XYLENE POWER LTD.

NUCLEAR FUSION ENGINEERING CONSIDERATIONS

By Charles Rhodes, P.Eng., Ph.D.

1) Nuclear fusion power cycles generally require the use of on-site automatic cryogenic hydrogen and helium iostope separation equipment;
 
2) It is desirable to avoid the use of metallic lithium in nuclear fusion power reactors because:
a) Lithium is highly flammable and its products of combusion are toxic;
b) Lithium reacts violently with water;
c) At the equipment operating temperature the lithium vacuum vapor pressure is high, which reduces spheromak lifetime;
d) Lithium has a low work function as compared to lead, which reduces spheromak lifetime;
e) The lithium atomic weight is too low for efficient reflection of H-2, H-3 and He-3 ions;
f) During equipment service it is difficult to prevent trace amounts of water vapor in the air chemically combining with exposed metallic lithium to form small amounts of lithium hydroxide. The lithium hydroxide melts at 462 degrees C and acts as a flux causing lithium to form a thin layer on the surface of electrical insulators;
3) For these reasons the liquid lead used for plasma compression should not contain metallic lithium.
 
4) Neutron absorption by the lithium forms radioactive tritium, which is a natural beta emitter with a half life of 12.6 years. Tritium tends to chemically bond to cool metal surfaces, making work place safety an ongoing issue;
 
5) The lithium in a nuclear fusion power plant will also produce radio active Be-7 which is a gamma emitter with a half life of 53 days;
 
6) A double isolated heat exchanger should be used to safely isolate the radio active liquid lead from the high pressure water/steam used for turbine propulsion for electricity conversion. The intermediate fluid can be liquid sodium.
 
7) The parasitic kinetic energy load of the nuclear fusion process may consume more than half of the energy that is generated. Hence the cooling water requirement per net electrical kWh produced is more than twice the cooling water requirement of a nuclear fission reactor. However, due to the higher operating temperature the heat sink can be a cooling tower instead of a lake or river. Similarly the turbogenerator, its condenser, its injection pump and its steam generator must be twice as large as for a nuclear fission reactor with the same net electricity output;
 
8) The science and technology involved in nuclear fusion power systems are beyond the scope of most existing undergraduate university course curricula. Hence employee training for work on nuclear fusion power systems is a challenge;
 
9) Nuclear fusion power systems involve both cryogenic and high vacuum equipment.
 
10) A practical issue, specific to PIF type nuclear fusion power systems, is noise and vibration. PIF fusion power systems are characterized by high energy pressure pulses comparable to the pressure pulses produced by the diesel engine of a large ship. The equipment needs acoustic and vibration isolation and needs physical separation from residential areas;
 
11) Once a nuclear fusion reactor has been in continuous service, two days must pass after equipment turn-off before the Pb-209 related beta decay radiation (half life = 3.3 hours) subsides enough to permit safe work by human beings on or near the liquid lead coolant. However, formation of Be-7 may be a much bigger parctical problem.
 
12) The operating temperature of metal surfaces in a nuclear fusion reactor is above the rated operating temperature of elastomer O-ring seals. Realization of reliable high pressure/high vacuum gasket seals that are rated for both the design operating temperature and level of vibration and that easily come apart for maintenance is a practical challenge;
 

CONSTRAINTS:
A potential problem is sputtering of neutral lead atoms into the plasma due to impact by high energy deuterium and tritium ions, which sputtering will occur if the liquid lead wall acceleration is not sufficient as the random plasma temperature rises. In this respect it is essential that there be sufficient radial acceleration of the liquid lead shell.

As shown on the web page titled ADIABATIC COMPRESSION the adiabatic compression heating process only works as long as the number of particles confined by the liquid lead shell is constant. If due to sputtering the number of enclosed particles significantly increases the plasma will fail to reach fusion ignition temperature. Furthermore, ions then diffusing out of the plasma will rapidly lose kinetic energy to the liquid lead wall.

It is essential that the inside shell surface have no lithium alloyed with it that can sputter off. The required lithium is introduced into the system by an non-alloy methodology.

If there are too few deuterium-tritium ions within the liquid lead shell the fusion pulse energy release will not be sufficient to sustain the on-going PIF process. If there are too many deuterium-tritium ions within the liquid lead shell these ions will not reach the required temperature. Injecting the correct number of deuterium-tritium ions into the liquid lead shell at the appropriate instant in time is critical.

Practical issues related to transport of the spherical pressure vessel limit its maximum outside diameter to about 5.0 m. In principle larger pressure vessels could be fabricated but roadway bridges and overpasses and railway tunnels impose size and weight constraints on moving larger outside diameter spherical pressure vessels.
 

ENGINEERING UNCERTAINTY:
1) The current nuclear fusion reactor designs relies on adiabatic compression of a preheated random plasma to achieve fusion conditions. Typically the plasma is preheated to an average particle kinetic energy of about 3.5 eV and then is adiabatically compressed to achieve an average ion kinetic energy of about 120 keV when:
Eph = (Ekld / 2).

There is an implicit assumption that the liquid lead in the plasma compression system is an ideal imcompressible fluid. In reality uncertainty relating to the behaviour of liquid lead at high pressures and non-adiabatic conditions in the range:
Rii< Ri < Rih
where:
0 < Ekl < (Ekld / 2)
causes some engineering uncertainty.

2) The plasma preheating energy is delivered to the adiabatic compression apparatus via a semi-stable deuterium plasma configuration known as a spheromak. The spheromak energy should be at least 3 kJ per spheromak to allow for energy storage and transfer losses and the spheromak lifetime must be at least 2.0 ms to allow complete closure of the liquid lead shell after spheromak injection but before the spheromak spontaneously randomizes. Therein lies a performance uncertainty. Typical experimental spheromak lifetimes are less than 1.0 mS while the best projected liquid lead shell formation time is about 1.8 mS. For the fusion reactors to function as intended the injected spheromak lifetime must be increased.

While the qualitative properties of spheromaks are known there is a lack of reliable quantitative data that can be used for the engineering design of fusion reactors. Hence, the production and experimental study of spheromaks is an important research topic. However, the special purpose equipment required for spheromak production and study together with a lack of funding is delaying this critical research. There is insufficient appreciation amongst R & D funding agencies of the key role of spheromaks and spheromak lifetime in the fusion power cycle. It is of great importance to experimentally determine the conditions under which a high energy deuterium spheromak has a life time of at least 2.0 milliseconds. Billions of dollars of hardware development hinge on this basic issue.

General Fusion Inc. in Burnaby, British Columbia is currently the leader in the field of high energy deuterium plasma spheromaks. This author believes that General Fusion Inc. has made sufficient technical progress to justify much more financial support from both governments and the private sector.

This author believes that the main issues affecting spheromak lifetime are the concentration of neutral gas atoms in the vacuum chamber, the electron impact ionization cross section of these neutral gas atoms, the velocity of the spheromak's free electrons and electron emission from the enclosure walls. However, this spheromak lifetime model remains to be experimentally confirmed.

3) The liquid lead acceleration methodology currently being pursued by General Fusion is unsatisfactory due to inadequate initial liquid lead radial velocity and inadequate energy coupling efficiency between recovered heat and the accelerated liquid lead. An intitial liquid lead radial velocity of about - 300 m/s with an energy coupling efficiency of about 70% is required to make this fusion system work as contemplated herein.
 

DEVELOPMENT RISKS:
1) From the perspective of this author a major area of development risk related to the PIF process lies in realizing the required compressed spheromak energy and lifetime.

As shown on the web page titled REACTION CHAMBER FORMATION the time required for formation of a closed liquid lead spherical shell is about 1.83 ms. This is the minimum time interval between the time of injection of compressed spheromaks into the pressure vessel sphere and the time when the spherical shell of radially convergent liquid lead is fully closed around the spheromaks, permitting injection of deuterium-tritium fuel into the spherical shell and subsequent adiabatic compression of the resulting random plasma.

For the PIF process to work as envisaged herein the compressed spheromak/FRC lifetime must be longer than the 1.83 ms time required to form and close the liquid lead spherical shell around the injected spheromaks. This extended compressed spheromak/FRC lifetime at a delivered spheromak energy of about 3000 J has yet to be experimentally demonstrated.

There could easily be a problem with the high energy compressed spheromak/FRC lifetime being too short, in spite of the contemplated measures intended to extend the compressed spheromak lifetime. As shown on the web page titled SPHEROMAK LIFETIME a compressed spheromak's lifetime is a function of the neutral gas molecule concentration of each atomic species, the species' electron impact cross sections and electron emission from the enclosure walls.

In the past lifetimes of dimensionally smaller compressed spheromaks of about 80 microseconds have been reported but crucial data relating to the corresponding parameters is missing. In this author's view the only way to conclusively resolve this spheromak/FRC lifetime issue is to build full size spheromak generators and plasma injectors and experimentally measure the high energy spheromak and FRC
lifetimes within an enclosure having an inner surface formed from 600 degree K flowing liquid lead.
 

2) Calculations indicate that to protect the spheromak generators from high velocity liquid lead the spheromak injector ports need ball valves that close in less than 7 ms to meet the time and dimensional constraints imposed by gun assembly of the liquid lead shell. The plasma injector neck inside diameter must be 0.6 m to allow injection of sufficiently large and energetic spheromaks without causing excessive field emission of electrons from the enclosure wall. Moving a magnetically coupled ball valve with a 0.6 m diameter port from full open to full closed in 7 ms is a significant mechanical challenge.
 

3) Another potential source of risk is dissipation of liquid lead alloy kinetic energy acquired due to vaporization of the central liquid lead during the fusion energy pulse. It may be difficult to prevent high velocity liquid lead droplets from eroding exposed interior surfaces of the spherical pressure vessel, the liquid lead guns and the spheromak injection ball valves.
 

4) Another source of risk is problems related to realizing and maintaining high vacuum seals in the presence of the high temperatures, high temperature swings and high mechanical impulse stresses prevailing in this energy system.
 

5) Another potential risk is that the system works as contemplated herein but the resulting fusion pulse rate and net fusion pulse power output are not sufficient for this method of energy production to be commercially viable.
 

This web page last updated January 20, 2015.