.S.A. (ORNL, 1968) but the development was abandoned in the early 1970s due to the expectations of low breeding, safety concerns regarding the large positive void reactivity characteristics and the lack of developmental experience. Those studies, however, expected a lower capital cost for the SCFBR compared with that of an LMFBR. In the 1980s, Schultz and Edlund (1984) presented a concept of the SCFBR with the PlUS-type configuration. They showed the negative coolant reactivity characteristics against voiding and flooding at the beginning of the fuel cycle. But the void reactivity should be proved as the negative value at the end of the fuel lifetime, since it tends to be positive due to the accumulation of fission products.
Note that the Chernobyl accident occurred at the end of the fuel cycle, which again is the worst possible time for such an accident to occur.
The rest of this paper is involved with the design of a supercritical water reactor that has a negative void coefficient throughout its entire fuel cycle.
The void reactivity is calculated using the CITATION code with the cross sections collapsed from the unit cell burnup calculation for the voided state in each core zone. In the calculational model (Fig. 1), the steam region was placed above the core for conservative estimation of the leakage. The neutron energy spectra for the operating and voided condition in the middle seed at the end of cycle is presented in Fig. 3. In the absence of coolant, the flux below 110 eV disappears. The negative coolant void reactivity, complete as well as partial, is proved by our novel concept of inserting thin zirconium-hydride layers between the seeds and blankets (Jevremovic et al., 1992a). The complete void reactivity is calculated for the whole core voided, including the blankets and reflector.
Well, its technical, but take my word for it, it's also very cool.
The last paper refers to examination of some potential bugs and concerns with the design however.
It is found that the present SCLWR-H design satisfies the coupled neutronic thermal-hydraulic stability criterion at full power normal operation. The coupled neutronic thermal-hydraulic instabilities may arise at low-power operations and careful startup design is necessary for the low-power low flow operating conditions. Our studies show that the coupled neutronic thermal-hydraulic stability criterion can be satisfied by decreasing the power to flow rate ratio at low-power operating conditions and during power-raising phase of constant-pressure startup of SCLWR-H. The coupled neutronic thermal-hydraulic stability is affected considerably by the presence of water rods due to moderator density reactivity feedback effects. The presence of water rods makes the resonant peak and the phase lag of the system larger. It increases the decay ratio and makes the system less stable. The present parametric studies show that increasing density coefficient of reactivity decreases the coupled neutronic thermalhydraulic stability, and decreasing power to flow rate ratio or decreasing core inlet temperature can improve the stability.
What this means is that the reactor has more problems operating at low power than at high power.
When the Chernobyl reactor exploded it was actually operating at low power. The comparison does not mean, however that this reactor suffers from the same instabilities as the Chernobyl reactor however, or that the design is unsatisfactory. It means that the issue needs to be evaluated and subject to design improvements.
Personally, I'm not sure that I'm too interested in this design, although I am always interested in supercritical systems of another type.
This type of reactor is part of the Gen IV nuclear program by the way, and may be developed to help save humanity from its own stupidity.