BITUMEN PYROHYDROLYSIS | RADWASTE MANAGEMENT
Pyrohydrolysis of bituminized waste drums
Long-term stability concerns over bituminized waste prompted
NUKEM Technologies to establish a novel pyrolysis process for treatment. Rainer Slametschka, Director Center of Competences at
NUKEM Technologies Engineering Services GmbH and Stefan Thiel, Research assistant, Technische Universität Bergakademie Freiberg, explain
IN THE PAST, BITUMEN WAS widely applied as matrix for many kinds of low and intermediate level radioactive waste, such as homogeneous conditioning of evaporator concentrates and sludge, homogenous embedding of spent ion exchange resins, heterogeneous embedding of metal technological waste and heterogeneous embedding of mixed waste like compacted drums, clothing, rubber, and plastics.
Bitumen was seen a long-lasting material providing
a safe enclosure for radioactive substances and good stability for the anticipated long-term storage and disposal. However, tests in France have shown that, over time, radiolysis can occur, causing the formation of gas bubbles within the bitumen matrix. This leads to acceptance problems in underground repositories due to risk of fire as well as the lack of long-term stability. Due to the characteristics of the bitumen matrix
reconditioning of these waste package present significant challenges, including the large volumes of bituminized waste, their high calorific value, the content of volatile radionuclides, the difficulties handling molten bitumen and the potential spread of contamination.
Using pyrohydrolysis to solve the bitumen challenge So far, no direct reconditioning process have been commercially established for bituminized waste packages. It has been suggested that the bitumen could be co- incinerated at existing facilities, but that would require the removal of the bitumen from the waste package prior to treatment. Additionally, that would still leave the original waste packages, having significant amounts of bitumen and radioactive material left inside, to be handled and treated somewhere. However, based on experience with the pyrolysis
and pyrohydrolysis process which was developed by NUKEM Technologies Engineering Services GmbH (NUKEM Technologies) and has operated successfully for decades, NUKEM Technologies decided to investigate its applicability for other waste types like bituminized waste. A typical application for this pyrohydrolysis process is the decomposition of consumed extractant tributyl phosphate (TBP) in a mixture with kerosene. In this case TBP is pyrolyzed together with calcium hydroxide in a stirred
bed system at temperatures around 500°C. The calcium hydroxide converts directly with the phosphate groups to calcium pyrophosphate, which contains all the radioactivity and is disposed of as intermediate level waste. The organic components pass into the pyrolysis gas, which is burned. The flue gas is further purified. TBP pyrolysis plants were built in France at La Hague, in Belgium at Mol and in Japan at Rokkashomura. The Belgian plant has now ceased operations as all the TBP from EUROCHEMIC’s operations has been successfully processed. The plant in Japan is currently in the testing phase. At the existing plants, as well as in corresponding pilot plants, multiple tests were carried out in order to expand the range of waste that can be processed in this way. Initial tests have shown that spent ion exchange resins (IEX) can also be decomposed with very good results by pyrolysis, resulting in an inert and chemically resistant product. The pyrolysate is produced as a free-flowing solid and not as a melt. It is therefore easy to handle and can be pressed or cemented depending on the interim and final storage conditions and activity inventory. Any other ingredients such as inorganic filter aids or
other organic substances do not interfere with the process flow, they are dried, calcined or also pyrolyzed. Gaseous products including hydrocarbons such as methane or propene, water vapor and, in small quantities, ammonia are part of the pyrolysis gas. This gas must be burned before being fed to an off-gas cleaning system equipped with HEPA filters.
The pyrolysis process showed volume reduction factors (VRF) of about 7 for a typical mixture of spent ion exchange resins. The addition of steam to the stirred bed reactor could increase this VRF to 20.
Building the demonstrator facility NUKEM Technologies together with DBI-Virtuhcon had demonstrated the ‘in-principle’ suitability of pyrohydrolysis for the treatment of bituminized waste packed in whole drums during testing at the University of Freiberg. From the results of these laboratory tests a concept for
the treatment of drums containing bituminized radioactive waste was developed and NUKEM Technologies set up an R&D project. This project supported implementation of an industrial scale pyrolysis process. The first step saw a U
www.neimagazine.com | August 2022 | 37
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