Nuclear Computational Science A Century in Review
Nuclear engineering has undergone extensive progress over the years. In the past century, colossal developments have been made and with specific reference to the mathematical theory and computational science underlying this discipline, advances in areas s
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Criticality Safety Methods G.E. Whitesides, R.M. Westfall, and C.M. Hopper
7.1 Introduction 7.1.1 Overview The objective of this chapter is to examine the history of nuclear criticality safety calculations. To this end, we will review the history of criticality safety concerns and look at the various approaches in dealing with this particular type of calculation. The criticality safety methods that are the subject of this chapter are those that are used to determine the criticality safety of the handling, transportation, and storage of fissile materials outside nuclear reactors. The object of criticality safety studies is defined in American National Standards Institute, Inc./American Nuclear Society Standard ANSI/ANS-8.1–1998 for Nuclear Criticality Safety in Operations with Fissionable Materials Outside Reactors, specifically: 4.1.2 Process Analysis. Before a new operation with fissionable material is begun, or before an existing operation is changed, it shall be determined that the entire process will be subcritical under both normal and credible abnormal conditions. 4.2.5 Subcritical Limit. Where applicable data are available, subcritical limits shall be established on bases derived from experiments, with adequate allowance for uncertainties in the data. In the absence of directly applicable experimental measurements, the limits may be derived from calculations made by a method shown by comparison with experimental data to be valid in accordance with 4.3. (i.e., 4.3 Validation of a Calculational Method).
7.1.2 Historical Background While we normally think of criticality occurrences as events that have taken place since the 1940s, we now know that one occurred many years ago. The first known
G.E. Whitesides (), R.M. Westfall, and C.M. Hopper Oak Ridge National Laboratory e-mail: [email protected]; [email protected]; [email protected]
Y. Azmy and E. Sartori, Nuclear Computational Science: A Century in Review, c Springer Science+Business Media B.V. 2010 DOI 10.1007/978-90-481-3411-3 7,
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“criticality” took place 1,800 million years ago in the Republic of Gabon [1]; hence, the issue of criticality outside a nuclear reactor has been with us for a long time. Presumably, from the perspective of personnel safety, we do not know the consequences of this occurrence. The most interesting aspect of the Oklo criticality is that the material has remained in place, without migration, for a very long time.
7.2 Nuclear Criticality Safety: The Early Years 7.2.1 The First Criticality Concerns In modern history, the origin of criticality safety studies began in the early 1940s. In determining whether a nuclear weapon could be built, the US government decided on three approaches to obtaining fissile material: electromagnetic separation of U-235, conversion of U-238 into Pu-239 by irradiation, and separation by gaseous diffusion. In the first approach, the batch-processing method, the amount of fissile material was relatively small and could not pose a criticality safety problem.
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