Aro, L.

Rantavaara, A.

Jukuri

Radiological impact of using forest biomass for energy and recycling  the ash    Aino H. Rantavaara1 and Lasse J. Aro2   1Radiation and Nuclear Safety Authority, P.O. Box 14, FIN-00881 Helsinki, Finland.  2 Finnish Forest Research Institute, Parkano Research Unit, Kaironiementie 54, FIN-003970 Parkano   INTRODUCTION   Today the development and research of bioenergy production is extensive due to the increasing cost of energy and the common goal to reduce atmospheric emissions of greenhouse gases from combustion of fossil fuels. Natural and artificial radionuclides found in forest biomass harvested for energy need consideration when the concepts for bioenergy are optimised. Also, the ash of biofuel is decisive to the radiation exposure of both workers handling the ash and the public using forests fertilised with the ash.   Currently, harvest residues received from thinning or final cutting of stands, and stump harvesting, are of prime importance to the delivery of biofuel from forests (Kuusinen and Ilvesniemi, 2008). The nutrient removal from forest with collected harvest residues and stumps, and the need to compensate such losses are of interest from the radioecological point of view.   Our study aims to clarify the effect of the use of forest biomass for energy on human exposure to radiation. Short and long-term consequences of the recurrent cycle of biofuel harvesting and ash fertilisation to radioactive contamination of forests were examined and are discussed. We also focus on the effect of biomass harvesting and recycling of ash on radionuclide budget and distribution in forests.   We review the chain of bioenergy production regarding radiation exposure to the public using forests and to workers involved in handling of ash. A case study demonstrates the changes in 137Cs activity of a spruce stand on mineral soil during rotation, with and without recycling of ash.    MATERIALS AND METHODS   The experimental studies, made in collaboration with energy industry and STUK, have resulted in data for activity concentrations of radiocaesium in forest biomass used as fuel in the form of woodchips, and ash (Vetikko et al.). In ash of wood by-products of forest industry, the artificial 90Sr and 137Cs and the natural radionuclides 40K, 210Pb, 226Ra, 232Th and 235U and also the solubility of most of these radionuclides have been determined (Rantavaara and Moring, 2001). The field experiments, in collaboration of STUK and METLA, have provided data on the effect of ash and mineral fertilisation on 137Cs contamination of stemwood and other fractions of trees (Aro and Rantavaara, 2002; 2004; Aro et al., 2002; Kaunisto et al., 2002; Rantavaara and Aro, 2003; Rantavaara et al., 2002; Rantavaara and Raitio, 2002). Ash fertilisation has a clear mitigating effect on the transfer of 137Cs from soil to trees and other vegetation (Rantavaara and Aro, 2003). This is because potassium in ash reduces the uptake of 137Cs, regardless of its origin. Therefore the reduction is significant for the 137Cs accumulated earlier in soil and the 137Cs of ash. The results obtained in these field studies are used here for calculation of radionuclide contamination in harvest residue, radiation exposure in forests, and changes in radioactive caesium budget of the stand.  Effect of ash returning to forest on human exposure from the Chernobyl-derived 137Cs was derived for a spruce stand with contamination level of about 40 kBq m-2. Human radiation exposure in forests, considering the recycling of biofuel ash is compared with the dose criteria for the public and for workers in the bioenergy production. The intervention level for radionuclide contamination of forest floor corresponding to acceptable exposure of workers and the public to external radiation can be derived for the suggested dose criteria.   RESULTS AND CONCLUSIONS  In undisturbed boreal forests radionuclide contamination of trees, particularly of stemwood and understorey vegetation, will decline after treatment of forest floor with potassium containing ash of tree biomass or mineral fertiliser. The effect appears slowly and lasts long. Reduction in solubility of most radionuclides during combustion of biofuel also has a mitigating effect on contamination of vegetation from radionuclides in ash and soil. Therefore, returning an acceptable amount of ash to forest will reduce the activity concentrations of biofuels from forests in the long term. The dosage should be related to the nutrient status of the forest.   Compared to not returning of nutrients to forests in the long term, the ash of forest biomass distributed to forest floor reduces contamination of trees and other vegetation as regards to most radionuclides. This fact suggests reducing doses to end users of wood, and also from ingestion of wild food received from forests (Levula et al., 2000).   Evaluation of other environmental consequences of harvesting forest biomass for energy, such as breaking the surface of forest floor (Kuusisto and Ilvesniemi, 2008), will reveal additional, but not necessarily significant, effects on the radionuclide dynamics in forests. To be acceptable, bioenergy is expected to be produced in a sustainable way (Stupak et al., 2007). Implementation of bioenergy concepts reviewed for sustainability may not reveal significant changes in our conclusions about radiation impact of bioenergy. Changes in human exposure will depend on the site conditions and silvicultural methods applied. Either decrease  − with recycling of ash, or increase of radiation exposure – if nutrient losses due to biomass harvesting are not compensated,  seems possible.   REFERENCES  Aro L, Rantavaara A., 2002. 137Cs activity in stumps and coarse roots of pine, spruce and birch trees as affected by fertilization. In: P. Strand, B. Borretzen, T. Jolle (Eds.): Proceedings from the International Conference on Radioactivity in the Environment, 1-5- September 2002 in Monaco, CD-appendix, 4 p.  Aro, L., A. Rantavaara, 2004. Effect of fertilisation on radiocaesium content of birch stands in a peat cutaway area. In: J. Päivänen, (Ed.): Wise Use of Peatlands. Proceedings of the 12th International Peat Congress, Tam-pere, Finland, 6-11 June 2004. Vol 2. Poster Presentations: 1211-1214. Aro L, A. Rantavaara, H. Raitio, V. Vetikko, 2002. Effect of fertilisation on 137Cs in understorey spruces on a dryish pine site. In: F. Bréchignac (Ed.): Radioprotection. Proceedings Volume I of the International Congress: The Radioecology – Ecotoxicology of Continental and Estuarine Environments ECORAD 2001, Aix-en-Provence, France, 3-7 September, 2001. Radioprotection – colloques , 37 (C1): CI-427-432. Kaunisto,  S., L. Aro and A. Rantavaara, 2002. Effect of fertilisation on the potassium and radiocaesium distribu-tion in tree stands (Pinus sylvestris L.) and peat on a pine mire. Environmental Pollution, 117(1): 111-119. Kuusinen, M., H. Ilvesniemi, 2008 (Eds.): Energiapuun korjuun ympäristövaikutukset. (Environmental effects of wood harvesting for energy). Tutkimusjulkaisuja, Tapion ja Metlan julkaisuja. (In Finnish).  Levula T., A. Saarsalmi, A. Rantavaara, 2000.  Effects of ash fertilization and prescribed burning on macronutri-ent, heavy metal, sulphur and 137Cs concentrations in lingonberries (Vaccinium vitis-idaea). Forest Ecology and Management, 126(3): 267-277. Rantavaara, A., L. Aro, 2003. Radioecology and management of contaminated forests. In: W. Paile (Ed.): Radiation Protection in the 2000s – Theory and Practice. Nordic Society for Radiation Protection. Proceedings of the XIII ordinary meeting, Turku/Åbo, Finland, August 25 - 29, 2002. STUK-A195. STUK, Helsinki, p. 443-449.  Rantavaara, A., L. Aro, J. Aarnio, 2002. Sustainable remediation of forests after 137Cs contamination. In: P. Børretzen, T.  Jølle, P. Strand, (Eds.): Proceedings from the International Conference on Radioactivity in the Environment, 1-5 September 2002, Monaco. Posters, CD-rom appendix, 5 p. Rantavaara, A.H., K.M. Moring, 1999. Contaminated tree biomass in energy production - potential need for radiation protection. In: I. Linkov, W.R. Schell (Eds.):  Contaminated Forests. Kluwer Academic Publishers, The Netherlands, 303-310. Rantavaara A., M. Moring, 2001. Puun tuhkan radioaktiivisuus (Radioactivity of wood ash). Report STUK-A177, Helsinki: Radiation and Nuclear Safety Authority, 40 +2 pp (In Finnish, summary in English).  Rantavaara A, H. Raitio, 2002. 137Cs and 90Sr root uptake of pine saplings in a managed forest. In: F. Bréchignac (Ed.): Proceedings Volume I of the International Congress: The Radioecology – Ecotoxicology of Continental and Estuarine Environments ECORAD 2001, Aix-en-Provence, France, 3-7 September, 2001. Radioprotection – colloques, 37 (C1): CI-565-568.  Stupak, I., A. Asikainen, M. Jonsell, E. Karltun, A. Lunnan, D. Mizaraite, K. Pasanen, H. Pärn, K. Raulund-Rasmussen, D. Röser, M. Schroeder, I. Varnagiryte, L. Vilkriste, I. Callesen, N. Clarke, T. Gaitnieks, M. Ingerslev, M. Mandre, R. Ozolincius, A. Saarsalmi, K. Armolaitis, H.-S. Helmisaari, A. Indriksons, L. Kairiukstis, K. Katzensteiner, M. Kukkola, K. Ots, H.P. Ravn, P. Tamminen, 2007. Sustainable utilisation of forest biomass for energy—Possibilities and problems: Policy, legislation, certification, and recommendations and guidelines in the Nordic, Baltic, and other European countries. Biomass and Bioenergy, 31: 666–684.  Vetikko V., T. Valmari, M. Oksanen, A. Rantavaara, S. Klemola, R. Hänninen, 2004. Energiateollisuudessa syntyvän puuntuhkan radioaktiivisuus ja sen säteilyvaikutukset. (Radioactivity of wood ash in energy industry and its radiation effects.) STUK-A200. (In Finnish, summary in English)   

Radiological impact of using forest biomass for energy and recycling the ash

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