Calculation of UO2 -PuO2-H2O Lattices

The problem of calculating UO2-H2O lattices is now well understood. There are many codes for evaluating the basic nuclear parameters that characterize aqueous reactor cores, both initially and during irradiation. However, the introduction into nuclear fuels of a high content of plutonium, in the form of UO2-PuO2 mixtures, causes serious disturbances, particularly in the thermal neutron spectrum. Considerable modifications have therefore to be made in the mathematical methods of calculation in order to take the physical phenomena fully into account. The derivation of a new formula and of the corresponding codes calls first for improvements in the theory and then for a comparison of experimental results with those obtained from calculations. The various codes used by BelgoNucléaire in aqueous reactor calculations (for example, PANTHER) are described briefly, and the particular characteristics and novel procedures that have been adopted to secure improved treatment of plutonium are pointed out. As part of a joint EURATOM -CEN-BelgoNucléaire programme, several attempts have been made recently to standardize these codes. The first such attempt consisted in applying these methods to the Hanford and Saxton plutonium lattices, the characteristics of which cover a very wide range of parameters. Analysis of the results confirms the validity of the mathematical model used while revealing certain imperfections of detail and a fairly marked sensitivity to the basic nuclear constants. Modifications have consequently been introduced, both in the numerical treatment and in the choice of constants, and these may be expected to yield the degree of precision generally considered desirable for thermal reactors, i.e. ± 1% for the multiplication factor; ± 4% for the power distribution; ± 1% for the reactivity during irradiation; ± 20% for the reactivity coefficients; ± 10% for reactor control. The codes described will be used in their improved form for making theoretical predictions regarding the various PuO2-UO2 lattices that are to be studied in 1967 as part of the programme of sub-critical experiments in the VENUS test facility. The comparison of theoretical and experimental results will yield a second set of standard codes for lattices having characteristics very close to those of the lattices intended for plutonium recycling in high-power thermal reactors. (author)