Calculational Methods for Interacting Arrays of Fissile Material: International Series of Monographs in Nuclear Energy

By A. F. Thomas and F. Abbey

Calculational Methods for Interacting Arrays of Fissile Material describes the methods used in assessing the criticality safety of interacting arrays of fissile materials. It demonstrates that the behavior of neutrons in an array can be divided into two parts which can, to a large extent, be treated independently. These are the neutron multiplication within units of the array and the transmission of neutrons between units. For the former the usual methods of neutron physics are applicable but used so as to place emphasis on the neutrons entering and leaving the unit. The latter is mainly a geometrical problem, being entirely so for an air-spaced array. This volume is comprised of four chapters and begins with an introduction to the practical aspects of the interaction problem affecting arrays of fissile materials. The discussion then turns to simple ""hand"" methods of calculation, paying particular attention to the general equilibrium conditions in interacting arrays, along with the Oak Ridge method, the Interaction Parameter method, and the PQR method. Finally, the application of Monte Carlo method to the study of the neutron economy of entire arrays is considered in terms of one particular computer code called GEM. The basis of neutron tracking in critical size calculations by GEM is analyzed. This book will be a valuable resource for nuclear engineers and scientists.

• Language: English
• Publisher: Elsevier Science
• Release date: Oct 22, 2013
• ISBN: 9781483156613

Book preview

(1968).

Introduction

CRITICALITY control is of particular importance in the safe design and operation of chemical and metallurgical plant processing or fabricating fissile materials, in the handling and storage of enriched fuel for reactors, and in the associated transport operations. Assessment of the effects of neutron interaction between different parts of the system is an extremely important element in this control. It has been shown(1) that the energy yield from a critical excursion in an interacting array of several fissile units will be higher than from a simple homogeneous system of the same initial excess reactivity.

In a few instances separate items of plant may be spaced far enough apart for it to be obvious that neutron interaction must be negligible, but in most practical cases such an arrangement will prove grossly uneconomic and a minimum safe spacing, or maximum size and number of units at fixed spacing, must be determined. Experimental determination, ideal in principle, is seldom feasible in practice for interacting systems, if for no other reason than that items of plant, etc., may not exist at the design stage. Hence, the criticality adviser uses calculational methods, supported by experiments on a few reference arrays, and it is these methods which are the subject of this monograph.

The treatment is aimed at the intending criticality specialist. It does not set out to provide a critical review of the considerable literature which exists on neutron interaction, or to draw comparisons between the many possible methods of calculation, each of which has advantages in its own special field. Rather it is intended to describe the basic principles involved as illustrated by a number of methods of calculation which have proved their worth in daily use at major establishments.

CHAPTER 1

The General Nature of the Interaction Problem

Publisher Summary

An interacting array may be defined as a system of two or more bodies incorporating fissile material that are close enough together for some neutrons to induce fissions in bodies other than those in which they had their origin. If the bodies concerned are individually net neutron sources, this possibility of neutron exchange will clearly give rise to an increase in reactivity and may result in the whole system becoming critical, even though each body would be well subcritical in isolation. Formally at least, the general reactor equations derived from the Boltzmann transport equation may be applied to any system of fissile material. A recent development accompanying the introduction of high-capacity computing machines has been the application of the Monte Carlo methods of calculation to interacting arrays. The advantages of the Monte Carlo technique for systems of complex geometry are being exploited in programs that give results of high accuracy for the expenditure of relatively modest amounts of machine time and with few limitations on the types of array, which may be studied.

AN INTERACTING array may be defined as a system of two or more bodies incorporating fissile material which are close enough together for some neutrons to induce fissions in bodies other than those in which they had their origin. If the bodies concerned are individually net neutron sources this possibility of neutron exchange will clearly give rise to an increase in reactivity and may result in the whole system becoming critical, even though each body would be well subcritical in