Page 45 - Radiochemistry and nuclear chemistry

34 Radiochemistry and Nuclear Chemistry

differences in the rates of dissociation of a proton (H +) and a deuteron (D +) from water,
and the rates of neutralization of these hydrated ions, and thus has a kinetic basis.
Depending on the physical conditions c~-values between 3 and 10 are obtained. For c~ =
6 it is necessary to electrolyze 2700 1 natural water (deuterium content 0.014 %) to produce
1 1 water containing 10 % deuterium, mainly as HDO. In a multistage process the hydrogen
gas is either burnt to recover energy, or used in a chemical process, e.g. in ammonia
synthesis by the Haber-Bosch process. Although this technique was used industrially in
Norway to produce ton amounts of pure D20, it is no longer considered economic except
for final purification of D20.

2.8.4. Gaseous diffusion

In a gaseous sample the lighter molecules have a higher average velocity than the heavier
molecules. In 1913 F. W. Aston in England showed that the lighter of the two neon
isotopes 2~ diffused through the walls of porous vessels somewhat faster than the heavier
isotope, 22Ne. In the gas the average kinetic energy of isotopic molecules must be the same,
i.e. 1/2MLV2L = 1/2MHV2H, where M H and M L are the masses of the molecules containing
the heavy and light isotopes to be separated. The maximum theoretical separation factor in
gaseous diffusion is given by

r = VLIV H = (MH/ML) lh (2.55)
The theory is more complicated, depending among other things upon the mean free path
of the gaseous atoms, the pore length and diameter of the separating membrane, and the
pressure difference over the membrane. If experimental conditions are carefully controlled,
this theoretical maximum can be closely approached.
235U is enriched through gaseous diffusion using the relatively volatile uranium compound
UF 6. In addition to its volatility, UF 6 has the advantage that fluorine consists of only one
isotope, 19F. For the isotopic molecules 235UF6 and 23sUF6, a value of 1.0043 is
theoretically possible for a (cf. (2.55)). The following conditions must be considered in the
technical application of the separation.
(a) The cells are divided into two parts by a membrane which must have very small pores
(e.g. l0 - 100 nm in diameter) in order to obtain isotopic separation. In order that large
gas volumes can flow through the membrane, millions of pores are required for each square
centimeter. Moreover, the membranes must have good mechanical stability to withstand the
pressure difference across them.
(b) UF 6 sublimes at 64~ which means that the separation process must be conducted
at a temperature above this.
(c) UF 6 is highly corrosive and attacks most materials. The membrane must be inert to
attack by UF 6. Water decomposes UF 6 according to the equation

UF6(g) + 2H20(g) = UO2F2(s ) + 4HF(g)