Developing Model with Breeze to Study Radon Migration and Mitigation in a Single Family House

A multi-cell computer model of a family house with cellar was developed with the ventilation and contaminant movement computer prediction program BREEZE. The model was used to simulate the movement of radon gas from the cellar to the living areas of the house. Field measurements were also carried out under various controlled ventilation strategies. The paper outlines the development of the computer simulations from a simple model to a more complex flow analysis. The results obtained from the simulation model and the measured values were compared. A parametric study has subsequently been carried out to investigate the movement of the gas for a range of natural and mechanical ventilation strategies. INTRODUCTION Computer models have been developed in domestic radon researches since the middle of the 80's. Parametric modelling has been used to study a variety of physical processes for a better understanding on mechanisms in this area or to examine the effectiveness of a proposed remedial measure to a given building before pragmatic application. Among those models, a complex model was developed in Lawrence Berkeley Laboratory, USA(L0weiro 1987; Loureiro et al. 1990) to simulate the process of soil radon entering a simplified house and examine quantitatively at the relationships between indoor concentration and other parameters. It helped us to gain a better knowledge on radon transport in soil and entry into houses. In pragmatic application, a preliminaly multi-cell model was developed with BREEZE to study the effects of substructure ventilation strategies on air movement and radon dispersion in a real house with cellar, which was geometrically complex from the view point of radon migration. The modelling suggested that both supplying and extracting ventilation would be effective when a mechanical ventilation system was installed in the cellar, which is commonly believed as the main radon source for the house(Ward et al. 1994). Further research found that the initial model was too simple to yield result consistent with that of the field measurement carried out later. It even did not bear the basic features of relationship between radon distribution patterns and air change rates. Firstly, the model did not correctly simulate the air flow through the staircase which was an important vertical route of indoor radon migrationffig. 2 a). The staircase was simplified as a vertical shaft started from the cellar to the top floor of the house (fig. 2 b). Although two open doors were at the sides of the shaft on the first and ground-floor respectively, the air flowed via these two floors carrying radon with it, until reached the top when no wind disturbed. The radon in the lounge came through the crack linking cellar and lounge directly. Consequently, tiiosl rooms were uncontaminalcd. which was rather different from the real situation. Secondly. a fixed feed rate was set in the cellar which was contrary to reality. The truth found and supported by researches is that radon entry from the surrounding soil depends on many factors including the 1995 International Radon Symposium V 6.1 differential pressure between the cellar and the outdoors(Nero 1987). The feed rate in the model should not be given to a constant value before calculation the pressures in the house. Lastly, the model failed to give a reversed radon distribution pattern found in the measurement, in which supplying ventilation resulted in higher radon levels on upperfloors than that in cellar. This pattern. which was also found in the testing basements in Princeton University by Cavallo and his colleagues, suggested that there was another radon entry route additional to the cracks linking cellar and ground floor. Obviously, a more precise modelling is needed which should be validated by the field measurement. The interpretation of measured results suggested that the features was because of geometric complexity of the house, whose ground floor covered the cellar and soil as well. This geometric complexity increased the difficulty in modelling the radon entry appropriately and consequently resulted in the shortcomings of the initial model. METHODOLOGY IN MODEL DEVELOPMENT The model of the house was developed in of two parts: structural modification and parameter adjustment. When adjustment of the parameters in the initial model failed to yield acceptable results, structural modification was made with further adjustment of the parameters in the next model. The process repeated until the calculated results came close to the field measured ones. The structural modification were aimed at modelling air flow in the staircase and radon entry from the soil while the parameter adjustment focused on the characteristics of the cracks linking the house with soil. General Features of Radon Distribution Pattern The article (Wang et al. 1995) discusses the results of the field measurement and outline the general features of the radon distributions varying with the air change rate in the cellar. The features can be summarised as follows . In the cellar, the radon levels were higher in de-pressusisation than those in pressurisation; The radon level rose as the speed of extracting fan slowed down and reached the maximum when the fan was running at speed 3 de-pressurisation. As the Can was slowed further, the radon level dropped the extracting was weak compared to stack effect. It decreased continuously in natural ventilated condition and pressurised condition as more outdoor air was introduced into cellar. In the living areas, the average concentration levels were lower in extracting ventilation than with supplying ventilation, when fan was running at same speed. All seven rooms monitored were almost evenly polluted. Interestingly, the average levels in living areas were higher than those in the cellar in pressurisation. Basic Assumption An assumption suggested in the article was that there were other routes for radon entry into the house beside the one into cellar (dashed lines in figure 1 ). The measured results can be reasonably interpreted with this assumption and the model could be developed on this basis. The simulation with final model also supports the assumption, which will be discussed later. 1995 International Radon Symposium V 6.2 Fig. 1 Schematic diagram of the air flow routes between the cellar and rooms on ground floor. The solid lines and ovals represent the air flow routes while the dished denote the possible air flow routes proposed by the assumption. MODEL DEVELOPMENT Staircase Modelling In the new geometry, instead of one single shaft linking all floors, three shafts were created. Each linked only two neighbouring floors. The air came out through the door when it reached the top of each shaft, then went via the landing into the door at the bottom of next shaft, finally arrived at the top floor(fig. 2c ). If the travelling air flow carried radon, it polluted the landings and hence polluted other rooms on the floor by either wind disturbance or stack effect. The route for vertical air movement in new model can be seen more clearly in figure 4. The air went from the cellar into shaft 1 which connected with shaft 1 on ground floor, then came out through the closed door. Via the hall into shaft 2 on the floor. The air then went up through shaft 2 linking ground floor with the first floor. On the first floor, the air turned out from the shaft door to the landing then went into the door of shaft 3 leading finally to top floor. a) Staircase and landing c) Two shafts in new model b) Single shaft in the old model Fig. 2 One section of Staircase on middle floor of the house. One single shaft in old model, in which air flow passing through that floor. Two shafts linking other floors separately. 1995 International Radon Symposium V 6.3 Crack linkine the Cellar with Outdoors The crack ( C8 in fig. 3 ) represents the leakage on the cellar's walls to outdoors. The crack provided an easier air inlet for a de-presswised cellar than the perimeter crack of the cellar door, where the stack effect created lower pressure outside the door. Without the crack, there would be a lower pressure with same power setting of extracting fan, thereby the radon level would vary in a larger range than observed in the field measurement when the fan changes from full power de-pressurisation to full power pressurisation. Cracks linkine livine Areas and Radon Source At least one route for radon entry into cellar should be created in the model. It was the crack (C4 in fig. 3) linking cellar with soil. Through it there was an air flow, which depended on differential pressure. It may enter the cellar from soil with radon or from the cellar into soil. Based on the assumption, other routes (C2 and C3 in fig. 4) were added in later structural modification. Sinele Dummv Cell In order to simulate radon feeding rate that depended on differential pressure, next to the cellar, a dummy cell was added, in which the feeding was constant. It had two air paths, one is crack linking it with outdoors and the other with the cellar. When the cellar was being de-pressurised, air with radon in dummy cell would be sucked in to the cellar and when it was being pressurised, the fresh air flow went from cellar into the dummy cell and resisted the radon entry to the cellar. During the de-pressurisation, radon entry rate depended on the radon level in the dummy cell. area of the crack, and pressure difference between these two cells. This model gave a good results bearing some basic features mentioned above. Pressurisation prevented the radon into cellar but pushed the radon in the dummy entering living areas via the crack linking dummy cell with the hall. This resulted in a higher radon level in living area than that of cellar. The model, however gave some unreasonable calculation as well. The radon level in the dummy cell is too sensitive to power change of the fan system so that it decreased dramatically when the fan was extracting at full power. This resulted in a lower radon level in cellar than the level when the fan was working in low power. Although a small decrease was normal according to the modelling of Loureiro if the soil permeability was high. a drop of 75% calculated by model seemed too much to be accepted. The reason is quite clear. all cells in BREEZE are free space for air movement, no pressure and concentration gradient. Hence the model with a single dummy cell can not simulate precisely the radon gas from soil underneath building. Further modification is necessary. 1995 International Radon Symposium V 6.4