This work presents the results of neutron spectrum measurements at central position of the IPEN/MB-01 reactor core. For this purpose has been used several activation foils (Au, Sc, U, Mg, Ti, Ni, and In Foils) that have been irradiated at central position of the reactor core (standard rectangular configuration 28 × 26 fuel rods) in the maximum power level (100 watts). After this one the foils were counting at HPGe solid state detector (gamma spectrometry). The nuclear reaction rates experimental results (saturated activity per target nucleus) is together with an initial approach to the neutron reactor spectrum (estimated by reactor physics cellular codes) the main data to obtain the most appropriated neutron spectrum at the irradiation position by interactive adjustment process using the unfolding SANDBP code. The results obtained were (5.1312±0.1609).10 9 n/cm 2 .s to the integral neutron flux, (9.8087±0.4974).10 8 n/cm 2 to the thermal neutron flux, (3.2272 ± 0.4378).10 9 n/cm 2 .s to the intermediate neutron flux and (1.9040 ± 0.6390).10 9 n/cm 2 .s to the fast neutron flux. These results are important and may be used to check and validate reactor physics codes and several nuclear data libraries.
One major objective of this work is to experimentally estimate the following kinetic parameters on the IPEN/MB-01 research reactor at São Paulo: the effective neutron delayed fraction, β e f f , the prompt neutron generation time, Λ, and the ratio β e f f /Λ. In order to achieve our goal, we will use a microscopic noise technique called Rossi-α method. This method is based on the statistical nature of the fission-chain process. Using a coincidence acquisition system, the rationale is to experimentally determine the probability distribution of detecting neutrons from the same chain. Through a least-squares fit of this distribution we estimate the prompt neutron decay constant α = (β e f f − ρ)/Λ. The α parameter will be measured for three sub-critical levels using three different source configurations, and the ratio β e f f /Λ is obtained via a extrapolation to ρ = 0. A specific acquisition system for Rossi-α measurements has been developed in order to achieve our objective. This system is based on a multichannel scaler controlled by virtual instruments that records the timing of all neutron events, allowing data analysis during the acquisition. The first measurements were performed using two BF 3 detectors positioned at the center of IPEN/MB-01 reactor core. In the future the same measurements described above will be performed using 3 He and photodiode detectors. The photodiode detectors have been developed by the Reactor Physics Division of IPEN. In this work we will present a preliminary set of Rossi-α measurements performed on the IPEN/MB-01 research reactor at São Paulo.
This experimental work aims to obtain the spectral indices 28p and 251) of the core of the IPENIMB-Ol nuclear reactor with a novice approach based on the direct gamma spectrometry of the irradiated fuel rods. The measurements were performed with high precision at the asymptotic central core region. The total and epithermal reaction ratios were obtained from the direct gamma spectrometry of a irradiated fuel element at the maximum neutron flux position. The gamma spectrometry was performed by a fuel rod scanning equipment with a collimator of 1 cm opening size. The data were acquired allowing the irradiation of the fuel rod to be bare or covered with cadmium sleeves of variable sizes. Correction factors were obtained both experimentally and from a calculation approach in order to correct the perturbations due to cadmium. Such calculations, carried out by the MCNP-4B, show the behaviour of these factors as a function of the cadmium sleeve length. The analysis shows that the correction factors are very sensitive to the nuclear data library used by MCNP-4 B; namely ENDF/B-V and ENDF/B-VI.5 libraries. The best results are obtained when the correction factors and the spectral indices are calculated with ENDF/B-VI.
This work presents the measures of the nuclear reaction rates along the radial direction of the fuel pellet by irradiation and posterior gamma spectrometry of a thin slice of fuel pellet of UO 2 at 4.3% enrichment. From its irradiation, the rate of radioactive capture and fission had been measured as a function of the radius of the pellet disk using a Ortec GMX HPGe detector. Lead collimators had been used for this purpose. Simulating the fuel pellet in the pin fuel of the IPEN/MB-01 reactor, a thin UO 2 disk is used, being inserted in the interior of a dismountable fuel rod. This fuel rod is then placed in the central position of the IPEN/MB-01 reactor core and irradiated during 1 h under a neutron flux of 5 × 10 8 n/cm 2 s. In gamma spectrometry, 10 collimators with different diameters have been used; consequently, the nuclear reactions of radioactive capture that occurs in atoms of 238 U and the fission that occurs on both 235 U and 238 U are measured in function of 10 different regions (diameter of collimator) of the UO 2 fuel pellet disk.
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