Exponential Pile Measurements

Lower value represents extrapolation from six-rod esq Ksnentlal pile measureicents higher value from single-rod exponential pile measurement. 

One very satisfactory method of determining critical size is to measure the Lapladan (A) in an exponential pile, i. e., a structure that is similar in all respects to the full size reactor contemplated, but is considerably smaller than that size required to m e the reaction seff-sustain-ing. A detailed discussion of how this is done can be found in the United States Patent of Enrico Fermi et al.. No. 2,708,656. In case the reactor is to be built in the form of a spherical structure employing uranium bodies of any shape or size imbedded in a heavy water (DsO) moderator, the following formula gives the critical overall radius ... 

It is clear that although the exponential pile and criticality experiments would be required ultimately in any large-scale reactor development program, it would be desirable to obtain some preliminary experimental verification of reactor calculations by means of other more modest tests. One experiment which appears to be eminently suited to this purpose is based on the pulsed neutron-beam technique. This technique has been applied by several investigators to the determination of the thermal-diffusion coefficient and macroscopic absorption cross sections of reactor materials.More recently, it has been used by E. C. Campbell and P. H. Stelson in the study of short-lived isomers and for the measurement of reactor parameters of multiplying media. The experiment con-... 

The material buckling for the mockup and condensed lattices were measured in an exponential pile. The measurement of the material buckling and (in PCTR) enabled a qualitative determination of the migration area. The migration area deduced for the condensed lattice is 300 cm for the mockup lattice, 646 cm. The ratio of the two values is 2.15 which is in good agreement with the ratio predicted theoretically to account for the effect of the voids on neutron diffusion. 

This will include buckling measurements using exponential piles neutron spectrum studies for heterogenous cells > development of computer codes and kinetic studies including axialog simulation studies. These studies will be performed for candidate fuel elements various... 

The measurements of k -l in the PCTR and those of from the mmonential piles have been compareif > by use of a calculated migration area M. The use of standard formulae for gave values of k -l from exponential bucklings which appeared to be systematically htyh by... 

The exponential assembly, which is supplied with neutrons from an external source, is perhaps the simplest of these facilities. The exponential facilities include the Sigma Pile, which is used for cross-section measurements and for the evaluation of diffusion parameters the Standard Pile, which is used as a basic standard of flux in the determination of counter efficiencies, one type of reactor power measurement, and the determination of cross sections, is also a device in this class. A third type of exponential equipment is the Subcritical Reactor in which a fissionable isotope is added to an assembly and in which neutron multiplication is made to occur. The a,cis made up of a sample volume of the reactor under consideration. 

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