4.0 METHODOLOGY
This evaluation uses a combination of subcritical reference data (LA-10860-MS) ², published reports (such as the International Handbook of Evaluated Criticality Safety Benchmark Experiments³), calculations, and previously approved Criticality Safety Evaluations (i.e., Ref. 1 and its reference documents) to demonstrate criticality safety for the application and removal of Capture Coating & InstaCote material containing up to 200 grams of fissile material. The primary basis for safety is fissile mass control, with volume and geometry of the process containers being factors in the overall safety basis, also. This evaluation relies upon controls and limits that were used to form parts of the safety bases for operations in Buildings 776, 777, and 779 as discussed in Ref. 1. These controls and limits are included in Section 7.0.

The computer code MCNP4A was used to calculate neutron multiplication rates using the recommended nuclear cross sections from the MCNP4A users manual.⁴ Generally, the recommended cross sections are those that were processed from the ENDF/B-V cross section set. Two IBM-PC clone type computers were used for these calculations. Both computers were verified with the standard set of 25 problems that are supplied with the MCNP4A code, and both were validated using a number of cases from the International Handbook of Evaluated Criticality Safety Benchmark Experiments, Volume I.b, "Plutonium Systems." One particular set of cases that was used for validation was the set from the thermal plutonium solutions group identified as PU-SOL-THERM-007; this set of experiments was selected because it was reported to be performed for plutonium which contained 4.57 wt% ²⁴⁰Pu, which is closer to the ²⁴⁰Pu content of materials evaluated in this document than is the ²⁴⁰Pu concentration in most other plutonium solution criticality experiments. The average calculated value of k_eff for the eight cases provided in PU-SOL-THERM-007 was 1.00752 for each computer over a range for k_eff values of 1.00157 to 1.01294; therefore, the decision was made not to apply a bias for the results of this evaluation.

The first set of cases that were run for this evaluation was for 200 grams of plutonium in homogeneous mixture with water to simulate the work that was presented in Ref. 1. From information shown in Ref. 2, the volume most likely to yield the highest reactivity was estimated to be 6.25 liters; for this reason, several problems were calculated using this pivotal volume. Subsequent calculations showed that the volume which resulted in the maximum reactivity depended on the amount of plutonium assumed to be present as well as the density of the material being analyzed, especially the hydrogen density. The second set of cases consisted of those in which 200 grams of plutonium in homogeneous mixture with an 8-weight-percent glycerin and 92-weight-percent water preparation (i.e., Capture Coating) was modeled; the glycerin-water preparation is to be supplied by Encapsulation Technologies, LLC. Because the author of Ref. 1 used only ²³⁹Pu as a conservative description of the plutonium source material and because that assumption provides an upper limit on the calculated reactivity for any problem in this evaluation, all cases problems were run with ²³⁹Pu, at least initially, to get an estimate of the upper limit for the specific problem scenario. A number of problems were run with the plutonium isotopic mixture for RFETS IDC-427⁵; the results from these runs are useful for a number of comparisons. For the third set of problems (i.e., Case C), one specific problem was run, not only with ²³⁹Pu as the fissile material, but also with each of the RFETS isotopic mixtures listed in Ref. 5--the results from these runs provide an insight into the degree of conservatism that results when ²³⁹Pu-only is assumed to be the source material. The Case C problems in which 200 grams of plutonium was mixed uniformly with dry InstaCote material, having a mass density⁶ of 1.629 g/cm³, also included problems for which the calculated results show the relative importance of specific metal impurities (both potential reflectors and potential poisons) in the InstaCote material. The inclusion of these metal impurities actually decreased the reactivity of the systems slightly. The effects of each, the reflectors and the poisons, are shown as well as the effects of all constituents combined. Since the reactivity of plutonium in mixture with the dry InstaCote material was shown to be greater than the reactivity of plutonium in mixture with water, the minimum critical mass of plutonium in mixture with the dry InstaCote material was determined; this took several iterative steps in which both the mass of plutonium and the volume of the mixture were varied. These minimum critical mass determinations are shown in Table 1 as Case D. Finally, since the minimum critical mass of plutonium in mixture with the dry InstaCote material was found to be less than twice the amount allowed in a glovebox prior to application of the Capture Coating & InstaCote mixtures and since this is the basis for one of the criticality controls listed below, then k_eff for two adjacent cylindrical volumes of the mixture, each containing 200 grams of plutonium was calculated. k_eff determinations for the two cylinders are shown in Table 1 as Case E. Because the thickness(es) and material type(s) of the containers permitted by RFETS for accumulation of the Capture Coating & InstaCote material was unknown, and because the evaluation is intended to retain conservatism, the volumes used for these two masses of materials were assumed to be equal and to exist without the benefit of neutron absorption by the containers or the spacing that such containers would provide. (All calculated results involving plutonium in mixture with the dry InstaCote material depended heavily on the hydrogen and carbon concentrations; therefore, the Galbraith Laboratory Manager was contacted⁷ to verify that the mass percentage of hydrogen, as well as other elements in the InstaCote material, was accurate. After reviewing the reported results for this material, including the calibration standards results, the Laboratory Manager stated that he could see no reason to doubt their validity.)

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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