Spain: Simulating Laminate Shielding Concepts
When neutrons interact with matter, they mainly interact with the nucleus (unlike gamma radiation that interacts with electron clouds) and can either be scattered or absorbed. To stop unwanted neutron interactions, e.g. with our bodies or with material that is not the intended target of a neutron beam, shielding solutions are used to make a safe work environment and obtain good quality measurements.
The initial energies of neutrons produced in a spallation source or research reactor are high and hence these neutrons are fast. To become useful in studies at scientific measurement stations, neutrons are moderated, i.e. slowed down. The scattering and moderation processes involve radioactive reactions that may release gamma radiation, atomic particles and fission products so these need to be shielded too.
Neutron shielding materials generally contain elements with a low atomic number like carbon, hydrogen and oxygen. These moderate fast neutrons when they collide with the atomic nuclei. Examples of these materials are paraffin, polyethylene, concrete and water. Sometimes these substances are interlaced with elements like boron, cadmium or gadolinium that can capture slower neutrons and produce minimal gamma radiation.
High atomic number elements like lead, bismuth and tungsten can also be incorporated into shielding layers to absorb any gamma radiation generated from the neutron interactions above.
Historically, the typical shielding at neutron facilities has been developed as homogenous mixtures of the above materials, which often lead to very big, bulky and expensive structures around the experiments. The reason to consider laminate shielding instead is that this type of shielding may be designed to be just as effective, but requires less material volume, so it does not take up so much space, is lighter, which minimizes the structural load, and is more cost-effective. The development of computing tools has enabled engineers to design far more cost-effective designs. However, performing a full shielding calculation is time consuming, both in terms of computing and workforce.
To provide the community with a practical tool for considering effectiveness of laminar shielding, Monica Huerta and Miguel Magán from ESS-Bilbao used Monte Carlo calculation methods to simulate the effectiveness of nine different neutron shielding materials, akin to the measurement of transparency at visible wavelengths of light.
They studied the following materials: concrete, high density concrete, concrete with Barium, concrete with Ba/B/Fe, Carbon Steel, Lead, Borated paraffin, Tungsten powder in paraffin wax and Boron Carbide in their simulations.
They simulated the effect of neutrons interacting with 10cm slabs of each material, placing the source 1mm into the slab. They used reflective surfaces to ensure the neutrons were either absorbed in the shielding material or exited the slab from the opposite side from where they entered meaning any neutron not stopped would be transmitted. The results now allow a designer to compare the transmission of neutrons through each material when developing shielding designs for new instruments. They can easily estimate the shielding performance of several layers of material depending on the energy distribution of the neutrons to be shielded and serve as an initial point for further investigation and optimization of their options.
Acknolwedgements: Peter Willendrup, DTU and Miguel Magán, ESS-Bilbao