M.H.Ahn, K.J.Lee Korea Advance Institute of Science and Technology 335 Gwahak-ro, Yuseong-gu, Daejeon , Republic of Korea

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1 The Methodology on Exposure Dose Evaluaton Modelng Related to Arbtrary Accdent n the Temporary Storage Faclty for Low and Intermedate Level Waste M.H.Ahn, K.J.Lee Korea Advance Insttute of Scence and Technology 335 Gwahak-ro, Yuseong-gu, Daejeon , Republc of Korea K.W.Cho Korea Insttute of Nuclear and Safety 34 Gwahak-ro, Yuseong-gu, Daejeon , Republc of Korea ABSTRACT For the establshment of arbtrary accdent scenaros n temporary storage faclty for LILW, the ntatng event was derved by master logc dagram (MLD) method based on the fault tree analyss (FTA), and then arbtrary accdent scenaros were developed by the event tree analyss (ETA) through the derved ntatng events. The man ntatng events led to the arbtrary operatonal accdent, whch s the droppng of a drum and fre, were derved from MLD method. The exposure effects resulted from release of radoactve materals related to arbtrary accdent n the temporary storage faclty for LILW can be dvded wth the nternal exposure caused by breathng and external exposure by radoactve lump. The 14 radonucldes for nternal and external exposure dose evaluaton were consdered: 3 H, 14 C, 55 Fe, 58 Co, 60 Co, 59 N, 63 N, 90 Sr, 94 Nb, 99 Tc, 129 I, 137 Cs, 144 Ce, and Gross α. Accordng to the U.S. NRC regulatory gude 1.145, the atmospherc relatve concentratons ( ) were derved by meteorologcal data measured n NPP ste: wnd speed, wnd drecton, and a measure of atmospherc stablty. For the evaluaton of nternal exposure, the breathng rate (m 3 /sec) descrbed n U.S. NRC regulatory gude 1.8 was consdered. Consequently, the nternal exposure dose by breathng can be derved by the combnaton wth the followng elements: the atmospherc relatve concentratons, release amounts of each radonuclde due to the arbtrary accdent n temporary storage faclty, breathng rate and dose converson factor by breathng. Smlarly, the exposure dose by radoactve lump can also be calculated by the contents consdered n the nternal exposure dose evaluaton except for the element of the breathng rate. INTRODUCTION In Korea, the low and ntermedate level wastes (LILW) generated from the operaton of NPPs have been gradually ncreased snce 1977 because of the absence of the repostory for the dsposal of LILW [1]. And then, the LILW has beng stored n the temporary storage faclty at each reactor stes. Fortunately, n 2005, the Gyeongju cty was selected by the resdent s vote n four compettve provnces for the LILW dsposal faclty. The repostory for the dsposal of LILW wll be operated n end of In opposton to many of researches on the dsposal of LILW, however, the rsk assessment on the temporary storage faclty has scarcely been conducted. Furthermore, the detals n regards of the safety analyss on ths faclty have not been consdered n the prelmnary and fnal safety analyss report because ths report focused on the nuclear reactor system rather than ths faclty. As a consequence of these stuatons, the number of the researches on the arbtrary accdents occurrng n the temporary storage faclty has not been enough. The objectve of ths study s to establsh the modelng method when the exposure dose for workers and publc wth regard to the temporary storage faclty of LILW s evaluated. For the establshment of arbtrary accdent scenaros n temporary storage faclty for LILW, the ntatng event was derved by master logc dagram (MLD) method based on the fault tree analyss (FTA), and then arbtrary accdent scenaros were developed by the event tree analyss (ETA) through the derved ntatng events. The man ntatng events led to the arbtrary operatonal accdent, whch s the

2 droppng of a drum and fre, were derved from MLD method. The exposure effects resulted from release of radoactve materals related to arbtrary accdent n the temporary storage faclty for LILW can be dvded wth the nternal exposure caused by breathng and external exposure by radoactve lump. The 14 radonucldes for nternal and external exposure dose evaluaton were consdered: 3 H, 14 C, 55 Fe, 58 Co, 60 Co, 59 N, 63 N, 90 Sr, 94 Nb, 99 Tc, 129 I, 137 Cs, 144 Ce, and Gross α. Accordng to the U.S. NRC regulatory gude 1.145, the atmospherc relatve concentratons ( ) were derved by meteorologcal data measured n NPP ste: wnd speed, wnd drecton, and a measure of atmospherc stablty. For the evaluaton of nternal exposure, the breathng rate (m 3 /sec) descrbed n U.S. NRC regulatory gude 1.8 was consdered. MASTER LOGIC DIAGRAM The Master Logc Dagram (MLD) s a basc method dentfyng the ntatng event. Ths method s based on the FTA but wthout the formal mathematcal propertes [2]. Furthermore, the FTA s expressed n logcal symbols wth regard to all cases n whch the expermental targets cannot be used anymore. Once the system falure was defned by top event n fault tree, a varety of pathways related to system falure can be deductvely pursued. The fault tree has the advantage of beng able to drect the analyss to ferret out falures, allowng the analyst to concentrate on one partcular system falure at a tme, and to provde an nsght nto system behavor [3]. In ths study the ntatng events needed for rsk-based accdent scenaros were deductvely derved by the MLD method based on the FTA. In order to develop the ntatng events, three accdent branches were consdered: falure when movng radoactve waste drums to transfer system (vehcles), falure when transferrng the drums to temporary storage faclty, and falure when lftng and managng the drums at temporary storage faclty. Each accdent branch s further dvded nto three knds of falure condtons: falure nduced by mechancal condtons, falure nduced by thermal condtons, and falure nduced by both mechancal and thermal condtons (Fg. 1-4). A varety of factors based on the comparatve revew of documents related to the management of the LILW n the temporary storage faclty, ncludng the ste drawng regardng the temporary storage faclty, and of smlar research conducted n other countres, were consdered n order to select the ntatng events n ths study. Fg. 1. Master logc dagram for selectng the ntatng events

3 Fg. 2. Falure when movng radwaste drums to the transfer system Fg. 3. Falure when transferrng radwaste drums to the temporary storage faclty

4 Fg. 4 Falure when lftng and managng radwaste drums to the temporary storage faclty ESTABLISHMENT OF THE INITIATING EVENT The rsk-based accdent scenaros concerned to LILW management n the temporary storage faclty could be readly derved by the ETA method. However, t could be taken over a long perod of tme and a great deal of to make whole event tree wth regard to all of ntatng events consdered n ths study. Accordng to ths reason, all ntatng events developed by the MLD could be elmnated by a flterng process f they are out of research scope, and could be grouped by a common event n case that they consst of duplcated events. The man ntatng events led to arbtrary operatonal accdents, the droppng of a drum and fre were derved from the MLD. Table 1 shows the ntatng events that wll be used by the headng event of the ETA for fnal rsk-based accdent scenaros. The ntatng event wth regard to the drop of drum s expressed wth the lftng falure and worker s error due to plng too many drums on vehcle and errors caused by an unsklled workman. Then, the ntatng event of fre s represented n both the gas generaton due to the droppng of a drum and collson wth some objects and fre ntated the wthn transport vehcle or the presence of combustve materals n the temporary storage faclty. Table 1. Lst of the ntatng events used n the event tree analyss Drop Fre Lftng falure Worker s error Gas generaton Fre - Tppng over due to errors caused - Gas generaton due to the by an unsklled workman - Fre ntated wthn the droppng of drums and collson - Droppng due to stackng too many transport vehcle wth some objects drums on the vehcle - Fre due to the presence - Gas generaton by the long term - lftng falure due to the mechancal of combustve materals storage of drums defects METHODOLOGY FOR THE EXPOSURE DOSE MEDING The man ntatng events led to the arbtrary operatonal accdent, whch s the droppng of a drum and

5 fre, were derved from MLD method. The exposure effects resulted from release of radoactve materals related to arbtrary accdent n the temporary storage faclty for LILW can be dvded wth the external exposure by radoactve lump and nternal exposure caused by breathng. The equaton (1) for the external exposure dose by radoactve lump conssted of followng parameters: dose converson factor n respect of each radonuclde, atmospherc relatve concentraton ( ), the release amount of each radonuclde, and constant. And then, the equaton (2) for the nternal exposure dose by breathng was also consttuted by smlar contents used n the equaton (1), except for consderng the breathng rate (m 3 /sec) [4]. DP DH 1 ( )( ) fqdfp 3600 Q ( 1) ( ) fqbrdfh Q (2) Where DP = external exposure dose (msv) = radonucldes; j = release tme = relatve ar concentraton (sec/m 3 ) Q = the amount of release wth respect to each radonuclde (Bq) DFP = dose converson factor for the external exposure (msv/hr per Bq/m 3 ) DH = nternal exposure dose (msv) Br = breathng rate (m 3 /sec) DFH = dose converson factor for the nternal exposure (msv/bq) The 13 radonucldes and gross α were consdered for the source term wth respect to the nternal and external exposure dose evaluaton: 3 H, 14 C, 55 Fe, 58 Co, 60 Co, 59 N, 63 N, 90 Sr, 94 Nb, 99 Tc, 129 I, 137 Cs, 144 Ce, and Gross α. Furthermore, accordng to the U.S.NRC regulatory gude 1.145, the atmospherc relatve concentratons ( ) were derved by meteorologcal data measured n NPP ste: wnd speed, wnd drecton, and a measure of atmospherc stablty. For the evaluaton of nternal exposure, the breathng rate (m 3 /sec) descrbed n U.S.NRC regulatory gude 1.8 was consdered. CONCLUSION The ntatng event analyss was used to dentfy the full spectrum of potental rsk related to LILW management n the temporary storage faclty. Furthermore, the ntatng events needed for the rsk-based accdent scenaros were deductvely derved by the MLD method based on the FTA. Then, the rsk-based accdent scenaros were developed by the ETA method through the derved ntatng events. In ths study three accdent branches were consdered to develop the ntatng events. One of them s the falure when movng radoactve waste drums to the transfer system. Another branch s the falure when transferrng radwaste drums to the temporary storage faclty. The thrd branch s the falure when lftng and managng radwaste drums at the temporary storage faclty. Each of these accdent branches s also dvded wth three knds of falure condtons: falure nduced by mechancal condtons, falure nduced by thermal condtons, and falure nduced by both mechancal and thermal condtons. The man ntatng events led to arbtrary operatonal accdents, the droppng of a drum and fre were derved from the MLD method. The ntatng event wth regard to the drop of drum s expressed wth the lftng falure and worker s error due to stackng too many drums on the vehcle and errors caused by an unsklled workman. Then, the ntatng event related to fre s represented n both the gas generaton owng to the droppng of a drum and collson wth some objects and the fre ntated wthn transport

6 vehcle or the presence of combustve materals n the storage faclty. The exposure effects resulted from release of radoactve materals related to arbtrary accdent n the temporary storage faclty for LILW can be dvded wth the nternal exposure caused by breathng and external exposure by radoactve lump. The 13 radonucldes for nternal and external exposure dose evaluaton were consdered. Accordng to the U.S. NRC regulatory gude 1.145, the atmospherc relatve concentratons ( ) were derved by meteorologcal data measured n NPP ste. For the evaluaton of nternal exposure, the breathng rate (m 3 /sec) descrbed n U.S. NRC regulatory gude 1.8 was consdered. Consequently, the nternal exposure dose by breathng can be derved by the combnaton wth the followng elements: the atmospherc relatve concentratons, release amounts of each radonuclde due to the arbtrary accdent n temporary storage faclty, breathng rate and dose converson factor by breathng. Smlarly, the exposure dose by radoactve lump can also be calculated by the contents consdered n the nternal exposure dose evaluaton except for the element of the breathng rate. REFERENCES 1. M.H. Ahn, S.C. Lee, and K.J. Lee, A Study on the Determnaton of Dsposal Prorty for Low and Intermedate Level Radoactve Waste (LILW) n Korea, Proc. of European Nuclear Conference (2007) 2. I.A. Papazoglou, O.N. Anezrs, Mater Logc Dagram: method for hazard and ntatng event dentfcaton n process plants, Journal of Hazardous Materals A97, pp (2003) 3. H. Kumamoto, E.J. Henley, Probablstc Rsk Assessment and Management for Engneers and Scentsts, IEEE Press (1996) 4. W.T. Hwang, etc., Basc Document for the Fundamental Emergency Plannng Zone of the HANARO Facltes, KAERI/TR-2831/2004, Korea Atomc Energy Research Insttute (2004)