Hironobu Unesaki

First Injection of Spallation Neutrons Generated by High-Energy Protons into the Kyoto University Critical Assembly

At the Kyoto University Research Reactor Institute, the world’s first injection of spallation neutrons generated by high-energy proton beams into a reactor core was successfully accomplished on March 4, 2009. By combining the fixed field alternating gradient (FFAG) accelerator with the A-core (Fig. 1) of the Kyoto University Critical Assembly (KUCA), a series of accelerator-driven system (ADS) experiments were carried out by supplying spallation neutrons to a subcritical core through the injection of 100MeV protons onto a tungsten target of 80mm diameter and 10mm thickness. In these experiments, the proton beams from the FFAG accelerator were generated at 30Hz repetition rate and 10 pA current. The neutron intensity generated at the tungsten target was around 1 10 s . The objective of these experiments was to conduct a feasibility study on ADS from the viewpoint of reactor physics, in order to develop an innovative nuclear reactor for a high-performance transmutation system with a capability of power generation or for a new neutron source for scientific research. The A-core employed in the ADS experiments was essentially a thermal neutron system composed of a highly enriched uranium fuel and a polyethylene moderator/reflector. In the fuel region, a unit cell is composed of a 93% enriched uranium fuel plate 1/1600 thick and polyethylene plates 1/400 and 1/800 thick. In these ADS experiments, three types of fuel rods designated as the normal, partial, and special fuels were employed. From the reason of the safety regulation for KUCA, the tungsten target was located not at the center of the core but outside the critical assembly, and the outside location was similar to that in previous experiments using 14MeV neutrons. As in previous ADS experiments with 14MeV neutrons, the introduction of a neutron guide and a beam duct is requisite to lead the high-energy neutrons generated from the tungsten target to the center of the core as much as possible. The detailed composition of the normal, partial, and special fuel rods, the polyethylene rod, the neutron guide, and the beam duct was described in Refs. 3–5). To obtain the information on the detector position dependence of the prompt neutron decay measurement, neutron detectors were set at three positions shown in Fig. 1: near the tungsten target (position (17, D); 1=200 BF3 detector) and around the core (positions (18, M) and (17, R); 100 He detectors). The prompt and delayed neutron behaviors (Fig. 2), which were an exponential decay behavior and a slowly decreasing behavior, respectively, were experimentally confirmed by observing the time evolution of neutron density in ADS. These behaviors clearly indicated that neutron multiplication was caused by an external source: sustainable nuclear chain reactions were induced in the subcritical core by spallation neutrons through the interaction of the tungsten target and the proton beams from the FFAG accelerator. In these kinetic experiments, subcriticality was deduced from the prompt neutron decay constant by the extrapolated area ratio method. The difference between the measured results of 0.74% k=k and 0.61% k=k at the positions (17, R) and (18, M) in Fig. 1, respectively, from the experimental evaluation of 0.77% k=k, which was deduced from the combination of both the control rod worth by the rod drop method and its calibration curve by the positive period method, was within 20%. Note that the subcritical state was attained by the full insertion of C1, C2, and C3 control rods into the core. The thermal neutron flux distribution was estimated through the horizontal measurement of the In(n, )In Atomic Energy Society of Japan Corresponding author, E-mail: pyeon@rri.kyoto-u.ac.jp Present address: SR Center, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu-shi, Shiga 527-8577, Japan Present address: Institute of Nuclear Safety System, Incorporated, 64, Sata, Mihama-cho, Mikata-gun, Fukui 919-1205, Japan Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 46, No. 12, p. 1091–1093 (2009)

Static and Kinetic Experiments on Accelerator-Driven System with 14MeV Neutrons in Kyoto University Critical Assembly

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 is expected to be around 3%Δk/k, the validity of both measurements and calculations has been demonstrated up to 6%Δk/k for the current KUCA cores. The prompt neutron decay constant is obtained by the pulsed neutron method, and subcriticality by the area ratio method, using an optical fiber detection system developed at KUCA. The optical fiber detector has been assessed as a promising original tool for the measurements of the subcriticality and prompt neutron decay constant, through ADS kinetic experiments with 14MeV neutrons.

Preliminary Experiments on Accelerator-Driven Subcritical Reactor with Pulsed Neutron Generator in Kyoto University Critical Assembly

A series of preliminary experiments on an accelerator-driven subcritical reactor (ADSR) with 14 MeV neutrons were conducted at Kyoto University Critical Assembly (KUCA) with the prospect of establishing a new neutron source for research. A critical assembly of a solid-moderated and -reflected core was combined with a Cockcroft-Walton-type accelerator. A neutron shield and a beam duct were installed in the reflector region for directing as large a number as possible of the high-energy 14MeV neutrons generated by deuteron-tritium (D-T) reactions to the fuel region, since the tritium target is located outside the core. And then, neutrons (14MeV) were injected into a subcritical system through a polyethylene reflector. The objectives of this paper are to investigate the neutron design accuracy of the ADSR with 14MeV neutrons and to examine experimentally the neutronic properties of the ADSR with 14MeV neutrons at KUCA. The reaction rate distribution and the neutron spectrum were measured by the foil activation method for investigating the neutronic properties of the ADSR with 14 MeV neutrons. The eigenvalue and fixed-source calculations were executed using a continuous-energy Monte Carlo calculation code MCNP-4C3 with ENDF/B-VI.2 for the subcriticality and the reaction rate distribution, respectively; the unfolding calculation was done using the SAND-II code coupled with JENDL Activation Cross Section File 96 for the neutron spectrum. The values of the calculated subcriticality and the reaction rate distribution were in good agreement with those of the experiments. The results of the experiments and the calculations demonstrated that the installation of the neutron shield and the beam duct was experimentally valid and that the MCNP-4C3 calculations were accurately carried out for analyzing the neutronic properties of the ADSR with 14MeV neutrons at KUCA.

Reactor Noise Experiments by Using Acquisition System for Time Series Data of Pulse Train

An acquisition system for the time series data of a pulse train was developed to perform the reactor noise experiments. Using this acquisition system, some important reactor kinetic parameters can be obtained by using various data analysis methods for the reactor noise experiments, which facilitates to achieve new experimental knowledge about characteristics of them proposed so far through direct comparison among the results of various methods. In the present study, the prompt neutron decay constant αpat two near delayed critical states of a core constructed in the Kyoto University Critical Assembly were measured by four well-known data analysis methods; the Feynman-α, the Rossi-α (Type-I and Type-II) and the frequency analysis methods. The results showed that ap values measured by the Type-I Rossi-α and the frequency analysis methods agreed with those by the Feynman-α method considering both effects of the delayed neutrons and the counting loss of neutron counter, whereas it was impossible to obtain the ...

General formulae for the Feynman-α method with the bunching technique

Abstract Recently, the bunching technique has been widely utilized in the Feynman-α experiment using a multi-channel scaler (MCS) to measure the prompt neutron decay constant α p . Although the bunching technique enables us to perform efficient experiments, it was pointed out that an inherent count-loss process arises due to the channel advance time Δ between adjacent MCS channels. Through derivation of a Feynman-α variance-to-mean formula containing Δ by means of the multi-gate Pal-Bell equation, Yamane and Hayashi ([Yamane, Y. & Hayashi, Y. 1995]. Annals of Nuclear Energy, 22(8), 533) indicated that this count-loss process does not play any important roles when the channel advance time is much smaller than the dwell time T. However, the Δ / T ratio often becomes large in thermal systems at deep-subcritical states or fast ones, because the dwell time should be chosen to be much smaller than reciprocals of α p values for such systems. On the other hand, since the ratio of the dead time d of neutron detectors to the dwell time becomes also large when the Δ / T ratio is not small, the count-loss process due to the dead time cannot be neglected. Therefore, Feynman-α variance-to-mean and covariance-to-mean formulae containing both Δ and d were derived by means of the compound detection probabilities. Based on the covariance-to-mean formula, a new experimental technique was developed and examined at the Kyoto University Critical Assembly. The result of the examination indicated that one can measure exact α p values when Δ / T ratios are known, even though Δ / T and d / T ratios are not small.

Application of Multiband Method to KUCA Tight-Pitch Lattice Analysis

The multiband method has been applied to analyses of critical experiments related to the high-conversion core at the Kyoto University Critical Assembly in order to accurately treat the resonance self-shielding in heterogeneous cells. Three-band parameters were generated using the self-shielding table installed in the SRAC code, and used to calculate the cell-averaged cross sections. The k ∞ values calculated by this method have been compared to those by the VIM Monte-Carlo calculation, the SRAC fine group calculation, Dancoff factor method and/or Tones method self-shielding calculation. The k∞ values calculated by the present method agree with those by the VIM calculation within 0.3%Δk for all the cases considered.

Determination of Subcritical Reactivity of a Thermal Accelerator-Driven System from Beam Trip and Restart Experiment

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.

Research Project on Accelerator-driven Subcritical System Using FFAG Accelerator and Kyoto University Critical Assembly

The Research Reactor Institute of Kyoto University started the KART (Kumatori Accelerator-driven Reactor Test facility) project in fiscal year 2002 under the Contract with the Ministry of Education, Culture, Sports, Science and Technology of Japan. The purpose of this research project is to demonstrate the basic feasibility of accelerator-driven system (ADS), studying the effect of incident neutron energy on the effective multiplication factor in a subcritical nuclear fuel system. For this purpose, a variable-energy FFAG (Fixed Field Alternating Gradient) accelerator complex is constructed and coupled with the Kyoto University Critical Assembly (KUCA). This paper presents the present status of the project and some of the results from the task performed up to Fiscal Year 2005.

TPD-based evaluation of near threshold mono-energetic proton energies for the 7Li(p,n)7Be production of neutrons for BNCT

An evaluation of mono-energetic proton energies ranging from 1.885 MeV to 1.920 MeV was carried out to determine the viability of these near threshold energies in producing neutrons for BNCT via the (7)Li(p,n)(7)Be reaction. Neutron fields generated at these proton energies were assessed using the treatable protocol depth (TPD) and the maximum TPD (TPD(max)) as evaluation indices. The heavy charged particle (HCP) dose rate to tumour was likewise applied as a figure of merit in order to account for irradiation time and required proton current. Incident proton energies closer to the reaction threshold generated deeper TPDs compared to higher energy protons when no boron dose enhancers (BDE) were placed in the irradiation field. Introducing a BDE resulted in improved TPDs for high proton energies but their achievable TPD(max) were comparatively lower than that obtained for lower proton energies. In terms of the HCP dose rate to tumour, higher proton energies generated neutron fields that yielded higher dose rates both at TPD(max) and at fixed depths of comparison. This infers that higher currents are required to deliver the prescribed treatment dose to tumours for proton beams with energies closer to the (7)Li(p,n)(7)Be reaction threshold and more achievable proton currents of around 10 mA or less for proton energies from 1.900 MeV and above. The dependence on incident proton energy of the TPD, TPD(max) and the HCP dose rate to tumour with respect to the (10)B concentration in tumour and healthy tissues were also clarified in this study. Increasing the (10)B concentration in tumour while maintaining a constant T/N ratio resulted in deeper TPD(max) where a greater change in TPD(max) was obtained for proton energies closer to the (7)Li(p,n)(7)Be reaction threshold. The HCP dose rates to tumour for all proton energies also went up, with the higher proton energies benefiting more from the increased (10)B concentration.

Accelerator driven subcritical system as a future neutron source in Kyoto University Research Reactor Institute (KURRI) —Basic study on neutron multiplication in the accelerator driven subcritical reactor—

A basic study on the nuclear characteristics in the accelerator driven subcritical reactor (ADSR) was performed through a series of neutronics calculations in view of a future neutron source in Kyoto University Research Reactor Institute (KURRI) for the joint use program among researchers of Japanese universities. In this series of calculations, it was assumed that three kinds of monoenergetic neutrons were isotropically generated at the center of spherical and homogeneous cores with different moderator-to-fuel volume ratios in order to examine the spectrum mismatching effect between injected neutrons and fission neutrons born in the subcritical core. The results of calculations clearly showed the spectrum mismatching effect on the neutron multiplication in the ADSR.