2.7 Isotopes of hydrogen

Hydrogen has three isotopes ( [link] ) and unlike other elements these are given special names to differentiate then from the most abundant isotope. Tritium is radioactive with a half life of about 10 years.

Synthesis of deuterium compounds

Electrolysis of water

The electrolysis of hydrogen-1 water (H ₂ O) in the presence of an alkali results in the formation of hydrogen and oxygen.

In a similar manner the hydrolysis of deuterated water (D ₂ O) yields deuterium and oxygen.

However, the rate of electrolysis of D ₂ O is slightly slower than that of H ₂ O. Thus, the partial hydrolysis of water with a mixture of natural isotopes results in the slight enrichment of the water with D ₂ O. The level of enrichment in one step is less than 1%. In order to obtain high levels of D ₂ O (e.g., ca. 30%) it is necessary to reduce the original volume of water by 1/100,000 ^th .

Chemical equilibrium

Proton exchange reactions can be used to enrich compounds in deuterium. For example, the reaction of HSD with water shown in [link] has a slight preference for the formation of H ₂ S, i.e., K _eq = 1.012. Thus, bubbling HSD through water results in the enrichment of the water in HOD. However, about 30% enrichment is about the best that can be achieved by this method.

Fractional distillation

The boiling point of H ₂ O is (by definition) 100 °C, in contrast the boiling point of D ₂ O is 101.4 °C. Thus, it is possible to separate H ₂ O from D ₂ O by fractional distillation. This method provides the most suitable route to high isotopic enrichment and D ₂ O of 99.8% can be produced this way.

Uses of deuterium compounds

Possible nuclear fusion

The largest use of D ₂ O is as a moderator and heat exchanger for fission nuclear reactors, however, the biggest potential application will be if nuclear fusion is realized as a commercial process.

The fusion of two deuterium atoms to form a helium atom and energy would be one source of energy, [link] , however, deuterium-tritium fusion is the most promising, [link] .

The deuterium part of the fuel does not pose a great problem because about 1 part in 5000 of the hydrogen in seawater is deuterium. This amounts to an estimate that there is over 10 ¹⁵ tons of deuterium in the oceans. The tritium part of the fuel is more problematic since there is no significant natural source ( [link] ), and the tritium would have to be obtained by breeding the tritium from lithium.

Since a gallon of seawater could produce as much energy as 300 gallons of gasoline, there is clearly a large amount of energy that can potentially be realized through nuclear fusion. Unfortunately, this advantage is also a disadvantage since the temperatures attained are similar to the surface of the sun, which would vaporize any conventional container. Fusion experiments therefore use a magnetic field to contain the reaction. The shape of the field is like a bottle, hence the term “magnetic bottle”.