Determinação experimental de taxas de reação no 238U e 235U ao longo do raio da pastilha de UO2 do reator IPEN/MB-01

Abstract

This research presents and consolidates an alternative methodology for determining nuclear reaction rates along the radial direction of the fuel pellets which does not require high neutron flux. This technique is based on irradiating a thin UO 2 disk inserted into a removable fuel rod at the IPEN/MB-01 reactor core.Several gamma spectrometries are performed after irradiation using a HPGe detector. Six lead collimators with different diameters are sequentially alternated during this process, thus, the nuclear radioactive capture which occurs in 238 U and the fissions which occur in both 235 U and 238 U are measured according to six different radial regions of the fuel disk. Geometric efficiency corrections due to the introduction of collimators in HPGe detection system are determined by MCNP-5 code. The fission rate measurements are performed using the 99 Mo. This radionuclide was studied and proved ideal for these measurements because it is formed in linear behavior in the reactor core, have a high yield fission and emits low-energy photons.Measurements were performed irradiating UO 2 disks (with 4.3%enrichment) in the central position of the IPEN/MB-01 core at 100 watts power level during one hour. Some measurements were performed using a cadmium glove wrapped in the fuel rod to determine the nuclear reaction rates in the epithermal energy range. The experimental results obtained are compared with nuclear reaction rate calculations by means of MCNP-5 with ENDF/B-VII.0 data library showing discrepancies of up to 9% in 238 U capture rates and 14% for U fission rates for epithermal energies. Uncertainties regarding the nuclear capture rates have maximum values of 4.5% and the fission rates has maximum values of 11.3%.

Medidas de taxas de reação nuclear e de índices espectrais ao longo do raio das pastilhas combustíveis do reator IPEN/MB-01

“…O novo modelo permite que os anéis encobertos tenham a área aproximadamente constante e suas paredes estejam perfeitamente paralelas [30,31].…”

Nova metodologia experimental para medida de taxa de reação nuclear no combustível do Reator IPEN/MB-01 (Tipo Varetas)

“…Essa luva, ou cobertura de cádmio, foi utilizada em trabalhos anteriores [13,41] , com geometria de 0,05mm de espessura e 70mm de altura, como mostra a [46] .…”

“…E para certificar a total cobertura dos detectores de ativação, foi utilizado uma cobertura interna a placa, composta de dois discos de 0,5mm de espessura de cádmio. A (calibrado em potência [1] ), utilizando uma vareta combustível experimental desmontável, [46] .…”

“…O fator de normalização Fn é obtido pela Eq. 4.8 a seguir: sua estabilidade e qualidade são testados rotineiramente com fontes padrões, como também são realizados testes estatísticos como por exemplo o qui-quadrado [5,6,46] .…”

“…A Figura 4.14 mostra o diagrama esquemático com as características da eletrônica da bancada experimental associada ao detector HPGe [46] .…”