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VALIDATION OF THE MCNP CODE IN FAST NEUTRON SPECTRUM IN THE COUPLED FAST-THERMAL SYSTEM ‘HERBE’ M. PEŠIĆ, V. LJUBENOV Nuclear Engineering Laboratory ‘NET’, ‘Vinča’ Institute of Nuclear Sciences P.O. Box 522, 11001 Belgrade, Yugoslavia
[email protected] ;
[email protected]
ABSTRACT Validation of the well-known Monte Carlo code MCNPTM against measured fast neutron spectrum in the centre of the coupled fast-thermal system HERBE at the RB research reactor is shown in this paper. Fast neutron spectrum is measured in energy range from 2.5 MeV to 10 MeV by using SSB sandwich spectrometer system filled by 3He gas. Calculation of the neutron spectrum in the centre of the fast core of the HERBE System is carried out for the first time by the MCNP code in 3D geometry applying detailed 3D model of the HEU fuel slug developed recently. Satisfactory agreements in comparison of the HERBE criticality and neutron spectrum calculations to measuring ones are obtained. Key words: HERBE, MCNP, fast neutron spectrum measurement and calculation
1. Introduction An idea of design of a small low power fast reactor was initiated in the Vinča Institute of Nuclear Sciences during late seventies and in eighties. This reactor, called LASTA [1], is supposed to have the same introductory role in research of physics of fast reactors as the RB reactor [2] had in late fifties in study of thermal reactors in the country. Unfortunately, due to various reasons, construction of the LASTA reactor was never started. But, as one of the results of few years extensive studies in fast neutron fields, a coupled fast-thermal core at the RB research reactor, called HERBE System, was realised in beginning of nineties [3]. One of basic purposes of the HERBE System is application in validation of computer codes for calculation of reactor complex lattice cells in especially designed experiments in this coupled fastthermal core. The HERBE System is described elsewhere [e.g., 4]. Here, horizontal cross sections of the RB reactor and the HERBE fast zone are shown in Figures 1 and 2.
2. Methods 2.1. Experimental Methods and Data Evaluation The neutron detection in a 3He-filled semiconductor-sandwich detector [5] is based on the reaction 3He (n, p) T and simultaneously detection of the reaction products in the Si diodes. The pulses from the diodes are amplified and shaped in separate 'energy' channels and summed to produce a single pulse with height proportional to the energy of the incident neutron plus energy of the Q-value ████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████
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(0.764 MeV) of the reaction. A well-known measuring system of the 3He-neutron spectrometer [5], used for the HERBE fast neutron spectrum measurement and calibrated in the thermal neutron field, is shown in Figure 3.
Figure 1. Horizontal cross section of the RB reactor with the HERBE System
Figure 2. Horizontal cross section of the fast zone of the HERBE System A thermal neutron peak is measured for various resolving times and the best energy resolution for thermal neutrons (FWHM=93 keV at 0.53 MPa of 3He gas pressure) is obtained at 30 ns resolving time. The best resolving time for measurement of the fast neutron spectra (60 ns) is determined by measuring the signal-to-background ratio of the spectrum for different resolving times [6]. The computer code HE3 [7] is developed for evaluation of neutron spectra measured by this spectrometer. For the case of isotropic neutron flux the code is based on a geometric model of the infinite diode approximation applying analytic method and on the Monte Carlo simulation of the processes in real spectrometer geometry. The neutron cross-sections are taken from the ENDF/B-VI.2 ████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████
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data library. Neutron group cross-sections are calculated by averaging the point-wise neutron crosssection data with 1/E spectrum in 0.2 eV to 2.5 MeV neutron energy range and with the 235U fission spectrum over neutron energies higher than 2.5 MeV. The width of each energy group is 0.1 MeV. The detection efficiency of the 3He spectrometer depends on incident neutron energy, angular and energy distributions of the reaction products under the conditions imposed by the geometry of the detectors, the gas pressure, low level discriminator setting in the electronic system and the detector orientation in respect to the incoming neutrons. The detection efficiency obtained by the analytic method and the Monte Carlo simulation of processes with isotropic and anisotropic distributions of emitted particles in the centre of mass system are very similar above 2 MeV [6]. Measurements are performed in the mixed neutron – gamma-rays field of the vertical experimental channel (VCH) of the HERBE System at low reactor power. The spectrometer head is surrounded with 1 cm thick lead shield in aim to reduce high gamma-ray background. To obtain good statistical requirements of the experimental data, the acquisition time of 2.3 hours was necessary for each individual measurement of the foreground spectrum (detector head filled with 3He gas) and the background one (detector head filled with 4He gas at the same pressure). After the neutron and gamma ray background subtraction, the efficiency correction is applied. Average leakage rate of the 3He gas (3.23 ± 0.90 kPa per day), obtained experimentally, is included in data evaluation as well.
Figure 3. Block scheme of 3He-neutron spectrometer used for fast neutron spectrum measuring
2.2. Neutron Spectrum Calculation Initial results of calculations of the HERBE criticality [3], carried out by standard reactor diffusion and transport codes were not agreed satisfactory with experimental results. To improve models and calculation methods applied, new codes were developed or existing ones were modified to include large heterogeneous effects in the HERBE System due to existence of void similar (air zones) and neutron high-absorption (Cd zone) regions in the fast zone. Last calculations of criticality and neutron spatial and energy distribution inside the HERBE System were carried out recently [8] by using the MCNP4B code [9]. Detailed 3D geometry model of the RB reactor with coupled fast-thermal core HERBE, based on new 3D geometry model of the HEU fuel slug [10], is developed for the MCNP code. The latest information on compositions of materials ████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████
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utilised in the RB reactor are used. Neutron continuous-energy library VMCCS [11], developed in the Vinča Institute and TMCCS library for neutron scattering at thermal energies at H and D atoms connected in molecules of light and heavy water, respectively, are used with the MCNP code. Fast neutron spectrum in 0.2 MeV wide energy groups (bins) is calculated in the centre of the fast zone, in the air of the vertical experimental channel, at the half of the critical height – at the position of the neutron spectrometer 3He head. For that purpose, an air sphere with 100 cm3 is selected in the MCNP code as neutron track-length scoring media (F4 type tally). The code is run for 50 million neutron histories (!) in the KCODE option, after 15 initial cycles with 2000 neutron histories each, to obtain the initial spatial distribution of neutron sources from fission. This long calculation (about 15 days at PC with AMD K6 processor at 166 MHz and 32 MB RAM) is run in aim to reach (satisfactory low) statistical error (1 σ, p = 0.67) of group neutron flux less than 5% in the most of neutron groups (0.4 eV – 4 MeV). Statistical errors of calculation of the group flux in higher (than 4 MeV) energy groups were in range between 10% and 30%. The calculation also shows that there are none neutrons in the VCH centre with energies below Cd energy threshold (0.465 eV).
3. Results Value of the effective neutron multiplication factor (0.99924 ± 0.00011, p = 0.67, after 50 million neutron histories), obtained from the MCNP calculations of the HERBE System in the RB reactor for the critical height (142.60 ± 0.02 cm, at 20 oC, and molar fraction of 2.22% light water in heavy water) is in very good agreement with the measured experimental data (1.0000 ± 0.00005, p = 0.67). Calculated neutron spectrum in the centre of the HERBE fast zone by the MCNP code is shown in Figures 4 and 5. It is compared to measured neutron spectrum (condensed into 0.1 MeV bins) and to the previously calculated one (in four highest energy macro-group) by using 1D transport code AVERY, [3], with 26-group BNAB neutron cross section library data [12]. Neutron spectra are normalised to the same value of the area under measured neutron spectrum in range of 2.5 MeV to 10.5 MeV [13].
Figure 4. Measured and calculated neutron spectra in the centre of the fast core of the HERBE System (energy range 0.1 eV – 10.5 MeV) ████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████
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Figure 5. Measured and calculated neutron spectra in the centre of the fast core of the HERBE System (energy range 2.5 MeV – 10.5 MeV)
4. Conclusion The new calculated neutron spectrum in the centre of the HERBE System, by using the MCNP code with continuous neutron energy library in the 3D model of the RB reactor, is compared to the measured one (in fast neutron energy range). Results of this validation of the MCNP code calculation provide satisfactory overlapping, considering the total estimated error of the experiment and uncertainties in the models [6, 7, 10] introduced in the calculations and data evaluation. The spectrum measurement and calculation show no serious divergence over the measured neutron energy range. This is particularly true having in mind that the measurement is carried out in mixed neutron-gammaray radiation field.
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