Technology

A few equations will help to convey the nature and scope, of the technical challenges confronting the development of fusion-nuclear energy. The starting point is the basic requirement for scientific breakeven, i.e., where the nuclear-output energy equals the energy input needed to heat the reaction. This condition may be expressed as a modified "Lawson Criterion", which expresses a triple product of density, n, temperature, T, and energy-confinement time, τ , that is written as, 

where  k is  the Boltzman constant, E is the energy released per nuclear reaction, and < σv > is the number of fusion reactions produced per second. This triple product defines the "threshold" above which net energy may be expected. However, not the threshold where the reactor concept is feasible from an engineering standpoint, and of course the more challenging requirement for commercial viability. 

Thus, a fundamental requirement is to attain a fuel temperature that is sufficiently above what is needed to initiate the fusion reaction in the first place. This temperature threshold is fuel specific and, as is well known, is often compared to the temperature of the sun and stars; i.e., typically on the order of 100's - 1000's of millions of degrees Celsius (C); compare that to the hottest atmospheric temperatures ever recorded on Earth, in the range of 56 C.  Therefore, at  fusion temperatures it is simply not possible for the fuel to remain in contact with ordinary material surfaces. 

Thus, the requirement is for the fuel to be created in a physical volume that remains sufficiently removed from the walls of the confinement vessel. Moreover, as with any energy-rich burn, a considerable heat flux is radiated to, and a particle flux is incident on,  the inner surface of the chamber.  To be more quantitative, the first wall energy flux will be of the order of 10's of MW/cm^2, that is comparable to what is found commonly in the fuel rods used in nuclear fission.

Note, the convention commonly used by fusion scientists to describe the fuel temperature is in units of kilo-electron Volts, i.e.,  a thousand electron Volts. In these units the fuel temperature requirement is typically in the range of, T ~ 10's - 100's of keVs, and more.  

Figure 1 is a plot of values for the second fraction displayed in the above equation, that is,  the ratio of the temperature squared to the reaction rate. This plot illustrates the range where fusion will occur most readily for different fuel cycles; smaller values on the y-axis are preferred, and as shown a 50:50 mixture of deuterium-tritium fuel, DT, is characterized by the smallest ratio at the lowest temperature, T ~30-50 keV. This explains why DT fusion is the default fuel cycle most widely considered in fusion research. The next lowest ratios are for mixtures of deuterium-deuterium, DD, and deuterium-helium3, DHe3, followed by hydrogen-boron11, HB11, at ~400 keV.  Above 500 keV the ratio values are roughly equivalent for all of these aforementioned fuels. 

Indeed, if it were possible to technically produce and sustain the higher temperatures indicated in this plot, then the reactor concept could be used to burn any of the indicated fuels, including a  multitude of other fuel cycles that have not yet been mentioned, for example in the temperature range, T > 1,000's of keV. Thus, such a reactor concept would be trancendent in our ability to choose the "best" fuel for a specific purpose, mindful of myriad technical and social requirements. For the present discussion we are most interested in establishing the technical basis for nuclear fusion reactions in the range of 50-500 keV, which means DT, DD, DHe3, and HB11. 

One must keep in mind that all fusion concepts must be "driven externally", at least initially, in order to attain the  fuel densities and temperatures required above, as the fuel is subsequently burned up. This means that once the fuel source, or driver, is removed the fusion reaction would terminate with minimal risk of uncontrolled nuclear runaway and is the reason why fusion is generally described as "safe". 

By some estimates the replacement timescale could be as short as a year, or perhaps several. Thus, the larger the size of the reactor, the larger the amount of radioactive debris. And similarly,  the  greater the probability for unpredictable single point failures of the confinement vessel and other components.   

Among the fuels mentioned above,  HB11 is the least radioactive, as its output nuclear energy is produced in the form of three energetic helium nuclei (ions), called "alpha particles". Nevertheless, as radioactive by products are also produced in HB11 reaction, due to trace amounts of neutron-producing background fuels, and side-reactions occuring at levels millions of times reduced relative to the primary fuel cycle, the HB11 fuel cycle is still commonly referred to as an "aneutronic fuel." DHe3 is another actively considered aneutronic fuel cycle, limited by the availability of He3. 

Perhaps the most attractive feature of aneutronic fuels is their ability to produce their nuclear energy almost entirely in the form of charged particles,. Thus, the energy of these reaction particles needs to be recovered using sophisticated  methods of charged-particle deceleration. Such technologies do not yet exist in a practical form, although concepts have been tested with estimates for the closed-cycle efficiency to be of the order of 90%. In effect, "direct-energy conversion" comprises an inverse particle accelerator. For comparison, the efficiency of thermal conversion is typically in the range of 30%, perhaps a bit higher when a a combined cycle process of thermal conversion and heat extraction are considered. 

Even though aneutronic fuels have a potentially higher, output energy conversion efficiency, their overall energy gain is still much lower than for a neutron producing, thermal cycle. The energy gain factor is written as, 

Q = Eout/(Eheat + Eradiation + Elosses), 

where Eheat is the input heating energy, Eradiation is a radiation loss term, and Elosses is the loss of charged particles.  As stated, the  contribution attributed to  Elosses may be assumed to be efficiently recovered, hence to first order it is neglected in the calculation of Q. 

The result is that the gain factors for the preceding fuels are nominally in the range of: Q(DT) ~ 25; Q(DD) ~ 8, Q(DHe3) ~5, and Q(pB11) ~ 3.5.  Inevitably, as the Q factor decreases the room for design errors will also decrease, making it more difficult to optimize the reactor's energy output at  levels that are commercially competitive. This is especially true in recent years where there now exists a plethora of renewable energy options that are now in the commercial range of  10 - 20  cents/kW-hr. There exist many reasons for cost reductions in these renewable energy sources, but they are largely due to technology maturation that is the result of government subsidies, continuing investments, and technical innovations; a journey that fusion has only recently embarked upon. 

The main reason that aneutronic fuels have a lower energy gain factor  is because the atomic numbers for their fuel components are much larger than for the  lower atomic number, "hydrogenic" fuels, such as DT, or DD. Higher atomic number fuels produce copious amounts of radiated energy that cannot presently be efficiently captured and converted. This is evident from the radiation term that is present in the denominator of the preceding equation, the effect of which decreases Q as Eradiation increases. Nevertheless, a Q factor greater than unity can still be an attractive option for fusion, because of the high energy density for  fusion, its essentially zero greenhouse gas emissions, and its potential for the long term availability of its component fuels. 

In light of these tradeoffs perhaps the most attractive feature of aneutronic fuels may be their potential to also produce a scalable output energy; i.e.,  one where the  output power level covers a broad range, perhaps in the range of MW's to many GW's. Solar, wind, hydroelectric, oil, gas, etc. are also scalable concepts, because adding a larger surface area, or higher capacity generator, is technically straightforward. This is simply not possible for existing neutronic fusion reactor designs, as the miniumum output power levels for fusion appear to be in the range of many 10's of GWs, for the more commonly considered fuels. 

The reason that aneutronic fuel cycles are scalable is based on the physics descriptions for their particle confinement. When the fusion fuel temperature is in the range of 10's of keV, experiments demonstrate that the energy and particle confinement times are "anomalous," which means that the physics equations used to describe the particle confinement are less predictable, chaotic functions, accompanied by high levels of turbulence. However,  when the temperature of the confined fusion fuel particles are higher, for example T ~ 500 keV, the behavior of the confined particles follow physics descriptions that are described as classical, which means that the particle losses may be calculated "a-priori" from simple equations.

Perhaps the most important question to be asked at this point is: why has it taken so long for aneutronic fusion concepts to be developed? Of course there are a multitude of reasons primarily based on the technologies widely used in fusion research to produce and confine the fuel at such extreme temperatures. Innovations in these areas will play a critical  role in the eventual success of aneutronic fusion.  

For the past years, we have continued to refine that concept and features  that underly a revolutionary new fusion concept.  We are now at the point where it is appropriate to disseminate imporant features of this new technology. Interested parties are encouraged to contact us to learn more.