Thermal treatment and decontamination methods.

Transcription

1 Thermal treatment and decontamination methods. Sergey Mikheykin D-R PMU, Kozloduy NPP site Workshop on development of specific decontamination techniques for RBMK dismantlement and/or highly active material from contaminated areas from accident conditions, Visaginas, Lithuania, International Atomic Energy Agency

2 Decontamination in molten salt reactors Molten salt oxidation is a flameless thermal desorption process. The waste is introduced into a bath of molten salts, typically at temperatures between C. This has the effect of oxidizing the organic constituents of the waste. Carbon dioxide, nitrogen and water are produced. The end product is an organic-free salt residue which captures radionuclides, metals and other inorganics. The production of acid gas emissions is inhibited by the formation of the stable salts. Molten salt oxidation results in the complete destruction of organic material and efficiently captures ash and radioactive particles within the salt bath. The lower operating temperature, relative to other thermal technologies, may result in reduced fuel costs. Low levels of gaseous emissions are produced. 2

3 Radioactive Wastes Painted Carbon Steel Structures Molten Salt Reactor Salt Bath Temperature: 300 to 900 o C Spent Salt Rinse in H 2 O (l) Salt Dissolution Filtration Pickling in H 2 SO 4 Rinse in H 2 O (l) Off gas / stack emissions U 3 O 8 and metal oxides Crystallization Salt Recovered Releasing as metal scrap Disposal as Radioactive waste Disposal as Radioactive waste

4 Melting A particularly advantageous consequence of melting is its decontamination effect on Cs-137, a volatile element. The dominant remaining nuclide in the ingots (for most reactor scrap) is cobalt-60. It is possible to receive concentration of RN in the slag if use specific additives The secondary waste consists of the slag from segmenting and melting, as well as dust from the ventilation filters. This secondary waste only comprises between 1 to 4 per cent of the weight of the melted scrap. Melting tests at the Korean Atomic Energy Research Institute for aluminium showed that % of 60Co and up to 99 % of 137Cs can be removed from the ingot phase. As for stainless and carbon steel, most of the 60Co remained in the ingot phase. 4

5 Melting The most commonly used furnace type for melting metals from the nuclear sector is the induction furnace. It is an alternating current (AC) electric furnace in which the primary conductor generates, by electromagnetic induction, a secondary current that develops heat within the metal charge. The induction furnace has the added advantage over other furnace types that induction ensures the material in the furnace is fully homogenous. 5

6 Melting - Advantages Extensively proven technology. High volume reduction, typically 5:1 to 20:1 If recycling is possible, volume reduction factors (from a disposal perspective) of up to 100:1 are possible. The end product is typically homogeneous and stable with the remaining activity content bound in the metal.. 6

7 Melting - Energy Solutions Melting Facility Facility operated near Oak Ridge, TN started metal melting operations in about 1992 Uses an induction type furnace for melting CS, SS and Al with a typical charge size of 20 tons of material Re use as shielding blocks for R&D facilities and as waste disposal containers Other material released to clean scrap pile if it meets release criteria 7

8 Ecomet S The re-melting department includes the induction furnace, the system of exhaust ventilation and gas purification, the area for fluxes preparation, metal loading into furnace and metal casting into molds. 8

9 Facilities 9

10 Facilities 10

11 THERMOSORPTION DECONTAMINATION The process is based on thermal volatilization of radionuclides de to beat generated when exothermic metallic compositions powder metal fuel (PMF) fill layer on material surface is burned flameless for a few tens on minutes. The layer of slag produced by PMF combustion has an extensively developed surface and traps volatilized radionuclides. Temperatures are from 300 to 1500 C and higher The heat combustion of EMC is about MJ/kg. 11

12 THERMOSORPTION DECONTAMINATION While the temperature of the metal grows up to 600 C and higher the permeability of slag layer 3 increases and the oxygen flux is provided due to peripheral inflow of air which retains radionuclides under the layer of burning composition. While the combustion process ends this convection flow decreases as well as the temperature of peripheral areas of slag carcass. As a result radionuclides spread into the gap between slag and metal from the central part to peripheral areas where they are settled to the colder slag due to thermophoresis which occurs in the gap between warm metal and cold slag. Therefore the radionuclides release into environment not exceeds 1%. 12

13 THERMOSORPTION DECONTAMINATION This method may apply for horizontal surface of concrete, bricks and corrosive/painted surface of metal 13

14 Plasma decontamination During plasma decontamination processing, the main process generates powerful reactive plasma gas suitable for decontamination condition, which is optimized according to the decontamination target elements. The etchant gas selectively reacts with radioactive elements on contaminated metal surfaces and completely eliminates them by etching out. An ion-assisted etching technique is additionally used to enhance the decontamination rate, if necessary. The volume of the uniform plasma processing zone can be easily extended up to a few cubic meters, depending on the dimension of target waste. [Plasma decontamination Technology, WM2011 Conference, 2011, Phoenix, USA.] 14

15 Plasma processing 15

16 Laser The energy of the laser beam is absorbed by the surface which needs to be decontaminated. A micro-explosions of the surface s material then occurs. This technique is has come to be known as scabbling, and was pioneered by Li and co-workers The effect is believed to be caused by the rapid dehydration and evaporation of the crystal water in the cement matrix, rather than the decarbonization of the limestone ubiquitously present as aggregate in many concrete types, with thermal shock playing a contributory role. This phenomenon causes the removal of the contaminated material in a very efficient way is a dry cleaning process which creates a minimum volume of secondary waste. 16

17 Laser Ablation The process is more effective when used with large beam diameters typically mm, significantly larger than the average diameter of the aggregate used. Single pass removal depth of up to 8 mm has been achieved using a 4 kw Nd:YAG or a 5 kw CO 2 laser with a spot diameter of 80 mm. An irradiance of W.cm -2 was found optimum for the process. Industrial multi-kilowatt Nd:YAG and CO 2 lasers are capable of ablating concrete surfaces and effecting decontamination 17

18 Laser 18

19 Thank you! 19