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THE HAW PROJECT : DEMONSTRATION FACILITY FOR HIGH-LEVEL RADIOACTIVE WASTE DISPOSAL IN THE ASSE SALT MINE

Objective

SINCE 1968 THE GSF HAS BEEN CARRYING OUT RESEARCH AND DEVELOPMENT PROGRAMS FOR THE FINAL DISPOSAL OF HIGH-LEVEL DISPOSAL OF HIGH-LEVEL RADIOACTIVE WASTE (HLW) IN SALT FORMATIONS. THE HEAT PRODUCING WASTE HAS BEEN SIMULATED SO FAR BY MEANS OF ELECTRICAL HEATERS AND ALSO COBALT-60 SOURCES.
IN ORDER TO IMPROVE THE FINAL CONCEPT FOR HLW DISPOSAL IN SALT FORMATIONS THE COMPLETE TECHNICAL SYSTEM OF AN UNDERGROUND REPOSITORY IS TO BE TESTED IN A ONE-TO-ONE SCALE TEST FACILITY.
TO SATISFY THE TEST OBJECTIVES THIRTY HIGH RADIOACTIVE CANISTERS CONTAINING THE RADIONUCLIDES CS-137 AND SR-90 WILL BE EMPLACED IN SIX BOREHOLES LOCATED IN TWO TEST GALLERIES AT THE 800 M-LEVEL IN THE ASSE SALT MINE. THE DURATION OF TESTING WILL BE APPROXIMATELY FIVE YEARS.
FOR HANDLING OF THE RADIOACTIVE CANISTERS AND THEIR EMPLACEMENT INTO THE BOREHOLES A COMPLETE DISPOSAL SYSTEM CONSISTING OF TRANSPORTATION CASKS, TRANSPORTATION VEHICLE, DISPOSAL MACHINE AND BOREHOLE SLIDER WILL BE DEVELOPED ANS TESTED.
THE ACTUAL SCIENTIFIC INVESTIGATION PROGRAM IS BASED ON THE ESTIMATION AND OBSERVATION OF THE INTERACTION BETWEEN THE RADIOACTIVE CANISTERS AND THE ROCK SALT.
THIS PROGRAM INCLUDES MEASUREMENT OF THERMALLY AND RADIOLYTICALLY INDUCED WATER AND GAS RELEASE FROM THE ROCK SALT AND THE RADIOLYTICAL DECOMPOSITION OF SALT MINERALS. ALSO THE THERMALLY INDUCED STRESS AND DEFORMATION FIELDS IN THE SURROUNDING ROCK MASS WILL BE INVESTIGATED CAREFULLY.
THE PROJECT IS FUNDED BY THE BMFT AND THE CEC AND CARRIED OUT IN CLOSE COOPERATION WITH THE NETHERLANDS RESEARCH FOUNDATION (ECN). SINCE 1988 THE FRENCH AGENCE NATIONALE POUR LA GESTION DES DECHETS RADIOACTIFS (ANDRA) IS PARTICIPATING IN THE FIELD OF DOSE AND DOSERATE MEASUREMENTS AND OF LABORATORY IRRADIATION EXPERIMENTS AND IN SITU INCLINOMETER MEASUREMENTS. ALSO THE SPANISH EMPRESA NACIONAL DE RESIDUOS RADIOACTIVOS (ENRESA) IS PARTICIPATING IN THE SALT IRRADIATION PROGRAMME.
The thermal conductivity of rock salt affects the results of the temperature increase in a HLW repository. A special apparatus was designed and built at the Institut fuer Tieflagerung in Braunshsweig, Germany to measure the thermal conductivity of rock salt. This device was designed to measure the mean axial heat flow through 2 cylindrical rock specimens arranged above and below a round flat heating plate. The thermal conductivity of the rock salt samples was measured at a constant probe temperature. To cover the temperature range from 0 to 200 C, tests at different constant temperatures performed. The thermal conductivity was measured twice in 2 rock salt specimens from the Asse salt mine at each of five temperatures. The resulting relationship between thermal conductivity and temperature can best be approximated to the straight line function.

Rock salts samples from the Asse mine were analysed using ultrasonic velocity measurements in the frequency range from 0.25 to 1.5 MHz. Longitudinal and transverse velocities in 3 orthogonal directions were measured on each specimen taken from the test field. The mean density of the 127 specimens was calculated as 2174.57 kilograms per cubic metre. Dynamic elastic constants, calculated from sonic velocities, were found to have an average for the Young's modulus of 33.4 GPa and for the Poisson's ration of 0.29. Distinct local differences and significant changes of velocities with depth were not found.

The dilantancy of rock salt is an important material property. In nature, for example, the rock salt surrounding the excavated gallery of a shaft dilates radial to the axis of the excavation, starting from a uniform stress field. Dilantancy is associated with a change of porosity and permeability and may lead to partial or total failure.

In this research, studies of rock salt dilantancy were carried out using the compression mode. Triaxial testing machines were used. The specimens tested were all made from cores extracted from borehole in the Asse mine and did not include any anhydrite bands. The tests were performed in 2 phases. First, a homogenous stress field of 2.5, 5.0 or 10 MPa was gradually applied, and, second, the stress was increased until rupture took place. 10 tests were conducted.

Dilatancies of up to 3% were measured. The complete results can best be shown by plotting lines of constant density in an octahedral diagram. All the curves had the same character as the failure curve and touched it at nearly the same angle.

The main aims of the research were:
the determination of the temperature, stress and time ranges over which a steady state creep law could be reliably applied;
the determination of the steady state creep parameters of the rock salt in the Asse mine and the variation of these creep parameters in space.
These are essential input parameters for numerical models used to predict the behaviour of nulclear repositories. The creep tests were performed with uniaxial cells, specially designed for long lasting creep experiments. The cells were built to test large specimens of relatively weak materials, such as rock salt at normal stresses up to 20 MPa and temperatures up to 200 C.

Data provided by the constant force tests were used to evaluate the influence of transient creep on the steady state creep tests. Probes used for laboratory creep tests were taken in the process of drilling test and observation holes in the Asse mine. 15 specimen were tested under 37 different conditions. The temperatures were varied between 40 and 120 C, and normal stresses between 4.2 and 17 MPa were applied. The averaged creep parameters found were relatively close to the average of all the steady state creep parameters calculated previously for the older rock salt in the Asse mine.

Failure under triaxial stress conditions is of great importance to experiments in highly stressed rock salt. In the case of the high level radioactive waste (HAW) programme a failure criterion for the temperature range from 20 to 200 C and confining pressures from 1E5 to 7E7 Pa is of special interest. Failure conditions depend on the loading paths of the rock specimen and have to be expressed by invariants of stress or invariants of stress deviations. Two parabolic failure envelopes, one for extension and one for compression, are expected.

Triaxial compression tests and extension tests on cylindrical rock specimens from borehole W3 in the HAW test field, the Asse cavern and comparable rock salt from Gorleben salt dome were performed between 1987 and 1989. The results of this triaxial tests are summarised in the equation of the failure envelops where tau (oct), the octahedral shear stress, is equal to b (sigma(oct)/sigma(*)){p}.g(phi m). sigma(oct) is the octahedral normal stress (MPa), sigma(*) the normal stress (MPa), g(phi m) the stress pass factor (the way in which the final state of stress is approached) and b and p are the characteristic parameters. The stress exponent, P, was found to vary between 0.58 and 0.82 and was not constant for different loading parameters.

The aim of the research was to investigate the porosity and permeability of rock salt for use in developing a computer code on the diffusion and migration of generated and liberated gases in the vicinity of an emplacement borehole. Salt samples from different predrilling depths of the emplacement boreholes were taken.
The samples were enclosed in a rubber jacket and a steel piston to determine their permeability. They were pressurised hydrostatically to between 10 and 200 bar using oil. Nitrogen with a pressure up to 70% of the hydrostatic oil pressure was led through the piston and the salt sample. After determining the gas flow through the sample the permeability was calculated by the Darcy equation. At a number of oil and gas pressures, the gas flow through each sample was measured for 150 to 200 hours to get stationary conditions.

It was found that plastic deformation of the salt sample had taken place. The investigation gave permeabilities of between 1E-16 and 1E-20 metre squared. The parameters gas and petrostatic pressure have a big influence on theses results. At a constant hydroststic pressure of 10 MPa the gas permiability decreases within 30 days by a factor of 10 as a result of consolidation and plasticity of rock salt. By increasing the hydrostatic pressure by the factor of two, the permiability decreases by the factor of 10 to 100. The gas pressure has a comparitively small influence to the permiability.

Sufficient results exist concerning the porosity, permiability and the gas generation but only little information is available on the relation of Darcy and Knudsen flow, and the effects of adsorption/desorption.

Rock salt formations are candidates for the geological disposal of high level radioactive waste. The initial planned programme involved the HAW experiment in the Asse salt mine. This was terminated in 1992 by the German Government and resulted in the increase of laboratory experiments. In these experiments not only sythetic but also a large amount of rock salt samples have been irradiated.

The radiation induced formation of colloidal sodium, radiation damage, has been studied using various techniques such as: light absorption; measurement of hydrogen evolution and OCI formation on dissolution of the irradiated samples; microstructural analysis; differential thermal analysis.

One of the most important results was that a saturation of damage has been found for natural rock salt at a level that corresponds to 1.7 mol% of decomposition of the salt. It was also shown that the convesion factor between stored energy and defect concentrations should be around 80 J/g per mol% defects.

In some cases the experimental results have been compared with calculated results obtained from computer simulations of the experiments using the Jain-Lidiard or Jain-Lidiard related model. Computer simulations of the damage expected in a repository using this model show that the damage levels in rock salt, even very close to the HLW containers, will be limited to a few mol%. The specific stored energy in the rock salt in a repository is theefore expected not to be higher than 300 J/g.

The general conclusion concerning radiation damage development in the crystals of rock salt is that it will not constitute a aafety problem in the eventuality of geological disposal of HLW in deep boreholes.

Since rock salt will be exposed to neutron irradiation in a waste repository and because this subject was not covered in former research projects a theoretical study was carried out to estimate the rate of formation of radionuclides and the amount of secondary gamma radiation generated. Salt with different mineralogical compositions were modelled.

The main result was that for any given HAW canister with a defined source strength and a known energy spectrum, the nature of the neutron induced nuclear reactions can be confidently predicted. The neutron energy spectrum and the distribution of gamma quanta were estimated. The amount of these secondary quanta was not important compared to the activation of the rock salt. It was also found that the highest activities were found when trace minerals were distributed inhomogeneously throughout the rock salt.

Irradiation of rock salt leads to an increase in gas release. The influence of various parameters on gas yield was studied and the data revealed the following: increasing irradiation dose results in most cases in increasing gas yield:
the dose rate does not seem to influence the gas release between 1E2 and 1E5 gray per hour:
hydrogen cannot accumulate in an air containing gas phase;
carbon monoxide is radiolytically destroyed;
high irradiation doses increase the additional carbon dioxide yield released by postirradiation heating.

The aim of the research was to determine the total amount of gas, which can be released from a given volume of rock salt. This information is an important prerequisite for an assessment of the long term safety of a nuclear waste repository. Measurements were not performed on samples obtained from drillings in the Asse mine. The cores were sealed in gastight flasks under a nitrogen atmosphere immediately after drilling. The gas components were analysed using gas chromatograph.

In addition a computer model for the gas release was developed at RWTH Aachen. According to the model the major source of gas release is represented by gas adsorbed on intercrystalline surfaces. During the initial testing at the RWTH Aachen this computer model was successfully employed to calculate existing data on the gas release observed during a former in situ test in the Asse mine.

The objective of this research is the observation and recording of cataclastic effects around a test borehole by means of so called cross hole measurements. Fans of acoustic P-wave velocities are determined, providing information about the structural changes in the rock salt. The measuring system consists of 5 sets of rubber tubes installed in each time parallel bore holes. The transducers are vulcanized in the tubes at mutual distances of 500 mm. Each tube of 5 m length contains 10 piezoelectric transmitters or receivers (20 to 80 KHz). For registering of the acoustic signals, two 10-channel, 8-bit transient recorders are used. The sample frequency is 2 MHz, the measured spectrum has a length of 2 ms and is averaged 64 times. The transmitters are activated by a blockpulse of 250 V and a length of 10 us. Temperatures are measured in 3 points in each measuring tube. The measurements confirm that macrofracturing does not occur along the measuring lines. Close to the wall the average velocities are lower, due to structural changes in the salt. The temperature increase in the floor hardly influenced the temperature distribution in the wall and pillar. The general picture of the structural changes shows a tendency of gradually decreasing velocity values.

2 important characteristics of the high atomic waste (HAW) testfield are its high radiation level and its heat generation. To assess the effects of these characteristics the field has been provided with a large number of different measuring instruments, amongst others thermocouples (for temperatures), etc. The resulting large number of about 1000 signals to be monitored require the presence of an automated data collection system (DCS). A number of tasks are carried out by this DCS, namely the collection of data from about 1000 different signals, the monitoring and control of a variety of equipment needed for the data collection, the control of alarm limit values and the transmission of collected data from the HAW-field.
By control of the HAW field is meant the monitoring of the gap width between canisters and guiding tube and control of heater power for a stable field temperature. These control functions are based on collected equipment status data as well as on so called safety data, which consists of gas pressures in the tubes, tube temperatures and measured signal values of the gap width. For these safety data limit values are present. If the actual signal value exceeds such a limit value, an alarm is produced by the DCS.
The DCS consists of 2 dual parts hardware and software. Furthermore parts of the hardware are present at -800 m level (2 so called front end processors) as well as at 0 m level (2 so called host computers). For reasons of reliability the hardware system design is made redundant and at the -800 m level hardware equipment is manufactured to military specifications as well as to high temperature specifications. After some minor startup problems the system has operated satisfactory.

In the research program of the high atomic waste (HAW) testfield it was decided to investigate the irradiation effect on rocksalt of different composition. For this purpose the dummy canister (DC), which is intended for shielding of radiation has been designed in such a way that it can contain samples of rocksalt. In several bore holes the dummy canister will be placed on top of the radioactive canister stack that can be taken out at desired intervals. In each DC salt samples will be placed to be irradiated at different irradiation dose levels and a different temperatures representative for a repository.
A sample holder, which will be placed inside the DC able to withstand pressures up to 200 bar, has been developed. 240 of these sample holders have been produced. At the University of Utrecht the preparation of the salt samples is in progress together with texture analyses of Spanish salt samples.
A test to quantify the effect of the pressure produced by the sample holders, on the microstructure of the samples has been designed. This is necessary because the samples will first be placed under pressure after which they are transported to the mine. In the dummy canisters, the temperature and pressure will increase further. By experience it is known from the BMT (Brine Migration Test) that microstructural observations can not be regarded as hard proofs for recrystallization. Consequently a good record is needed of the position of the grain boundaries at the moment of the start of the experiment will tell if grain boundaries moved by a significant amount. The planned experiments will make it possible to compare samples before and after the pressure effect of the holder, and before and after the pressure and temperature effect of the canister.
B.1. ELABORATION OF THE TEST PLAN AND THE SUPPORTING DOCUMENTS FOR THE LICENSING PROCEDURE.
B.2. DEVELOPMENT AND PROCUREMENT OF THE TECHNICAL COMPONENTS FOR HANDLING AND EMPLACEMENT OF THE RADIOACTIVE CANISTERS.
B.3. PROCUREMENT AND INSTALLATION OF THE DATA COLLECTION SYSTEM.
B.4. MINING OF THE TEST FIELD, DRILLING OF THE BOREHOLES, INSTALLATION OF THE MEASURING EQUIPMENT AND PREPARATION FOR THE EMPLACEMENT OF THE HLW CANISTERS.
B.5. TEST DISPOSAL INCLUDING OPERATION OF ELECTRICAL TESTS FOR COMPARISON AND ASSESSMENT OF THE TECHNICAL COMPONENTS.
B.6. IN SITU MEASUREMENTS OF RELEASED WATER AND GAS FROM THE SALT, OF THERMALLY INDUCED STRESS AND DEFORMATION IN THE ROCK MASS, AND PERFORMANCE OF SEISMIC AND ULTRASONIC MEASUREMENTS.
B.7. ACCOMPANYING AND COMPLEMENTARY LABORATORY INVESTIGATIONS TO ENSURE THE TRANSFERABILITY OF THE RESULTS TO OTHER SITES.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

GSF - FORSCHUNGSZENTRUM FUER UMWELT UND GESUNDHEIT GMBH
Address
Ingolstaedter Landstrasse 1
85764 Neuherberg
Germany

Participants (1)

ECN
Spain
Address

Madrid