6.0 EVALUATION AND RESULTS
Once all the loose material and other material has been removed from the glovebox and the requirements for criticality safety evaluation(s) for "Out of Commission Gloveboxes and/or Ancillary Equipment " in the affected building(s) have been met, Capture Coating & InstaCote may be applied to the walls, floors and remaining equipment to further remove the remaining fissile material. Capture Coating & InstaCote are types of aqueous and organic coatings (as described in the Introduction to this document) which are sprayed onto the contaminated surfaces in two separate stages, allowing each layer to dry before proceeding with the decontamination process. Once the Encapsulation Technologies layer of glycerin and water and the InstaCote layers have dried, the stripcoat paint layers may be peeled from the surface taking removable surface contamination with it. The first coating, which is supplied by Encapsulation Technologies, is composed of 8 wt% reagent grade glycerin (1.16750 g/cm³)⁸ and 92 wt% reagent grade, deionized water⁹, having a combined calculated density of 1.0099 g/cm³. Reference 10 lists glycerin as commercial glycerol with the stoichiometric formula of C₃H₈O₃. The second coating is comprised of hydrocarbon polymers and has a measured⁶ density of 1.629 g/cm³ when fully dry and cured. As each coating dries, vapors or gases are released to the glovebox atmosphere. According to Galbraith Laboratories, Inc.’s Laboratory Manager, who analyzed the two constituent parts of the InstaCote material (i.e., Parts A and B) as well as the dry, cured InstaCote material, the dried coating has the following nominal elemental composition: 65.00 wt% C, 9.08 wt% H, 4.88 wt% N, and 19.83 wt% O. Part B, the soft resin mixture, of the InstaCote material contained a somewhat higher concentration of hydrogen than did the dry, cured InstaCote material --the effect of this higher concentration of hydrogen will be discussed below. Galbraith Laboratories also analyzed both the Part B and the dry InstaCote material for 13 selected metal impurities; the list of metal impurities for which results were reported included beryllium, aluminum, iron, nickel, lead, uranium, lithium, boron, cadmium, gadolinium, samarium, lutetium, and hafnium. The first six of these impurity metals were selected because they are good reflecting materials and could, therefore, increase the reactivity of the system. The last seven of these impurity metals were selected because they are good neutron poisons and could, therefore, decrease the reactivity of the system. Appendix A contains the results of all of these laboratory analyses of the elemental constituents of Parts A and B, as well as the results for the dry and cured InstaCote material. The data reported in Appendix A was supplemented by telephone calls to the Galbraith Laboratories, Inc.’s Laboratory Manager, who affirmed or responded with the following additional information:

For analyses of C, H, N, and O, the results are to be interpreted as the percent by weight of the original sample as received, e.g., 65.00% of the weight of the original sample of dry InstaCote is carbon, etc. The precision of these measurements were each +2 percent at the 1-s (or 68 percent confidence level).

For analyses of C, H, N, and O, the sum of the results are recognized to be not exactly equal to 100 percent, but are within +2 percent of 100 percent.

For analyses of the 13 selected metal impurities, the precision of each measurement was +10 percent at the 1-s (or 68 percent confidence level) unless the result was listed as a less than (i.e., <) value, which indicated that the result was below the detection limit of the measurement technique. For measurement results that were below the detection limit, the uncertainty of the measurement is unknown; i.e., laboratory personnel have not been asked to evaluate uncertainties associated with results that are below the detection limits.

For analyses of the 13 selected metal impurities, the results were reported in units of "ppm"; the Laboratory Manager confirmed that these units are weight fractions and can be replaced by the units "mg/kg" or "micrograms/gram" of the original sample. The original samples were ashed to produce the results for metal impurities.

The conservative procedures that were followed for this criticality evaluation for use of measured results that were received from the analytical laboratory together with their associated uncertainties were as follows:

For the results from C, H, N, and O measurements, the hydrogen weight percent was adjusted upward by 2 percent of the reported result (i.e., up to the 68 percent confidence level), and then all measured results for C, H, N, and O were normalized such that their sum was 100 percent. For example, the dry InstaCote material was measured to contain 9.08 percent by weight hydrogen; therefore, the hydrogen weight percent was multiplied by 1.02 (yielding a product of 9.262 weight percent hydrogen), the sum of the measured results, including this adjustment for hydrogen, was found to be 98.972 percent. Each result for C, H, N, and O was then divided by 0.98972 to give the weight percent used for criticality calculations.

Positive results for metallic impurities, which were found only for aluminum, iron, and lead, were increased by 10 percent--in other words adjusted upward to the upper 68 percent confidence level. Analytical laboratory results for metallic impurities which were less than the detection limits were doubled in absolute value for the purpose of assessing their effect on system reactivity. For the dry InstaCote material, examples of these adjustments are as follows: lead was adjusted from 1.7 ppm to 1.87 ppm, and boron was adjusted from 5 ppm to 10 ppm. The effects of impurities (both reflector and poison materials) are discussed below.

Calculations were performed for uniformly mixed plutonium-water systems, for uniformly mixed plutonium-glycerin-water systems, for uniformly mixed, dry and cured plutonium-InstaCote material, and for uniformly mixed, plutonium in Part B (which is the soft resin portion of the InstaCote material) to determine the bounding case. Final calculations were performed for two cylinders, each containing 200 grams of plutonium in mixer with dry InstaCote material. The results of all of these calculations are presented in Table 1. Calculations were performed using the MCNP4A computer code together with the nuclear cross sections recommended in the MCNP4A manual⁴ (usually processed from the ENDF/B-V nuclear data sets). Results from these calculations show that the maximum reactivity for 200 grams of plutonium in water occurs for a plutonium water volume of 6 to 7 liters, and the results show that the maximum reactivity for 200 grams of plutonium in a mixture of 8 weight percent glycerin and 92 weight percent water occurs in approximately the same volume as for the plutonium-water system. These calculations show that the uniformly mixed plutonium-glycerin-water systems were somewhat less reactive than uniformly mixed plutonium-water systems for the same amount of plutonium content; this is primarily a result of the fact that the hydrogen concentration is slightly less in the plutonium-glycerin-water systems than it is in the simpler plutonium-water systems.

Calculations were performed for pure ²³⁹Pu (conservative, yet unrealistic cases), since Ref. 1 was prepared with this assumption, and calculations were performed for various RFETS plutonium isotopic mixtures. As expected, the results for the pure ²³⁹Pu cases were always higher than for any of the RFETS mixtures. As shown in the results for Cases C, D, and E, the RFETS plutonium isotopic mixtures designated as "Aged Plutonium" and "RFRAG plutonium" were found to yield the highest neutron multiplication rates of any of the realistic isotopic mixtures. The hydrogen concentration (and therefore the hydrogen atom density) in the InstaCote part of the Capture Coating & InstaCote was greater than the hydrogen concentration in water, and the Laboratory’s analysis of the soft resin portion (i.e., Part B) of the InstaCote material contained a slightly higher hydrogen concentration than did the dry, cured InstaCote material. Since hydrogen is a much better moderator than carbon, nitrogen, or oxygen, then the dry and cured plutonium-InstaCote material case was found to be the limiting realistic case--that is, the soft resin Part B material is never used without being mixed together with Part A material which contains a much lower weight percent of hydrogen; consequently, Case E for the soft resin Part B material is an unrealistic case for which results are presented only for completeness.

Since the plutonium-InstaCote material systems were found to be more reactive than plutonium-water systems for the same amount of plutonium, a search was made to determine the minimum critical mass (MCM) of plutonium in dry InstaCote. Reference 2 (Figures 31 and 32) shows that the minimum critical mass of Pu uniformly dispersed in a water system is 520 grams in an approximately 17-liter-volume sphere that is fully reflected by a thick water reflector. (It is noted that while systems with carbon moderation will have greater minimum critical masses than water moderated systems, they may be more reactive at low fissile material concentrations.) Reference 2 also provides a formula for estimating the MCM of the plutonium-InstaCote material systems based on data from plutonium-water systems; however, there are two unknowns (the volume and the mass needed to determine density) and only one equation. These calculations (Case D) show that the MCM for the plutonium-InstaCote material systems is approximately 270 grams and that this 270 grams must be uniformly dispersed in a volume of approximately 6 liters of dry InstaCote material which is surrounded by a thick water reflector in order to achieve criticality. In order to distinguish smaller differences such as the difference in reactivity between the Aged Pu and the RFRAG mixtures and the most precise volumes for the MCM, the number of neutron histories were doubled as compared to the number used for other cases. Typically, k_eff results which have calculated uncertainties less than 0.00100 were generated by doubling the number of neutron histories.

Previous nuclear criticality evaluations for other RFETS facilities¹ found that controls for an optimally water-moderated, uniformly mixed plutonium-water system that allowed up to 200 grams of plutonium material inside each glovebox was sufficiently restrictive to ensure nuclear criticality safety. In this evaluation, calculations show that use of the InstaCote materials could result in more reactive systems when mixed with plutonium than those containing only plutonium and water; however, the 200-gram glovebox limit is still seen to be sufficiently restrictive to ensure nuclear criticality safety. Two 4-liter cylindrical volumes (adjacent to each other and without the presence of a container), each having their height equal to their diameter and each containing a mass of 200 grams of the RFETS RFRAG plutonium mixture homogeneously mixed with InstaCote material, were found to be safely subcritical, having a calculated k_eff of approximately 0.963. The effects of both the neutron absorption by the container walls and spacing between containers may very well reduce the value of k_eff to less than 0.95. While the margin of safety remaining when 200 grams of plutonium is allowed in mixture with dry InstaCote material is not as great as it is for 200 grams in mixture with water, 200 grams of plutonium uniformly mixed in any of the materials was still found to be safely subcritical.

The application of Capture Coating & InstaCote to the walls, floors, and equipment in a glovebox will not produce hazardous reactivity levels in the system. It would form a thin layer of reflector over an even thinner layer of fissile material. A Master-Lee representative¹¹ stated that the average thickness of dry, cured InstaCote material over coated objects is approximately 50 mils, but that the range could extend to 70-90 mils. One applicable criticality scenario associated with the use of Capture Coating & InstaCote material would be to collect 270 grams of material and then force it to form into a compact spherical shape. This scenario can be shown to meet the requirements of the double contingency principle through the application of the following controls to ensure that less than 200 grams of Pu will be allowed to be stripped off the surfaces at one time. This is accomplished by the following two methods.

First, if the glovebox is scanned and has an aggregate total of 200 grams or less at the upper 95% confidence level for the box, then the entire glovebox may have the coating applied and removed in 4-liter containers.

A second method may be used if the glovebox assay scan indicates that the aggregate total is greater than 200 grams at the upper 95% confidence level. Using the highest areal density assay reports, an area of Capture Coating & InstaCote which may be peeled and collected at one time can be determined. The size of the area that may be stripped from the glovebox is determined by dividing 200 grams by the highest areal density reported for the glovebox. The uncertainty of the measurement must be accounted for in the areal calculation. For example, if the assay scan indicates that the highest area in the glovebox is 50 g/ft² + 5 g/ft², then the area that may be stripped is 200 grams / 55 g/ft² = 3.6 ft² . This area will vary for each glovebox, but will ensure that the probability of having 270 grams of fissile material is extremely small. The stripcoat will be peeled off the surface such that only the intended section of coating is removed. The coating will be scored to peel only the intended section which will be subsequently measured by weighing to confirm that less than 200 grams fissile material has been removed. Once a section, which potentially contains 200 grams has been removed from the glovebox, another section may be peeled and removed. An accumulation of Capture Coating & InstaCote may be subdivided and placed into containers with a maximum of 4 liters volume. This control on material accumulation precludes the formation of either the optimal 17-liter (12.5 inch diameter) Pu-water sphere or the 6-liter Pu-InstaCote sphere, which is the necessary volume for fully water reflected systems to become critical. This control will prevent criticality in the bounding case of the dry and cured plutonium-InstaCote material mixture. Restricting the accumulated material to a maximum 4-liter volume provides the necessary controls to establish double contingency.

The material may be removed from the glovebox in any number of 4-liter containers provided that the maximum mass value and volume restrictions are not violated. With controls on the gram quantity (i.e., the mass), on the individual container volumes, and with restrictions on storage, the use of Capture Coating & InstaCote is shown to meet the requirements of the double contingency principle. Operations external to the glovebox, such as fissile material handcarry, cart transfer, and generation of waste drums are addressed in other evaluations specific to each building and are not addressed within this evaluation.

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