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Effects of Stabilization Temperature on Surface Area and Grain Size of Representative Plutonium Material s

Jeremy Boak, RRES-SA Deborah J . Dale, C-AAC Phillip Gary Eller, NIS- 7 Los Alamos National Laborator y

Plutonium Futures - The Science Conference Albuquerque, NM JUly 6-10, 200 3

o Los Alamos NATIONAL LABORATOR Y

Los Alamos N a t io n a l Laboratory, an affirmative action/equal opportunity employer, is o perated by th e U niversity of C ali forn ia fo r th e U. S . Departm en t of E nergy und e r contract W -7 4 05-ENG-36 . By acc e ptance o f t his article, the pu b lish e r recogni zes that the U. S. Gove rn men t retain s a n on e xclusi ve, roya lty-free l i cense to publish or reproduce the published form o f this contributi on, o r t o a ll ow o t he rs t o do so , f o r U .S. Govern ment pur poses. Los Alam os National Laboratory requests that the p ublishe r i dentify thi s a r ticle as wo rk pe rfo r m ed u nder the a us p ic es of the U.S. Department of Energy. L os A lamos N ational Labora t ory strongly s up p o rts a cadem ic f re e dom and a re searcher's right to p ubli s h ; as an instit ution, however, t he Lab o ratory d o es n o t endo rse the vie wp oint of a publicati on o r g ua ra ntee it s te ch nica l co rr ectness . Fo rm 836 (8/ 00)

Effects of Stabilization Temperature on Surface Area and Grain Size of Representative Plutonium Material s Boak, Jeremy, Dale, Deborah, and Eller, P . Gary, Los Alamos National Laboratory Introductio n Calcination at 400-1000°C is used throughout the Department of Energy (DOE) comple x

to stabilize plutonium material for transportation and storage . The objectives of this stabilization are to remove moisture and other potentially water-producing phases, and to ensure that readsorption will not occur before material is placed in welded containers . Such moisture may threaten the integrity of containers through pressurization with radiolytically generated hydrogen . It is also considered valuable to reduce the fine (respirable) fraction of the material to mitigate potential impact of accidents . I Two properties are measured to support the adequacy of stabilization at various temperatures - specific surface area (SSA), a dominant control on the readsorption of moisture, and particle size distribution, from which the respirable fraction is derived . Prior studies suggested that SSA and respirable fraction decrease with increasing calcination temperature .2'3 These studies were performed on carefully prepared, pure laboratory material, derived from various processes . Even so, the reduction in SSA and particle size is strongly dependent on the preparation history of the sample . Reductions in both measures are reasonably explained by agglomeration of finer grains at elevated temperatures. The Materials Identification and Surveillance (MIS) program at Los Alamos National Laboratory (LANL) has measured these properties on approximately thirty items representing the material being stabilized in response to Recommendation 94-1 of the Defense Nuclear Facilities Safety Board .4 Measurements were made before stabilization and after stabilization at temperatures of 600 °C, 800 °C, and 950 °C . We evaluated these data as part of an effort to determine whether lower temperature calcination of Rocky Flats electrorefining oxides containing chloride salts would be equivalent to the 950 °C stabilization mandated by DOE-STD-3013 1 . Results SSA measurements in the MIS database were performed using the Brunauer-EmmetTeller (BET) method. The data are summarized in Table 1, which lists averages over MIS items . Low averages and standard deviations for 800 °C reflect the small number of items calcined at this temperature and analyzed for SSA . Although calcination does generally reduce the SSA, even pure oxides show variable reductions above 700 °C, and chloride-bearing material shows little reduction above this temperature . The chloridebearing material uniformly shows SSA below 5 m2/g for calcination above 600 °C, a value for which moisture content is likely to be below the limit set in DOE-STD-3013 . However, most chloride-bearing items already had low SSAs, presumably due to previous calcination at unknown temperature . The chloride salts, which melt in the range 600-800 °C, may also enhance agglomeration of such material, resulting in lower SSA .

Table 1 : Average Specific Surface Area (± One Standard Deviation) of MIS Samples Stabilized at Various Temperature s Calcination SSA (All Item s) SSA (Chloride Item s) Temperature ( °C) As Received 6.368 t 11 .39 4 .837 t 4 .980 600

2 .241 ± 2 .071 1 .677 ± 0 .856

800

0 .907 ± 0 .683 0.502 ± 0 .17 6

950

1 .487 f 1 .429 1 .316 f 1 .142

Particle size data reported by Dale et a15 were performed in a Lasentec Lab-Tec 1000 particle size analyzer using - 1 gram samples in 100 ml of water . The MIS data show that calcination generally, although not uniformly, reduces respirable fraction (either the