Effect of ventilation rate on ²²⁰Rn concentration distribution in a model house

In recent year, the dose associated with thoron and its decay products has gained considerable attention in India. This is mainly due to increasing awareness of thoron issue in coastal regions of India commonly known as High Background Radiation Areas (HBRAs) as well as in thorium and ²³²U handling occupational facilities. Thoron profiling is an important aspect for thoron dosimetry in these environments. Thoron (²²⁰Rn) is being a short lived (∼ 55 sec) element; it has been presumed that ²²⁰Rn concentration distribution pattern would not be significantly affected by ventilation rate. That is why higher value of ²²⁰Rn concentration was assumed near to the source such as wall etc. while beyond a particular distance no significant value of ²²²Rn concentration could be observed. To investigate the effect of ventilation rate on ²²⁰Rn concentration distribution, a controlled experimental study was carried out in laboratory model house at ISS, Germany at different ventilation rate of 0.5 h^-1, 4.5 h^-1 and 9.0 h^-1. The model house of dimensions 2.8 m x 1.5 m x 1.8 m has one door and two windows in one of the long sides, is made of from unburned adobe bricks which are plastered with thick clay layer homogeneously enriched with powered granite. For obtaining the ²²⁰Rn concentration profile, ²²⁰Rn concentration was measured at different positions using sampling cup and indigenously developed portable thoron monitor (STM). It has been observed that ²²⁰Rn concentration profile itself varies with the ventilation rate. At the ventilation rate of 0.5 h^-1 a gradient in ²²⁰Rn concentration profile was observed. ²²⁰Rn concentration was found higher near to the wall and decreasing towards the centre. On the contrary, at high ventilation rate of 4.5 h^-1 and 9.0 h^-1, a distorted ²²⁰Rn concentration profile was observed. This is due to the turbulent mixing caused by high ventilation rate. Hence, it can be emphasized that turbulent mixing plays a crucial role in deciding ²²⁰Rn distribution pattern in the confined environment. (author)