Research and Mining Activities for Nuclear Waste Management in Hungary

(Extended abstract)

 

István Szűcs, Dr.^a

 

^aMECSEKÉRC Ltd.

Pécs, Hungary

 

1.          Introduction

According to Article 40 of the Act No. CXVI. on Nuclear Power from 1996, the Government is liable to execute the tasks of final disposal and interim storage of Hungarian radioactive wastes and decommissioning of the nuclear facility. The related activities are financed by the Central Nuclear Financial Fund, which is a separated state financial fund. The manager of the Fund is the Hungarian Atomic Energy Agency, which established, the Public Agency for Radioactive Waste Management (PURAM), to accomplish the related tasks. The ongoing research programmes include two issues: the near surface facility for low and intermediate level radioactive wastes (L/ILW) in the Morágy Granite Formation (MGF) and the deep geological disposal of spent fuel (SF) and high level radioactive wastes (HLW) in the Boda Claystone Formation (BCF), both carried out and financed in form of mid- and long-term plans and investigation programmes.

2.          Waste inventory of HLW and conditioned L/ILW

The sources of the national radioactive waste are diverse. The highest amount of waste is produced by the only one Hungarian nuclear power plant, the Paks Nuclear Power Plant (Paks NPP), with its four VVER–440 reactors, generating approximately half of the nation’s electricity. Paks NPP was planned to produce energy for 30 years. Due to its important role in the Hungarian energy production and the lack of substitutive sources, a 20-year period of extension in the operation is planned but still not decided.

Small amounts are produced by research facilities like the training reactor of the Budapest University of Technical and Economical Sciences, Institute of Nuclear Technology and the research reactor in the Atomic Energy Research Institute of the Central Physical Research Institute.

 

Table 1. The amount and origin of Hungarian nuclear waste

WASTE TYPE	AMOUNTS
	30 yrs operation time	50 yrs operation time
HLW [1]
spent fuel (Paks NPP)	11 266 pieces	18 706 pieces
spent fuel (other sources)	3 225 pieces	3 225 pieces
operational wastes	173 m3	263 m3
decommissioning wastes	247 m3	247 m3
Püspökszilágy RWTDF a	100 m3	100 m3
conditioned L/ILW [2]
solid waste	2 547 m3	ND
ion exchange synthetic resin	639 m3	ND
evaporation residue	16 067 m3	ND
other liquid wastes	1 649 m3	ND

^a Radioactive Waste Treatment and Disposal Facility

3.          The research projects

(a)          The BCF programme for the final disposal of spent fuel and high-level radioactive waste started as a preliminary characterization programme between 1989-1992, which was followed by the Alfa Project (1993-1995) and the Short Term Project (1996-1999). These latter two projects were carried out underground in the so-called Alfa shaft, which reached the BCF through an investigation shaft driven from the sandstone block of the former uranium mine near the city of Pécs. The technical co-ordinator of the underground research was MECSEKÉRC Ltd., the official controller was PURAM. The results were summarized in a 10-volume report published by MECSEKÉRC Ltd. in 1998. In 1999 the shafts and tunnels of the uranium mine were filled back, therefore the access to the Alfa shaft was blocked irrecoverably. The two phase (the site selection and the site characterization phase) Middle Term Project, started in 2003, is planned to have been completed by 2008.

(b)          The MGF programme for the final disposal of low- and intermediate-level radioactive waste started with site selection in 1993 and ended in 1996. The results of this period were published in the Annual Report of the Geological Institute of Hungary (MÁFI), 1996/II and were followed by the site characterization running till 1999. The ground-based exploration was carried out in 2002-2003 under the responsibility of Bátatom Ltd., a consortium of four institutes and companies: Geological Institute of Hungary (MÁFI), ETV-Erőterv, Golder Associates Hungary and MECSEKÉRC Ltd.

4.          The main goals of the MGF research programme

The research activities for the subsurface investigation phase started in 2004 have different goals:

(a)          To find and locate a suitable rock body for the repository. This means that the investigations focus on the expected repository level after the establishment the inclined tunnels. To reach the goal all necessary measures shall be taken, including additional studies from the ground surface.

(b)          To characterize the selected rock to provide data for the design and construction of the repository and the safety analysis. It is necessary to have a preliminary layout to position the characterization boreholes in a reasonable way to avoid disadvantageous hydraulic connections within the repository volume.

(c)          To better understanding the geology, tectonics, geotechnics and hydrogeology of the site. The access tunnels will provide a good opportunity to study these topics in the actual environment. Their construction can be regarded as a training phase to learn how the rock mass actually behaves and influences the surroundings.

5.          Investigation of MGF at Bátaapáti

The complementary survey at Bátaapáti was carried out in the scope of the geological exploration aimed at the final disposal of low- and intermediate-level radioactive waste. The measurements were related to the drilling activity, in 2002-2003, and were made in the centralpart and its closely connected area.

The conditions of the area from a geophysical aspect are unfavourable. The physical parameters of the loess, which is a 40-60 m thick cover on the granite body, makes it hard to apply both electromagnetic and seismic methods to investigate the granite mass and the Bátaapáti site.

The results of geophysics are unusable without geological or hydrogeological explanation. Fortunately, examinations conducted by other methods in the area produced a large amount of data, thus facilitating the geophysical interpretation.

The geophysical methods used did not image the geological structures directly but the physical variations of the rock were explored. Parameters measured by geophysical methods  usually represent average values of a space domain determined by the resolution.

5.1      Seismic  tomography

Seismic tomography is an image reconstruction technique. If measured data are line integrals of the observed physical quantity, the distribution of the physical quantities of the inner structure can be determined from measurements carried out along the boundary of the given domain. Such a kind of connection between wave propagation types and the reciprocal of the velocity, and between the logarithm of the amplitudes and the absorption, is known from seismic studies. The distribution of velocity and absorption can be determined by seismic tomography when the propagation times and amplitudes between shot points and geophones are measured along ray paths crossing each other. To get a reliable profile of adequate resolution the observed area must be covered uniformly by a multitude of rays in conformity with direction and number.

At the Üveghuta Site seismic tomographic measurements were carried out between adjoining pairs of boreholes in the technically executable depth ranges. To calculate the velocity propagation parameters in the granite only the data from the boreholes were used. If the sources or receivers were to be placed into the low-velocity loess layers the tomographic data system would be charged with considerable errors. This is because the thickness of the loess can be determined at only one cell precision and this time delay is comparable with the total runtime in the granite. The starting model and the boundary conditions for the SIRT (Simultaneous Reconstruction Technique) computer algorithms were provided by PSQ and PQ seismic borehole data (where P and S are seismic body waves, Q is the quality factor). The computation is based on the modification of the wave propagation parameters along raypaths, which cross each other in the space domain between boreholes until the misfit between computed and measured parameters is minimal.

The resolution of tomography between boreholes is direction-dependent, especially in the case of large borehole distances because of the partial absence of near vertical rays; consequently steep elements are not imaged. Another inherent characteristic of imaging is that the accurate velocity of a small-sized, low-velocity structure is not mapped adequately by the tomography: it is “smeared” because the rays do not cross the given structure (Fermat’s principle).

The results of the tomographic measurements at the site show some spots or stripe-like low- or high-velocity granite bodies. The structures are considered to be 2D because of the lack of 3D data. Most of the low velocity bodies can be observed at rather shallow depths. In boreholes Üh-23-Üh-2-Üh-22-Üh-3, where the geometry was the most favourable, the velocity and absorption tomographic sections resolved even steep dipping elements [3] [4]. It can be inferred from resolution parameters that tomographic spots and forms of zones do not necessarily display the peculiarities of the parameters recognized in drill-core or well logs. With this method changes comparable to the wavelength can be observed. These changes are caused by the granite material, the fissures in the granite, the fissure infillings, the direction of fissures etc.

Experiences at the Üveghuta Site show that seismic velocity and absorption are less affected by the rock stresses and the rock material, but they depend definitely on the rock-mechanical conditions.

The results of velocity and absorption tomography should be interpreted together: their data along the boreholes are in good correlation with smoothed, averaged well-log data principally with electric resistance, acoustic and seismic velocity sections.

5.2      Imaging the granite inside

Geophysical information with the best resolution is provided by the well-logging applications because they made measurements in the immediate vicinity of the observed material. In spite of the good resolution of the methods, the measured parameters do not generally correlate, even in closely spaced boreholes. For the macro-level spatial description of the extent of the granite body, the number of boreholes and the interpretation of well-logging data are insufficient altogether.

There is a special character of the granite body beyond what can be observed in the boreholes and that is the variability in the scale between boreholes. This can be imaged with the best resolution by seismic cross-hole methods.

Lacking other possibilities the tomographic results were evaluated as phenomena with clear changes in the plane of the boreholes, while the changes of the actual granite show 3D features. This is verified by seismic tomographic sections which were measured in the nearby planes of Üh-3-Üh-23, and Üh-3-Üh-22-Üh-2-Üh-23. Comparing the suitability of methods for extension of the attributes to the area inside and between the boreholes, the magnetotelluric method, 3D seismic first break tomography and S-wave reflection profiling are suitable from point of view of the order of resolution at the investigated area. The integrated interpretation of the borehole tomography outlined the inner structure of the granite. The granite bodies are defined by seismic tomography (Fig. 1).

 

Fig. 1.   Absorption tomographic section between boreholes

5.3      Conclusions

The methods described contributed fundamentally to the investigation of the surface and to the knowledge of the spatial characteristics of the granite. The granite in the boreholes, and in its extended surroundings, can be well characterized by seismic tomography. The weakened zones between the bodies could have importance from the point of view of water conduction and mining activity based on “design as you go” practice.

References

[1]     Takáts, F., Baksay, A., Back-end stratégia elemeinek tisztázása, a várható hulladék mennyiségeinek számítása (Report on the elements of the back-end strategy and the potential inventory of spent fuel and high level radioactive waste in Hungary). – Manuscript, TS Enercon, Budapest, Hungary, (2004)

[2]     Takács, T. et al., Összegző biztonsági jelentés, (BA-04-18), (Safety Case, Summary), Hungary (2004)

[3]     Prónay, Zs., Tőrös, E., Jelentés a szeizmikus sebességtomográfiáról (Report on the seismic velocity tomography). Manuscript, Eötvös Lóránd Geophysical Institute of Hungary, Budapest, Hungary, (2003)

[4]     Szűcs, I., Menyhei, L., Gacsályi, M., Jelentés az üveghuta körzetében 2002-2003-ban végzett abszorpciós tomográfiai mérések feldolgozásáról (Report on the processing of the seismic absorption tomography conducted in the vicinity of Üveghuta in 2002-2003). – Manuscript, Geopard Kft., Pécs, Hungary, (2003)