Campbelling Mode Research Articles

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 is proposed 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. A theoretical model of the signal is used to calculate the calibration coefficient. The model's input parameters come from statistical distribution of individual pulses. Supporting measurements were made in the CEA Cadarache zero power reactor MINERVE and results are compared to an empirical Campbelling mode calibration. The tested fission chamber calibration coefficient is roughfly 2 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$10^{-26}$</tex></formula> <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm A}^{2}$</tex></formula> /Hz/(c/s). Both numerical and empirical methods give consistent results, however a deviation of about 15% was observed.

A Monte Carlo simulation of the fission chambers neutron-induced pulse shape using the GARFIELD suite

A computation route that simulates the neutron-induced charge spectrum and pulse shape of a fission chamber is presented. It is based on the GARFIELD suite, and makes use of the MAGBOLTZ and SRIM codes. It allows the simulation of the signal in the current and Campbelling modes. Computations made with several fission chambers exemplify the possibilities of the route. A good qualitative agreement is obtained when comparing the results with the scarce experimental data available to date. After a further experimental qualification, this route will improve the design of fission chambers by assessing its overall sensitivity.

Experimental Verification of the Fission Chamber Gamma Signal Suppression by the Campbelling Mode

For the on-line monitoring of high fast neutron fluxes in the presence of a strong thermal neutron component, SCK·CEN and CEA are jointly developing a Fast Neutron Detector System, based on <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">242</sup> Pu fission chambers as sensors and including dedicated electronics and data processing systems. Irradiation tests in the BR2 reactor of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">242</sup> Pu fission chambers operating in current mode showed that in typical MTR conditions the fission chamber currents are dominated by the gamma contribution. In order to reduce the gamma contribution to the signal, it was proposed to use the fission chambers in Campbelling mode. An irradiation experiment in the BR2 reactor with a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">242</sup> Pu and a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">235</sup> U fission chamber, both equipped with a suitable cable for measurements in Campbelling mode, proved the effectiveness of the suppression of the gamma-induced signal component by the Campbelling mode: gamma contribution reduction factors of 26 for the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">235</sup> U fission chamber and more than 80 for the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">242</sup> Pu fission chamber were obtained. The experimental data also prove that photofission contributions are negligibly small. Consequently, in typical MTR conditions the gamma contribution to the fission chamber Campbelling signal can be neglected.

Research Activities in Fission Chamber Modeling in Support of the Nuclear Energy Industry

Fission chambers are widely used in the nuclear industry. As an example, they play a major role in the control of any fission reactor and are thus regarded as a key component for ensuring their safety. They are also employed in the material testing reactors for monitoring irradiations. We have recently started a research program, the objective of which is to improve the performance of those neutron detectors in terms of lifetime, calibration, and online diagnosis. In this paper, we present several studies carried out in order to model the signal delivered by a fission chamber. First, the simulation of the deposit evolution allowed us to select the most appropriate fissile material for a given spectrum and fluence. Second, we studied the impact of the bias voltage and filling gas characteristics on the charge collection time. Finally, the simulation of a pulse signal prior to amplification showed how it is important to have a satisfactory knowledge of the energy for creating ion pairs to accurately assess the signal in current or Campbelling mode.

상용 S/W를 이용한 소형가스터빈엔진 회전체의 동적 구조해석 및 검증

The accurate prediction of dynamic characteristics of high speed rotors, such as gas turbines, is important to avoid the possibility of operating the machinery near the critical speeds or unstable speed regions. However, the dynamic analysis methods and softwares for gas turbines have been developed in the process of producing many gas turbines by manufacturers and most of them have seldom been disclosed to the public. Recently, commercial FEM softwares, such as SAMCEF, ANSYS and NASTRAN, started supporting some rotordynamics analysis modules based on 3-D finite elements. In this paper, the dynamic analysis method using commercial S/W, especially ANSYS, is attempted for the small-size gas turbine engine rotor, and the analysis capability and limitations of its rotordyamics module are evaluated for further improvement of the module. As the preliminary procedure, the rotordyamic analysis capability of ANSYS was tested and evaluated with the reference models of the well-known dynamics. The limitations in application of the rotordynamics module were then identified. Under the current capability and limitations of ANSYS, it is shown that Lee diagram, a new frequency-speed diagram enhanced with the concept of <TEX>$H{\infty}$</TEX> in rotating machinery, can be indirectly obtained from FRFs computed from harmonic response analysis of ANSYS. Finally, it is demonstrated based on the modeling and analysis method developed in the process of the S/W verification that the conventional Campbell diagram, Lee diagram, mode shapes and critical speeds of the small-size gas turbine engine rotor can be computed using the ANSYS rotordynamics module.

Absolute calibration of microfission chamber in JT-60U

We have conducted calibrations of a microfission chamber, which was installed between the vacuum vessel and the toroidal field coils, by both a Cf-252 neutron source and real deuterium plasmas in JT-60U. The detector employs both pulse counting and Campbell (mean square voltage) modes in the electronics to cover a wide dynamic range of the neutron source strength. The pulse counting and Campbell modes were calibrated by Cf-252 and deuterium plasmas, respectively. Point efficiencies, counts per neutron from a point at a single angle, were measured for 27 locations of the neutron point source in toroidal scan. The efficiencies were influenced by the various components such as the vacuum vessel, port, and graphite tiles. The point efficiencies can be integrated and averaged with angle to provide toroidal line efficiencies. The line efficiencies of the microfission chamber and the nearest neutron monitor of the U-235 fission chamber were 5.38x10(-9) and 1.77x10(-8), respectively. Then the calibration for the Campbell mode was also performed by using a real deuterium plasma. The detection efficiency in the Campbell mode was about three-tenths of that of the nearest neutron monitor, which is consistent with the calibration result obtained by using a Cf-252 neutron source for pulse counting mode.

Fusion Neutron Flux Monitor for ITER

Neutron flux monitor (NFM) as an important diagnostic sub-system in ITER (international thermonuclear experimental reactor) provides a global neutron source intensity, fusion power and neutron flux in real time. Three types of neutron flux monitor assemblies with different sensitivities and shielding materials have been designed. Through MCNP (Mante-Carlo neutral particle transport code) calculations, this extended system of NFM can detect the neutron flux in a range of 104 n/(cm2·s) to 1014 n/(cm2·s). It is capable of providing accurate neutron yield measurements for all operational modes encountered in the ITER experiments including the in-situ calibration. Combining both the counting mode and Campbelling (MSV; Mean Square Voltage) mode in the signal processing units, the requirement of the dynamic range (107) for these NFMs and time resolution (1 ms) can be met. Based on a uncertainty analysis, the estimated absolute measurement accuracies of the total fusion neutron yield can reach the required 10% level in both the early stage of the DD-phase and the full power DT operation mode. In the advanced DD-phase, the absolute measurement accuracy would be better than 20%.

Development of in-vessel neutron monitor using micro-fission chambers for ITER

A neutron monitor using micro-fission chambers to be installed inside the vacuum vessel has been designed for the International Thermonuclear Experimental Reactor (ITER). The monitoring system needs to be insensitive to the changes of the plasma position and the profile, and the locations behind upper and lower outboard blankets were selected as appropriate based on the neutron transport calculations with the Monte Carlo code for neutron and photon transport (MCNP). Employing both pulse counting and Campbelling modes in the electronics, the ITER requirement of 107 dynamic range with 1 ms temporal resolution will be accomplished. The system meets the 10% accuracy required for the fusion power monitor. A set of a U235 micro-fission chamber with 12 mg UO2 and a fissile-material-free “blank” detector to eliminate noise issues arising from γ rays, etc. were fabricated based on the design. The vacuum leak rate of the chamber with the mineral insulated (MI) cable, resistances between the central conductor and outer sheath, and mechanical strength up to 50 G acceleration were tested to meet the design criteria. The output signals for γ rays were measured with the Co60 γ-ray irradiation facility at Japan Atomic Energy Research Institute (JAERI)-Takasaki and the influence was estimated to be less than 0.1% of the signals for neutrons. Excellent linearities between count rates, square of Campbelling voltage, and neutron fluxes were confirmed in the temperature range from 20 °C (room) to 250 °C with the Fusion Neutronics Source (FNS) facility of JAERI. The influence of the surrounding material was studied with the shielding blanket mock-up, and it was verified that the chamber provides an effective response although the sensitivity was enhanced by slow-downed neutrons. As a result, it was concluded that the present micro-fission chamber is applicable to ITER power monitoring.

Neutron flux monitoring system for ITER-FEAT (abstract)

The concept of the neutron flux measurements for International Thermonuclear Experimental Reactor ITER-FEAT is discussed. In spite of the fact that ITER-FEAT has reduced fusion power with respect to ITER-FDR, the requirements for neutron flux monitors are similar—wide dynamic range (seven orders), good temporal resolution (1 ms), and high accuracy (10%). It is clear that fission chambers are the most suitable detectors for this application. However high neutron intensity of the fusion plasma and hard requirements lead to a more sophisticated detection system than the ordinary fission chamber. Another problem is an absolute calibration of the detectors. We propose a neutron flux monitoring system, which consist of microfission chambers placed inside the ITER vacuum chamber, three wide range fission chambers placed outside the vacuum chamber, natural diamond detector based compact neutron monitors placed inside the channels of the neutron cameras, and a compact neutron generator for calibration. Microfission chambers could be installed in the standard plugs with other detectors (vacuum x-ray diode, magnetic probe). 235U could be used as well as threshold fission materials (238U, 237Np, 232Th). In the last case the fission chamber will be covered by a boron shield to reduce the changes in the sensitivity. Wide range fission chambers will operate in both pulse count mode and Campbell mode. High linearity is provided by count mode. Temporal resolution of 1 ms is provided by the count mode at low neutron flux and by the Campbell mode at high flux. The nonlinearity of the fission chamber during the switch from count mode to Campbell mode will be corrected by another fission chamber with low sensitivity operating in count mode. Compact neutron flux monitors placed inside neutron cameras will consist of up to ten natural diamond neutron counters with sensitivity to DT neutrons doubled by properly installed poliethilen radiators. Such monitors provide DT neutron flux profile measurements with dynamic range (three orders), temporal resolution (1 ms), and accuracy (10%). The full system could be calibrated by compact moveable neutron generator with neutron yield 1011 neutron/s, which operate in continuous mode. All elements of the system are commercially available, except for the neutron generator and diamond detector based monitors. The neutron generator and multidiamond detector based DT neutron monitors are now under development. An existing prototype of the neutron generator operates with a yield of one order less. Performance and major characteristics of the proposed systems of neutron flux monitoring will be discussed from the point of view of their application for neutron flux measurements and control of deuterium and deuterium–tritium phases of the ITER-FEAT operation.

Neutron monitor using microfission chambers for the International Thermonuclear Experimental Reactor

We are designing microfission chambers, which are pencil size gas counters with fissile material inside, to be installed in the vacuum vessel as neutron flux monitors for the International Thermonuclear Experimental Reactor (ITER). We computed the neutron and gamma flux around the shielding blanket by a two-dimensional neutron calculation, in order to find suitable locations for microfission chambers. We found that the U238 microfission chambers are not suitable because the detection efficiency will increase up to 50% during the ITER lifetime by breeding U239. We propose to install U235 microfission chambers on the front side of the back plate in the gap between adjacent blanket modules and behind the blankets at ten poloidal locations. One chamber will be installed in the divertor cassette, just under the dome. Employing both the pulse counting mode and Campbelling mode in the electronics, we can accomplish the ITER requirement of 107 dynamic range, with 1 ms temporal resolution, and eliminate the effect of gamma rays. An in-vessel neutron monitor will be affected by changes of the detection efficiency due to the change in the plasma position and neutron source profile. Here we demonstrate by neutron Monte Carlo calculation with three-dimensional modeling that we avoid those detection efficiency changes by installing microfission chambers at several poloidal locations inside the vacuum vessel.

Uranium-233 fission detectors for neutron flux measurement in reactors

Fission counters have been developed indigenously, using 233U as a sensing material for reactor startup applications. The first model has a counting sensitivity of 0.003 cps/nv in the pulse mode, 1×10−15 A/nv in the dc mode, and 6.4×10−29 A2 Hz/nv in the Campbell mode. A more sensitive counter with about 75 mg of 233U showed an average sensitivity of 0.02 cps/nv. A proposal for 233U in-core miniature dc fission chambers is discussed. A design for high sensitivity (0.5 cps/nv) U-233 pulse counter with transmission line-type readout is also proposed.

Absolute calibration of tokamak fusion test reactor neutron detectors for D-T plasma operation

The two most sensitive TFTR fission-chamber detectors were absolutely calibrated in situ by a D-T neutron generator (∼5×107 n/s) rotated once around the torus in each direction, with data taken at about 45 positions. The combined uncertainty for determining fusion neutron rates, including the uncertainty in the total neutron generator output (±9%), counting statistics, the effect of coil coolant, detector stability, cross calibration to the current mode or log Campbell mode and to other fission chambers, and plasma position variation, is about ±13%. The NE-451 (ZnS) scintillators and 4He proportional counters that view the plasma in up to 10 collimated sightlines were calibrated by scanning the neutron generator radially and toroidally in the horizontal midplane across the flight tubes of 7 cm diam. Spatial integration of the detector responses using the calibrated signal per unit chord-integrated neutron emission gives the global neutron source strength with an overall uncertainty of ±14% for the scintillators and ±15% for the 4He counters.

Chromosomal Gene Integration and Enhanced Xylanase Production in an Alkalophilic Thermophilic Bacillus Sp (NCIM 59)

Chromosomal integration and xylanase gene amplification were demonstrated for the first time in an alkalophilic thermophilic Bacillus sp. (NCIM 59). The integrants were characterized by larger zone of xylan clearance than the parent culture and hybridization with vector (pUC8) DNA. Repeated transformation strategy was used for further amplification of the xylanase gene. The results of Southern blot analysis indicated the occurance of homologous recombination in the 6.5 kb xylanase gene region of the genomic DNA and suggested a non campbell mode of recombination. The integrants were checked for xylanase production up to ten subcultures and consistently showed two fold higher xylanase activity than the parent strain with the maximum xylanase productivity (U/ml/h) at 16 h.

In situ calibration of TFTR neutron detectors

We report results of the TFTR fission detector calibration performed in December 1988. A NBS-traceable, remotely controlled 252Cf neutron source was moved toroidally through the TFTR vacuum vessel. Detection efficiencies for two 235U detectors were measured for 930 locations of the neutron point source in toroidal scans at 16 different major radii and vertical heights. These scans effectively simulated the volume-distributed plasma neutron source and the volume-integrated detection efficiency was found to be insensitive to plasma position. The Campbell mode is useful due to its large overlap with the count rate mode and large dynamic range. The resulting absolute plasma neutron source calibration has an uncertainty of ±13%.