Dworkin50 measured the enthalpy increment of UF4 from room temperature up into the liquid phase. These results are in very good agreement with unpub­lished results obtained by Cordfunke and reported by Fuger37 (Figure 9).

Based on the results from both studies, the enthalpy is expressed as:

HT — H298(kJ mol-1) = -35.058 + 0.1145T + 10.27745 10-6T2 + 4.13159 10-3T-1 (298 — 1309 K) in solid state

HT — H298(kJmol-1) = -39.413 + 0.166Г

(1309 — 1400 K) in liquid state with T(K)

Dworkin deduced from his measurements the entropy and enthalpy of fusion at the fusion temperature

Tfus = 1309 K AfusH° = 46986 J mol 1 Af = 35.98 JK—1 mol-1

This enthalpy of fusion is higher than the approximate value calculated by Khripin5 from the differential ther­mal analysis in the UF3-UF4 binary system (AfusH0(UF4,s, 1309 K = 43 514 ± 2kJ mol-1).

The experimental data of Dworkin for the enthalpy of fusion has been recommended by Guillaumont et al. in their review, although the vapor pressure data for UF4(cr, l) are more consistent with a smaller value (36 kJ mol-1).39

2.06.3.2.3 Preparation

Two ways are industrially used to obtain UF4 from UO2 powder:

• the precipitation of UF4 in HF aqueous solution,

• the reaction of UO2 with gaseous HF at 573-773 K: UO2 + 4HF! UF4 + 2H2O.

As a consequence, the impurities in UF4 are usually oxygen based, such as UO2F2, UO2, or H2O.

The large density variation between the fluoride and the oxide tends to hinder the reaction because swelling occurs and the porosity is easily blocked dur­ing the conversion. The kinetics of hydrofluorination will then depend much on the specific surface of the

UO2. A model has been developed that takes into account the grain and pellet sizes.51 Impurities such as sodium can also be detrimental because they may favor the sintering. The NaF-UF4 eutectic melts close to 893 K.

UF4 can be obtained in a rotating furnace, in a fluidized bed, or in a moving bed reactor where the UO2 has been pelletized as described in Harrington and Ruenhle.33 If the hydrofluorination temperature is maintained below 750 K, the effect of sodium is light.

2.06.3.2.4 Uses

UF4 can be used to produce UF6 (see previous section) or U metal using magnesium reduction in a bomb:

2Mg + UF4 ! U + 2MgF2

In all cases, the oxygen residual level in UF4 will be very important. It will affect the conversion rate to U metal or UF6.

Mixtures of fluoride salts with UF4 are candidates as fuel carrier of molten salt reactors52 (see Chapter 3.13, Molten Salt Reactor Fuel and Coolant).

2.06.3.2 UFx (4 <X< 6) — Intermediate Fluorides