Experimental Determination of the Neutron Characteristics of UO2-PuO2-H2O Lattices

As part of the investigation, in the VENUS test facility, of the variably moderated core of the BR3/VULCAIN reactor, a fuel assembly consisting of 37 UO2-PuO2 pins (94% natural UO2, 6% PuO2 ) was substituted for one of the enriched (to 7% 235U) UO2 fuel assemblies constituting the reactor core. Experiments were carried out with the object of refining the mathematical models for calculating the performance of this special assembly; inter alia, the fission density distribution and the changing ratio of the effective cross-sections for fission in the 233Pu and 235U were measured. Using the same critical facility, the authors are carrying out a critical experiment related directly to the problems of plutonium recycling in pressurized light-water thermal reactors. Three types of fuel are being used: UO2-PuO2 with 3% 235U and 1% fissile plutonium, UO2-PuO2 with 2% 235U and 2% fissile plutonium, and UO2 with 4% 235U. The two UO2-PuO2 mixtures have completely different isotopic contents of 240Pu: 7% and 17%. In the first part of the experimental programme, a study is being made of regular lattices in cores having two co-axial cylindrical zones: a UO2-PuO2 zone and a UO2 zone. Particular attention is being paid to investigating the region on either side of the interface separating the two zones, where the neutron spectrum reflects the characteristic energy distributions in each of the two lattices. The experimental results are to be used in calibrating the computational methods. In the second part of the experimental programme, parts of the core of the SENA power reactor will be simulated with a view to studying the problems of reloading one third of the core with mixed UO2-PuO2 fuel. Among the experimental techniques employed in these various experiments emphasis is given to those most specifically related to the presence of plutonium: fission density measurements by direct counting of the fission products formed in the mixed fuel as a result of irradiation, and measurements of spectral indices by means of fission detectors and detectors having resonances close to those of the isotopes of plutonium. The optimum conditions for applying these techniques are described and the effect on the fuel cycle of uncertainties in the values for the spectral indices (ratios of cross-sections) is discussed. A brief reference is finally made to a programme of sub-critical experiments conducted hitherto with two batches of UO2 fuel enriched to 5% and 7% 235U, and having the purpose of studying possible methods of exciting a sub-critical lattice by means of an external neutron source in pulsed and modulated operation. The aim of this programme is to ascertain to what extent these methods, which employ a quantity of fuel considerably less than the critical mass, can help in determining the characteristics of a lattice. The advantages of such methods are particularly obvious in the case of plutonium fuels. (author)