Stabilize volatile species

DOE-STD-3028-2000

c) Stabilize volatile species

The principle intermediary in the purification of 233U oxides from thorium fuel is

uranyl nitrate UO2(NO3)2 generally appearing as the dihydrate UO2(NO3)22H2O

[Storch 1999]. This compound melts at 184C (363 F) and denitration begins

above 300C (572F) [Ullman 1996]. Thus, stabilizing to 750C (1382F) will be

sufficient to drive off any residual nitrates.

Process histories indicate no known significant quantities of organics in existing

233

U oxides. Any organics used in the separation of 233U from fission products

(for example, in the Purex or Thorex processes) would be decomposed and

driven off below 750C (1382F). Examples of such materials include tributyl

phosphate (boiling point = 289C [552.2F]), diethyl benzene (boiling point =

200C [392F]), and kerosene (boiling point = 175C -325C [347-617F]).

d) Convert to U3O8

U3O8 is the most attractive storage form because it is the most stable uranium

oxide (the others convert to U3O8 upon heating). Thus, if materials are to be

treated, then converting to U3O8 is desirable.

e) Enhance particle size

Enhancing particle size is desirable because it reduces the inhalation hazard of

powdered material. However, it was not included as a primary objective because

the two sealed containers provide multiple barriers between the hazard and

targets (workers, public, and the environment). Thus, enhancing particle size

would be defense-in-depth rather than a fundamental requirement. Furthermore,

the effect of heating on particle size is highly dependent on the method of

preparation of the oxide [Clayton 1961]. As an example, much of the oxide

currently in inventory was prepared at ORNL by heating pregranulated

ammonium diuranate in a furnace for 6 hours at 800C (1472F) or greater (not

including preheating to reach temperature or cool down). Samples of this

material indicate that it has a particle size distribution with greater than 90% of

the particles being less than 10 microns (3.28E-5 ft) in equivalent spherical

diameter. In fact, greater than 90% at less than 10 microns (3.28E-5 ft) was a