Method to calibrate fission chambers in Campbelling mode

Fission chambers are neutron detectors which are widely used to instrument experimental reactors such as material testing reactors or zero power reactors. In the presence of a high level mixed gamma and neutron flux, fission chambers can be operated in Campbelling mode (also known as 'fluctuation mode' or 'mean square voltage mode') to provide reliable and precise neutron related measurements. Fission chamber calibration in Campbelling mode (in terms of neutron flux) is usually done empirically using a calibrated reference detector. A major drawback of this method is that calibration measurements have to be performed in a neutron environment very similar to the one in which the calibrated detector will be used afterwards. What we propose here is a different approach based on characterizing the fission chamber response in terms of fission rate. This way, the detector calibration coefficient is independent from the neutron spectrum and can be determined prior to the experiment. The fissile deposit response to the neutron spectrum can then be assessed independently by other means (experimental or numerical). In this paper, the response of CEA made miniature fission chambers in Campbelling mode is studied. We use a theoretical model of the signal to calculate the calibration coefficient. Input parameters of the model come from statistical distribution of individual pulses. Supporting measurements have been made in the CEA Cadarache zero power reactor MINERVE. Results are compared to an empirical Campbelling mode calibration

New neutron detector based on Micromegas technology for ADS projects

A new neutron detector based on Micromegas technology has been developed for the measurement of the simulated neutron spectrum in the ADS project. After the presentation of simulated neutron spectra obtained in the interaction of 140 MeV protons with the spallation target inside the TRIGA core, a full description of the new detector configuration is given. The advantage of this detector compared to conventional neutron flux detectors and the results obtained with the first prototype at the CELINA 14 MeV neutron source facility at CEA-Cadarache are presented. The future developments of operational Piccolo-Micromegas for fast neutron reactors are also described

Thermal neutron activation experiments on Ag, In, Cs, Eu, V, Mo, Zn, Sn and Zr in the MINERVE facility

The MAESTRO experimental program has been designed to improve nuclear data uncertainty on a large range of materials used for detection, absorption, moderation and structures in LWRs. It consists of pile-oscillation and neutron activation experiments, carried out in the MINERVE low power facility. For this program, the core configuration has been designed to be representative of HZP (Hot Zero Power) conditions of a typical PWR. Samples of high purity elements have been manufactured with severe technological constraints to reach a target accuracy of ±2% (1σ) on the measurement. This paper presents a preliminary analysis of activation experiments, based on TRIPOLI4 Monte-Carlo calculations and various nuclear data libraries

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

Correction factors to apply to fission rates measured by miniature fission chambers in various neutron spectra

CEA develops and makes use of miniature fission chambers (MFCs, with radius down to 1.5 mm) for reactor physics conducted in experimental reactors such as EOLE and MINERVE zero power reactors (CEA Cadarache). When measuring fission rate, it is known that the neutron spectrum in the irradiation channel can be modified by the detector and the detector fixture. So the result of the measurement does not give a direct access to the desired quantity (fission rate, neutron flux,etc.) To overcome this problem, it is possible to make use of Monte Carlo calculations based on a detailed modeling of the detector. It could then be included in the 3D reactor model but this leads to large and time consuming calculations. In this case, measurement results can be combined directly with calculated values to produce the desired quantity. Another possibility is to calculate correction factors to apply to the biased measurement, i.e. to perform two-step calculations. Those factors depend on the detector geometry, the neutron spectrum and the fissile isotope at stake. A method to determine those factors is presented in this paper. The previously calculated neutron spectrum is fed to a simplified calculation route that includes only the detector and its close environment. Correction factors are obtained from two calculations results (with and without the detector fixture). In this case, the measured fission rates are corrected before being further processed. This paper details a parameter study on the impact of MFC parts and its environment (cable, connector) on the observed fission rate. Precise models of CEA-made MFCs have been developed for that purpose and used to produce correction factors for various fissile isotopes and neutron spectra. It is shown that fission rates can be greatly underestimated because of neutron radiative capture in MFC parts close to the fissile coating (9% in the worst case). The impact on standard reactor physics measurements is then discussed

On the feasibility to perform integral transmission experiments in the GELINA target hall at IRMM

International audienceShielding experiments are relevant to validate elastic and inelastic scattering cross sections in the fast energy range. In this paper, we are focusing on the possibility to use the pulsed white neutron time-of-flight facility GELINA to perform this kind of measurement. Several issues need to be addressed: neutron source intensity, room return effect, distance of the materials to be irradiated from the source, and the sensitivity of various reaction rate distributions through the material to different input cross sections. MCNP6 and TRIPOLI4 calculations of the outgoing neutron spectrum are compared, based on electron/positron/gamma/neutron simulations. A first guess of an integral transmission experiment through a 238U slab is considered. It shows that a 10 cm thickness of uranium is sufficient to reach a high sensitivity to the 238U inelastic scattering cross section in the [2-5 MeV] energy range, with small contributions from elastic and fission cross sections. This experiment would contribute to reduce the uncertainty on this nuclear data, which has a significant impact on the power distribution in large commercial reactors. Other materials that would be relevant for the ASTRID 4th generation prototype reactor are also tested, showing that a sufficient sensitivity to nuclear data would be obtained by using a 50 to 100cm thick slab of side 60x60cm. This study concludes on the feasibility and interest of such experiments in the target hall of the GELINA facility