PCTR Measurements

The results of the PCTR measurements are given In Table 2.6.2. Actually, two lattice arrangements were studied, one vas as exact a copy of the actual lattice as possible, the other vas a condensed lattice In that the large steam vent and othbr voids In the stack vere removed, and the lattice spacing coapressed to maintain the same graphite-to-uranlum ratio. 

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S appears that the standard derivation of 1 and r should be modified to provide agreement between eiq onenflat and PCTR measurements. 

Measurements have been comjdeted on a 190 rod duster of 1/2-in. UOa rods on an 8-in. pitch with aluminum pressure and calandria tubes for comparison with a similar lattice measured in the PCTR.. The k > results for 99.7% D 0 purity are 1.059 t 0.005 for ttie PLATE measurement and 1.052 0.003 for the PCTR measurement, hi addition, a measurement was made in PLATR at a purity of 98.3% Dd>. The result of this measurement was 1.0391 0.005. A simple purity correction was made to the 98.3% result assuming no change in the flux distribution. The corrected result, 1.055 1 0.007, was in good agreement with the measured value at the higher purity. 

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... 

D. D. Lanning,et iL, The Nuclear Parameters of Some Grafriiite, Natiinil Uranium Lattices Measured in the PCTR, Proc. Con/, on Graphite Reactors, BNL-489, p. 85 (1958). 

Analysis and Measurement of PuAI-Poly-ethylene Systems In the PCTR / D. G. 

A series of experiments with hydrogen-moderated plutonium systems has been performed in the Physical Constants Testing Reactor (PCTR). The boron concentration required to reduce the infinite multiplication factor, k , to unity was measured for various Pu-240 concentrations, fuel plate thicknesses, and moderator-to-fuel ratios. The measured values of the boron concentra-Uon were compared to theoretical predictions to check the accural of calculational methods and parameters. The principal components for these experiments were Pu-Al (M wt% Pu), pure polyethylene, and borated lyetlqrlene (0.90 wt% natural boron). All three materials were in the form of disks, 1.960-in. in diam and 0.020-in. thick. 

Masterson et al. reported a value of 8.0 O.S g lUer for the limiting critical concentration of Pa in water. This value was determined by the well-known null reactivity method in the PCTR facility sd Hanford. As recently pointed out by Clark, large corrections were necessary in this eiqperiment for qtectral perturbations caused by Hie stainless-steel tanks containing the plutonium solutions in the reactor core. The correction for the stainless steel was determined in Hie original experiment by making measurements with varying thicknesses of steel containers and extrapolatiiig the results to a sero wall thickness. 

Measurements Made lii The Physical Constant Test Reactor (PCTR)... 

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. 

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