A. Kies*, F. Massen
Centre Universitaire, L-1511 Luxembourg, kies@cu.lu
(Conf. LGL Munsbach Lux. Nov. 1999)

Abstract

Radon concentrations have been measured several times over two week periods in the drainage gallery under the water reservoir of a pumping storage power plant. For this purpose, the artificial ventilation was stopped. The aim was to investigate a possible influence of the changing water loads on the radon levels. As expected radon concentrations in the gallery were influenced mainly by airflow pattern caused by differences in inside-outside air densities; to a lesser extend by variations in atmospheric pressure. After treatment of the data it could be shown that radon levels are also affected by the daily, often irregular variations of water levels that depend on the momentary energy demand. Measurements performed in a borehole drilled into the bedrock gave much better results and show a clear influence of the water load on the radon concentrations.

Introduction

Radon as an omnipresent radioactive gas can be used as tracer gas in geology, hydrogeology and geophysics where it accounts for an interesting alternative to other investigation tools. The present contribution relates of the results of underground radon monitoring in connection to local stress and strain changes. Radon measurements were initiated under a large water basin in order to investigate on the possibility of an influence of changing water loads and thus changing volumetric strains. 
The transport of radon is affected either by motion of a carrier fluid along pressure gradients or by diffusion along concentration gradients. We have to keep in mind that diffusion is responsible of the transport on a small scale whereas the motion of fluids along pressure gradients effects the transport on a large scale (Schery, 1984). The radon specific mechanism has to be considered: due to the short half-life of 222Rn (3.82 days) its mobility in the rocks by diffusion is limited to small distances in the meter range. To be effective, radon needs a carrier fluid: the air in the microfractures and the pores or drainage tubes in the present case. The elastic properties and porosity in the vicinity of the galleries and the compressibility of the pore fluids may define the magnitude of strain-induced variations (Holub, 1981; Trique, 1999).
After an investigation of the influence of earth tides on underground radon concentrations (Kies, 1997), we were interested in a further evaluation showing the possible influence of changing loads on radon levels. A pumped storage facility to the North of Luxembourg gave us this possibility.



Fig. 1: Topographical map of the Vianden pumped power station area (heights in meters)

The Vianden storage power station

The electric storage power station is situated in the Devonian part in the North of the Grand Duchy of Luxembourg. As pumping storage plant it serves as a buffer unit within the West-European electricity network in order to balance energy consumption and production. The lower basin contains an effective volume of some 10 Mm3 with water levels ranging between 219 m and 228.5 m. The upper reservoir consists of two basins with a useful water capacity of 3.0 Mm3 (basin I) and 3.8 Mm3 (basin II) respectively, occupying an area of 50 ha. The minimum and maximum water highs vary in between 494.0 m and 510.3 m equivalent of a water level variation of 16.3 m. The connection between basin II and I in general is open such that the instantaneous water levels are nearly the same. For a reported investigation period, due to routine works, basin I was empty. 
The maximum performance of the power station is 1.2 GW with a maximum discharge rate of 430 m3/h. the maximum pump discharge is 263 m3/s. Fig. 1 shows a topographic map of the power station area. A geophysical network with gravimeters and tiltmeters has been added in order to study gravitation effects and microkinimatical processes due to the displacement of large water masses and changing water loads. It has been shown that vertical and horizontal gravitation effects due to the water displacements amount up to more than 600 (Gal in the close vicinity of the basins and that gravitationally induced vertical displacements of equipotential surfaces (levelling) distinctly exceed 0.1 mm locally (Bonatz, 1995 and 1997). Unfortunately, during our investigation period the geophysical devices were not in operation.
The determination of the load induced kinematical processes of the upper basins, as well as the geophysical observation of the relevant environment, is part of the safety control tasks to be carried out. Any observed physical quantity related to stress/strain effects is an interesting parameter to be measured. The aim of the investigations is to analyse the possibility to use, for the site of Vianden, radon as a tracer gas for the study of variable stress-strain situations.
One major difficulty of the use of radon as a tracer gas, is the extreme sensibility of the radon concentrations in the gas phase to air movements. Prior to any study, a detailed investigation on temperature and sometimes air pressure-induced variations of radon concentrations has to be done. 
The two basins of the upper reservoir are formed by rockfilled embankments, which surround the plateau of Mount Saint-Nicolas and follow almost perfectly the natural contour. Under the waterproofed bottom of the reservoir is located a drainage system which collects the seepage waters so that there discharge rate may be measured. 

Experimental set-up

The underground radon measurements were performed in the narrow tunnel gallery underneath the upper basin II. Normally an active ventilation system blows air from the entrance into the gallery; air outflow is observed through the large tubes into a circular tubing system surrounding the basins and, for two tubes, to the outer air. The active ventilation induces very low radon concentrations and makes any investigation based on airborne radon impossible. For some 2-week periods, forced ventilation had been stopped. During these periods, at the most remote parts of the gallery situated under the mid-part of basin II, continuous radon measurements were performed with Alphaguard ionisation chambers. 
Temperature in the gallery is uniform; a meteorological station located near the side monitors temperature, atmospheric pressure, humidity, rainfall, wind direction and speed. A Ryttmeier level meter records the data of the water level of the basins.
In order to get rid of the effect of ventilation and also of meteorological factors, recently radon concentrations are measured in a 1.2 meter deep vertical borehole drilled through the concrete the floor of the gallery into the bedrock.

Results and discussion

Radon measurements in the gallery

Under forced ventilation conditions radon concentrations in the gallery are useless. The shut off of the forced ventilation has as a consequence very slow air movements whose direction and intensity depend on the sign and amount of the difference of air densities between outside and inside. 
Without forced ventilation, radon concentrations oscillate around 300 Bq/m3. During the 4 investigation periods, these transient concentrations never lasted for a long time; in each period radon bursts were observed where radon levels went up to 35 kBq/m3. This situation happened always when air densities were lower outside than inside, or when outside temperatures were higher than the constant 11 °C in the gallery. The radon spikes occurred after neutral pressure conditions. The explanation of the radon highs can be done in focusing on pressure driven flow mechanisms, due to atmospheric pressure decrease and above all the sucking effect of air moving out of the tubes into the gallery.
Wind and rain did not affect indoor radon. As mentioned before, radon spikes happened in periods of decreasing atmospheric pressure. During these periods, the most important factor governing radon concentration was outside-inside density difference drho (or the difference of outside-inside temperature (t).

In order to show a possible influence of the water load, the confounding factor (( is very hard to manage. He induces over long time periods very high dynamics that mask possible second or third order influences. Furthermore, the variations of drho are of the same 24h time period as the energy demand or the changes in the water levels of the basin. Taking a time period without forced ventilation and without high dynamics, only two contributions are supposed to govern changing radon levels: drho and water level variation dh. These two factors act independently, therefore a simple model states that the difference in radon concentration (c is proportional to the product of two variables drho_max and dh.

dc = a*dh * drho_max

 a is a constant, dh denotes the amount of decreasing water level, drho_max is the maximum variation between outside-inside air densities at chosen periods where drho is negative (t°ex > t°int).

For this model we made the assumption, gathered from earlier investigations (Barnet 1997), that radon concentrations increase with decreasing load.
The results of this computation show with a reasonable accuracy that in Vianden changing loads influence radon concentrations. These influences are very low, most of the time they are masked by exterior confounding factors. They confirm that increasing load inhibits slightly radon migration out of the bulk material into the voids; on the other hand, radon has a greater mobility with decreasing strain.

Dose-rate measurements in a bore-hole

In order to get rid of the influence of exterior confounding factors, recently we have the opportunity to use a 1,2 m deep borehole drilled through the concrete covering the bottom of the gallery into the Devonian schist bedrock. Intercomparisons with radon monitors in the radon room at the Centre Universitaire and in the gallery showed a very good correlation between radon concentrations and the dose rate given by an Aware geiger-counter. Therefore it was decided to instrument the borehole with this counter. One advantage of its use is the ease of installation and the lack of active pumping with the resulting pressure perturbations. The used geiger-counter has a sensibility and resolution similar to that of the Alphaguard radon monitor. Under forced ventilation the dose rate in the gallery is 120 nSv/h, without ventilation a minimum of 150 nSv/h and a maximum of 2800 nSv/h during radon bursts. In the borehole the dose rate is oscillating around 500 nSv/h. In a first investigation period, a defective packer prevented to tighten the borehole. No correlation between dose rate and variation in air densities or water level could be seen during this period. 


Fig. 2: Dose-rate in the borehole and water level (arbitrary origin) in the reservoir II 

The borehole being tightened with a packer, a good anti-correlation between water level and dose rate or radon concentration can be seen (Fig. 2). Decreasing water levels or the decrease of vertical load in the bulk of the rocks may open supplementary pathways for radon and induce an increase of radon transport through the rocks. The result of the first measurement campaign in the borehole showed that temperature has no influence on the dose rate; furthermore all borehole measurements were done under forced ventilation conditions. The borehole is drilled into the bedrock and not into the slacks of the fill material, this could be the explanation of the better possibility to show the effect of changing water loads. As the gallery is connected through numerous tubes to the slacks of the fill material, the response to variable stress on air borne radon in the gallery and in the borehole can be different. It is important to notice that during this measurement period, only the reservoir II situated directly above the gallery was filled.




Fig. 3: Dose-rate in the borehole and water level (arbitrary origin) in the reservoir II

Figure 3 shows the results of the measurements in the same borehole under the same conditions but this time with the two reservoirs II and I filled in operation. The anti-correlation of radon versus water level has been changed into a strong correlation. Furthermore the amplitude of the effect is much higher: variations of the radon concentrations are now 8 times higher. It seems that increasing loads from the reservoir I open radon pathways, whereas radon pathways are inhibited due to the water pressure of reservoir II. In normal use, the first effect, being much more important, masks the second. Only by chance for a measuring period, reservoir I was empty and the latter effect could be documented.


Fig. 4: Geology of the site of the Vianden pumped storage power plant, with main fracture directions (Bintz 1964).

An explanation of this discrepancy in response may be given by the geological structures of the bedrocks underlying the reservoirs. As seen on the geological map of Fig. 4, the reservoir II is mainly situated on Lower Emsian E1a schist whereas reservoir I on Upper Praguian (Siegenian) Sg3 schist (Bintz 1964). Reported are many fractures and fissures that could be observed during the levelling works prior to the construction of the reservoirs. Fig 4 gives the main directions of these cracks and fissures (Bintz 1964). The opposite behaviour concerning the influence of load on radon concentrations can be due to the difference in the consistence: coarse sandy clayschist for the Upper Pragian and more clayey schist for the Lower Emsian rocks with a more pronounced layering. Furthermore there is a possible difference concerning the number of cracks and fractures. Clearly, under the reservoir II, an increase of the load of reservoir I opens in the top layers of the bedrock supplementary entry pathways for radon, due to reversible elastic shear stress situations. Further investigations, done in different locations under the reservoirs paired with geophysical investigations may give a clearer view of the rocks affected by reversible strain situations. 

Conclusion

Radon measurements in the air of a gallery under the upper reservoir of the pumped storage station in Vianden (Luxembourg) showed high radon dynamics influenced by inside-outside temperature differences in periods when forced ventilation was turned off. In some rare periods of low radon dynamics it could be shown that an increase of water level or an increase of stress keeps pace with slightly decreasing radon concentrations in the air of a gallery underlying the reservoir. This effect, suggested in the air of the gallery, was better shown by measurements in a radon tight borehole drilled at the bottom of the gallery into the Devonian bedrock. 
Variable water levels in the sole reservoir above the gallery show an anticorrelation of water load and radon concentrations. A changing water level in both reservoirs had a much higher influence on the radon concentrations with a phase shift of 180°: now radon concentrations experience variations in phase with the water level. For the interpretation of the observations the fact that the two reservoirs are situated on different geological structures has to be considered.
Unfortunately, until now, no simultaneous measurements of radon concentrations and other geophysical parameters have been performed. Further measurements are planned, in parallel with other geophysical investigations.

Acknowledgement

The authors would like to express their cordial thanks to the management, specially to director Weis, and the co-workers of the Vianden power station, as well as to the Société Electrique de l'Our (SEO), Luxembourg.


References

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