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Criticality and Thermal Analyses of Separated

E. Bakker

University of Nevada, Las Vegas:4505 Maryland Parkway Box 4027, Las Vegas, NV, 89154, [email protected]

Abstract – and americium pose special problems in the chemical preparation of spent for transmutation. Once separated from the other actinides, the can lead to nuclear fission with the subsequent release of a large amount of . A criticality code was used to determine keff for varying quantities of Cm2O3 and Am2O3 held within spherical or cylindrical containers. These geometries were investigated both in air and in . Recommendations are made on the maximum amount of Cm2O3 and Am2O3 that can be safely stored or handled before encountering criticality. Several isotopes of curium and americium also generate a significant amount of by . If kilogram quantities are stored in a container, for example, the material may heat to an equilibrium temperature that exceeds its melting temperature. The heat generation of curium and americium present even more restriction on the of that can safely be contained in one location.

INTRODUCTION that may be placed in a receptacle of certain The objective of this thesis is to devise a geometry. suitable storage plan for several mixtures of the actinides found in spent , specifically PREVIOUS RESEARCH curium and americium oxide, by analyzing their Americium is a man-made element. It was criticality and properties. We will produced by Glen Seaborg and a group of determine the amount of material that can be scientists by neutron bombardment of safely stored in a container while the material (1). It is produced inside a by awaits transmutation. This information will be of 241Pu. It is most commonly used based on the characteristics of the fuel including in smoke detectors and is available for its initial enrichment, fuel , and decay commercial sale by contacting Oak Ridge time. These three factors can be varied in order National Laboratory via the U.S. DOE to investigate various scenarios of spent fuel Programs website. It is packaged in a welded storage. Once the components of the stainless steel contain meeting special form characteristic fuel are determined, criticality requirements in units of grams and in americium tendencies will be analyzed by modeling that dioxide powder form (2). specific fuel in various geometries and Curium is also a man-made element created calculating the effective neutron multiplication from the beta decay of 242Pu. It, too, is shipped factor (keff) for that configuration. The keff value in a welded stainless steel container meeting determines whether or not the spent fuel can special requirements in the form of curium oxide sustain a nuclear fission . If a in milligrams (2). “Since only milligram occurred outside the carefully amounts of curium have ever been produced, designed interior of a nuclear reactor, the results there are currently no commercial applications could be devastating. Great care must be taken for it, although it might be used in radioisotope in order to prevent this type of accident from thermoelectric generators in the future. Curium is happening. primarily used for basic scientific research (3).” Next, the heat transfer tendencies of the Opperman, E.K., et al, in 2001, outlines spent fuel were investigated by using the requirements for shipment and transportation of radioactive , as well as many other different mixtures of including properties of the material, to calculate the americium and curium. This report states that maximum temperature achieved inside the “americium and curium will require a packaging storage container. Conduction, convection, and with gamma and neutron shielding, and heat radiation heat transfer are considered. The removal capability” (4). purpose of this analysis is to avoid melting of the material, storage container, and surroundings. Methodology The completion of this analysis will show the The largest source of high level nuclear maximum amount of curium or americium oxide waste comes from commercial nuclear

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(5). is created in a reactor, an oxide are to be separated from spent fuel. The intricate assembly designed to sustain a nuclear melting temperature can be exceeded if kilogram fission chain reaction. inside quantities are stored. A heat transfer analysis of the core of a reactor gets used up and must be curium and americium containers can be used to removed and disposed of. determine the temperatures that will be attained There are several types of nuclear reactors and whether melting will occur. The heat that have been tested and developed, but the generated by decay within a container of curium most widely used in the world today is the - or americium will be lost by heat transfer from water reactor (6). It uses water to moderate, the surface of the container. Eventually, the reflect, and cool the reactor at high pressures. container will reach an equilibrium temperature There are two types of LWRs in use today; the where the generated heat will balance the heat pressurized water reactor (PWR) and the boiling- lost from the container surface. The “worst water reactor (BWR). case” container is a sphere. For a sphere, the Both of these types of reactors are fueled by surface available for heat transfer is minimal for a fission reaction. A neutron strikes the fissile a given of material. The analysis of a material and releases fission fragments, sphere provides a conservative view of how fast radiation, and which go on to produce the container will heat up and will over predict further fission reactions. The released by the maximum temperature attained when the fission reaction depends on the amount of compared to a cylindrical container. neutrons traveling through the core. The amount The change in heat stored within the container is of neutrons is controlled by control rods which equal to the heat generated by decay, the heat should be positioned in the reactor very carefully lost by radiation heat transfer, and the heat lost at all times. by convection heat transfer. This balance is given by Eq. (1) DESCRIPTION OF THE ACTUAL

d[]T 4 4 mcp = g′′′V −σεA(Ts −T∞ ) − hA(Ts −T∞) RADDB was used to determine the dt composition of the spent nuclear fuel. It is a light (1) water reactor radiological database developed at

Oak Ridge National Laboratory (ORNL) that where Ts is the surface temperature of the sphere. breaks down the characteristics of spent nuclear The temperature will rise until radiation heat fuel removed from a commercial reactor based transfer and convection removes as much heat on its initial enrichment, burnup, and decay time. from the surface of the container as is generated This program provides interpolated data from by decay. At this point, the temperature of the ORIGEN, another code developed by ORNL. It material will reach a maximum, or equilibrium, lists the radioactive totals, elemental temperature at the center of the geometry. compositions, and individual isotope Convection may be due to a forced air flow concentrations of all of the materials present over the container or it may be due to the natural within a characteristic fuel. For this research, a currents that arise in stagnant air flow. characteristic fuel of 50,000 MWd/MTHM Convection in this analysis will not be forced. burnup, 4.26% enrichment, and a 10 year decay Convection heat transfer is defined in terms of time was used. Once the isotopic concentration the Nusselt number, a dimensionless group that of the fuel was calculated in terms of g/MTIHM provides the ratio of heat lost by convection to and W/MTIHM, the neutron diffusion equation the heat transferred by conduction within the was solved using a neutron criticality code curium. (SCALE4.4a) to determine the effective neutron The results of the both the criticality and the multiplication factor (keff) for varying quantities heat transfer analysis are compared and the of curium and americium oxide held within analysis that most limits the maximum allowable cylindrical containers. These geometries were radius of the container becomes the limiting investigated both in air and in water. Based on analysis, providing recommendations for the the value of keff, recommendations are made on maximum allowable container radius. the maximum amount of curium and americium oxide that can be safely stored or handled before RESULTS encountering nuclear criticality. Each material was investigated under both Decay heat generation also presents the criticality and heat transfer analysis. Table I significant problems if curium and americium

ATALANTE 2004 Nîmes (France) June 21-25, 2004 2 P2-03 and III shows results from the criticality analysis for a Cm2O3 and Am2O3 mixture, respectively. TABLE V. Criticality Radius and Mass for The heat transfer results for these mixtures are Cylindrical Waste Form shown in Tables II and IV. The heat transfer 92%/8% Mix Radius (cm) Mass (kg) analysis proved to be more limiting and the Cylinder recommendation for the size of the container was Criticality based on this analysis. This study concludes that Air 10 737.9 materials such as americium and curium oxide Water 9.3 593.54 are dangerous and great care must be taken in handling and storing them. The containment of a TABLE VI. Heat Transfer Radius and Mass for relatively small amount of these materials can Cylindrical Waste Form create criticality and heat generation concerns. 92%/8% Mix Radius (cm) Mass (kg) A mixture of 92% americium oxide and 8% Cylinder Heat curium oxide in air, for example, can have a Transfer semi-infinite cylinder radius of only 1.4 cm Air 1.4 2.02 before it begins to melt. This corresponds to a Water 2 5.88 mass of only 2.02 kg. The results of the 92% americium oxide and 8% curium oxide mixture for both analyses is shown in Tables V and VI. REFERENCES This work was sponsored by the Advanced Cycles Initiative through the Harry Reid Center 1. Foundation for Blood Research Home Page. at the University of Nevada, Las Vegas. http://www.fbr.org (accessed November

2003). TABLE I. Criticality Radius and Mass for 2. Department of Energy Isotope Program Cylindrical Waste Form Home Page. http://www.ne.goe.gov Cm2O3 Radius (cm) Mass (kg) (accessed November 2003). Cylinder 3. Jefferson Lab Home Page. Criticality http://www.jlab.org (accessed November Air 5.4 125.59 2003). Water 3 21.53 4. Opperman, E.K. et al, Transportation Packages to Support Savannah River Site TABLE II. Heat Transfer Radius and Mass for Missions. Westinghouse Savannah River Cylindrical Waste Form Site Scientific and Technical Information Cm2O3 Radius (cm) Mass (kg) (2001). Cylinder Heat 5. Nuclear Waster Disposal Home Page. Transfer http://carbon.cfr.washington.edu/esc110/200 Air 1.55 2.74 3Winter/projects/133/index.html (accessed Water 2.25 8.37 October 2003). 6. Lamarsh, J.R., Baratta, A.J. Introduction to TABLE III. Criticality Radius and Mass for . 3rd ed. Upper Saddle Cylindrical Waste Form River, NJ: Prentice Hall (2001). Am2O3 Radius (cm) Mass (kg) Cylinder Criticality Air 11.2 1032.28 Water 10.5 850.58

TABLE IV. Heat Transfer Radius and Mass for Cylindrical Waste Form Am2O3 Radius (cm) Mass (kg) Cylinder Heat Transfer Air 1.6 3.01 Water 1.9 5.04

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