Lithium-6 Deuteride (Li-6-D)and Lithium-6 tritide (Li-6-T),

[Axil]

This post addresses the fusion of Deuterium in a FF reactor to produce thermal neutrons as a means to implement a fusion/fission hybrid. In previous posts, I had wondered if heavy water could be used to thermalize fast neutrons from D-D fusion. I think Lithium-6 Deuteride is far better in this regard.

Background

Lithium hydride (LiH) is the compound of lithium and hydrogen. It is a colourless crystalline solid, although commercial samples appear gray. Characteristic of a salt-like, or ionic, hydride, it has a high melting point of 689 °C (1272 °F). Its density is 780 kilograms per cubic metre. It has a standard heat capacity of 29.73 J/mol.K with thermal conductivity that varies with composition and pressure (from at least 10 to 5 W/m.K at 400 K) and decreases with temperature.

LiH has the highest hydrogen content (in mass percentage) of any saline hydride. The hydrogen content of LiH is three times that of NaH because lithium is lighter than sodium, making LiH of interest for hydrogen storage.

The corresponding lithium-6 deuteride, formula Li-6-D, is the fusion fuel in thermonuclear weapons. In warheads of the Teller-Ulam design, LiD is compressed and heated by the explosion of the fission primary to the point where fusion occurs. Lithium-6 deuteride, unlike tritium, is non-radioactive. It should be noted, as was discovered when the Castle Bravo nuclear test ran away, that the isotope lithium-7 which makes up the bulk of natural lithium is also subject to neutrons as is lithium-6, when bombarded by fast neutrons.

Lithium-6 deuteride can also be used as a storage vessel for use in hydrogen vehicles. Li-6-D can be made by heating lithium-6 and deuterium gas (from electrolyzing heavy water) in a hermetically sealed container to 600-700 C.

Application

It may be possible to form a self regulating and self annealing solid blanket of Lithium-6 Deuteride in a FF core to thermalize neutrons produced using fusion of deuterium, tritium, and lithium.

At a temperature of 700C, Lithium-6 Deuteride (Li-6-D) and Lithium-6 tritide (Li-6-T), vaporizes into deuterium, tritium, and lithium-6. The heat from the formation of the plasmoid and associated fusion will form a pocket of deuterium, tritium, and lithium gas vapor that the plasmoid can use to fuel a FF fusion reaction. The area at and near the core wall will be at a lower temperature (less than 500C) due to active cooling of the core wall. This will form a thick solid coating of Li6D / Li6T on the inside surface of the reactor core wall. This solid Li6D /Li6T layer will thermalize fast neutrons produced by the FF fusion reaction. A solid/vapor boundary will form in a solid sphere of Li6D which will fully enclosed a vapor pocket that will center on the plasmoid and provide fusion fuel to variety of fusion reactions as follows:

D-T fusion.

D-D fusion

D-He3 fusion

T-T fusion

He3-He3 fusion

He3-T fusion

D-Li6 fusion

p-Li6 fusion

He3-Li6 fusion

The magnitude of the cross sections of these various fusion reactions will be approximately in the order listed above, from most probable to least probable. The solid Li6D will protect the first wall of the FF reactor from fast neutrons, energetic alpha particles, and high speed electrons as well as EMF radiation.

Moderation

A good neutron moderator must efficiently thermalize colliding fast neutrons. This thermalization process occurs by neutron-nucleus elastic collisions. It can be shown from a simple consideration of Newton’s laws that maximum energy loss per collision occurs when the target nucleus has unit mass, and tends to zero energy loss for heavy target elements. Low atomic weight (Z) is thus a prime requirement of a good moderator. The maximum energy is always lost in a head-on collision. However, elastic collisions occur at many scattering angles, and thermalization takes place over numerous collisions. Therefore, a useful quantity in characterizing the scattering properties of a moderator is the average logarithmic energy loss per collision, E, which is independent of energy. Values of E and n, , the average number of collisions to thermalize a 2 MeV fast neutron to 0.025 eV, are given as follows:

Scattering properties of some nuclei 

	

	Element                               z                             E                  n

	

	Hydrogen                             1                            1.000               18

	Deuterium                            2 0                           .725               25

	Helium                               4 0                           .425               43

	Lithium                              7 0                           .268               67

	Beryllium                            9 0                           .208               87

	Carbon                              12 0                           .158              114

	Oxygen                              16 0                           .120              150

	Uranium                             23 8                          0.0084            2150

[Brian H]

Axil wrote:
…
A good neutron moderator must efficiently thermalize colliding fast neutrons. This thermalization process occurs by neutron-nucleus elastic collisions. It can be shown from a simple consideration of Newton’s laws that maximum energy loss per collision occurs when the target nucleus has unit mass, and tends to zero energy loss for heavy target elements. Low atomic weight (Z) is thus a prime requirement of a good moderator. The maximum energy is always lost in a head-on collision. However, elastic collisions occur at many scattering angles, and thermalization takes place over numerous collisions. Therefore, a useful quantity in characterizing the scattering properties of a moderator is the average logarithmic energy loss per collision, E, which is independent of energy. Values of E and n, , the average number of collisions to thermalize a 2 MeV fast neutron to 0.025 eV, are given as follows:

…

But you’re using that bad word: thermalize. When you’ve got your heat, then what? You must run it through a steam generator, and you’ve just reversed a primary advantage of FF: no boilers and turbines!
Also, you’ve introduced that horrible concept of a containment vessel and its walls, with all the associated mass, expense, and materials degradation issues that implies.

The FF idea is to avoid neutron and heat flux, not maximize and exploit them. I get the image, when talking of FF “hybrid” designs, of turning a sportscar into an armored vehicle. Possible, but it makes for a lousy sportscar and worse armored car.

[Author]

Posts