Volumetric Radioactivity Viewed as Surface Radioactivity for Free Release Assessment Purposes

Transcription

1 s / f WSRC-MS Volumetric Radioactivity Viewed as Surface Radioactivity for Free Release Assessment Purposes by W. L. Boettinger Westinghouse Savannah River Company Savannah River Site Aiken, South Carolina m m 00: VtifS ~~~ A document prepared for BENEFICIAL REUSE 1998 at Knoxville, TN, USA from 8/3/98-8/6/98. DOE Contract NO. DE-AC09-96SR18500 This paper was prepared in connection with work done under the above contract number with the U. S. Department of Energy. By acceptance of this paper, the publisher and/or recipient acknowledges the U. S. Government's right to retain a nonexclusive, royalty-free license in and to any copyright covering this paper, along with the right to reproduce and to authorize others to reproduce all or part of the copyrighted paper.

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4 Beneficial Reuse 98 August 3-6,1998 Knoxville, Tennessee W. L. Boettinger Savannah River Site (803) ( Abstract Volumetric Radioactivity viewed as Surface Radioactivity for Free Release Assessment Purposes. As a part of the SRS Beneficial Reuse Program, stainless steel radioactive scrap metal is melted, pour into ingots, and roll into sheets. The sheets are then fabricated into boxes and barrels for beneficial reuse. The melting activity is a partial decontamination process. Certain isotopes separate from the melted steel, while others stay in solution. Cobalt-60 is the primary constituent, which remains in solution, and becomes the major contributor to the volumetric radioactivity of the finished products (boxes and barrels). There is currently no de minimis free release level for volumetrically radioactive material. However, under certain circumstances, pathway analysis can be used (and have been used) to free release volumetrically radioactive material. This paper presents an analysis using empirical data derived from over sixty melts, to demonstrate that the implied surface radioactivity for specific beneficial reuse products is within free release limit. The approach can be applied to other recycled metal products. Background As part of the Stainless Steel Beneficial Reuse Demonstration fbnded by the DOE Oflice of Science and Technology Development radioactive scrap metal (RSM) was melted, poured into ingots, and then rolled into sheet metal. The sheet metal was used to fabricate a number of different kinds of containers such as boxes and barrels. The metal was contaminated with a number of radioactive isotopes, some of which stayed in the liquid (melt). Upon solidification the soluble radioactive isotopes became volumetrically distributed throughout the metal. The major isotope causing volumetric contamination was ( There is currently no criteria allowing the fkee release of volumetrically contaminated material, no matter how minute the contamination level. There is, however, a fkee release criteria for surface contamination. The free release level for surface contamination is 5000 d p d 100 cm2. 1 -i f L

5 Model used for comparison Though there is no volumetric free release limit a comparison with the surface free release limit can be made by assuming the volumetrically contained radioactive isotope(s) all migrate to the surface of the item in which they are contained. The resultant or implied surface activity level then can be calculated and compared to the surface free release levels. Calculational Approach To simplify the comparison, the volumetrically contaminated metal was assumed to be rolled into sheet metal with the contamination equally dispersed throughout the metal. The radioactive contamination was then assumed to migrate to the surface, half to each side of the sheet. The original data (specific activity) was provided in terms of picocuries per gram. The conversion equation used to convert to Implied Surface Activity is: ISA = (a) x (d) x (d2)x (100) x (c) Where: ISA = the Implied Surface Activity in dpd100 cm2 a = the volumetric (specific) activity in pico-curies per gram d = the density in grams/cm3 t = the thickness of the sheet metal in cm c = conversion factor (2.2 dpdpico-curie) g c L Notice the ISA, in the equation above, will vary directly with the thickness of the sheet metal; the thicker the metal, the higher the ISA. Number of heats analyzed A nominal two-ton melter was used to pour the ingots, which were subsequently rolled into sheet metal. Numerous pours or heats were required due to the limited size of the melters and the efficiency of the fabrication cycle. The fabrication cycle efficiency is predicted by the equation: zn P = Fq ( 1-0 (N-l) Where: P = Product (output) F = Feed (input) q = Emciency N = Number of remelts 3

6 For the manufacture of drums, the over all fabrication cycle efliciency is less than 40%, which means for any given heat, 60 % of the metal is recycled into another heat. The Co60 contamination level of a group of 67 heats is shown in the Attachment Table. Product evaluation Hundreds of 55 gallon and 85 gallon drums were fabricated fiom RSM. The drums had a metal thickness between 18 gage (0.127 cm.) and 14 gage (0.198 cm.). The ISAs for 18 gage and 14 gage drums fabricated fiom the 67 heats displayed in the Attachment Table were calculated. The information is presented graphically in Figures 1 and 2. The free release limit for surface contamination is 5000 dpd100 cm2. If ISA were to be used as a Free Release criterion for volumetric contaminated items 55 of the 67 heats could be fiee released if the drums were 18 gage while 45 of the 67 heats could be fiee released if the drums were 14 gage. FiPure 1 18 Gage metal Implied Surface Acyivity (dpmlloocm2) as a Funcb'on of Heat Number 15ooo (SO00 dpd100 cm2) loo00 0 4

7 .- Figure 2 14 Gage Metal Implied Surface Activity (dpmlloocm2) as a function of Heat Number ooo ree Release Limit loo00 f 5ooo Recommendation It appears reasonable that metal containing volumetric contamination should be less of a concern than metal having the same level of surface contamination, since the sucface contamination would be more likely to be spread. For this reason any volumetric standard for fkee release should allow for contamination levels higher than the equivalent implied surface contamination levels. For metals a volumetric fiee release limit of five to ten times the equivalent implied surface level would be reasonable. 5

8 a', Attachment Table: - used make Drums 6