Metal foams found to excel in shielding X-rays, gamma rays, neutron radiation

May lead to better shielding for nuclear reactors and space travel
July 20, 2015

Lightweight composite metal foams like this one have been found effective at blocking X-rays, gamma rays and neutron radiation, and are capable of absorbing the energy of high impact collisions — holding promise for use in nuclear safety, space exploration, and medical technology applications (credit: Afsaneh Rabiei, North Carolina State University)

North Carolina State University researchers have found that lightweight composite metal foams they had developed are effective at blocking X-rays, gamma rays, and neutron radiation, and are capable of absorbing the energy of high-impact collisions. The finding holds promise for use in nuclear power plants, space exploration, and CT-scanner shielding.

“This work means there’s an opportunity to use composite metal foam to develop safer systems for transporting nuclear waste, more efficient designs for spacecraft and nuclear structures, and new shielding for use in CT scanners,” says

Afsaneh Rabiei, a professor of mechanical and aerospace engineering at NC State, first developed the strong, lightweight metal foam made of steel, tungsten, and and vanadium for use in transportation and military applications. But she wanted to determine whether the foam could be used for nuclear or space exploration applications — could it provide structural support and protect against high impacts while providing shielding against various forms of radiation?

So she and her colleagues conducted multiple tests to see how effective it was at blocking X-rays, gamma rays, and neutron radiation. She then compared the material’s performance to the performance of bulk materials that are currently used in shielding applications. The comparison was made using samples of the same “areal” density – meaning that each sample had the same weight, but varied in volume.

Better than lead and non-toxic

The researchers found that the high-Z foam was comparable to bulk materials at blocking high-energy gamma rays, but was much better than bulk materials — even bulk steel — at blocking low-energy gamma rays; it outperformed other materials at blocking neutron radiation; and was better than most materials at blocking X-rays. It was not quite as effective as lead, but with the advantages of  being lightweight and more environmentally friendly.

“However, we are working to modify the composition of the metal foam to be even more effective than lead at blocking X-rays, and our early results are promising,” Rabiei says. “And our foams have the advantage of being non-toxic, which means that they are easier to manufacture and recycle. In addition, the extraordinary mechanical and thermal properties of composite metal foams, and their energy absorption capabilities, make the material a good candidate for various nuclear structural applications.”

The research paper was published in Radiation Physics and Chemistry. It was supported by DOE’s Office of Nuclear Energy under Nuclear Energy University Program.

Abstract of Attenuation efficiency of X-ray and comparison to gamma ray and neutrons in composite metal foams

Steel-steel composite metal foams (S-S CMFs) and Aluminum-steel composite metal foams (Al-S CMFs) with various sphere sizes and matrix materials were manufactured and investigated for nuclear and radiation environments applications. 316 L stainless steel, high-speed T15 steel and aluminum materials were used as the matrix material together with 2, 4 and 5.2 mm steel hollow spheres to manufacture various types of composite metal foams (CMFs). High-speed T15 steel is selected due to its high tungsten and vanadium concentration (both high-Z elements) to further improve the shielding efficiency of CMFs. This new type of S-S CMF is called High-Z steel-steel composite metal foam (HZ S-S CMF). Radiation shielding efficiency of all types of CMFs was explored for the attenuation of X-ray, gamma ray and neutron. The experimental results were compared with pure lead and Aluminum A356, and verified theoretically through XCOM and Monte Carlo Z-particle Transport Code (MCNP). It was observed that the radiation shielding effectiveness of CMFs is relatively independent of sphere sizes as long as the ratio of sphere-wall thickness to its outer-diameter stays constant. However, the smaller spheres seem to be more efficient in general due to the fine fluctuation in the gray value profile of their 2D Micro-CT images. S-S CMFs and Al-S CMFs are respectively 275% and 145% more effective for X-ray attenuation than Aluminum A356. Compared to pure lead, CMFs show adequate attenuation with additional advantages of being lightweight and more environmentally friendly. The mechanical performance of HZ S-S CMFs under quasi-static compression was compared to that of other classes of S-S CMF. It is observed that the addition of high-Z elements to the matrix of CMFs improved their shielding against X-rays, low energy gamma rays and neutrons, while maintained their low density, high mechanical properties and high-energy absorption capability.