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In the previous study, the theoretical formulae of two time-domain reactor noise techniques, i.e., the auto-covariance and the variance-to-mean function techniques, were derived (Kitamura et al., 2018). These two techniques analyse the temporal fluctuation in continuous neutron detector current signals arising from ionization chambers such as the fission chamber. They are hence inherently insensitive to the count-loss effect that sometimes brings serious difficulties to the conventional time-domain techniques that analyse the number of detector pulse signals. With regard to these two techniques, the experimental conditions under which they successfully measure the subcriticality through determination of the prompt neutron decay constant were clarified. However, for mathematical simplicity and the sake of insight, the previous study was performed on the basis of a theoretical model neglecting delayed neutrons. The formulae of these two techniques are hence re-derived by taking delayed neutrons into consideration. Using the formulae thus derived, the delayed neutron effect in these two techniques is discussed.

The conventional time-domain reactor noise techniques analyse the number of neutron detector pulse signals, so that they sometimes encounter serious difficulties owing to the count-loss effect due to the dead time of detector systems. To avoid all the difficulties coming from the count-loss effect, a novel time-domain technique was recently proposed (Pal and Pazsit, 2015). This technique analyses the auto-covariance function of continuous current signals arising from ionization chambers such as the fission chamber, so that it is inherently insensitive to the count-loss effect. In the present study, a different time-domain technique that analyses the integral values of current signals is proposed. With regard to these two techniques, the experimental conditions under which they successfully measure the subcriticality through determination of the prompt neutron decay constant are clarified.

The rotation method is a novel method for detecting nuclear materials using a neutron source such as californium. In this method, while a neutron source is rotated rapidly nearby a measurement object, neutron measurement is carried out by synchronizing the rotation motion. If the object contains a nuclear material, as the rotation speed increases, the larger deformation of time distribution of neutron counts is observed, which in turn resulted to the detection of the nuclear material. In addition to its features of low cost and portability, this method is capable of detecting uranium that emits very few spontaneous fission neutrons. This study presents the fundamental principle of this method and its effectiveness for detecting nuclear materials through the experimental verifications.

A Neutron/Gamma-ray combined inspection system for hidden special nuclear materials (SNMs) in cargo containers has been developed under a program of Japan Science and Technology Agency in Japan. This inspection system consists of an active neutron-detection system for fast screening and a laser Compton backscattering gamma-ray source in coupling with nuclear resonance fluorescence (NRF) method for precise inspection. The inertial electrostatic confinement fusion device has been adopted as a neutron source and two neutron-detection methods, delayed neutron noise analysis method and high-energy neutron-detection method, have been developed to realize the fast screening system. The prototype system has been constructed and tested in the Reactor Research Institute, Kyoto University. For the generation of the laser Compton backscattering gamma-ray beam, a race track microtron accelerator has been used to reduce the size of the system. For the NRF measurement, an array of LaBr3(Ce) scintillation detectors has been adopted to realize a low-cost detection system. The prototype of the gamma-ray system has been demonstrated in the Kansai Photon Science Institute, National Institutes for Quantum and Radiological Science and Technology. By using numerical simulations based on the data taken from these prototype systems and the inspection-flow, the system designed by this program can detect 1 kg of highly enriched 235U (HEU) hidden in an empty 20-ft container within several minutes.

Recently, a modified Feynman-alpha technique for the subcritical system driven by periodically triggered neutron bursts was developed. One of the main features of this technique is utilization of a simple formula that is advantageous in evaluating the subcriticality. However, owing to the absence of the theory of this technique, this feature has not been fully investigated yet. In the present study, a theory of this technique is provided. Furthermore, the experimental conditions under which the simple formula works are discussed to apply this technique to the subcriticality monitor for the accelerator-driven system.

In the accelerator-driven system (ADS), the effective delayed neutron fraction (βeff) is a requisite for converting the subcriticality from dollar units to pcm units. To evaluate the accurate βeff value in ADS, measurement of βeff complements its calculation methodology and the nuclear data on delayed neutrons. Subcriticality measurements are carried out by the pulsed neutron source method in the Kyoto University Critical Assembly, and the neutron noise analyses are conducted by the Rossi-α method with the pulsed shape of the spallation neutron source. The value of βeff is deduced with the combined use of measured subcriticality in dollar units and correction parameters by MCNP6.1 together with JENDL-4.0 and JENDL/HE-2007. A comparison between the calculated and the measured βeff represents the acceptable accuracy within the subcriticality range of around keff = 0.93 in the ADS operations. Here, the applicability of the measurement methodology based on the Rossi-α method is demonstrated by varying...

In an accelerator-driven system (ADS), the effective delayed neutron fraction βeff is required for subcriticality measurements in order to convert measured reactivity in dollar units into pcm units...

A wide-range neutron flux measurement instrument is developed herein for monitoring the total neutron emission rate and yield of the Large Helical Device (LHD) during deuterium experiments implemented from March 2017 in the National Institute for Fusion Science (NIFS), Japan. The instrument is designed for and installed on the Neutron Flux Monitoring (NFM) system, which measures the counting rate using a 235U Fission Chamber. By combining the pulse counting and Campbell methods, the Digital Signal Processing Unit (DSPU) realized a wide dynamic range of over six orders of magnitude from 1 × 103 counts/s (cps) to 5 × 109 cps. This study explains and discusses how the instrument is developed, including topics from the predevelopment activities to the verification test at the Kyoto University Critical Assembly (KUCA). Experimental results in the LHD using the finished products suggest that the NFM system works well during deuterium experiments.

The Feynman-alpha method has been frequently employed to measure the subcriticality of a neutron multiplying system through determination of the prompt neutron decay constant. This method, however, often encounters a difficulty with unstable multiplying systems where the neutron population is temporally varying and drifting owing to perturbations due to removal or replacement of fuels and absorbers. In the unstable multiplying systems, the delayed-neutron contribution is remarkably enhanced and makes it difficult to extract the prompt neutron decay constant from the prompt-neutron contribution. The linear difference filter methods that are variants of the Bennett variance one were hence developed to mitigate this kind of difficulty by reducing the delayed-neutron contribution. In the present study, further new variants are proposed. It is shown that the present methods with correlation indices consisting of more than three successive gates can reduce the delayed-neutron contribution more effectively than the conventional ones.

We have proposed a new selective isotope transmutation method using photonuclear reactions with quasi-monochromatic γ-ray beams. This method is based on the fact that the particle threshold of a long-lived fission product (LLFP) such as 93Zr, 107Pd, or 79Se is lower than those of stable isotopes of the same chemical element. Therefore, this method has the excellent advantage that LLFPs cannot, in principle, be produced newly even if the target materials include stable isotopes in addition to LLFPs. Furthermore, this method is effective for 126Sn, 135, 137Cs, 90Sr, and 3H. The nuclear data involved and suitable γ-ray sources are discussed. Laser Compton scattering γ-ray sources and neutron capture γ-rays in nuclear reactors are candidates for this method.

Experiments on the thorium-loaded accelerator-driven system (ADS) were carried out at the Kyoto University Critical Assembly to reevaluate the accuracy of reactor physics parameters, including prom...

The Simmons-King and the Sjostrand (or the area-ratio) methods have been widely applied in measurement of the subcriticality of reactor systems driven by the pulsed neutron source and operated in the stationary state. In the present study, a theory-based investigation is conducted to examine the time-response of these two methods after perturbations in various parameters. As a result, the prompt neutron decay constant determined by the Simmons-King method shows a good trackability to the subcriticality. In the Sjostrand method, calibration-free determination of the subcriticality is achieved, although it needs a long delay due to an asymptotic behaviour. Furthermore, it is found that the latter method can quickly detect the perturbation in neutron yield of the pulsed neutron source. Therefore, it is expected that a strong on-line monitoring tool for the subcritical reactor system driven by the pulsed neutron source can be developed by combining these two methods.

Detection of hidden special nuclear materials (SNMs) used for nuclear explosives such as 235 U is important issue for nuclear security to counter terrorist threats. The interrogation systems used in a port and an airport has been developing in the world, and the active neutron-based interrogation system is the one of the candidates for this purpose. We are developing an active neutron-based interrogation system combined with radiation detectors and a D-D neutron source usable in seaports and airports. The D-D neutron source shown in Figs. 1 and 2 is a compact and light-weight portable discharge-type fusion neutron source called IECF (Inertial Electrostatic Confinement Fusion) device [1] . It provides 2.45 MeV mono-energetic neutrons whose production rate is more than 5x10 7 n/s in CW mode without using radioisotope such as tritium. An mportant advantage of IEC comes from the use of "gas target" and this enables stable high-power operation of IEC devices to produce copious amount of D-D neutrons in a compact system. We adopted new Threshold Energy Neutron Analysis (TENA) method and neutron and gamma-ray noise analysis method based on the variance-to-mean value method in the present interrogation system.

At the Kyoto University Critical Assembly, a series of reactor physics experiments on the accelerator-driven system (ADS) coupled with the fixed-field alternating gradient (FFAG) accelerator are ca...

For developing an accelerator-driven system (ADS), many studies, such as those on accelerator development, subcritical system neutronics, materials irradiation effects, and thermal hydraulics, are required. In the Research Reactor Institute at Kyoto University (KURRI), ADS is planned as a future neutron source. Recent activities of relevant ADS studies at KURRI are introduced herein.

Experimental studies on the thorium-loaded accelerator-driven system (ADS) were conducted at the Kyoto University Critical Assembly. Mockup experiments were carried out in both the critical and sub...

Since the early 1960s, many studies on criticality safety evaluation have been conducted in Japan. Computer code systems were developed initially by employing finite difference methods, and more recently by using Monte Carlo methods. Criticality experiments have also been carried out in many laboratories in Japan as well as overseas. By effectively using these study results, the Japanese Criticality Safety Handbook was published in 1988, almost the intermediate point of the last 50 years. An increased interest has been shown in criticality safety studies, and a Working Party on Nuclear Criticality Safety (WPNCS) was set up by the Nuclear Science Committee of Organisation Economic Co-operation and Development in 1997. WPNCS has several task forces in charge of each of the International Criticality Safety Benchmark Evaluation Program (ICSBEP), Subcritical Measurement, Experimental Needs, Burn-up Credit Studies and Minimum Critical Values. Criticality safety studies in Japan have been carried out in cooperat...

A series of power spectral analyses for a thermal subcritical reactor system driven by a pulsed 14 MeV neutron source was carried out at Kyoto University Critical Assembly (KUCA), to determine the prompt-neutron decay constant of the accelerator-driven system (ADS). The cross-power spectral density between time-sequence signal data of two neutron detectors was composed of a familiar continuous reactor noise component and many delta-function-like peaks at the integral multiple of pulse repetition frequency. The prompt-neutron decay constant inferred from the reactor noise component of the cross-power spectral density was consistent with that obtained by a pulsed neutron experiment. However, the reactor noise component of the auto-power spectral density of each detector was hidden by a white chamber noise in the higher-frequency range and this feature resulted in a considerable underestimation of the decay constant. For several runs with a low pulse-repetition frequency, furthermore, we attempted to infer t...

The Kyoto University Research Reactor Institute is engaged in research on an accelerator-driven system (ADS) using the fixed field alternating gradient accelerator at the Kyoto University Critical Assembly. With the aim of achieving the high-energy neutrons anticipated in the actual ADS experiments, we propose the use of a new target concept (the multi-target) in this study. The conceptual design of the multi-target is numerically carried out, and the high-energy neutrons generated by the injection of high-energy protons at the multi-target are successfully measured using a liquid organic scintillator in the experiments. The conceptual design and effectiveness of the multi-target is confirmed by the obtained neutron spectrum and neutron yield in both the experimental and calculated results.

Neutron spectrum analyses of spallation neutrons are conducted in the accelerator-driven system (ADS) facility at the Kyoto University Critical Assembly (KUCA). High-energy protons (100 MeV) obtained from the fixed field alternating gradient accelerator are injected onto a tungsten target, whereby the spallation neutrons are generated. For neutronic characteristics of spallation neutrons, the reaction rates and the continuous energy distribution of spallation neutrons are measured by the foil activation method and by an organic liquid scintillator, respectively. Numerical calculations are executed by MCNPX with JENDL/HE-2007 and ENDF/B-VI libraries to evaluate the reaction rates of activation foils (bismuth and indium) set at the target and the continuous energy distribution of spallation neutrons set in front of the target. For the reaction rates by the foil activation method, the C/E values between the experiments and the calculations are found around a relative difference of 10%, except for some reactions. For continuous energy distribution by the organic liquid scintillator, the spallation neutrons are observed up to 45 MeV. From these results, the neutron spectrum information on the spallation neutrons generated at the target are attained successfully in injecting 100 MeV protons onto the tungsten target.

Static and kinetic experiments on an accelerator-driven system have been conducted at the Kyoto University Critical Assembly. A small neutron detector using wavelength shifting fiber that can be inserted into a narrow space has been developed for measuring static and kinetic parameters. This study has found that the new detector type can be applied to kinetic experiments such as subcriticality measurements with the use of a D–T pulsed neutron source.

In this study, we characterized the Cerenkov radiation generated from optical fibers as fundamental research for the development of a fiber-optic Cerenkov radiation sensor to verify spent fuel. We measured the amount of Cerenkov radiation according to OH content and numerical aperture of the optical fiber. Also, we measured the Cerenkov radiation as functions of the length and the diameter of the optical fiber. As a result, the silica optical fibers with high numerical aperture and high OH content were efficient and stable for gathering the Cerenkov radiation generated in the optical fiber. Also, the intensity of the Cerenkov radiation generated in the optical fiber was proportional to the irradiated length and to the square of the irradiated diameter.

Reaction rate experiments on the accelerator-driven system (ADS) are conducted by combining a critical assembly of a solid-moderated and -reflected core with a pulsed neutron generator. Neutrons (14 MeV) generated from the accelerator are injected into a subcritical system and the reaction rates are measured by the foil activation method to obtain neutronic spectrum data. The numerical calculations are executed by MCNPX with ENDF/B-VI.8, JENDL-3.3 and JENDL/D-99 libraries to evaluate the reaction rates of activation foils set in the center of the core. For the ADS experiments with 14 MeV neutrons, the C/E values between the experiments and the calculations are found to be well within the relative difference of about 30% in all foils up to subcriticality 1.05%Δ k / k . The reaction rates do not depend on the subcriticality level in cases of 115 In, 56 Fe (purity 99.99%), 27 Al, whereas subcriticality dependence is observed in 93 Nb. In the critical experiments carried out in the A, B and C cores, special mention should be made of the remarkable effect of the composition rate of 56 Fe material. Thus a remarkable improvement is observed in the accuracy of experimental and numerical reaction rates, demonstrating the importance of material impurity for subcritical experiments.

The effects of silicon cross section and neutron spectrum on the radial uniformity of a Si-ingot are examined experimentally with various neutron spectrum conditions. For the cross section effect, the numerical results using silicon single crystal cross section reveal good agreements with experiments within relative difference of 6%, whereas the discrepancy is approximately 20% in free-gas cross section. For the neutron spectrum effect, the radial uniformity in hard neutron spectrum is found to be more flattening than that in soft spectrum.

Significant axial variation of radial uniformity is observed in Si-ingot neutron transmutation doping in the flux screening method, and leads to non-uniform resistivity distribution for a certain part of Si-ingot. This axial variation of radial uniformity is caused by the installation of a partial neutron screen which decreases the reaction rates differently in the center and surface at the region not surrounded by the partial neutron screen. For the improvement of the specific distribution of radial uniformity in the axial direction, a new concept of axial reflector is introduced to partly change the reaction rate at a certain region of Si-ingot, and neutron irradiation experiments are carried out at the heavy water neutron irradiation facility in the Kyoto University Research Reactor. Based on the experimental and numerical results, the new axial reflector is proved to be effective for improving the axial variation of radial uniformity.

At the Kyoto University Critical Assembly (KUCA), spallation neutrons generated by high-energy proton beams are injected into the thorium-loaded systems on March 2010. By combining the Fixed Field Alternating Gradient (FFAG) accelerator with the thorium-loaded system at KUCA, a series of the ADS experiments is carried out under conditions whereby the spallation neutrons are generated at a tungsten target by 100 MeV protons at an intensity of 30 pA. Prompt neutron behavior in the time evolution is observed and thorium fission reactions are attained through the experiments and calculations, respectively. And the effects of neutron leakage and spectrum softening are experimentally observed through the neutron multiplication and reaction rate analyses. From the experimental and numerical analyses, in the future, experimental conditions need to be improved to attain further neutron multiplication using the variation of fuels (thorium, highly-enriched and natural uranium) and moderators (graphite, polyethylene, aluminum and beryllium).

In a series of Feynman-α correlation measurements for a thermal Accelerator-Driven System (ADS) with 14MeV neutrons at the Kyoto University Critical Assembly (KUCA), an unstable accelerator condition such as a drift of beam current has been frequently observed. Neutron source instability caused by such unavoidable beam-current instability resulted in a divergent variance-to-mean ratio and, consequently, the correlation analysis failed. Nevertheless, we attempted to apply a difference-filtering technique to the correlation analysis to reduce the influence of the above instability. The present attempt resulted in consistent prompt-neutron decay constants with those obtained in a previous pulsed neutron experiment. The application of the filtering is expected to enhance the robustness of Feynman-α analysis against various instabilities of accelerator operation in actual ADS.

The usefulness of the measurements of the backscattering neutron and 2.22 MeV capture γ-ray from hydrogen in the landmine detection method is described in this paper. When the soil moisture content is increased, the reaction rates of both the neutron scattering reaction and capture reaction are increased. However, the backscattering neutrons are more influenced than the capture γ-rays by the soil moisture before the reaction with the detector. The facts that the backscattering neutron method is useful in the dry soil case and that the capture γ-ray method is effective in well-wet soil case are confirmed by the experiments and the calculations. The landmine detection efficiency is improved in various soil moisture conditions by combining the backscattering neutron method together with the capture γ-ray method. The effectiveness of the pulse mode operation was confirmed numerically.

An experimental technique based on an accelerator-beam trip or restart operation is proposed to determine the subcritical reactivity of an accelerator-driven system (ADS). Applying the least-squares inverse kinetics method to the data analysis, the subcriticality can be inferred from time-sequence neutron count data after these operations. A series ofbeam trip and restart experiments with 14 MeV neutrons were carried out in a thermal ADS of Kyoto University Critical Assembly (KUCA), to demonstrate the applicability of the proposed technique. The subcriticalities evaluated using neutroncounters far from the DT target were consistent with those obtained in a previous pulsed neutron experiment. However, a counter placed close to the target significantly overestimated the subcriticality. The present technique is expected to be available for subcriritcality measurement at startup and shutdown of various ADSs.

6LiF small neutron detectors with an optical fiber have been used to measure 6Li(n,α)T reaction rate distributions at thermal research reactors and accelerator facilities. In the present study, we developed an experimental method for the measurement of tritium production rate (TPR) of 6Li using this small detector in deuterium-tritium (D-T) neutron fields. Reaction rate measurements with the detector were conducted in the D-T neutron fields at the Fusion Neutronics Source (FNS) facility. From the results, we determined that this detector can be used to measure the TPR distribution in soft neutron spectrum fields such as in a Be assembly. It is difficult to obtain 6Li(n,α)T reaction rate separately in hard neutron spectrum fields such as in a Li2O assembly, because many kinds of charged particle production reactions need to be taken into consideration. However, a time-dependent reaction rate measurement method combined with the 6LiF detector and the ZnS detector is effective to separate the 6Li(n,α)T react...

To investigate the characteristics of a reactor and a neutron generator, a small scintillation detector with an optical fiber with ThO2 has been developed to measure fast neutrons. However, experimental facilities where 232Th can be used are limited by regulations, and S/N ratio is low because the background counts of this detector are increase by alpha decay of 232Th. The purpose of this study is to develop a new optical fiber detector for measuring fast neutrons that does not use nuclear material such as 232Th. From the measured and calculated results, the new optical fiber detector which uses ZnS(Ag) as a converter material together with a scintillator have the highest detection efficiency among several developed detectors. It is applied for the measurement of reaction rates generated from fast neutrons; furthermore, the absolute detection efficiency of this detector was obtained experimentally.

In this paper, we describe a detector system for finding land mines in the soil using 10.83 and 2.22 MeV capture γ-ray detections by neutron capture reactions with nitrogen and hydrogen, respectively. There are certain difficulties in land mine detection, including high background radiation levels during measurement and a low detection efficiency of high-energy γ-rays. To overcome these difficulties, a methodology involving simultaneous measurements using anticoincidence and coincidence methods and a combination of BGO and NaI(Tl) scintillators is proposed. The background radiation level is suppressed using this methodology and the detection efficiency of high-energy γ-rays is improved. Consequently, it should be possible to detect land mines buried in the soil with a high efficiency.

The neutron multiplication parameters: neutron multiplication M , subcritical multiplication factor k s , external source efficiency φ * , play an important role for numerical assessment and reactor power evaluation of an accelerator-driven system (ADS). Those parameters can be evaluated by using the measured reaction rate distribution in the subcritical system. In this study, the experimental verification of this methodology is performed in various ADS cores; with high-energy (100 MeV) proton–tungsten source in hard and soft neutron spectra cores and 14 MeV D–T neutron source in soft spectrum core. The comparison between measured and calculated multiplication parameters reveals a maximum relative difference in the range of 6.6–13.7% that is attributed to the calculation nuclear libraries uncertainty and accuracy for energies higher than 20 MeV and also dependent on the reaction rate distribution position and count rates. The effects of different core neutron spectra and external neutron sources on the neutron multiplication parameters are discussed.

At the National Cancer Center, proton irradiation experiments have been conducted using the cyclotron accelerator to measure the neutron spectrum by the foil activation method. The experimental results demonstrate that high-energy neutrons are bombarded by injecting high-energy protons onto the tungsten target. The agreement between the results of the experiments and the MCNPX calculations with ENDF/ B-VI is around 20% in the relative difference of the C/E (calculation/experiment) values in some of the 209 Bi(n, xn) 210-x Bi reactions. Here, the activation foil 209 Bi is useful in obtaining experimental neutron spectrum information on nuclear spallation reactions of the tungsten target by the high-energy protons ranging between 150 and 235 MeV. The current experimental data could contribute to evaluating the accuracy of the numerical simulation methodology of reaction rates and the uncertainties of cross sections of 209 Bi, as experimental benchmarks.

The subcritical multiplication factor ks and the external neutron source efficiency φ ∗ are important parameters in the accelerator-driven system (ADS) performance assessment. The theoretical relation between ks and the effective multiplication factor keff in a subcritical system is discussed in different cases of subcritical system. On the basis of the theoretical background, the dependence of ks and φ ∗ on subcriticality, source position, and energy is numerically investigated using a simple thermal subcritical model. For the sake of experimental evaluation of ks and φ ∗ , the ADS experiments have been carried out in the subcritical systems combined with 14 MeV pulsed neutrons of the Kyoto University Critical Assembly (KUCA). The ks and φ ∗ parameters are successfully measured by utilizing the reaction rate distribution obtained by the optical fiber detectors in the subcritical system, within a relative difference of less than 7% and 12% for ks and φ ∗ , respectively, between measured and calculated values for most studied cases.

Recently, the neutron irradiation for large diameter silicon (Si)-ingots of more than 8" diameter is requested to satisfy the demand for the neutron transmutation doping silicon (NTD-Si). By increasing the Si-ingot diameter, the radial non-uniformity becomes larger due to the neutron attenuation effect, which results in a limit of the feasible diameter of the Si-ingot. The current evaluation method has a certain limit to precisely evaluate the radial uniformity of Si-ingot because the current evaluation method does not consider the effect of the Si-ingot diameter on the radial uniformity. The objective of this study is to propose a new evaluation method of radial uniformity by improving the conventional evaluation approach. To precisely predict the radial uniformity of a Si-ingot with large diameter, numerical verification is conducted through comparison with the measured data and introducing the new evaluation method. A new concept of a gradient is introduced as an alternative approach of radial uniformity evaluation instead of the radial resistivity gradient (RRG) interpretation. Using the new concept of gradient, the normalized reaction rate gradient (NRG) and the surface normalized reaction rate gradient (SNRG) are described. By introducing NRG, the radial uniformity can be evaluated with one certain standard regardless of the ingot diameter and irradiation condition. Furthermore, by introducing SNRG, the uniformity on the Si-ingot surface, which is ignored by RRG and NRG, can be evaluated successfully. Finally, the radial uniformity flattening methods are installed by the stainless steel thermal neutron filter and additional Si-pipe to reduce SNRG.

In the subcritical nuclear reactor, the subcriticality is closely related with the neutron flux distribution. Since the relationship between them is complex, the subcriticality measurement by conventional methods involves some difficulties, particularly when the subcriticality is large. A new subcriticality measurement method is proposed for the steady subcritical state. By this method, the relative distribution of the change in neutron flux between before and after the perturbation is numerically calculated. On the other hand, the amplitude of the perturbation is estimated from the count rates of neutron detectors. By this combination of the calculation and experiment, the subcriticality can be accurately estimated, to the extent that the perturbation is qualitatively known beforehand. One of the advantages of this method over the conventional modified neutron source multiplication method is that numerical calculations are only executed on unperturbed states. In addition, the completeness of the eigenfunctions is not assumed. The validity of this method is confirmed through the numerical calculation.

Basic experiments are carried out in the Kyoto University Critical Assembly (KUCA) to study the nuclear characteristics of the accelerator-driven systems (ADS) and to establish a new neutron source for research. A method for measuring the neutron multiplication in the subcritical system is proposed by utilizing the reaction rate distribution obtained by the optical fiber detector. A comparison between the measured and calculated neutron multiplication shows agreement within a relative difference of 5% in most cases, through introducing certain correction factors.

Basic research on the Accelerator-Driven System (ADS) with thermal neutron spectrum has been promoted by the Kyoto University Research Reactor Institute. At the Kyoto University Critical Assembly (KUCA), various experiments on thermal ADS with a pulsed spallation source are planned. In such an ADS, neutron flux distribution may be sensitive to the injection of neutrons, and the high sensitivity results in various spatial effects. In this study, a pulsed neutron experiment with 14MeV neutrons was carried out in a thermal ADS of KUCA, to determine the λ-mode eigenvalue separation, which is a quantitative indication of spatial effects. An original data-processing technique was applied to infer prompt-neutron decay constants of fundamental and higher modes from neutron count decay data, and then the eigenvalue separation around 13%Δk/k was obtained from these decay constants.

Neutron spectrum experiments on the Accelerator-Driven System (ADS) are conducted by combining a critical assembly of a solid-moderated and solid-reflected core with a Cockcroft-Walton-type accelerator. Neutrons (14 MeV) generated from the accelerator are injected into a subcritical system and the reaction rates are measured by the foil activation method to obtain neutronic spectrum data. The numerical calculations are executed by MCNP-4C3 with JENDL-3.3 and JENDL/D-99 libraries to evaluate the reaction rates of activation foils set in the core center and at the target. In the present study, the measured and calculated (JENDL/D-99) reaction rates in all the activation foils at the target reveal around a difference of 10% in C/E values, while a bigger discrepancy between the results of the experiments and the calculations is observed in the center of the core. On the other hand, a special mention is made of the fact that the reaction rate analyses for the neutron spectrum in the subcritical systems demonst...

In preparation for connecting a Fixed Field Alternating Gradient (FFAG) accelerator atthe Kyoto University Critical Assembly (KUCA), static and kinetic experimental and numerical analyses of Accelerator-Driven System (ADS) are envisaged. Improvement of source multiplication by the implementation of a neutron guide is expected to affect the characteristics of the ADS core. The effects on reactivity and neutron flux distribution are investigated by measurements of the subcriticality and the reaction rate distributions of indium wire, respectively, using the KUCA core and the current 14MeV pulsed neutron generator. A comparison of the results of static measurements with those of calculations provides an estimation of MCNP-4C3 precision with JENDL-3.3 and ENDF/B-VI.2 for the basic research on ADS with 14 MeV neutrons at KUCA. The neutron guide has proved to be experimentally effective in improving external source when it is extended into the fuel region. Since the reactivity range of the actual ADS operation ...

A fast reactor cycle scheme that incorporates a thoria-based minor actinide-containing cermet fuel is given. The present cermet fuel consists of an oxide solid solution of Th and minor actinides and Mo-inert matrix. It has been proposed as a high-performance device that can enhance minor actinide incineration in a fast reactor cycle. It is used in an independent small sub-cycle, whereby dedicated cycle technologies are adopted. Two-step reprocessing process was proposed for the present cermet fuel; it consists of a pre-removal of Mo-inert matrix and an actinide recovery. A preliminary test for the pre-removal of Mo-inert matrix was carried out using a surrogate cermet fuel. Burnup characteristics of a fast reactor core loaded with the cermet fuel were investigated by using neutronic calculation codes. It was revealed that a heterogeneous composition of Mo-inert matrix in inner and outer cores may lead to an effective transmutation of minor actinides and a flattened power density. It was concluded that the present cermet fuel was potentially promising as a high-performance incineration device of minor actinides for fast reactors.

Current results are described on the research and development of the advanced humanitarian landmine detection system by using a compact discharge-type fusion neutron source called IECF (Inertial-El...

Current results are described on the research and development of the advanced humanitarian landmine detection system by using a compact discharge-type fusion neutron source called IECF (Inertial-Electrostatic Confinement fusion) devices. With a 50 mm-thick water-jacketed IEC device (IEC20C) of a 200 mm inner diameter, it can produce 107 neutrons/s stably in CW mode for 80 kV and 80 mA. Ample 10.8 MeV γ-rays produced through (n, γ) reaction with nitrogen atoms in the melamine (C3H6N6) powder (explosive simulant) are clearly measured by a BGO-NaI-combined scintillation sensor with distinct difference in cases with and without melamine. This proves feasibility of the identification of the buried landmines.