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Cold Crucible Technology for Waste Treatment

#1713


Mastering Technology and Equipment Based on Induction Melters with “Cold” Crucible for conditioning Active Metal Waste and Thermal Insulation Materials Arising at Operating and Decommissioned NPP and Radiochemical Plants to Storage and final Disposal

Tech Area / Field

  • ENV-RWT/Radioactive Waste Treatment/Environment

Status
3 Approved without Funding

Registration date
14.12.1999

Leading Institute
All-Russian Scientific Research Institute of Non-Organic Materials named after A. Bochvar, Russia, Moscow

Collaborators

  • Idaho National Engineering and Environmental Laboratory, USA, ID, Idaho Falls\nCEA / DCC / CEN Valrho, France, Marcoule\nLos-Alamos National Laboratory, USA, NM, Los-Alamos\nLawrence Livermore National Laboratory, USA, CA, Livermore\nEnergoTech LLC, USA, VA, Arlington

Project summary

The expiration and, particularly, decommissioning of NPP and equipment of radiochemical plants result in high scope of active metal waste (AMW) and thermal insulation materials (TIM). The management of this waste entails many problems in storage and disposal.

For instance, the waste arising from the current operation of the Leningrad Nuclear Power Plant (RF) constitutes 1200 t/year AMW of which intermediate and high activity level waste amount to 250 t/year.

The decommissioning of 1 NPP unit of 1000 MW electric power results in up to 40 thousand ton AMW. The major mass of the waste has the specific activity levels not more than 0.5 Ci/t (5ґ10-4 Ci/t), in other words, it belongs to the low- and intermediate activity level waste.

The radioactive contamination with AMW from nuclear reactors and process equipment of reprocessing plants is caused by both the activated products of steel components (60Co, 54Mn, 51Cr, 65Zn, 55Fe, 59Fe, 63Ni) and the products of nuclear fuel fission (134Cs, 137Cs, 90Sr, 106Ru, 144Ce). The short-lived radionuclides 58Co, 51Cr, 59Fe almost fully decay upon 1 year holding.

The reprocessing of active metal waste, specifically, expensive stainless steel, Zr, Ni and Ti alloys with the view of decontamination, compaction and re-usage is cost- and ecology-effective.

It is known that in Germany, USA, Sweden and Japan the low-activity stainless steel is re-used after liquid decontamination and remelting in electrical arc or induction furnaces.

The remelting of AMW can be favourable in several aspects:

– a factor of 4-6 reduction of waste and, correspondingly, storage and burial facility volumes;

– conversion of some AMW to intermediate and low level waste with the resultant simplification and lower cost of the storage;

– simplified measurement of specific and total activity of AMW (particularly, for large sizes and volumes of waste when only random control is feasible);

– the possible radioactive contamination of the environment is almost fully eliminated due to the uniform distribution and reliable immobilization of radionuclides in the metal matrix.

The numerous experiments with actual low activity level waste established that the process of the induction furnace melting efficiently purifies the steels from 90Sr, 144Ce and other REE, 65Zn, 137Cs, 235U, 238U, 239Pu as well as from other TUE.

In practice it has been demonstrated that the application of electrical-arc and air-induction (with a ceramic crucible) installations gives rise to serious problems in collecting and reprocessing dust and slag; the gas supply and clean-up system is cumbersome and expensive (20-25 mln $ US).

The Process of AMW melting in induction furnaces with ceramic crucibles and pouring the metal into moulds has a substantial disadvantage, i.e., the low service-life (Ј 50 melting runs) of packed crucibles and moulds resulting in additional non-reprocessible secondary waste which aggravates the operation of facilities.

Their alternative is an induction furnace with a “cold” crucible (IMCC).

In 1991 the French scientists reported the work launched in 1982 by COGEMA to develop IMCC process and facility aimed at reducing the amount and decontamination cost of high activity level stainless steel and Zr-alloy claddings of fuel rods in presence of a fluoride flux.

In Japan during the recent five years investigations are under way of the feasibility of IMCC for melting active metal waste.

Based on the acquired results the French, Russian and Japanese researchers came to the conclusion that the industrial-scale application of the ISM-CC technology and equipment for AMW is promising.

In RF in the framework of ISTC Projects N143-94 and N898-98 financially supported by the PNC in Japan the specialists at SSC RF VNIINM have undertaken investigations and the work is in progress to master the technology and equipment for the induction slag melting of high activity level metal waste as chopped Zr-alloy and stainless steel claddings of fuel rods in a cold crucible.

A demonstration facility of “ISMW-CC” with a cold crucible 130 mm dia and 500 mm high was installed and is being demonstrated in a VNIIM rig; it produces ingots up to 50 kg in mass.

It follows from the experience gained in Russia, France and Japan in melting low and high activity level metal waste using IMCC rigs and commercial facilities that by the melting of steels in furnaces of this kind with the addition of oxifluoride fluxes the waste volume reduction factor is reaced equal to 5-6, the factor of metal decontamination from Cs and Sr is >98% while that for a-emitting nuclides is >98%. The IMCC process used for AMW (without liquid pre-decontamination) removes not only non-immobilized contaminants from the surface of waste but also contaminants that penetrated deep into the metal or oxide film.

Compared to electric-arc and induction furnaces with ceramic crucibles the main merits of the IMCC process as applied to AMW are:


– compactness and low material intensity of equipment;
– long life-time of crucibles (10 years);
– compact and simple gas clean-up system (the process is implemented in a small furnace volume filled with an inert gas);
– low amounts of introduced fluxes (3-5% of the metal mass) and resultant slags;
– no moulds or other casting devices;
– high quality of decontaminated metal;
– feasibility of remote control and management of the melting and dismantling processes;
– feasibility of resultant slag reprocessing by cementation or vitrification.

The storages at NPP and radiochemical plants accomodate significant amounts (dozens of thousands of cubic metres) perse thermal insulation materials (glass fibre, mineral wool, basaltic fibre as low activity level waste (10-6-10-4 Ci/kg).

The melting of that waste efficiently reduces its volume and promotes its environmentally safe storage.

The chemical composition of thermal insulation materials is close to that of some matrix materials used to solidify liquid active waste. This fact ensures the preparation of mechanically strong and chemically stable final materials by the process of thermal insulators melting.

As for as thermal insulation materials, of the highest promise here is the application of induction melters with a “cold” crucible (IMCC). The design and process features of IMCC allow melting perse materials in a wide temperature range (1200-2000 °C) without any problems relating to the stability of the crucible material.

The advantages of the IMCC type melters cover small overall sizes and the high specific throughput as well as the feasibility of the remote replacement of the crucible and the remote process management.

In the framework of ISTC Project №376-96 at SSC RF VNIINM investigations are under way to study the process of the high activity level liquid waste vitrification in the induction melter with the “cold” crucible. Making use of the experience gained and the laboratory-scale facilities the preliminary experiments were implemented that evidence that fibre glass subjected to melting in the IMCC facility reduces its volume by 30-40 times to produce a glass log the rate Cs and Sr nuclide leaching from which is <1ґ10-7 g/cm2 day.

The objective of this project (being the evolution of the work on ISTC Projects №№376 and 898) is to master (optimize) the technology and equipment based on induction melters with a “cold” crucible as applied to commercial melting conditions for low and intermediate activity level metal waste and thermal insulation materials arising at NPP and radiochemical plants.

In the framework of this project financially and technically supported by the Japan institutions (JNC, RANDEC, JAERI) larger-scale experiments and design activities shall be implemented with the view of mastering the technologies and developing the designs of commercial facilities intended for induction-slag melting metal waste (the throughput of 3-5 t metal/day) as well as melting thermal insulation materials (the throughput of 0.5-1.0 m3/h). The implementation of the Project assumes involving the creative staff currently successfully implementing the work on ISTC Projects №№ 376-96 and 898-98. Among those people there are 2 doctors and 8 candidates of sciences. During the recent 5 years the Project participants published 20 reports and papers submitted to International Conferences, reports on R&D and procedures applied to this sphere of the investigations.

Currently, the application for the patent is undergoing an examination by experts, the application was filed as a result of the implementation of the ISTC Project.

In compliance with the objectives and tasks of ISTC the suggested project deals with the resolution of the problems of the safe management of radioactive waste arising from NPP operation and decommissioning as well as from reprocessing spent nuclear fuel and waste from the special material production.

The project related developments will promote the evolution of the applied R&D and design work in the field of the environmental protection and the integration of the Russia’s specialists to the International Scientific Community.

The results of the Project can be commercially used for the creation and operation of plants for compacting radioactive waste which will make it possible to reduce substantially the sizes of storage facilities and promote the environmentally safe and cost-effective reprocessing of waste.

Role of International Collaborator

The following is assumed to be carried on: share information on Project implementation, joint scientific and technical workshops and working group meeting, joint participation in international conferences and symposia on problems of radioactive waste reprocessing.


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