When assessing shielding levels, a distinction must usually be made between gamma radiation and neutron radiation. DIN 25413 [18] classifies shielding concretes by the proportion of elements they contain. How much shielding concrete provides against gamma rays depends directly on the bulk density of the concrete and the proportions of elements it contains by weight. In other words, the higher the weight of concrete, the more shielding it provides. With neutron radiation, how much shielding concrete provides depends on what chemical elements it contains. As well as using additives with crystal water content, the proportion of light elements, such as hydrogen, is particularly important, (or boron compounds are also used, such as boron carbide, borax frit, colemanite, boron calcite) which are particularly good at ‘trapping’ fast neutrons.

Raw density specifications are generally based on dry weight or dry raw density. DIN 25413 defines different compositions of concrete mixes and the main element proportions involved, such as O, C, Si, Ca and Al or it recommends a so-called average composition. For heavyweight concrete, this standard also specifies different kinds of concrete and characteristic proportions of elements, depending on the heavy aggregates used (haematite, magnetite, ilmenite, barytes, limonite and serpentine). Under DIN EN 206-1 [19] and/or DIN 1045-2 [20], heavyweight concrete has a dry specific gravity in excess of 2.6 t/m3. However, DIN 25413 refers to an older definition of heavyweight concrete. Radiation-proof concrete made with heavyweight aggregates therefore generally has a raw density over 2.8 t/m3.

Heavyweight concrete is considerably more expensive to bring in than standard concrete. Because of the largely angular aggregates and higher density involved, it does not pour nearly as well as standard concrete, and it requires more careful mixing to ensure that components of different density do not separate. For notes on this, and an overview of heavy aggregates, see the DBV code of practice for radiation protection concretes [21].

Heavyweight concrete as radiation protection concrete was already being used in the first nuclear facilities in Germany in the 1960s, at the Jiilich research centre (research reactors DIDO and MERLIN). So-called ball scrap concrete (with cast-iron granulate), for example, with a specific gravity of 5.6 t/m3, had been used.

In more recent plants, however, additives such as magnetite, serpentine, haematite or barytes or in some cases granulated iron additives had been used, as they are easier to work.

A cement content of 340kg/m3 (CEM III/B 32.5), a water content of 170kg/m3, with 1410 kg/m3 of haematite 0/6, with 1680 kg/m3 haematite 6/25 and 150kg/m3 sand 0/8 has been used to give a specific gravity of 3.6 t/m3, for example. Using 890 kg/m3 haematite 0/8 instead of sand 0/8 and 1920 kg/m3 haematite and an extra 1350 kg/m3 iron granulate can give a bulk density of approx. 4.5 t/m3.

Heavyweight concrete as radiation-proof concrete is mostly required in the immediate environment of the reactor pressure vessel as part of the bioshield. In the support area of the reactor pressure vessel (known as the skirt area) of the bioshield in the containment at Gundremmingen a heavyweight concrete with specific bulk densities of 2.7-4.2 t/m3 was used, for example.

What is particularly important in construction terms is the dry specific bulk density of a normal concrete to be designed for radiation protection purposes. Using normal quartz gravel as aggregate can only reliably give a dry specific gravity of 2.2 t/m3. If shielding requires a higher specific bulk density, it must be borne in mind that special aggregates will be required. These may have to be brought by considerable distances, which could increase costs.