The Neutron Spectrum in a UO₂ Tube at Different Temperatures

We have carried out some experiments and calculations on the neutron spectrum in a UO₂ tube at various temperatures., The tube, 65.6-mm o.d., 32.3-mm i.d., and 300-mm high, was canned in steel and mounted in a container which was placed in the central vertical channel of the reactor Rl. The tube was heated by the fission energy which was enough to keep it at a maximum temperature of 750°C. The lowest temperature was 50°C., With the fast chopper, we measured the spectrum from a lead scatterer in the center of the tube. By multiplying this spectrum with some correction factors, we got the spectrum in the scatterer, which is the final result from the measurements., The increase of the temperature from 50 to 750°C gave an appreciable hardening of the neutron spectrum. This hardening will increase the reaction rate in ²³⁹Pu by 7.5% if one normalizes to equal reaction rate in a l/v absorber. Only a minor part of the value 7.5% depends on the heating of the steel and the lead., In the calculations we used the THERMOS code with the effective-width model for deuterium. All other materials, including U02, were treated with the free-gas model. The energy ranged from 0.00025 to 3.06 eV., The experimental and computed spectra agreed reasonably well in the thermal region both at 50 and at 750°C. The calculated epithermal ratio and, to some extent, the spectrum in the joining region were sensitive to the value of the cell radius used in the calculations. So it is difficult to make a definite statement about the computational method for these energies. We have, however, no reason to suspect that the method would produce wrong results., The ratio between the high-temperature spectrum and the low-temperature spectrum appeared to be rather insensitive to the value of the cell radius even for epithermal energies, and we found a remarkable agreement with the measured ratio. Therefore, we had no reason to replace the free-gas model for UO2 by some other model - although the free-gas model is, of course, somewhat unphysi-cal., Some THERMOS calculations were also made on the influence of hot steel and hot lead and on the influence of elevated moderator-temperature - 220°C instead of the value 33°C used in the experiments.,