The Final Neutrons

The final neutrons from Canada’s primary source supported clean energy, food security, human health, and fundamental science.

Source: Canadian Neutron Beam Centre (CNBC)
Image: A neutron image of the plaque in which the final neutrons of the NRU reactor are captured, as explained in the article, and overlaid with labels of each neutron beamlines’ designation. (CNBC)

While a group of distinguished guests assembled in the control room to witness the historic final shutdown of the NRU reactor on 2018 March 31, the committed and passionate researchers, technicians, and technologists of the Canadian Neutron Beam Centre (CNBC) along with the last visiting researcher, were busy out on the main floor conducting experiments on the beamlines until the very end.

Since February 2015, when the final shutdown of the NRU reactor was announced, the CNBC team has strived to extract as much value and impact from the reactor as possible in its final years of operation. In other words, the goal was to ‘cross the finish line running’, and that is what they achieved.

The instruments were collecting data until the end, except one that had completed its final measurements just hours before. During the last minutes of the reactor’s operation, the neutron experts took a memorial neutron image that captured the last neutrons exiting the reactor as it shut down. The neutron image shows the layout of the six neutron beamlines around the core of the NRU reactor (a further explanation of the image and how this was done is provided below).

Most of these beamlines still perform among the best in the world, in terms of their quantity and quality output for science and engineering. Among these high performers were a powder diffractometer, polarized triple-axis spectrometer, neutron reflectometer and stress mapping diffractometer. Some parts of Bertram Brockhouse’s original instrument from the 1950s were still in use on one beamline.

The final neutron beam experiments (listed counter-clockwise around the image)

N5 Triple-Axis Spectrometer – The final study was undertaken in collaboration with the University of Saskatchewan’s Global Institute for Food Security, whose goal is to strengthen resilience of crops to climate change, and ultimately to avoid potential food shortages in the future. The study took neutron images of plant roots grown in both heavy water and light water, and of soil inoculated to track migration of nematodes (roundworms that inhabit most soils).

L3 Stress-Scanning Diffractometer – The final study was in support of life extension of nuclear reactors in collaboration with Canadian Nuclear Laboratories (CNL). The study examined how hydrogen that can be absorbed into alloys in a reactor as the reactor ages, thereby helping the industry to better understand and prevent defects from forming in the alloys over many years of operation. Specifically, hydrogen solubility measurements in Zr-2.5Nb (an alloy commonly used in Canadian nuclear power reactors) were completed to further develop models of delayed hydride cracking.

E3 Triple-Axis Spectrometer – The final experiment, in collaboration with CNL’s Radiobiology and Health branch and University of Ottawa, was the last of a series of ground-breaking experiments to investigate the effects of neutron exposures on human blood at the genetic level.  These experiments produced new scientific knowledge about the resilience of DNA to radiation.

D3 Reflectometer – The last neutron reflectometry experiment was in support of CNL’s development of technology that can provide continuous power in outer space or other remote locations, using tritium betavoltaics: one of a family of nuclear batteries which convert natural radioactive decay from a radioisotope into electricity. The experiment was to determine whether a nanometer-thin cap layer could prohibit the unwanted hydrogen desorption in a tritium betavoltaic battery.

C5 Polarized Beam Triple-Axis Spectrometer – The final experiment was in collaboration with Montana State University. This experiment was a discovery-driven study of exotic magnetic materials. Specifically, the experiment studied the magnetic excitations in a new low-dimensional magnetic system, NiTa2O6. The results obtained will help to validate theoretical models and lead to a better understanding of the magnetic interactions in this system.

C2 High Resolution Powder Reflectometer – The final experiment was in collaboration with Canadian Nuclear Laboratories, in support of research identified as important following the accident at the Japanese Fukushima Daiichi Nuclear Power Station, caused by the 2011 earthquake and tsunami. The experiment studied the effects of additives in uranium oxide toward development of nuclear fuels that are more accident tolerant.  The neutron diffraction data will build a better understanding of the structural properties of fuel with these additives and aid the design of composite fuel with enhanced safety properties: improved thermal conductivity and fission gas retention.

A neutron image of a plaque of the layout of the neutron beam lines around the NRU reactor core. White: NRU reactor core and beamlines. Light shade: The radiation shielding around the reactor and the beamline equipment.

About the final neutron image

The image is of a stylized plan view of the NRU core, the biological shield that protects personnel, the graphite column and the six neutron beamlines around the outside. Beam channels emanate from the core into the instruments where the beams are turned through angles to research materials placed in the neutron beams.

Much like an x-ray image, neutron images are created when an object casts a shadow on a sensitive film or imaging detector.  The shadow is caused by neutrons not reaching the detector because they were scattered by or absorbed the material.

The plaque was made using three materials chosen for their varying levels of neutron absorption properties. The plaque was designed so that the core and beamlines would appear bright, like an intense light bulb and beams of light.  This was achieved by letting neutrons pass though the plaque without being absorbed, producing the white core and beam lines. Some of the neutrons were absorbed in nickel alloy plates to produce the light and medium shades that represent the shielding materials around the reactor and graphite column, and the beamline equipment. The shielding absorbs undesired neutrons and gamma rays to protect personnel and enable high-quality measurements.

Left and center: The two different nickel alloy plates used to create the image. Right: The two nickel plates stacked to illustrate how they combine to create contrast in the image.

Photograph of the completed assembly.  Visible is an aluminum plate with white lettering of gadolinium. The nickel plates that create the image of the layout are mounted behind the aluminum.  The neutrons are absorbed by the gadolinium lettering, but elsewhere pass through the aluminum plate to the nickel plates behind.

Finally, the dark lettering was created using strongly absorbing gadolinium that filled engraved lettering in the plaque.

The plaque was placed in the beam and successive digital images acquired during the last 14 minutes of full-power operation and as NRU was shutdown.  Therefore, neutrons that were captured in the plaque during the shutdown—some of the last neutrons produced by NRU and delivered to the neutron scattering instruments—are now preserved within that plaque, with the most captured in the words CANADIAN NEUTRON BEAM CENTRE and NATIONAL RESEARCH UNIVERSAL REACTOR.