The Safety Case for Transporting Spent Nuclear Fuel

Transcription

1 The Safety Case fr Transprting Spent Nuclear Fuel Charles Krs 1, Dr Ruth Weiner 2 1 NECSA, PO Bx 582, Pretria, 0001, Suth Africa 2 Sandia Natinal Labratries, PO Bx 5800, MS 0718 Albuquerque, NM, , USA Abstract The transprtatin safety case fr transprting spent nuclear fuel is a requirement fr licensing. It has bth qualitative and semi-quantitative aspects. The qualitative aspects include transprtatin regulatins, radiatin dse limits, rle f the transprtatin package in transprtatin, transprtatin package certificatin prcess, training, emergency respnse, the perfrmance f the transprtatin package in accidents, and the evaluatin f past transprtatin accidents. The quantitative aspects supprt the qualitative descriptins. Radiatin dses accrued by members f the public and by wrkers are calculated using the cde RADTRAN. Dse frm bth rutine, incident-free highway transprtatin and frm highway transprtatin accidents are part f the safety case and will be cmpared with bth backgrund dses and the regulatry safety criteria. The radiatin dses frm rutine transprtatin are calculated fr: - The maximally expsed member f the public. - Dses t vehicle escrts. - Dses t vehicle crew Cllective dses t ppulatins are calculated fr representative rutes. Cllective dse depends n the number f peple affected as well as n the extent f the radiatin frm the surce t which reference grups are expsed. Accidents invlving lss f gamma shielding and lss f cnfinement integrity are discussed, as are accidents in which there is n impact n the carg. Keywrds: transprt safety assessment, spent fuel transprt, RADTRAN, radiatin dse, cllective dse INTRODUCTION This paper discusses the experience gained in licensing the transprting f spent nuclear fuel thrugh a spent nuclear fuel repatriatin prject in Suth Africa during The paper will fcus n the requirements f a transprt safety case and demnstrate hw this was applied in the case f this prject. The Natinal Nuclear Regulatr (NNR) is the cmpetent authrity respnsible fr the regulatin f transprt f nuclear material in Suth Africa. As such, the safety case fr the transprting f spent nuclear fuel needs t be submitted t and apprved by the NNR prir t the transprt actin taking place. DEFINING THE SCOPE OF THE ASSESSMENT The NNR requires thrugh lcal legislatin and in terms f the IAEA transprt regulatins [1] fr the peratr t demnstrate cmpliance t safety criteria. A graded apprach is applied where the transprt f small quantities and ad-hc transprt actins can demnstrate cmpliance thrugh simplified methds. Typically fr excepted packages, extensive use is made f the IAEA Advisry Material [2] and mre fcus is placed n nrmal cnditins f transprt, cmmensurate with the risk. The NNR requires cmplete safety cases fr the transprt f radiactive material t be prepared and submitted fr apprval. Transprt safety cases need t address the risks assciated with nrmal as well as accident cnditins f transprt. It was recgnised that the transprt f spent nuclear fuel was

2 regarded t be nt nly a large quantity f material but als a sensitive issue fr which the regulatr had t ensure the safety f the public. Therefre a cmplete safety case was prepared. QUALITATIVE ASPECTS The safety case cntains aspects f a qualitative nature mst f which is very prescriptive in its requirements. In Suth Africa requirements are a cmbinatin f lcal and internatinal requirements: - Transprtatin regulatins [1] where a graded apprach is applied, fr example the cntent limits fr packages, package design criteria, - Radiatin dse limits fr expsure f members f the public, set in lcal legislatin but aligned with the recmmendatins f IAEA Basic Safety Standards [3], - Rle f the transprtatin package t ensure safety by means f the transprtatin package certificatin prcess which shuld adequately demnstrate cmpliance thrughut design, manufacture and maintenance, - Prcedural arrangements including thse fr training, emergency respnse, preparatin, cnsigning, lading, carriage, in-transit strage, unlading and receipt f radiactive material and packages, - Security prvisins as required by lcal and internatinal authrities. QUANTITATIVE ASPECTS The safety case cntains quantitative aspects which supprts the qualitative descriptins and facilitates the demnstratin f cnfrmance. The cmmn denminatr is the radiatin dses accrued by members f the public and wrkers during the transprt peratin. The Transprt Regulatins [1] defines three general severity levels: - Rutine cnditins f transprt (incident free); - Nrmal cnditins f transprt (minr mishaps); and - Accident cnditins f transprt. The cnsequence f these severity levels (r cnditins f transprt) needs t be quantified fr the transprt peratin. This was achieved by the use f the RADTRAN cmputer mdel develped by Sandia Natinal Labratries. RADTRAN 6.0 [4] is an internatinally accepted prgram and cde fr calculating the risks f transprting radiactive material bth deterministic and prbabilistic. Almst all input parameters are user-defined, therefre the user needs a certain familiarity with the apprpriate values and the use f the prgramme. A simplified diagram f the cde structure is given in Figure 1. Thrugh a cllabrative agreement between the Suth African and US gvernments, a capacity building prgramme was launched t equip Suth African analysts in RADTRAN t perfrm this quantitative assessment as part f the licensing requirements (safety case). The utput f the cde is highly dependent n the selectin f apprpriate input parameters. Much reliance was placed n default parameters [4] and internatinally available studies [5]. Ntwithstanding, the challenge f any cmputer mdel is the balance between realism and cnservatism. ROUTINE TRANSPORT DOSES Risks assciated with rutine cnditins (incident free) f transprt are limited t external expsure scenaris. The radiatin dse rates fr spent fuel shipments are measured befre each shipment and kept within regulatry limits. The radiatin dse frm this external radiatin t any member f the public during rutine transprtatin, including stps, is barely discernible cmpared t natural backgrund radiatin. Principally, since n accidents are anticipated, n lss f any radiactive material and therefre n cntaminatin is anticipated.

3 Expsure scenaris are limited t: - Wrkers invlved in the transprt peratin (drivers, handlers) and - Members f the public in the vicinity f the peratin r n rute. Figure 1: Simplified layut f RADTRAN cde structure Occupants f vehicles that share the rute with the radiactive shipment als receive a radiatin dse frm the spent fuel cask. The cllective dse t ccupants depends n the average number f ccupants per vehicle and the number f vehicles per hur that pass the radiactive shipment in bth directins. Any rute can be divided int as many sectins as desired fr dse calculatin; e.g., the dse t residents f a single huse r city blck. Hwever, as a practical matter, rutes are divided int rural, suburban, and urban segments accrding t the ppulatin per square kilmetre (ppulatin density). Given the cmplexity f calculating dses frm fr example mving vehicles, RADTRAN prvided a suitable slutin. Input parameters fr rutine cnditins f transprt in RADTRAN are: - Package external dse rate, which can either be the limits f the Transprt Regulatins (cnservative) r actual measured/mdeled dse rates; - Crew details, ie number f crew, distance frm package, shielding factrs; - Vehicle and package dimensins, fr use in calculating expsure distances using inverse square law frmula fr handlers; - Vehicle speeds t calculate expsure times frm mving vehicles; - Vehicle external dse rate fr dse calculatins; - Rute characteristics, ie distances, ppulatin densities (RADTRAN assumes the expsed ppulatin is in a 800 meter wide band n either side f the rute), vehicle densities, persns per vehicle;

4 - Stp characteristics fr expsure scenaris during stps at re-fuelling, cmpulsry stps, etc; - Handling characteristics fr expsure scenari during handling (nrmally ccupatinally expsed individuals). RADTRAN calculates the dses assuming a prbability f 100% and the results is nrmally a very small external radiatin dse, presented as: Results f such accidents are presented in the fllwing frms: - Cllective external dse t residents alng rute; - Cllective external dse t public at stps; - Cllective external dse t urban nn-residents; - Cllective dse t ccupants f vehicles sharing rute; - Occupatinal external dses; and - Maximum individual in-transit dse. TRANSPORT ACCIDENT DOSES The calculatin f risks assciated with transprt accident is mre cmplex. Given that the deterministic dses might exceed the criteria due t ver-cnservatism in assumptins, RADTRAN prvides fr the prbabilistic assessment f these dses. The different types f accidents that can interfere with rutine transprtatin f spent nuclear fuel are: - Accidents in which the spent fuel cask is nt damaged r affected. Minr traffic accidents ( fender-benders, flat tires), resulting in minr damage t the vehicle. These are usually called incidents. Accidents which damage the vehicle and r trailer enugh that the vehicle cannt mve frm the scene f the accident under its wn pwer, but which d nt result in damage t the spent fuel cask. Accidents invlving a death r injury, but n damage t the spent fuel cask. - Accidents in which the spent fuel cask is affected. Accidents resulting in lss f lead gamma shielding but n release f radiactive material. Accidents in which there is a release f radiactive material. Radiactive materials released int the envirnment are dispersed in the air, and sme depsit n the grund. If a spent fuel cask is in a severe enugh accident, spent fuel rds can tear r be therwise damaged, releasing fissin prducts and very small particles f spent fuel int the cask. If the cask seals are damaged, these radiactive substances can be swept frm the interir f the cask thrugh the seals int the envirnment. Release t the envirnment requires that the accident be severe enugh t fail the blts that hld the cask lid, disldge the lid, fail the seals, damage the fuel rds, and release the pressure in the rds. There must be psitive pressure t sweep material frm the cask t the envirnment. Even if the blts and seals fail, if the fuel is in a clsed canister in the transprtatin cask, n radiactive material will be released. When material is swept frm the cask and released int the envirnment, it is dispersed by wind and weather. The dispersin is mdelled using the accident mdel in RADTRAN 6, which is a Gaussian dispersin mdel. The release wuld be at abut 1.5 meters abve grund level, since the cask is sitting n a truck r railcar. The gas sweeping frm the cask is warmer than ambient, s that release is elevated. The maximum air cncentratin and grund depsitin are 21 m dwnwind frm the release. The dispersin was mdelled using neutral weather cnditins (Pasquill stability D, wind speed 4.7 m/sec). It was repeated using very stable meterlgy (Pasquill stability F, wind speed 0.5 m/sec) but the difference was negligible, because f the relatively lw elevatin f the release. The

5 maximally expsed individual wuld be lcated directly dwnwind frm the accident, 21 meters frm the cask. Input parameters fr accident cnditins f transprt are: - Radinuclide inventry which, fr spent fuel, can be very elabrate thus requiring a screening analysis t determine mst imprtant radinuclides based n relative dse cntributin t be included; - Accident rate (rute characteristic) similar t rutine cnditins f transprt but includes accident statistics and land farming fractins; - Accident specific parameters such as the cnditinal prbability f accident (severity fr Type B cntainers) btained frm available studies [5], release, aersl, respirable fractins, particle settling velcities; - Meterlgical parameters in case f an accident which frtunately prvides fr the selectin f wrst weather cnditins (stability class) in the absence f knwn weather cnditins. Prvided that the parameters were selected apprpriately, the utput f these calculatins will yield results fr the varius kinds f accidents pssible fr example accidents invlving a release and/r dispersin f material. Results f such accidents are presented in the fllwing frms: - Number f expected accidents (per link); - Cllective dse and/r ppulatin risk frm inhalatin, resuspensin, grundshine and cludshine; - Maximum dse and/r risk fr individual; - Dses and dse risks per radinuclide and - Critical grup dses and dse risks Additinal infrmatin available in RADTRAN and utside the scpe f this paper is a lss f lead shielding (LOS) mdel, an ecnmic mdel and a recently added uncertainty analysis capability. INTERPRETATION OF RESULTS The safety case presented t the cmpetent authrity needs t include the aspects addressed in this paper. The safety case fr the repatriatin f spent fuel frm Suth Africa included all the required aspects including a cask validatin reprt, prcedures, detailed assessments f the actins in the wrk prcedures and a cmplete transprt safety assessment. The transprt safety assessment was perfrmed using RADTRAN cmputer cde and the mst imprtant utcmes are summarised: - The radinuclide screening assessment cnfirmed the 10 mst imprtant nuclides wuld be representative f >99.9% f the radilgical impact; - The spent fuel was ld (ver 26 years since remval frm cre) and significantly less material were laded int the casks than it was designed fr; - Dses frm rutine cnditins f transprt were extremely lw (as a result f the lw activity f the spent fuel) with the maximum individual in-transit dse calculated t be less than 3.0E-03 Sv; - Ntwithstanding the availability f a prbabilistic mdule in RADTRAN, it was pssible t demnstrate deterministically (assuming a prbability f 1 fr the accident t ccur) cmpliance t dse criteria fr accident cnditins. The explanatin fr this is the lw activity f the spent fuel as a result f its age. CONCLUSIONS The safety case fr the transprt f spent fuel has several aspects fr which cmpliance t regulatry requirements needs t be demnstrated.

6 The qualitative aspects are mstly administrative in nature. The quantitative aspects supprt the qualitative aspects t demnstrate cmpliance. Thrugh the use f the RADTRAN cmputer cde it was pssible t demnstrate that fr nrmal and accident cnditins f transprt the dses and risks were extremely lw and in cmpliance t regulatry requirements. ACKNOWLEDGEMENTS Dr Ruth Weiner, Sandia Natinal Labratries, fr training, guidance and supprt. US Department f State fr funding the presenter s attendance t IRPA. REFERENCES [1] IAEA Safety Requirement TS-R-1, Regulatins fr the Safe Transprt f Radiactive Material, 2009 [2] IAEA Safety Guide TS-G-1.1, Advisry Material fr the IAEA Regulatins fr the Safe Transprt f Radiactive Material, 2008 [3] IAEA Safety Series 115, Internatinal Basic Safety Standards fr Prtectin against Inizing Radiatin and fr the Safety f Radiatin Surces, 2003 [4] RadCat 3.0 User Guide, SAND P, Ruth F. Weiner et al, Sandia Natinal Labratries, 2009 [5] Reexaminatin f Spent Fuel Shipment Risk Estimates, Sprung J.L. et al, U.S. NRC, NUREG/CR-6672, SAND , 2000