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LONG TERM DIFFUSION IN ROCK- NATURAL ANALOGUE

Objective

DIFFUSION THROUGH ROCK IS IMPORTANT AS A MECHANISM BOTH FOR RADIONUCLIDE TRANSPORT IN ROCK WITHOUT WATER FLOW AND FOR RETARDATION OF TRANSPORT IN FLOW THROUGH FRACTURED ROCK. LABORATORY MEASUREMENTS OF DIFFUSIVITY AND POROSITY OF VARIOUS ROCKS OF INTEREST ARE BEING MADE BUT, BECAUSE OF THE LOW VALUES BEING OBTAINED, MEASUREMENTS CANNOT BE OBTAINED OVER MORE THAN CENTIMETRE DISTANCES IN CONVENIENTLY SHORT TIMES.
IT IS UNKNOWN WHETHER OR NOT THE PORES THROUGH WHICH DIFFUSION OCCURS IN CENTIMETRE TEST SPECIMENS ARE CONTINUOUS THROUGH GREATER THICKNESSES OF ROCK. THE OBSERVED POROSITY MAY BE EITHER ALL OF OPEN, INTER-CONNECTED PORES OR PARTLY OF PORES CLOSED AT ONE END. IF SOME OF THE PORES WHICH ARE CONTINUOUS THROUGH THE CENTIMETRE SPECIMENTS TERMINATOVER GREATER DISTANCES, A MODEL ASSUMING ALL THE MEASURED POROSITY TO BE OPEN IS INVALID.
THE PRINCIPLE OF THE METHOD TO BE USED IS TO MEASURE THE DIFFUSION OF CHLORIDE FROM THE SEA INTO ROCKS WHICH HAVE BEEN BELOW WATER LEVEL IN HARBOURS AND SEA WALLS FOR KNOWN, LONG TIMES. CORES OF THESE ROCKS WILL BE OBTAINED AND SECTIONED. ANALYSES OF THESE SECTIONS FOR CHLORINE WILL PROVIDE THE REQUIRED DATA.
THE OBJECTIVES OF THIS CONTRACT ARE THEREFORE TO STUDY DIFFUSION WHICH HAS OCCURED NATURALLY OVER LONG DISTANCES AND TIMES AND TO TEST THE VALIDITY OF THE DIFFERENT MODELS COMPATIBLE WITH THE SHORT-DISTANCE LABORATORY DATA.
Solute diffusion profiles for chlorine, bromine, fluorine and sulphate ions have been measured in a granite block which was immersed in the sea at Falmouth, Cornwall for 30 years. Leachable concentrations of chlorine and bromine were found to be higher in the block than in quarry samples of granite, which demonstrated that solutes from the sea water had diffused into the block.
The chlorine and bromine ion profiles within the block were flat, implying that equilibrium had been reached between the seawater and granite porewater. Concentration profiles for fluorine and sulphate ions indicate that these species have high concentrations at the margins of the block (to depths of up to 15 centimetres) and are in the process of diffusing outwards into the surrounding seawater.
This preliminary experiment has demonstrated the potential for the measurement of solute migration in granite, as a result of the rock having been immersed in seawater.
B.1. IDENTIFY A SITE WHERE A GRANITE BLOCK, FROM A KNOWN LOCALITY, HAS BEEN IMMERSED IN SEA WATER FOR A KNOWN LENGTH OF TIME.
B.2. OBTAIN PERMISSION FROM THE RELEVANT AUTHORITIES TO RETRIEVE THE GRANITE BLOCK.
B.3. LIFT BLOCK FROM THE SEA AND DRILL THROUGHT IT TO OBTAIN CORES.
B.4. CUT THE CORE INTO SECTIONS AND MEASURE THE AMOUNT OF CHLORIDE WITHIN THE PORE WATER OF EACH SECTION.
B.5. CALCULATE THE EFFECTIVE DIFFUSION COEFFICIENT OF THE GRANITE FROM ANALYSIS OF THE CHLORIDE CONCENTRATION PROFILE WITHIN THE BLOCK.

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Coordinator

United Kingdom Atomic Energy Authority
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353,Harwell
OX11 0RA Didcot - Oxfordshire
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