Overview of recycling technologies for decommissioned materials. Lessons learned during the dismantling of a small PWR reactor

Full text: SCK CEN is dismantling its 11 MWe PWR reactor. The reactor was shutdown in 1987 after 25 years of operation and the dismantling started in 1990. For the management of the low radioactive materials, we apply a strategy promoting the minimisation of the production of radioactive waste and hence the maximisation of the production of recycled materials while keeping the costs as low as possible. The recycled materials are either reused in the non- nuclear industry as raw materials (metal scrap industry or building industry for the concrete) or recycled in the nuclear industry for specific applications (reuse of metals for fabrication of shielding, potential reuse of concrete for production of 'radioactive mortar'). The clearance of radioactive materials and their reuse require the strict respect of procedures and specifications. In our case, the Health Physics department under supervision of the Competent Authority establishes the procedures. This procedure is still a case by case practice but the legislation in Belgium is progressively put in place. For the recycling in the nuclear industry, we must respect the specifications of the end-user. Up to now, we have recycled low radioactive metals for the fabrication of shielding in the USA, so we had to respect the specifications of the melting facility and to obtain the authorisations for the transport abroad and for the transfer of property. Besides the radioactive waste route, we are using several evacuation routes for the dismantled materials: Evacuation of the cleared metals (iron, stainless steel, copper, electric motors...) to a local scrap dealer; Evacuation of metals to the Studsvik melting facility situated in Sweden: after clearance by the Swedish Authority, the non radioactive materials are sent to a local scrap dealer and the secondary radioactive waste is sent back to Belgium and conditioned by Belgoprocess. This technology further decontaminates the metals and allows performing an accurate determination of the radionuclides content of the ingots; We are developing a recycling route for the low radioactive concrete. The basic idea is to perform a pre-treatment (crushing and sieving) of the radioactive concrete so that it can be reused as aggregates for the fabrication of radioactive grout. This grout is then used for the conditioning of metallic radioactive waste. We demonstrated that it is technically feasible to prepare a good quality grout using radioactive heavy concrete as raw material mixed with fresh cement. We use several technologies to reach the clearance or recycling criteria. For metals, we use mainly: Simple manual washing techniques or cleaning in an ultrasonic rinsing bath; Manual polishing with metal grinding and polishing machines; Stripping of electric cables to separate the contaminated insulation from the clean copper; Abrasives decontamination in our Wet Abrasive Decontamination unit (ZOE unit) which can treat pieces up to 3m long and 3t or in an automatic dry sand blasting unit operated by Belgoprocess which handles smaller pieces; Hard chemical decontamination with the MEDOC Cerium process: MEDOC is used for stainless steel and carbon steel heavily contaminated up to 30,000 Bq/cm260Co. The MEDOC installation has a capacity of about 20 m2/batch, which corresponds to 0.5 to It of metals. For the components contaminated by primary water, it must be noted that we performed, shortly after the reactor shutdown, a full system chemical decontamination that allowed to strongly reduce the dose rate of the components. For concrete, we use mainly scabbling or shaving techniques for the controlled removal of some mm of contaminated concrete. For deeper contaminated concrete, we use a remotely controlled jackhammer or diamond sawing techniques. One of the key issues is the correct measurement of the low levels of remaining radioactivity of most often quite complex pieces. For the measurements of pieces of simple geometry, we use the classical Health Physics β and α monitors. Two measurements of 100 % of the surface have to be performed by two independent controllers including a certain time interval between both measurements and using two different apparatus. The residual radioactivity must be below 0.4 Bq/cm2 for β emitters and 0.04 Bq/cm2 for α emitters. The difficulty is to measure materials of complex geometry and to avoid the clearance of materials presenting still a 'hot spot'. A first possibility is to send the materials to a melting facility, which will homogenise the product. A second possibility is to reduce the size of the object. We follow a procedure based on the size of a 200-l drum. To prove that the batch is quite homogeneous, it is first divided into batches of about 15 to 20 kg which are individually measured in a shielded 4 π plastic scintillator. This gross gamma measurement is then combined with a gamma spectrometry measurement of the whole filled drum with a 'Canberra-Q2' system. The gross gamma counting allows rejecting the pieces presenting 'hot spots' (i.e. very localised contamination levels above the authorised clearance limits) and the spectrometry of the drum allows proving that we respect the clearance values (based on isotopic values). The experience gained so far will be detailed: successful clearance of contaminated concrete from the reactor building, clearance of contaminated structural materials, of contaminated primary loop piping, of various tanks and auxiliaries, melting for recycling or for clearance of various pieces. (author)