Pile Oscillator Research Articles

Innovative Hybrid Pile Oscillator Technique in the Minerve Reactor: Open Loop Versus Closed Loop

Pile oscillator techniques are powerful methods to measure small reactivity worth of isotopes of interest for nuclear data improvement. This kind of experiment has long been implemented in the Minerve experimental reactor, operated by CEA Cadarache, Saint-Paul-les-Durance, France. A hybrid technique mixing reactivity worth and flux variations surrounding the oscillator is presented here. It was made possible after developing high-sensitivity miniature fission chambers introduced next to the irradiation channel. A test campaign, called MAESTRO-SL, took place in 2015. Its objective was to assess the feasibility of the hybrid method and investigate the separation of mixed neutron effects, such as fission and capture or scattering and capture. The experimental results are presented and discussed in this paper, which focus on comparing two measurement setups, one using a power control system (closed-loop technique) and another one in which reactor power is free to drift (open-loop technique). First, it is demonstrated that the open loop is equivalent to the closed loop, and both techniques bring similar results. It is shown that measuring the flux change around oscillated samples provides valuable information on partial neutron cross sections. Second, uncertainty management and methods’ reproducibility are discussed. As expected, both techniques can achieve similar performances. In this paper, uncertainty as low as 0.007 pcm for samples’ reactivity worth and $1.2 \times 10^{-4}$ for local flux change were achieved.

Innovative hybrid pile oscillator technique in the Minerve reactor: open loop vs. closed loop

Pile oscillator techniques are powerful methods to measure small reactivity worth of isotopes of interest for nuclear data improvement. This kind of experiments has long been implemented in the Mineve experimental reactor, operated by CEA Cadarache. A hybrid technique, mixing reactivity worth estimation and measurement of small changes around test samples is presented here. It was made possible after the development of high sensitivity miniature fission chambers introduced next to the irradiation channel. A test campaign, called MAESTRO-SL, took place in 2015. Its objective was to assess the feasibility of the hybrid method and investigate the possibility to separate mixed neutron effects, such as fission/capture or scattering/capture. Experimental results are presented and discussed in this paper, which focus on comparing two measurements setups, one using a power control system (closed loop) and another one where the power is free to drift (open loop). First, it is demonstrated that open loop is equivalent to closed loop. Uncertainty management and methods reproducibility are discussed. Second, results show that measuring the flux depression around oscillated samples provides valuable information regarding partial neutron cross sections. The technique is found to be very sensitive to the capture cross section at the expense of scattering, making it very useful to measure small capture effects of highly scattering samples.

A hybrid pile oscillator experiment in the Minerve reactor

Pile oscillator techniques are powerful methods to measure small reactivity worth of material of interest for nuclear data improvement. In the Minerve reactor, operated by CEA Cadarache, experiments are being held using a reactivity oscillator method based on the measurement of the global reactivity perturbation associated to samples motion in the core center. The technique has recently been extended by introducing new miniature fission chambers close to the oscillation location, in order to measure local neutron flux perturbations. New detectors and associated acquisition systems were developed for that purpose by CEA. First results are presented and discussed in this paper. Data processing techniques are presented in details which demonstrate that global and local perturbations can be unraveled. Experiments also demonstrate that measuring the local flux depression around oscillated samples indeed provide valuable information regarding partial cross sections. The preliminary results are found to be very sensitive to capture cross section at the expense of scattering cross section, making the technique very useful to measure small capture effects of highly scattering samples and improve nuclear data knowledge, as foreseen in the future CEA’s ZEPHYR critical facility to be built in Cadarache around 2026.

Americium-241 integral radiative capture cross section in over-moderated neutron spectrum from pile oscillator measurements in the Minerve reactor

An experimental program, called AMSTRAMGRAM, was recently conducted in the Minerve low power reactor operated by CEA Cadarache within the frame of the CHANDA initiative (Solving CHAllenges in Nuclear Data). Its aim was to measure the integral capture cross section of 241 Am in the thermal domain. Motivation of this work is driven by large differences in this actinide thermal point reported by major nuclear data libraries. The AMSTRAMGRAM experiment, that made use of well characterized EC-JRC americium samples, was based on the oscillation technique commonly implemented in the Minerve reactor. First results are presented and discussed in this article. A preliminary calculation scheme was used to compare measured and calculated results. It is shown that this work confirms a bias previously observed with JEFF-3.1.1 (C/E-1 = −10.5 ± 2%). On the opposite, the experiment is in close agreement with 241 Am thermal point reported in JEFF-3.2 (C/E-1 = 0.5 ± 2%).

Transfer Function Measurements in JOYO by Pile Oscillator Method

Using the pile oscillator technique, the reactor transfer functions of the experimental fast reactor “JOYO” were examined to ascertain reactor stability and to elucidate the reactor feedback mechanism. Measurements were made in the frequency range of 0.001 to 3 Hz at various reactor power levels. Major results were as follows: 1. Satisfactory agreement obtained between the zero-power transfer function measurements and calculations made by one point reactor model with six delayed neutron groups.2. Reactor stability confirmed from measurements of at-power frequency responses; large negative feedback effect (-0.851$/δ/φ at 50MWt) observed.3. Feedback transfer function and transfer function between neutron flux and outlet temperature at the central fuel subassembly found expressible in approximation by first-order lag model with a time constant of about 3.5 s.

The stepwise time dependent backward Kolmogorov equations with delayed neutrons in counting statistics II. Covariance

This paper completes work dealing with the neutron counting statistics in a zero power reactor subject to an arbitrary sequence of stepwise variations of all the parameters. Using the one-velocity, point reactor model with delayed neutrons and following the backwards Kolmogorov formalism, expressions for the mean value, variance and covariance of the count distribution have been obtained. Knowledge of these moments allows the estimation of kinetic parameters of a reactor together with their statistical errors under any time variable condition concerning critically, source intensity and detector efficiency. Numerical examples relating to a rod drop and to a sinusoidal pile oscillator experiments are reported for illustration.

Transfer function measurements in JOYO by pile oscillator method.

Using the pile oscillator technique, the reactor transfer functions of the experimental fast reactor “JOYO” were examined to ascertain reactor stability and to elucidate the reactor feedback mechanism. Measurements were made in the frequency range of 0.001 to 3 Hz at various reactor power levels. Major results were as follows: 1. Satisfactory agreement obtained between the zero-power transfer function measurements and calculations made by one point reactor model with six delayed neutron groups.2. Reactor stability confirmed from measurements of at-power frequency responses; large negative feedback effect (-0.851$/δ/φ at 50MWt) observed.3. Feedback transfer function and transfer function between neutron flux and outlet temperature at the central fuel subassembly found expressible in approximation by first-order lag model with a time constant of about 3.5 s.

Material worth measurements with a fuel-filled pile oscillator rod

A method to determine the reactivity worth of material samples within an undisturbed environment by a pile oscillator is described. In contrast to the usually performed pile oscillator measurements the oscillator rod consists of a tube filled with the material of the environmental core in which the sample is embedded. By this method, perturbations of the real and adjoint spectra, and of the angular neutron distribution are considerably reduced. A simple, theoretical model that describes the kinetic behaviour of the reactor during the oscillation is presented. The method and the theoretical model have been investigated on the Fast-Thermal Argonaut Reactor STARK. Results show the benefits of the proposed procedure and the applicability of the model.

Determination of the lumped heat-transfer and reactivity coefficients of a research reactor

This work presents a method of determining the lumped heat-transfer coefficient and the temperature coefficients of reactivity of a nuclear reactor. No knowledge of the component temperatures is required. The method is thus particularly attractive in systems where temperatures cannot be monitored, or where uncertainties in temperature variations can produce large errors in experimental menults. A mathematical model is developed for the reactor, which is assumed to consist of lumped fuel, moderator, and reflector regions. Simplified forms of this model are also developed: a two-region model for fuel and moderator, and a one-region model for fuel only. The equations for these models are expressed as functions of the feedback response of the reactor. The high frequency feedback response is normally a function of only the fuel temperature coefficient of reactivity and the heat-transfer capability from the fuel. The experimental high frequency response characteristic can thus be applied to the one-region mathematical model to determine the fuel reactivity coefficient and the heat-transfer capability. In a similar manner, the experimental medium frequency response of the reactor can permit a determination of the fuel and moderator feedback coefficients, and the low frequency response the determination of the reflector coefficients. The model was applied to a swimming pool reactor, with the frequency response measured with a pile oscillator over the frequency range 0·03 to 150 rad./sec. Excellent agreement was obtained between the mathematical model and the experimental response over the entire frequency range.

Calibration of an epithermal local pile oscillator

A description is given of the calibration of a local pile oscillator for epithermal neutron absorption. The nuclide 197Au is used as a standard in a gold-lead alloy. First the neutron absorption of the standard is determined by activation analysis and compared to computations. Thereafter the calibration is performed by oscillating these standard samples. A linear relationship is found between the number of absorbed neutrons in the sample and the oscillator results.

The local pile oscillator as a device for measuring epithermal neutron absorption

A local oscillator is described which is modified for operation in the epithermal region. Reduction of the results to effective resonance integrals, and determination of the cadmium cut off energy are discussed. Attention is given to the construction of the detector and associated electronics.

Dipole Effect of Pile Oscillator

A higher order effect was found in the transfer function measurements on the Argonaut Reactor at the Argonne National Laboratory. The gain and phase were altered when the direction of rotation of the pile oscillator was reversed. This effect can be explained by considering higher spatial modes and the dipole characteristics of the oscillator. The dipole represents the finite dimensions of the pile oscillator, which is a corrective term when the monopole effect is dominant.

MEASUREMENT OF THE YIELDS OF 135I AND 140Ba IN THE FISSION OF 233U, 235U, 239Pu, AND 241Pu

Fission yields of 135I and 140Ba have been measured relative to the yields from 235U for samples of different fissile materials irradiated in the NRU reactor in a flux of 2.7 × 1014 neutrons/cm2 sec for 24 hours. The relative 135I contents of the samples were determined from the reactivity effect of the daughter 135Xe in the Chalk River Pool Test Reactor using a pile oscillator. The relative concentrations of 140Ba were determined by gamma-ray counting of the daughter 140La. The fractional yields, normalized to 0.0625 for both fission products in 235U fission, are:[Formula: see text]In the fission of 233U and 239Pu the cumulative yields to 135I and 140Ba are approximately 20% and 7% less than the mass-spectrometrically-measured cumulative yields to 135Cs and 140Ce respectively. The differences are attributed to direct formation of 135Xe and 140La.

Dipole Effect of Pile Oscillator

A higher order effect was found in the transfer function measurements on the Argonaut Reactor at the Argonne National Laboratory. The gain and phase were altered when the direction of rotation of the pile oscillator was reversed. This effect can be explained by considering higher spatial modes and the dipole characteristics of the oscillator. The dipole represents the finite dimensions of the pile oscillator, which is a corrective term when the monopole effect is dominant.

Instrumentation at the Los Alamos Critical Assembly Laboratory

A summary is presented of the various types of electronic instrumentation used at the Los Alamos Critical Assembly Laboratory. Also discussed is automatic data handling equipment as well as the nature of command and readout signals used on remote control critical assembly machines. Equipment is described for performing prompt neutron lifetime measurements by the Rossi-? method, pile oscillator technique, and the power fluctuation experiment.

The measurement of the effective resonance integrals of uranium and thorium using the pile oscillator operating inside a cadmium tube

The feasibility of determining resonance absorption integrals by performing pile oscillator measurements inside a cadmium tube has been studied. It was concluded that reliable results could be obtained with this technique for strong resonance absorbers, but that large corrections were necessary for weakly absorbing materials, particularly those of low atomic weight. The technique has been used to determine the effective resonance integrals of uranium and thorium metal and oxide rods of various sizes.

Application of the Pile Oscillator to Large-Neutron-Dose Measurement

Application of the Pile Oscillator to Large-Neutron-Dose Measurement

Paired-chamber type pile oscillator

A pile oscillator, which is improved from a conventional one with the intention of eliminating the effect from the fluctuation of the reactor power by using a method of compensation, is reported. The signal-to-noise ratio is increased by a factor of about l00. The maximum sample size in this oscillator is 17 mm in diameter and about 100 mm in length. The attainable standard deviation of the measured value is less than 0.5 x l0-/sup -3/ cm/sup 2/ in the whole macroscopic cross section EPSILON /sub a/V of the sample. (auth)

Pile oscillator measurements of highly irradiated plutonium

Samples of plutonium alloyed with aluminium have been irradiated up to a maximum neutron dosage of 1·7 n/kb in the NRX reactor at Chalk River. At this irradiation over 80 per cent of the original 239Pu is destroyed. The resultant changes in cross-section have been measured with the pile oscillator in Gleep. The results are in agreement with theoretical predictions based upon present world nuclear data for plutonium isotopes as compiled by Westcott (1958) . This agreement gives added confidence in the ability to predict reactivity changes due to plutonium formation or destruction in power reactors.

Pile oscillator measurements of resonance absorption integrals

Measurements of the resonance absorption integral cross-sections of some thirty-five materials are reported. The effective cross-sections of the materials concerned were measured by the Dimple pile oscillator in both a pure thermal neutron spectrum and in one with an appreciable epithermal component, the difference between the cross-sections found in these two cases leading directly to the resonance absorption integral. Experiments have also been performed to measure the amount of self-screening in many of the samples. The cross-sections are compared with other experimental results, and also with estimates of the resonance integrals deduced from resonance parameter data.