Incineration of Radioactive Waste

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9. Incineration of Radioactive Waste

9.1 Introduction

The title of this chapter must not be misunderstood: it does not mean incineration of radioactive substances. Radioactive waste for processing by incineration consists of combustible materials such as fabrics and polymers which have become contaminated with a radioactive substance. When such waste is incinerated the radioactive contaminant remains in the ash, thus its volume is greatly decreased and its manageability improved. In fact volume reduction by a factor of about 100 is expected when radioactive waste is incinerated [1]([1] ‘Incineration of radioactive waste’ NUKEM Technologies GmbH 2007 (available online in pdf form)). The temperature dependence of radioactive processes is extremely weak so they are not accelerated in incineration which is a chemical process providing safe destruction of a substance previously containing radioactive substances.

9.2 Units and amounts

The becquerel (Bq) will be used as the unit of radioactivity throughout this discussion: for those more at home with the curie the conversion is given as a footnote.

Now:

1 Bq = 1 disintegration per second
or equivalently
1Bq = 1 s^-1

Now Bq is an infinitesimal degree of radioactivity, orders of magnitude lower than that in the air we breathe which is largely due to Radon. The following figures for levels of radioactivity in various substances and media are taken from [2]([2] http://www.world-nuclear.org/education/ral.htm).

The information in the third row of the table could have been re-expressed 103Bq kg^-1 or, more conventionally, 1 Bq g^-1. The radioactivity so expressed is the specific activity and it can be on a volume basis. Clearly when there is incineration the actual activity does not change but the specific activity does, a point which will feature later in this chapter.

9.3 Classifications of radioactive waste

Obviously such classifications differ from place to place and are subject to review. Sources including [3]([3] http://www.enviroserv.co.za/pages/content.asp?SectionID=508) state that the threshold below which a substance can, for waste disposal purposes, be seen as having nil radioactivity is a specific activity of 100 Bq g^-1 and a total activity of 4000 Bq. By way of perspective, in the days when watches worked mechanically and lacked any electrical power they sometimes had on the dial paint containing radium and such a watch would release [4]([4] http://www.hps.org/publicinformation/ate/q1697.html) at a few thousand becquerels. A watch weighing 25 g the radium within which had radioactivity 5000 Bq would therefore have a specific activity:

5000/25 = 200 Bq g^-1

The half-life of whatever radioactive process is taking place and whether the emitted particles are α or β are relevant to waste classification, and distinction on this basis is made in an International Atomic Energy Agency document [5]([5] http://www-pub.iaea.org/MTCD/publications/PDF/Pub1419_web.pdf) which gives 400 to 4000 Bq g^-1 for the upper limit on the definition of lowlevel waste (LLW) for isotopes emitting α particles and having long half lives. For radioactive isotopes of long half life with β and γ emission the value defining LLW can be 10⁴ Bq g^-1 or higher. With LLW and intermediate level waste (ILW, more fully discussed below) the concentration of radionuclides is such that their heat release into the waste of which they are a part is negligible.

One sometimes (e.g. [6]) encounters the term very low level waste (VLLW) and this is on the basis that the level is below that formally defining a radioactive substance in certain UK legislation. Reference [6]([6] Kemp R., O’Riordan T. ‘Planning for radioactive waste disposal’ Land Use Policy 5 37-44 (1988)) gives 0.4 Bq g^-1 for this. The same source defines LLW as having 4000 Bq g^-1 for an α-emitting waste and as 12000 Bq g^-1 for a waste emitting β and γ. The distinction between LLW and ILW is a little blurred, but for an α-emitting waste ≈ 10000 Bq g^-1 would be a reasonable threshold value.

HLW is defined as radioactive waste which releases energy at 2kW m^-3 or more [7]([7] ‘Fundamentals of the management of radioactive waste: An introduction to the management of higher-level radioactive waste on nuclear licensed sites’ HSE, UK (2007)). This figure is set in context in the calculation in the shaded area below which uses Cs¹³⁷ as an example.

9.4 The performance of a typical radioactive waste incinerator plant

It should first be noted that when radioactive waste is incinerated the fuel is whatever substance is holding the radioactive substance in low concentration. Emission of gases such as SO_x, NO_x have to be controlled according to local standards. Also an incinerator might be for radioactive waste only or for simultaneous treatment of radioactive and non-radioactive waste in which case the term ‘mixed’ is used. The German company Nukem manufactures incinerators for radioactive waste disposal [1], [8]([8] ‘Treatment centres for radioactive waste’ NUKEM Technologies GmbH 2007 (available online in pdf form)) and have such installations in countries including (in addition to Germany) Japan, Taiwan and Slovakia. A typical Nukem incinerator burns wood, paper, fabrics, rubber and other ‘conventional’ wastes at 50 kg per hour with bulk densities of 140 to 250 kg m^-3. The payload of the incinerator can have concentrations up to 10¹⁰ Bq m^-3 of α-emitting radionuclides and up to 10¹² Bq m-3 of β- and γ-emitting radionuclides. Using the mean of the bulk density range given, that is 195 kg m^-3, these figures convert to 50 kBq g^-1 for α-emitting radionuclides and 5 MBq g^-1 for the β- and γ-emitting radionuclides. When these are compared with values defining LLW and ILW in this chapter it is clear that the capabilities of the incinerator under discussion go beyond either. This means that the incinerator could take wastes having these levels of radioactivity, or wastes themselves having higher levels blended with uncontaminated waste to bring the radioactivity levels of the bulk to within the incinerator specifications.

The ash yield from the incinerator under discussion [1] will be about 8%, and this will have a specific activity on a weight basis:

100/8 = 12.5

times higher than that of the initial contents of the incinerator. It will however have a much smaller volume and the ash residue can be distributed at a landfill at whatever proportion is necessary to bring the specific activity into the VLLW range or even lower than that. That is what is meant by the improved ‘manageability’ of radioactive waste by incineration referred to at the beginning of this chapter.

The raison d’etre of a radioactive waste incinerator is entrapment of the radioactive isotopes in the ash and loss of any ash as particles of micron size (‘fly ash’) in the post-combustion gases would clearly represent a degree of loss of function of the incinerator. This point is examined in the calculation below.

We approximate the composition of the non-radioactive waste which is the fuel to that of cellulose, formula C6H10O5. This has formula weight 162 g, and a routine calculation of the amount and composition of flue gas following combustion of 1 kg of it follows.

Continuing into the boxed area below:

The value in the calculation of 1000 kBq m^-3 greatly exceeds the background level in the atmosphere which, largely by reason of the radon, is about 1 kBq m^-3 (≈ 1 Bq g^-1). Flue gas from the incinerator will of course be diluted on discharge to the atmosphere but even so control of particle emission is required. This often uses a fabric filter [11]([11] ‘Design and Operation of Radioactive Waste Incineration Facilities’ Safety Series No. 108, International Atomic Energy Agency, Vienna (1992)).

9.5 Concluding remarks

The way in which incineration can render radioactive waste safe has been outlined in this chapter. Where a suitable incinerator is in operation waste can be imported for disposal, and obviously the ‘import’ becomes in effect an export in that revenue is raised. There are proposals for this in the US and a reader going to [9]([9] http://leanweb.org/news/latest/company-to-import-and-export-nuclear-waste-through-port-of-neworleans-2.html) and to [10]([10] http://www.nuclearwastenews.com/view_article.asp?artID=5730) will be introduced to the pros and the cons.