The Criticality of Periodically Boron-Poisoned Enriched Uranium Solution Systems

Nuclear safety engineers must evaluate the criticality potential of a variety of plant problems. Many of these involve an essentially unreflected system containing uranium solution and a fixed nuclear poison. Measured critical parameters for such a system at solution concentrations of 52.2- and 141.5-g U/liter, together with those reported previously at 450.8-g U/liter, provide the engineer information over a wide range of concentrations normally encountered in industrial applications. The uranium was enriched to 93.24 wt% ²³⁵U. The fixed poison was 1.02 wt% natural boron alloyed in stainless-steel plates. Critical solution heights were measured for various numbers of nearly uniformly spaced vertical plates within the 106.6-cm-diam experimental tank. The simplest cases studied involved no poison, resulting in low critical heights. As plates were added, the critical height increased until a sufficient number were present that even an infinitely tall tank would have been subcritical. The actual finite plate height permitted a third type of experimental result: the critical parameters of an unpoisoned uranium solution slab on top of a highly poisoned solution region. Experimental data at all three concentrations compared with results from Monte Carlo and neutron transport computer codes are found to predict critical heights consistently in excess of measured values. A nuclear safety engineer may safely apply these calculational methods to similar plant situations provided an ∼20% reduction in either the solution height or plate spacing— whichever is appropriate—is made to account for the theory/experiment difference. Boron-containing plates are compared with borosilicate glass Raschig rings as fixed nuclear poisons for large-volume solution storage. Neither is clearly superior to the other considering the poison volume percent required for criticality. Nuclear safety engineers may safely apply these experimental poison plate data to standard ringpoisoned systems involving a high-concentration uranium solution provided a 2% increase in the boron density is made to account for uncertainties in the comparison.