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IN-SITU STUDY OF RADIONUCLIDE DIFFUSION IN CLAYS BY MEANS OF THE AUTOLAB PROBE.

Objective

THE STORAGE OF NUCLEAR WASTES IN A DEEP GEOLOGICAL FORMATION CALLS FOR THAT A DEPENDABLE KNOWLEDGE OF THE SITE SAFETY IS GAINED. SIGNIFICANT DATA ON THE RADIONUCLIDE DIFFUSION THROUGHOUT ARGILLACEOUS MATERIALS SHOULD BE ACQUIRED BY IN-SITU OBSERVATIONS AND MEASUREMENTS IN A DRILL-HOLE, PROVIDED THAT THE CONDITIONS OF THE RECEPTOR ENVIRONMENT ARE SAFEGUARDED.

IN THE FIELD OF THE IN-SITU MEASUREMENTS, TWO FAMILIES OF INSTRUMENTS PRESENTLY EXIST OR ARE UNDER DEVELOPMENT :

A) EQUIPMENT MAKING IT POSSIBLE A DIRECT MEASUREMENT OF THE ENVIRONMENT CHARACTERISTICS, SUCH AS PH, EH, CA++ CONTENT, TEMPERATURE, RADIO-ACTIVITY, RESISTIVITY. THOSE PROBES MAKE IT POSSIBLE TO OBTAIN A LOGGING OF THE PREVIOUSLY-MENTIONED PARAMETERS IN A DRILL-HOLE CHROMATOGRAPHY PROBE, FOR THE INDIRECT MEASUREMENT OF CHEMICAL ELEMENTS PRESENT AS TRACES IN THE DRILL-HOLE, IS PRESENTLY BEING DEVELOPED.

B) THE SECOND CLASS OF PROBES IS RELATED TO THE EXPERIMENTATION IN A DRILL-HOLE TO OBTAIN THE VELOCITY OF RADIONUCLIDE MIGRATION IN THE FOURTH BARRIER. THE VELOCITY OF THE WATER WHICH IS THE VECTOR OF THE RADIOACTIVE POLLUTION MAY VARY IN WIDE PROPORTIONS.
FIRST, THERE IS THE DOMAIN OF THE CONVECTION-DISPERSION WHICH IS THAT OF THE FORALAB PROBE PRESENTLY USED, SECOND, THE DOMAIN OF THE DIFFUSION WHERE THE VERY SLOW PROGRESS OF THE PHENOMENA REQUIRES THAT THE IN-SITU EXPERIMENT COMPLIES WITH SPECIAL SPECIFICATIONS.

C) REPRESENTATIVENESS OF THE IN-SITU EXPERIMENTS

THE REPRESENTATIVENESS OF THE IN-SITU TESTS IS RELATED TO THE DRILL-HOLE WATER CHARACTERISTICS BECAUSE, AS THE HOLE IS BEING DRILLED, WATER IS USED WHICH IS ENTIRELY FOREIGN TO THE ENVIRONMENT. MOREOVER, THE HOLE MAY ALSO FILL ITSELF WITH WATER FROM A HIGHER-LYING WATER TABLE.

FROM THE CHEMICAL POINT OF VIEW, HOWEVER, AS CHEMISTRY IS THE CRITERION FOR THE RADIONUCLIDE CONDITION, AN EQUILIBRIUM IS GRADUALLY GAINED BETWEEN THE FOREIGN WATER AND THE GEOLOGICAL ENVIRONMENT WITH WHICH IT IS IN CONTACT.

1. AUTOLAB IS A PROBE DESIGNED FOR MEASURING THE DELAY TERM OF THE REDIONUCLIDE IN-SITU IN THE DRILL-HOLE CLAYS.

2. SPECIFICITY OF THE AUTOLAB PROBE
THE PROBE IS DESIGNED FOR DIFFUSION MEASUREMENTS, WHICH ENTAILS PARTICULAR CHARACTERISTICS FOR GRAPPLING THE SLOWLINESS OF THE PHENOMENA.
THE SPECIFICATION DATA REQUIRED FROM THE PROBE ARE AS FOLLOWS :
- OPERABILITY IN A DRILL-HOLE WITHOUT TIME LIMITATION (IN PRACTICE, 6 MONTHS),
- COMPLETE AUTONOMY, NO SUPERVISION, NO POWER FEED,
- GOOD EXCHANGE BETWEEN THE DRILL-HOLE WATER AND THE INSIDE OF THE PROBE UNDER STATIC CONDITIONS (NO PUMP OPERATING)
- NO POLLUTION OF THE ENVIRONMENT

3. DESCRIPTION OF THE PROBE
THE PROBE IS COMPOSED OF A REACTION CHAMBER, OF 5 L. CAPACITY (65 MM DIA., 1500 MM HIGH), CAPABLE OF CONTAINING 10 SAMPLES (APPROXIMATELY 10 MM DIA., 50 MM HIGH).
THE TRACER IS CONTAINED IN A GLASS BULD WHICH WILL BE RUPTURED BY A WEIGHT (MESSENGER) SLIDING ALONG THE CARRIER CABLE.
THE REACTION CHAMBER MAY BE PUT IN COMMUNICATION WITH THE DRILL-HOLE BY WINDOWS (36 SQ.CM SURFACE) WHICH MAY BE ACTUATED USING THE DRILL-HOLE PRESSURE. IT SUFFICES FOR THIS TO LOWER THE PROBE BY A FEW METERS, WHICH ACTS UPON A HYDRAULIC VALVE HELD CLOSED BY A SPRING SET AT A PRESSURE CORRESPONDING TO THE DRILL-HOLE DEPTH; THE VALVE, WHEN OPENING, PERMITS THE ACTION OF THE HOLE WATER ON A PISTON OPENING OR CLOSING THE WINDOWS.

4. EXPERIMENT
THE PROBE WILL BE USED FOR MEASURING THE IN-SITU MIGRATION OF RADIONUCLIDES IN CLAYS.

A) EQUILIBRIUM STAGE
IN A FIRST STAGE THE ARGILLACEOUS MATERIAL SAMPLES (IN 10 MM DIA., 50 MM HIGH TUBES) ARE LEFT TO COME INTO AN EQUILIBRIUM WITH THE DRILL-HOLE WATER. THE WATER DIFFUSES INTO THE MATERIAL; A DIFFUSIVE FRONT OF EQUILIBRIUM IS FORMED AND WILL SLOWLY PROGRESS FOR SEVERAL MONTHS.

B) EXPERIMENTAL STAGE
WHEN THE FOREGOING STAGE IS COMPLETED, THE WINDOWS GET CLOSED, THE TRACER (DOUBLE TRITIUM-LANTHANIDE OR ACTINIDE MARKING) IS RELEASED FROM THE GLASS BULB USING THE MESSENGER (THIS ALSO PUTS AN ELECTRI CELL-DRIVEN AGITATOR INTO ACTION)
THE TRACERS ARE DIFFUSED INTO THE CLAY. A SECOND DIFFUSION FRONT IS FORMED, OFFSET WITH RESPECT TO THE FIRST ONE AND PROPAGATING IN A ZONE ALREADY SETTLED TO A CHEMICAL BALANCE WITH THE DRILL-HOLE WATER.

5. TESTS PLANNED
- QUALIFICATION OF THE PROBE ON THE AURIAT SITE
SAMPLES ARE SELECTED WITH VARIOUS SAND CONTENTS AND VARIOUS NATURES OF ARGILLACEOUS MATERIALS.
THE TRACERS WILL BE TRITIUM AND EUROPIUM.
THE EXPERIMENTS WILL TAKE PLACE :
* IN THE GRANITIC SITE OF PARTENAY
* IN THE SALINE SITE OF GOERLEBEN
* IN THE ARGILLACEOUS SITE OF MOL
USE WILL BE MADE OF TRITIUM, OF AN ACTINIDE OR, IF UNAVAILABLE, OF A LANTHANIDE.
THE NUMBER AND COMPOSITION OF THE SAMPLES WILL BE DETERMINED FOR EACH PARTICULAR CASE. FILLING MATERIALS MAY BE USED, AS NECESSARY.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Commissariat à l'Energie Atomique (CEA)
Address
Centre D'études De Cadarache Sere-ders
13113 Saint-paul-lez-durance
France