Shielding evaluations were performed in support of developing the Safety Analysis Report for Packagings (SARP) for the 9975 Shipping container. The objective of these evaluations was to demonstrate compliance with the performance requirements specified in federal regulations for each content envelope.

Ten content envelopes are defined for shipment in the 9975 Shipping Container including combinations of plutonium and uranium as metal or oxide with and without impurities. Shielding evaluations were performed to demonstrate compliance with the performance requirements specified in 10 CFR 71.47, 10 CFR 71.57, 49 CFR 173.403, and 49 CFR 173.441 for each content envelope. According to these regulations, dose rate limits for an undamaged package are 200 mrem/h at the accessible surface of the package, 10 mrem/h at 1 meter from the accessible surface of the package, and 1000 mrem/h at 1 meter from the surface of a damaged package after a hypothetical accident.

Sources material isotopic compositions were first decayed to allow for in-growth of daughter isotopes using the ORIGEN-S Module of the SCALE Code System. The radiation source terms were then characterized using RASTA, and the MCNP 4C code package was used for subsequent three dimensional Monte Carlo transport calculations to determine absorbed dose rates outside the 9975 shipping container. The calculations performed are grouped into two sets: Normal Conditions of Transport (NCT) and Hypothetical Accident Conditions (HAC).

The 9975 shipping container consists of two concentric cylindrical stainless steel containment vessels, two metal product cans, a 35-gallon steel drum, cane fiberboard insulation, bearing plates, lead shield, and aluminum honeycomb spacers. Neither the materials nor the geometry is specifically designed to provide significant neutron shielding. Neutron attenuation is primarily provided by distance between the source and points external to the package, whereas photons are significantly absorbed by the lead shield.

The content envelopes authorized to be transported in the 9975 shipping container are:

Seven content envelopes (C1-C7) have the impurities homogeneously mixed and are shown in Table I. Other calculations have been done for the three content envelopes (C8-C10) with impurities heterogeneously mixed with the source material. Content Envelope C1 is the isotopic composition to be used in the other envelopes when using a uranium loading.

The 9975 package is designated to carry plutonium metal or oxide. The DOE Standard 3013 permits reduced plutonium content and correspondingly higher impurity content. Because of the potential for high ²³⁸Pu and ²⁴¹Am content, the magnitude of the generated neutron field (from (a, n) reactions in light element impurities) is significant. The determination of impurity limits is discussed below.

The limiting amount of impurities that can be included in a 9975 shipping container was established by calculating the neutron source strength generated by various combinations of plutonium metal and impurities. The impurities considered are the fourteen elements (aluminum, beryllium, boron, carbon, chlorine, fluorine, lithium, magnesium, neon, nitrogen, oxygen, phosphorous, silicon, sodium) that are subject to significant a-n reactions. The content with source strength exceeding the value that would generate 200 mrem per hour at the surface of the 9975 shipping container was chosen as the limit. This limit (for NCT) permits shipping the 9975 shipping container without additional restrictions.

The results of these analyses based on most radioactive contents indicate that the 9975 package complies (with a significant margin of safety) with the federal regulations for nonexclusive use. However, evaluation of plutonium with homogeneously mixed high impurity contents packaged within the 9975 failed to satisfy the federal regulations for non-exclusive use. Calculated dose rates also can exceed the regulatory requirements for exclusive use shipment. The evaluation of the effects of adding impurities to the non-assayed content envelope demonstrate that the requirements of 10 CFR 71 for radiation dose rate limits are met under both normal and hypothetical accident conditions, if the impurities included with the plutonium payload are within specified bounds.

* Uranium uses the composition given by C1.