Discussion on the Standardization of Shielding Materials – Sensitivity Analysis of Material Compositions

. The overview of standardization activities for shielding materials is described. We propose a basic approach for standardizing material composition used in radiation shielding design for nuclear and accelerator facilities. We have collected concrete composition data from actual concrete samples to organize a representative composition and its variance data. Then the sensitivity analysis of the composition variance has been performed through a simple 1-D dose calculation. Recent findings from the analysis are summarized.


Introduction
In radiation shielding design and safety analysis, it is necessary to employ appropriate calculation conditions of shielding material, e.g., material density and composition. Concrete, lead and iron are widely used as shielding materials for nuclear power plants and accelerator facilities. Among them, the concrete composition cannot be universally specified in design phase, because it varies greatly depending on the locality of aggregate and a mixture ratio of aggregate and cement.
In Japan, the composition data listed in American National Standards Institute [1] or other reports [2,3] are often quoted for concrete composition. However, it is not clear whether they can represent the actual samples at Japanese construction sites. In fact, the importance and difficulties of this issue has long been widely shared in Japan.
In 2014, a working group was established in order to standardize shielding materials under the Standards Committee of Atomic Energy Society of Japan. Then we have discussed representativeness of the material composition and implication of the composition variance in shielding performance, especially for concrete. In this paper, our approach to standardize material composition is described, and calculation models and results of sensitivity analysis of concrete compositions are shown.

Basic approach for standardization of material composition
Our approach to standardize concrete composition is as follows. First, we collect concrete composition data used in the present shielding calculation and actual sample data to organize the representative composition and variance of composition. Then neutron and gamma ray dose calculations are performed to evaluate influences of the composition variation on shielding performance. Lastly, influences of other variations, such as a moisture within concrete, mixture non-uniformity and trace elements contributing to induced activity, are considered.

Sensitivity analyses
It is needed to prepare sensitivity analyses data so that shielding designers can reffer the influence of the fluctuation of concrete composition and water content on the shielding design. Fig. 1 shows sensitivity analysis method in this paper.

Method
Influences of variation of concrete composition on a shielding calculation were evaluated. Table 1 shows Evaluate influence of the concrete composition fluctuation due to the concrete mixture on the dose rate.
Evaluate influence of difference among the typical concrete compositions on the dose rate.
Make recommendations of (1) standard concrete composition (2) applicable condition and safety margin (3) standard concrete water content.
basic sensitivity analysis conditions. Neutron and gamma dose calculations were performed by using one dimensional SN code ANISN [4] with MATXSLIB-J33 library [5]. As shown in Fig.2, 1-D simple sphere geometry was used for the sensitivity analysis so that influences of concrete composition can be shown clearly. A hollow concrete sphere having inner radius 500cm was used to avoid appearing the distance attenuation nearby source. A radiation source of 1cm diameter is placed at the center of the sphere. Representative concrete composition and its elemental variance data derived from actual samples was used in this calculation. Composition of F02HT is shown in table 2. As radiation sources 60 Co for gamma and 235 U for neutron were used in this paper. In neutron dose calculation, it is also considered dose by secondary gamma rays.

Influence of the concrete composition with realistic density
Although calculation with measured elemental composition with ±3sigma carried out, further calculation was carried out with restricted density between ±3sigma of the analyzed an actual concrete density since concrete densities shown in table 1 were too varied (about 8%) to evaluate effect of concrete composition. Concrete density used for radiation shielding was usually controlled to be desinged value. In this calculation, density restricted concrete composition derived with actual concrete data was emplyed. Table 3 shows density restricted concrete composition of F02HT (about 2.2%). Dose attenuation calculation model is shown in Fig.2 (same as first calculation). Fig.6, Fig.7 and Fig.8 show neutron, secondary gamma and total dose atteuation rate with U-235 source. Neutron dose attenuation rate at 250cm depth of -3sigma concrete is about 2 times as large as that of +3sigma. As a result of this calculation, it can be concluded that neutron dose rate variation due to the density fluctuation by concrete mixing is at most within 2 times.

Sensitivity analysis with U-235 neutron source
It is important to set water content in concrete appropriately when neutron transport calculation is carried out because neutron is easily slowing down by low atomic numbered elements (typically hydrogen in water). In this paper, three cases of water content in concrete were evaluated. The cases were absolute dry (0g/cm 3 free water), 0.04g/cm 3 free water and 0.1g/cm 3 free water in concrete.  Fig.2 (same as first calculation) Fig.9, Fig.10 and Fig.11 show the result of this calculation. Neutron dose attenuation rate is worse with decreasing the water content in concrete because neutron is easily slowing down in rich water content. Secondary gammas generation rate increases with increasing water content in concrete but dose rate of poor water content is higher than dose rate of rich water content because of neutron slowing down by water.  Fig.11. Total dose attenuation rate of F02HT with dinsity reconfigured.

Sensitivity analysis with gamma source
Dose rate sensitivity analysis for water content with gamma source was carried out. Co-60 was set as a gamma source. Fig. 12 shows the dose attenuation rate for these gamma rays. Since increase of water content brings increasing concrete density, gamma dose rate is well attenuated with rich water content concrete. Fig.13 shows the gamma dose attenuation for concrete mass thickness of F02HT. Fig.13 indicates that dose attenuation rate depends on its density since water content in concrete make a little contribution to the gamma dose attenuation rate by mass thickness.

Conclusions
The overview of standardization activities for shielding materials was described. As a first step, important data to determine radiation shielding concrete composition was obtained. Variation and behaviour of dose attenuation rate for gamma ray and neutron caused by variances of the concrete and the water content of the concrete indicates that neutron dose rate is influenced by light elements and gamma dose rate is influenced by concrete density. In the determination of the standard material composition, it has to present the consideration for variation of composition, the effect and dose caused by concrete density for neutron and gamma ray, consideration for water content and conservativeness of the shielding calculation with the standard concrete composition.