THE METHOD OF MEDIUM AND LOW LEVEL RADIOACTIVE WASTE
PLACEMENT INTO LARGE DIAMETER WELLS IN CLAY
GEOLOGICAL FORMATIONS
I.A.Sobolev, L.B.Prozorov, A.V.Tkatchenko, V.V.Martianov
Mos SIA "Radon"
ABSTRACT
Today the problem of reliable isolation of radioactive waste is getting more and more actual, and the ecological safety requests are getting more and more hard. The near surface RAW storage facilities are used for low and intermediate level radioactive waste (LLW/ILW) placement. The Russian long-term experience of low and intermediate level of activity waste placement has shown, that the set of engineering and natural factors influences on protective properties of near surface facility (engineering barriers) This fact essentially reduces reliability of RAW isolation. Besides the use of near surface facilities causes exception of large ground areas. These problems can be decided with placing radioactive wastes into large diameter wells, drilled in clay geological formations.
INVESTIGATION
Testing of large diameter well technology for the constant LLW/ILW isolation in morein clay is carried out in Mos SIA "Radon" now.
The experimental site is located on a territory of "Radon" enterprise. Prospecting drilling revealed that the geological structure is of moreine clay up to 62 m.
Fig. 1. "RADON" enterprise geological profile.
The Cretaceous age aquifer which consists of sands with clay seams (in the upper part) and of various granulometric structure sand (in the lower part), occurs under the moreine clay layer. The water of the aquifer is high-pressure. The static level depth of underground water is 41.8 m. The local perched water in sand lens occurs in depth of 45.7-48.2m. The lens contains pressure head waters. The depth of a static level is 11.4 m.
The hydrogeological researches have not shown hydraulic connection between the lens and the aquifer. Moreine clay filtration coefficient (Kf) is 0.003 m/day. 137Cs and 90Sr distribution coefficients (Kd) are 1200ml/gms and 150ml/gms.
Large moreine clay thickness and its practical homogeneity, high sorption ability and low permeability allow making the conclusion about their suitability for final LLW/ILW isolation
The drilling of large diameter well was conducted in 1997. The depth of the well was 42m. The drilling diameter was 1900mm. (Fig.2)
The drilling was made with drilling rig MB-50 developed on the basis of serially produced drilling rig UBV-600. The normal circulation was conducted with clay solutions. The clay solution availability ensured stability of well walls. The well was casedwith a metal pipe (d=1.5m) after drilling. The space between a pipe and morein clay was cemented in. The large diameter well drilling and its equipment was for 2 months. RAW placement will begin after winter.
The packagess (drums) with radioactive wastes should be placed in the well using special equipment. Then bentonite-and-concrete solution should be pulled into the well. (The bentonite-and-concrete solution has superseded a clay solution forming a protective covering around of waste packagess.)
The experiments on determination of physical properties of bentonite-and-concrete solution were conducted.
The solution has been stiffeneing 3 weeks and has turned in bentonite-and-concrete stone having the next properties:
It was prognosticated that after packages the thickness of protective covering has made:
The large diameter well should contain 96 packagess or 19,2 m3 solid wastes. After completion of the large diameter well the protective cover will be made. Thus, the well RAW storage will have three levels of a long-term protect:
Fig. 2. Lithological profile and large diameter well design.
The realization of complex research works for study of RAW influence on a geological environment was provided.
Now research work is carried out on two main directions:
Six wells (depth 40m each and diameter 140mm each) were drilled for realization of research works. Three wells are used for determination mode of clay massive deformation and others are used for radiation monitoring controle. The recording detectors were installed in these wells.
The recording detectors will allow receiving information about seismic condition of the clay massive and about its stability
Radiation monitoring is made with the purpose of radiation control of RAW in the wells and of geological environment(clay layer, aquifer). The radiation monitoring of RAW will be made on special channels located inside of large diameter well. The radiation monitoring of clay layer and aquifer will be conducted with the help of of monitoring wells. Main controlled variables are the 137Cs and 90Sr contents in interstitial water, volumetric radioactivity of interstitial water and gamma-radiation intensity on depth of monitoring wells and channels. The developed system allows inspecting RAW condition in the large diameter well and geological environment.
Main parameter of any RAW depository efficiency is the level of storage safety.
The drilling both of monitoring wells and large diameter well has allowed receiving necessary input data for radioactive migration process simulation.
The conducted accounts of migration process have shown, that even at full destruction of the protective covering of a large diameter well, the depth of radioniclides penetration in clay in vertical direction will make 6,3m during decrease of radioactivity to a safe level (400 years).
The depth of radionuclides penetration in horizontal direction will be on two order less, than in vertical direction.
CONCLUSION
Using large diameter well will allow increasing safety of their storage and saving ground squares assigned under RAW facilities.
RAW placement in the large diameter well will allow ensuring a multi barrier guard of environment within 400 years. The time of drilling and equipment of the well has made 2 months. 96 packagess (drums) containing radioactive waste can be placed in the large diameter well.
REFERENCES