Steve J. Hensel

Analysis of Pressurization of Plutonium Oxide Storage Vials During a Postulated Fire

The documented safety analysis for the Savannah River Site evaluates the consequences of a postulated 1000 °C fire in a glovebox. The radiological dose consequences for a pressurized release of plutonium oxide powder during such a fire depend on the maximum pressure that is attained inside the oxide storage vial. To enable evaluation of the dose consequences, pressure transients and venting flow rates have been calculated for exposure of the storage vial to the fire. A standard B vial with a capacity of approximately 8 cc was selected for analysis. The analysis compares the pressurization rate from heating and evaporation of moisture adsorbed onto the plutonium oxide contents of the vial with the pressure loss due to venting of gas through the threaded connection between the vial cap and body. Tabulated results from the analysis include maximum pressures, maximum venting velocities, and cumulative vial volumes vented during the first 10 minutes of the fire transient. Results are obtained for various amounts of oxide in the vial, various amounts of adsorbed moisture, different vial orientations, and different surface fire exposures.

Analysis of Powder Airborne Release Fractions for Vessel Ruptures

The Department of Energy handbook for airborne releases from nonreactor nuclear facilities bases its bounding airborne release fraction (ARF) for pressurized powders on tests conducted at Pacific Northwest Laboratory (PNL). An analysis is presented that correlates the ARF from these tests. The amount of powder that becomes airborne is correlated in terms of an adjusted airborne release fraction (AARF) equal to the product of the powder entrainment from the powder bed and the ratio of the total vessel volume to the volume occupied by the powder bed. Powder entrainments and release fractions at low pressures are correlated using a fluidized bed analogy. The analysis shows that the entrainment is enhanced by a sonic shock if the pressure prior to the rupture exceeds approximately 33 psig. A secondary, three-dimensional shock is predicted to occur at an initial pressure of approximately 332 psig. A correlation based on this analysis is used to predict the ARF for ruptures of vessels containing plutonium oxide. It is assumed that the oxide is pressurized by hydrogen that is radiolytically generated from adsorbed moisture.Copyright

HYDROGEN FLAMMABILITY MITIGATION IN A CONTAINMENT VESSEL USING A RECOMBINER

Plutonium oxide packaged in a 9975 Primary Containment Vessel (PCV) is evaluated in terms of preventing a flammable gas mixture due to hydrogen generation. Hydrogen is generated via radiolysis of adsorbed moisture on the plutonium oxide. A recombiner is placed in the PCV to recombine hydrogen and oxygen at concentrations to prevent hydrogen flammability. A detailed hydrogen diffusion analysis which evaluates expected and bounding conditions in order to demonstrate that hydrogen concentrations will remain below 5% by volume within PCV is presented.Copyright

ANALYSIS OF THE RATE OF ADSORPTION OF MOISTURE ONTO PLUTONIUM OXIDE POWDERS

Rates of adsorption of moisture onto plutonium oxide powders exposed to air are modeled. The moisture contents of these powders must be limited to minimize the radiolytic generation of flammable hydrogen gas when the plutonium oxide subsequently is stored in containment vessels. The pressure in the vessels is related to the amount of moisture adsorbed. Moisture adsorption rates are modeled for powders in two different containers used by the Savannah River Site (SRS) HB-Line facility, a B vial and a product can. The adsorption models examine the effects of the powder layer fill height, gas mixing conditions above the powder layer, and ambient relative humidity. Moisture distribution profiles are calculated to enable the evaluation of the effect of sampling location on the measured moisture content. The adsorption models are applied using the COMSOL Multiphysics® finite element code. The COMSOL® models couple moisture diffusion with thermal conduction and radiation. The models incorporate an equilibrium adsorption isotherm and a detailed model for combined radiation and conduction heat transfer in the powder, both developed at Los Alamos National Laboratory. The COMSOL® adsorption rate calculations are successfully benchmarked using an analytical, one-dimensional ash and pore diffusion model.Copyright

Moisture Adsorption Considerations for Packaging Plutonium Oxide

Compliance with DOE-STD-3013, Stabilization, Packaging, and Storage of Plutonium-Bearing Materials, requires plutonium oxide to be stabilized at high temperatures such that the material is non-reactive and has less than 0.5% wt. moisture adsorption [1]. Since plutonium oxide is known to readily adsorb moisture, compliance with the standard requires a sample of oxide material to be measured for moisture adsorption. The measurement is typically performed using thermogravimetric analysis (TGA). The sample must be representative of the actual oxide material in order for the TGA measurement to be valid. Obtaining a representative sample from an oxide powder container may be done using a core sampler or a grab sample method that accounts for potential spatial distribution of the oxides. A further complication with moisture sampling is that the plutonium oxide typically continues to adsorb moisture from the glove box ambient air for many hours or even days until equilibrium is reached. In typical oxide material handling operations, the material, both product and sample, is canned and bagged out prior to reaching a moisture level in equilibrium with the ambient relative humidity. In fact, given the strict moisture requirement for DOE-STD-3013 compliance, it is highly undesirable to allow for equilibrium moisture adsorption to be achieved. Given the dynamic nature of moisture adsorption, a technical basis for obtaining a representative sample is important for DOE-STD-3013 compliance. The technical basis not only includes how the sample is obtained, but more importantly, must account for all handling once the sample is physically separated from the product. This paper provides an analytical basis for moisture adsorption to define handling controls that assures a representative oxide sample is obtained.Copyright

Analysis of the Pressure Transient and Burst Pressure for Exposure of the 9975 Primary Containment Vessel to Fire

Bare shipping package containment vessels can be utilized to stage plutonium oxide at the Savannah River Site. Pressurization and subsequent release could occur due to a hypothetical facility fire. Pressurization due to adsorbed moisture on the plutonium oxide and plastic packaging materials could result in rupture of the containment vessel. The containment vessel was evaluated to determine rupture pressure when subjected to the fire conditions. The rupture pressure is compared with pressures developed due to radiolytic gas generation.Copyright

Benchmarking of Improved DPAC Transient Deflagration Analysis Code

The transient deflagration code DPAC (Deflagration Pressure Analysis Code) has been upgraded for use in modeling hydrogen deflagration transients. The upgraded code is benchmarked using data from vented hydrogen deflagration tests conducted at the HYDRO-SC Test Facility at the University of Pisa. DPAC originally was written to calculate peak deflagration pressures for deflagrations in radioactive waste storage tanks and process facilities at the Savannah River Site. Upgrades include the addition of a laminar flame speed correlation for hydrogen deflagrations and a mechanistic model for turbulent flame propagation, incorporation of inertial effects during venting, and inclusion of the effect of water vapor condensation on vessel walls. In addition, DPAC has been coupled with CEA, a NASA combustion chemistry code. The deflagration tests are modeled as end-to-end deflagrations. The improved DPAC code successfully predicts both the peak pressures during the deflagration tests and the times at which the pressure peaks.

FLAMMABILITY ANALYSIS FOR ACTINIDE OXIDES PACKAGED IN 9975 SHIPPING CONTAINERS

Packaging options are evaluated for compliance with safety requirements for shipment of mixed actinide oxides packaged in a 9975 Primary Containment Vessel (PCV). Radiolytic gas generation rates, PCV internal gas pressures, and shipping windows (times to reach unacceptable gas compositions or pressures after closure of the PCV) are calculated for shipment of a 9975 PCV containing a plastic bottle filled with plutonium and uranium oxides with a selected isotopic composition. G-values for radiolytic hydrogen generation from adsorbed moisture are estimated from the results of gas generation tests for plutonium oxide and uranium oxide doped with curium-244. The radiolytic generation of hydrogen from the plastic bottle is calculated using a geometric model for alpha particle deposition in the bottle wall. The temperature of the PCV during shipment is estimated from the results of finite element heat transfer analyses.

Hydrogen Concentrations During Storage of 3013 Oxide Samples

As part of a surveillance program intended to ensure the safe storage of plutonium bearing nuclear materials in the Savannah River Site (SRS) K-Area Materials Storage (KAMS), samples of these materials are shipped to Savannah River National Laboratory (SRNL) for analysis. These samples are in the form of solids or powders which will have absorbed moisture. Potentially flammable hydrogen gas is generated due to radiolysis of the moisture. The samples are shipped for processing after chemical analysis. To preclude the possibility of a hydrogen deflagration or detonation inside the shipping containers, the shipping times are limited to ensure that hydrogen concentration in the vapor space of every layer of confinement is below the lower flammability limit of 4 volume percent (vol%). This study presents an analysis of the rate of hydrogen accumulation due to radiolysis and calculation of allowable shipping times for typical KAMS materials.