Fast Neutron Source Reactor YAYOI Research Articles

Analysis of the propagation of neutrons and gamma-rays from the fast neutron source reactor YAYOI

The skyshine effect is crucial for designing appropriate shielding. To investigate the skyshine effect, the propagation of neutrons was measured and analyzed at the fast neutron source reactor YAYOI. Pulse height spectra and dose distributions of neutron and secondary gamma-ray were measured outside YAYOI, and analyzed with MCNP-5 and JENDL-3.3. Comparison with the experimental results showed good agreement. Also, a semi-empirical formula was successfully derived to describe the dose distribution. The formulae can be used to predict the skyshine effect at YAYOI, and will be useful for estimating the skyshine effect and designing the shield structure for fusion facilities.

Fast neutron shielding and neutron transport studies at YAYOI

The fast neutron shielding experiments at the fast neutron source reactor YAYOI of the University of Tokyo were summarized. The experiments were grouped into the following four types; 1. (1)|experiments for shield design of fast breeder reactor, 2. (2)|fast neutron transmission experiments, 3. (3)|fast neutron streaming experiments, 4. (4)|neutron skyshine experiment. The experimental data are useful for shield design and assessing the accuracy of the calculational methods. The accuracy of numerical solution of the neutron transport equation by the discrete ordinates method were analysed. The effect of the uncertainty of the neutron cross sections and approximations in the computational method on the errors were assessed. The errors of multigroup approximation was dominant in the iron shield calculation.

Fast Neutron Radiography at reactor YAYOI

Fast Neutron Radiography (FNR) has been studied with use of the fast neutron source reactor YAYOI of the University of Tokyo. Three kinds of imaging techniques have been applied to the FNR. A CR39 nuclear track detector has been used successfully in the first FNR experiments at YAYOI. This is called as a CR39 track-etch method. The etched out images on the CR39 sheet usually have a low optical density. In order to perform an imaging technique with high resolving power and high contrast, a combination method of a thin type of luminescent converter and X-ray film (a film method) has been developed for the FNR. The converter is made of mixture of PMMA and ZnS(Ag), the thickness of which is 40 μ m. The system could observe the gap of 100 μ m. In the procedure with use of the CR39, it took long time, about 24 hours, to get images. And it took about 3 hours to get imaging results by the film method. A fast neutron television method (FNR-TV) was proposed for shortening the time of taking the images and for taking real time images. The same technique as a current thermal neutron television system was applied to the system while different luminescent converter was used. The prototype converter was made of mixture of polypropylene resin and ZnS(Ag). Static images and real time images were obtained successfully by the fast neutron television method.

Measurement of decay heat of fast neutron fission products

Experimental study on decay heat of fast neutron fission products using the fast neutron source reactor YAYOI at the University of Tokyo is reviewed in this paper. The decay heat was measured on 233U, 235U, 239Pu, 238U and 232Th using beta- and gamma-spectroscopic method. Their results were compared with other data and results of summation calculations using some fission product decay data files, and also have been used in the benchmark experiments for the improvement of fission product decay data files.

Characterization of the YAYOI fast neutron radiography field

Fast Neutron Radiography (FNR) has been studied with the use of the fast neutron source reactor YAYOI of the University of Tokyo. Three kinds of imaging techniques have been applied to the FNR. A CR39 nuclear track detector called the track-etch method has been successfully used in the first FNR experiments at YAYOI. The etched out images on the CR39 sheet usually have a low optical density, and took about 24 hours to get images. In order to perform an imaging technique with a high resolving power and high contrast, a thin type of luminescent converter and X-ray film method (film method) has been developed for the FNR. The converter is made of a mixture of PMMA and ZnS(Ag). the thickness of which is 40 μm. The system could observe a gap of 100 μm, and took about 3 hours to get results. To make the required imaging time shorter, a television system of fast neutron radiography (FNR-TV) was proposed. In the system, the same technique as a current thermal neutron television system was applied while using a different luminescent converter. The prototype converter was made of a mixture of polypropylene resin and ZnS(Ag). A method of determining L and D was proposed in this paper. Experimental result of L agreed with the geometric dimension of L, while the value of D will be discussed in another paper.

Correction of fast neutron scattered components from fast neutron radiography images

Fast Neutron Radiography (FNR) results have been successfully obtained by the use of the fast neutron source reactor YAYOI of the University of Tokyo. However, an effect of contrast distortion in the FNR images was observed. The intensity of scattered fast neutrons at the center of an object image was analyzed with a simple model of an iron cylindrical sample. Analytical results agreed well with experimental results.

A study on the mechanism model of luminescence in fast neutron television converter

For a real time imaging technique using a fast neutron radiography (FNR), a fast neutron television system (FNR-TV) was studied using the fast neutron source reactor YAYOI at the University of Tokyo. In order to obtain a good quality image with the FNR-TV, it is essential to develop a high sensitive converter to detect fast neutrons. A sensitive prototype converter made of polypropylene (PP) resin and ZnS(Ag) mixture was recently developed. It was called a PP converter. In this paper, the mechanism models of luminescence for fast neutrons and gamma-rays in the PP converter are presented as well as the comparison with the experimental results. The analytical results agree well with the experimental results.

A study on the development of a fast neutron television converter

Experiments on fast neutron radiography (FNR) were carried out using the fast neutron source reactor YAYOI of the University of Tokyo. For a real-time imaging technique using FNR, for a fast neutron television system (FNR-TV), it is essential to develop a highly sensitive luminescent converter in order to obtain good quality images. A new converter was recently developed and tested. It showed an about six times higher sensitivity compared with the first prototype converter. In this paper, the characteristics of the converter are discussed as well as the clearer images of a FNR-TV when using the converter. The FNR-TV images were obtained with 500 frames integrated. A small hole (1.0 mm in diameter and 10 mm in depth) in an acrylic plate was well imaged through a 50 mm thick iron block. A 0.15 mm thick aluminum spacer in the calibrated fuel pin was also discernible.

Television Imaging System for Fast Neutron Radiography Using Baby Cyclotron.

Abstract A television imaging system for fast neutron radiography (FNR-TV) developed using the fast neutron source reactor YAYOI was applied to the baby-cyclotron based fast neutron source to get images of thick objects quickly. In the system the same technique as a current television imaging system of thermal neutron radiography was applied, while the luminescent converter was used to detect fast neutrons. Using the CR39 track etch method it took about 7 h to get an image, while the FNR-TV only 20 s enough for taking the same object. However the FNR-TV imaging result of the simulation model of a large explosive device for the space launch vehicle of H-2 type was not so good as the image taken with the CR39 track etch method. The reason was that the luminescence intensity of the FNR-TV converter was a quarter of that in the YAYOI.

Fission-product decay heat for fast-neutron fissions of U-238 and Th-232

Abstract Beta- and gamma-ray decay heat of fission products caused by fast neutron fissions of U-238 and Th-232 was measured using spectroscopic method at the fast neutron source reactor YAYOI. Time dependent energy spectra of beta- and gamma-rays from the samples of U-238 and Th-232 were separately obtained using scintillation spectrometers. The spectral data were processed in the form of the pulse function of the decay heat. The measured decay heat was found to contain the contribution from neutron capture products of U-238 or Th-232. The contamination was calculated and subtracted from the measured data. The results showed good agreement with the calculations based on JNDC 1.5 File except for the beta-ray decay heat of Th-232.

Neutron streaming through narrow and long annulus.

Neutron streaming in a very narrow and long stainless steel annulus which was 3 mm wide and 2,480 mm long, was measured by TLDs and a neutron dose rate meter at the fast neutron source reactor YAYOI. The attenuation was very strong near the entrance and became very weak beyond 900 mm. The weak attenuation was due to the streaming of neutrons through the annulus. The two-dimensional discrete ordinates transport calculation with a forward biased S418 quadrature set failed to predict the attenuation beyond the point where the most-forward ray from the entrance intersected the wall of the annulus. It was practically impossible to analyze the streaming problem by the Sn code with more detailed Sn quadratures. The analytic expression describing the direct component agreed fairly well with the experiment far from the entrance of the annulus.

｀２３５´Ｕ，｀２３９´Ｐｕおよび｀２３３´Ｕの高速中性子核分裂による核分裂生成物からのγ線崩壊熱の測定

Gamma decay heat released from fission products (FPs) has been measured for fast-neutron fissions of 235U, 239Pu and 233U using the radiation spectrometry method. The sample irradiations were for 10 and 100s in the fast neutron source reactor YAYOI of the University of Tokyo. Spectral data for γ-ray were obtained at post-irradiation time intervals ranging 11-25, 950s using a NaI (Tl) scintillation detector. The data were processed to the form of γ-ray energy release rates per fission for each set of time-interval parameters. The standard representation of the decay heat following fission pulse (in cooling times ranging 19-24, 000s) was provided from the γ-ray energy release rates. The uncertainties (1σ) of the decay heat data were about 5% for 235U and 239Pu, and about 7% for 233U.The data obtained from the present experiments are compared with three summation calculations using JNDC, TASAKA and ENDF/B-IV decay data libraries, and with other experimental results. The summation calculation results using the JNDC FP decay data library are in better agreement with the present data than the other calculations.

Method of Experimental Power Reactor Noise Analysis without Use of High Power Reactor

A method is proposed for conducting power reactor noise analysis without recourse to an actual high power reactor. The basic concept is to simulate the power reactor noise by integrating the different elements constituting the actual reactor noise, such as the random noise-generating force, the zero-power reactor transfer function, and feed- back loops between the reactor power and the noise-generating force. For the simulation study, a nucleate boiling noise generator and a single feed-back loop were divised to permit experiments with flux-related heater input using the fast neutron source reactor YAYOI of the University of Tokyo. This report discusses problems encountered in applying the proposed method to the simulation of power reactor noise, and presents some of the results obtained: Two kinds of change in amplitude were observed in the normalized auto power spectral density of the neutron flux, emanating form: (a) fluctuating displacements of the boiling zone bottom boundary, and (b) fluctuating num...