Neutron tomography of axially symmetric objects using 14 MeV neutrons from a portable neutron generator

Abstract

In nuclear boiling water reactor cores, the distribution of water and steam (void) is essential for both safety and efficiency reasons. In order to enhance predictive capabilities, void distribution assessment is performed in two-phase test-loops under reactor-relevant conditions. This article proposes the novel technique of fast-neutron tomography using a portable deuterium-tritium neutron generator to determine the time-averaged void distribution in these loops. Fast neutrons have the advantage of high transmission through the metallic structures and pipes typically concealing a thermal-hydraulic test loop, while still being fairly sensitive to the water/void content. However, commercially available fast-neutron generators also have the disadvantage of a relatively low yield and fast-neutron detection also suffers from relatively low detection efficiency. Fortunately, some loops are axially symmetric, a property which can be exploited to reduce the amount of data needed for tomographic measurement, thus limiting the interrogation time needed. In this article, three axially symmetric test objects depicting a thermal-hydraulic test loop have been examined; steel pipes with outer diameter 24 mm, thickness 1.5 mm, and with three different distributions of the plastic material POM inside the pipes. Data recorded with the FANTOM fast-neutron tomography instrument have been used to perform tomographic reconstructions to assess their radial material distribution. Here, a dedicated tomographic algorithm that exploits the symmetry of these objects has been applied, which is described in the paper. Results are demonstrated in 20 rixel (radial pixel) reconstructions of the interior constitution and 2D visualization of the pipe interior is demonstrated. The local POM attenuation coefficients in the rixels were measured with errors (RMS) of 0.025, 0.020, and 0.022 cm(-1), solid POM attenuation coefficient. The accuracy and precision is high enough to provide a useful indication on the flow mode, and a visualization of the radial material distribution can be obtained. A benefit of this system is its potential to be mounted at any axial height of a two-phase test section without requirements for pre-fabricated entrances or windows. This could mean a significant increase in flexibility of the void distribution assessment capability at many existing two-phase test loops.

A powerful pulsed “point-like” neutron source based on the high-current ECR ion source

“…To compare the presented results with performance of existing systems for fast-neutron tomography reported by other authors it is reasonable to refer to pure scientific studies [2][3][4] and commercial developments [11,12]. In [2][3][4]neutron generators with a total yield of 10 7 − 10 8 neutrons per second were used and that is much lower than the suggested generator based on high-current gasdynamic ion source could provide. Improvement of experimental schemes used in [2][3][4] with sufficiently more powerful neutron source could lead to a very important results for fast-neutron tomography evolution.…”

“…In [2][3][4]neutron generators with a total yield of 10 7 − 10 8 neutrons per second were used and that is much lower than the suggested generator based on high-current gasdynamic ion source could provide. Improvement of experimental schemes used in [2][3][4] with sufficiently more powerful neutron source could lead to a very important results for fast-neutron tomography evolution. An absolutely different situation could be observed around commercial devises.…”

“…However, these facilities are very expensive and complicated in operation, which hinders the spread of neutron tomography as a routine technique. Another approach for neutron imaging is a fast-neutron radiography [1][2][3][4][5]. In this case there is no need for neutron thermalization and radiation from a point-like sources could be applied instead of parallel beams, which means higher efficiency of neutron utilization for an image production.…”

A Fast Neutron Radiography System Using a High Yield Portable DT Neutron Source

“…This early neutron imaging work focused on thermal neutrons, which were either produced by a reactor, or were produced by moderating the neutrons from a neutron generator. Fast neutron radiography (FNR), directly using the 14.1 MeV neutrons arising from the deuterium–tritium (DT) reaction, has been reported using lower-yield portable neutron generators by Andersson [ 6 ], and high-yield neutron generators that are not portable, by Wu [ 7 , 8 ]. With incremental improvement in DT set up reported by Bishnoi [ 9 ] and Kam [ 10 ].…”

A MLEM-TV-MRP Algorithm for Fast Neutron Computed Tomography Reconstruction of High Statistical Noise and Sparse Sampling

“…For example, a plastic scintillator detector array had been used instead of a conversion screen and a CCD camera. But, the change came at the cost of sacrificing spatial resolution [13]- [15]. In this study to shorten the total measurement time, the methods, reducing measurement time of single projection (an neutron photography) and numbers of projection are employed at the same time.…”