Switzerland: Developing special materials for fast neutron shielding
Uwe Filges, Christina Klauser, and Masako Yamada at the Paul Scherrer Institute in Villigen, Switzerland have explored shielding materials that can efficiently shield neutrons at neutron sources. This is one of the tasks of Work Package 8 E-tools.
Heavy concrete has good shielding characteristics and is regularly used for shielding. Thus, different concrete compositions were investigated in this project. However, concrete is not a suitable shielding material for instruments where the shielding dimensions must be very precise because concrete usually shrinks over time by higher than 0.25%. Therefore, alternative material available on the market was also investigated.
A promising candidate was something called mineral cast that is composed of quartz, sand and epoxy. This material is stable over time, and shrinks by less than 0.03%. Additionally, the epoxy binding material which has a high hydrogen content was expected to show good shielding properties for fast neutrons. The standard mineral cast is available on the market from Rampf (www.rampf-group.com) and has already shown better shielding characteristics than normal concrete.
The full investigation explored 7 different concrete compositions and 14 different mineral cast compositions. To make the concrete effective at shielding neutrons, the mass density needs to be over 4.7 g/cm3 and contain 5 wt% Boron-Carbide. The latter was achieved by substituting some of the sand in the concrete recipe with boron carbide of equivalent grain size.
Furthermore, they wanted to see if a composition with non- or low-magnetic properties could also be made. The special heavy concretes tested are not available on the market so they prepared a sample with the help of Alpha Beton AG (Switzerland). The proportions of the ingredients, which included barite, magnetite, hematite, Fe- granulate and boron carbide, were varied to make the different mixtures.
As for the mineral cast, pre-investigation with the standard one showed that a neutron absorber, such as boron carbide, needs to be added to reduce the activation of the material. The samples were thus adapted, in collaboration with Rampf, by varying the percentage weights of boron carbide and sand and by making samples with and without superplasticizer and with and without basalt.
Measurements of all the material samples were conducted at ICON or BOA beamlines at PSI. They were measured in terms of thermal neutron transmission, epithermal neutron transmission, fast neutron transmission and activation decay.
The neutron detector system used was the BSS (After being determined for D8.1 See our BSS article). The neutron transmissions of the mineral cast were measured with the set of three spheres. On the other hand, all the spheres were used for the concrete samples. Activation decay was measured with a Ge-detector suitable for gamma spectroscopy.
The most promising heavy concrete composition had the highest mass density. The measured attenuation was 2.4 times higher than standard concrete. This means that a 20cm thickness of this concrete reduced the fast neutron flux by a factor of around 520, compared to a thickness of 48cm that would be required on a block of normal concrete.
As for the mineral cast, the best results achieved for thermal neutrons were obtained for the samples that had a high content of boron-carbide i.e. 3 wt%. For epithermal neutrons, the mass density also played a role. For fast neutrons only small differences were observed but a couple of the samples did exhibit better shielding characters than the others.
The best heavy concrete and mineral cast materials were then compared to each other and also to standard concrete. These newly developed materials had shielding performances similar to each other but both performed better than the standard concrete. An attractive consideration is that the mineral cast is cheaper. It has a similar price to standard concrete whereas heavy concrete is 5 times more expensive.
Overall, though, the potential for using lighter, thinner and cheaper shielding for the same amount of protection is starting to be realized.
The researchers now plan to produce some more mineral cast samples with higher epoxy content expecting that the shielding performance against fast neutrons will be improved.
Acknowledgements: Masako Yamada, PSI