Comper, W.

Diefenbacher, W.

Korthaus, E.

Smailos, E.

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Mai 1976Abteilung Behandlung radioaktiver AbfälleAbteilung Reaktorbetrieb und TechnikRadi!llllysis Elrlirll Tem~terait1!Jne Ef'hlltts im CES® [jfUndiSr~mlmd StiGlrage ~f Bit1!Jm:ernE. Smailos, \IV. Diefenbacher, E. Korthaus, W. ComperKFK 2329GESEllSO~AFTFÜRKERNFORSCHUNG M.B.H.KARlSRUHEAls Manuskript vervielfältigtFür diesen Bericht behalten wir uns alle Rechte vorGESELLSCHAFT FÜR KERNFORSCHUNG M. B. H.KARLSRUHEK ERN F 0 R S C H U N G S ZEN T RUMKFK 2329PWA-Nr. 43/76A R L S RUH EAbteilung Behandlung radioaktiver AbfälleAbteilung Reaktorbetrieb TechnikRadiolysis and Temperature Effects in Case ofUnderground Storage of Bitumenx )E. SmailosW. DiefenbacherE. KorthausW. ComperGesellschaft für Kernforscnung mbH, Karlsruhex) Paper presented at the NEA Seminar on the Bituminizationof Low and Medium Level Radioactive Wastes, Antwerp,18th - 19th Mai, 76AbstractOn conservative assumptions limit values were determinedfor the specific activity in the bitumen products, whichsafely avoid under the conditions of storage in a non-ventilated prototype cavity both the formation of aignitable radiolytic gas/air mixture and intolerableheating of waste products over the entire storage time.Depending on the filling factor of the cavity and on theage of fixed fission products, the limit values of thespecific activity in the waste products allowing to avoidan ignitable gas/air mixture in the cavity range from0.09 Ci/l to 0.78 Ci/I. The respective limit values ofspecific activity allowing to avoid intolerable heatingof wastes (> 70o C) range from 0.3 Ci/l to 0.7 Ci/I,depending on the age of fission products and on type offilling.Radiolyse- und Temperaturprobleme bei der Endlagerung vonBitumenproduktenKurzfassungUnter konservativen Annahmen wurden Grenzwerte für die spe-zifische Aktivität in Bitumenprodukten bestimmt, bei denenunter den Bedingungen der Lagerung in einer unbelüftetenPrototyp-Kaverne sowohl die Bildung eines zündfähigen Radio-lysegas/Luft-Gemisches als auch eine unzulässige Erwärmungder Abfallprodukte während der gesamten Lagerzeit mit Si-cherheit vermieden wird.Die Grenzwerte der spezifischen Aktivität in den Abfallpro-dukten zur Vermeidung eines zündfähigen Gas/Luft-Gemischesin der Kaverne betragen je nach Füllungsgrad der Kaverneund Alter der fixierten Spaltprodukte 0,09 Ci/l bis 0,78 Ci/i.Die entsprechenden Grenzwerte der spezifischen Aktivität zurVermeidung einer unzulässigen Erwärmung der Abfälle (> 70o C)liegen je nach Alter der Spaltprodukte und Einlagerungsartzwischen 0,3 Ci/l und 0,7 Ci/i.Table of Contents:1. The Problem and the Objectives of theInvestigations 12. On the Formation of Radiolytic Gases fromBituminized Wastes 12.1 Model Assumptions and ComputationalMethods2.2 Results3. On the Heat Generation from BituminizedWastes3.1 Model Assumptions and Methods ofCalculation3.2 Results4. Discussion5. Conclusions6. References1488912131511. THE PROBLEM AND THE OBJECTIVES OF THE INVESTIGATIONSIn cooperation between Gesellschaft für Strahlen- und Umweltfor-schung mbH Munich (GSF) and Gesellschaft für Kernforschung mbHKarlsruhe (GfK) a prototype cavity of 10,000 m3 volume is beingconstructed in the Asse 11 salt mine for trial storage of mediumlevel wastes. Fig. 1 shows a drawing of the cavity. Under thetechnique of filling envisaged here the fixed waste products arelowered about 918 m from the surface down to the unloadingfacility by rneans of a shaft hoisting equipment and introducedinto the cavity frorn this level in a free fall of about 70 m atthe maximum.Prior the disposal of bituminized radioactive wastes in theprototype cavity GfK had performed investigations into nuclearsafety. It was studied in this context, whether radiolysis andheat problems respectively, arise in the prototype cavity as aresult of radiolytic decomposition of bitumen and decay heat ofthe fixed fission products, which set a limit to the specificactivity of the bitumen products when stored in the cavity.The aim i8 pursued in the investigations of defining the limitvalues for the specific activity in bituminized wastes in whichunder the conditions of storage inanon-ventilated prototypecdvity both the formation of an ignitable radiolytic gas/airmixture and intolerable heating of waste products during theentire per iod of storage are safely avoided. As a maximumadmissible temperature for the bitumen products was assumed thesofting point of bitumen Mexphalt 15 (70oC).2. ON THE FORMATION OF RADIOLYTIC GASES FROM BITUMINIZED WASTES2.1 Model Assumptions and Computational MethodsThe following assumptions were made for the calculations:2Type of WasteFixed LWR-fission products (33,000 MWd/t, 30 MW/t); bitumen Mex-phalt 15/NaN03 products (50 wt. % salts), density 1.5 kg/l;specific activity of products at the time of filling 0.1 Ci/lup to 1 Ci/li age of fission products (at the time of filling)0.5 year, 1 year, 2 years.Cavity and Type of FillingCavity volume: 10,000 m3 ; filling rate 500 m3/yri filling time5 yrs and 10 yrs, respectivelYi type of filling instantaneousifilling factor 25 vol % and 50 vol. %, respectively.Moreover, the following conservative assumptions were made:- The cavity is a completely sealed system (no ventila-tion of the cavity) .- All the radiolytic gases formed by radiolysis of bitu-men are released from the waste products into the at-mosphere of the cavity.To calculate the cumulative amounts of hydrogen, which is theprincipal component of radiolytic gases (besides hydrogen CH4and C2Hx are formed reaching about 15 % of the hydrogen volume),the integral absorbed radiation dose of bitumen/NaN03 mixturesduring storage in the prototype cavity was calculated with a "SPALT"integration program /1/ as a function of the storage time fordifferent specific activities of the products and different agesof the fission products. The program allows to calculate theinventory of fission products and the activity as a functionof the irradiation and cooling times. The equations are solvednumerically. Total absorption of ß and r radiation was assumedin the calculations. The yields of fission products from 235ufission by thermal neutrons (14 MeV) were taken from Meek andRider /2/, the half lives and the ß andTenergies of isotopesfrom Lederer et al /3/ and the activation cross sections from3h sting equipmentmachinery houseshafthoisting ca gedru m--+------r-....Ii........."--hinged lid-926mE(Y')(Y')neck of the cavity-995mcavi ty(10,000 m3 )Fig. 1 Sketch of prototype cavity4BNL /4/ and W. Seelmann-Eggebert et al /5/, respectively. Thecalculated values for the absorbed radiation dose together withthe results of investigations by Kluger /6/ on the formationof radiolytic hydrogen under irradiation of bitumen/NaN03 mixturesshowing that the hydrogen formation rate is 3 . 10-3 cm3 H2/Mrad. gwere used to calculate with the computer program and as afunction of storage time the accumulated hydrogen amounts presentin the cavity, which are directly proportional to the absorbedradiation doses.2.2 ResultsIt appears from the calculations that with increasing specificactivity of the products and with growing age of fission pro-ducts present in the wastes (assuming the same specific initialactivity) the absorbed radiation dose and, consequently, theformation of hydrogen increase. The results of calculations ofthe integral absorbed radiation dose in the bitumen productsand of the resulting hydrogen formation in a 175 1 drum filledwith bitumen products have been represented in Fig. 2 as afunction of the storage time. The specific activity of the pro-ducts is 0.1 Ci/l for 0,5 year, 1 year, 2 years old fissionproducts. Fig. 3 shows the amounts of hydrogen accumulated inthe cavity during storage of bitumen products as a function ofthe storage time for a filling factor of the cavity of 50 vol. %(~5,000 m3 of waste) . The specific activity of products is0.1 Ci/l for 0.5 year, 1 year und 2 years old fission products.It is evident from Fig. 3 that the hydrogen formed by radiolysisof bitumen/NaN03 mixtures having a specific product activityof 0.1 Ci/l and with the fixed fission products up to 2 yearsof age will not give rise to an ignitable radiolytic gas/airmixture in the prototype cavity.The limit values derived for the specific activity, taking intoaccount all radiolytic gases (H 2 , CH 4 , C2Hx ) and the absorbeddose in the bitumen products in which, according to the assumptions5made here, ignitable radiolytic gas/air mixtures are not formedduring the entire per iod of disposal in the prototype cavity,have been indicated in Table I as a function of the age offission products in the wastes and of the filling factor of thecavity.t<11I/lo"01:.D....oI/l.DCl::: 105C'l<11-CIntegr. absorb. dose --o-.2.J...r.s...c~.I \ )~. ~__ ~.-!1.r_obltumen/NaN031 SOwt~/oNaN03) j /1:/ 0-........ 0.5 yrspee. ae tivity: 0.1 Cil! .......... ,P"fission produet age: O. Syr .~~1 yr ;f'/2 yrs//+l total storage time( fission product age+storage l.....co--t:IE....o--IN:::c104 +--~-.,--.-..,.....,....,..,......----r-.........--.,-...,..,.--.-r.,.,---.-~.........--,--,r-r-r..,.,----,-----,---r-.-,...-.r"1""'" 10-110-1 100 101 102 103+l storage time [yearsJ--Fig. 2 Integral absorbed dose in bitumen/NaN03 mixturesand hydrogen formation in a 175 1 drum filledwith bitumen products0'\1 year0.5 yearbitumen / NaN0 3 (50 wt,% NaNO 3 )spec. activity : 0.1 eil!fission product age: 0.5 yrJ 1 yr J 2 yrsfilling rate: 500 m3/ yearfilling time: 10 yearsfilling foctor: 50 vol.%175150200i-- __ ~wer bound ~hyd rogen igniti~ _o2 yearsCGICi'Ie 75"'C>..c.W 5025i 125~100...-.......--.0-1 ~II I I1I I 11I I I IIIII'~1 101 10 2 103storage time [yeors]---Fig. 3 Amounts of hydrogen accumulated in the prototype cavity7Table ILimit values for the specific act andradiation dose in the bitumen/NaN03 mixturescorresponding to 4 vol. % H2 .integral absorbedwt.% salts)Age of Fission Cavity Filling Limit Values of Limit Values ofProducts Factor Specific Activity Absorbed Dose un-til Total Decay/..Yr~.7 {vol. ~7 {Ci/!? /..i-aqj'0.5 25 0.78 2.6·1071 25 0.43 2.6 · 1072 25 0.26 2.6 · 1070.5 50 0.26 8.6 · 1061 50 0.14 8.6 · 1062 50 0.09 8.6 · 10683. ON THE HEAT GENERATION FROM BITUMINIZED WASTES3.1 Model Assumptions and Methods of CalculationThe following model assumptions were made for the investi-gations:Type of WasteFixed LWR-fission products (33,000 MWd/t, 30 MW/t); bitumenMexphalt 15/NaN03 products (50 wt. % salts), density 1.5 kg/l;specific activity of wastes at the time of filling 0.1 Ci/l upto 1 Ci/i; age of fission products (at the time of filling)0.5 year, 1 year and 2 years.CavityDiameter 22.8 m, height 36 m (corresponding to the maximumdimension of the cavity); compact filling (up to the respectivelevel); heat conductivity of filling 0.31 W/moC (correspondingto the waste product leaving aside the sheet metal drums); noair gap provided between the filling and the cavity wall; initialtemperature of the surrounding salt 37 oC.Type of FillingInstantaneous or step by step in constant single steps extendedover 10 years.Three different non-steady-state heat conduction programs wereused for the temperature calculations. The case of instan-taneous filling was first studied using an analytical formulafor heat propagation based on the "WÄRMELEIT" computer pro-gram /7/ with the cavity geometry approximated by a rectangularparallelepiped having the same cross section. The temperatureat the cavity wall was assumed to be constant. More precisecalculations of this case by means of a numerical two-dimensionalheat conduction program in the "TEFELD" cylinder geometry /8/showed that the calculations based on the first computer programfurnished maximum temperatures which were too low by only about 5%.9For the case of step by step fill comprising a greater numberof individual steps a special program was developed /9/ allowingto simulate this process, thus simulating also continuousfilling in an approximation. By this the heat transportfrom plane layers of constant thicknesses is explicitly calcula-ted in the axial direction only while the radial heat removal wastaken into account by introducing a term into the system ofequations. This term was fitted by means of results from respec-tive calculations based on the "WÄRMELEIT" program. The heatevacuation from the surface of the respective filling was consi-dered by assuming a constant heat transfer coefficient of air(~= 2 W/m2 oe). The fission product inventory and the thermalpower of fission products as a function of the irradiation andcooling times were calculated with the "SPALT" computer program/1/ also used to calculate the absorbed doses in the products.The fission product yields obtained in 235u fission with thermalneutrons and the decay data of isotopes were taken from theliterature /2, 3, 4, 5/.The calculations are conservative for the following reasons:Assumption of a compact filling results in a higherheat power density than will be encountered in practice.The approximation used of the true shape of the cavity(prolate ellipsoid with circular cone superimposed)underestimate the heat dissipation to the saltoThe effective heat conductivity of the cavity fillingis underestimated since the influence of the drum ma-terial on heat dissipation has not been taken intoaccount.3.2 ResultsThe results of investigations show that with growing age of thefission products (with the specific initial activity remaining10the same) i.e. with increasing integral absorbed dose and withincreasing specific activity of the waste products the tempera-tures rise in the cavity.As an example of results Fig. 4 shows the time curve of themaximum temperature in the cavity for specific waste activi-ties of 0.1 Ci/l and 0.3 Ci/I, an age of fission products of0.5 yr and instantaneous as weIl as step by step filling.The limit values of specific activity in the bituminized wastes,derived from the temperature calculations, which ensure thaton the model assumptions made here the maximum tolerable tem-perature of 70°C ist not exceeded during the whole storage time,have been indicated in Table 11 as a function of the age offission products. Table 11 shows that there is no major differencebetween the instantaneous and the step by step fillings as re-gards the limit va lues of the specific activity. However, in thecase of step by step filling the maximum temperature does notoccur in the center of the cavity as in case of instantaneousfilling, but at the upper end, the more so, the lower the age offission products iso551 I-"-"5bitumen I NaN03age of the fission products: 0.5 yr.A =0.31 W/moC,,:>c,\\\ ./ -- -0-...~ß/ ""-/ "///---0-- instantaneous fiHing-- step-by-step fiUing (10 eps)35-1 i I I I i I i i I I I i I i I i i J1 5 101storage time [y~ars]---....tEE...--...~ 50GJa...':'......Cfa...<IJc.. 45EGJ-Fig 4 Maximum temperature in the prototype cavity as a functionof storage time12Table 11Limit values for the specific activity in the bitumen/NaN03mixtures (50 wt.% salts) corresponding to a maximum temperatureof 70°C.age of fission limit va lues of specific activityproducts LCi/l)Lyrs]instantaneous step by stepfilling filling0.5 0.6 0.71 0.45 0.452 0.35 0.34. DISCUSSIONIt appears from the investigations that the formation of hydrogenand heat generation from fission product bearing wastes stronglydepend on the age of fission products, i.e. on the integralabsorbed dose. To be able to fix limit values for the specificactivity in wastes in case of storage in the prototype cavitythe knowledge is necessary of the age of fission products atthe time of filling. The values indicated here for the specificactivity in the wastes are applicable only to fission productsfixed in bitumen originating in LWR fuel elements. They are con-servative values since pessimistic assumptions were made forthe calculations.On ac count of various external influences (e.g. ventilation ofthe shaft and of the upper half of the neck of the cavity asweIl as variations in atmospheric pressure) an exchange of airin the cavity and, consequently, a considerable dilution ofradiolytic gas concentration in the cavity will certainly take13place in the prototype cavity. As regards radio is this willentail an increase in specific activity in the wastes. Theoreti-cal studies performed by the Gas Institute of KarlsruheUniversity /10/ on behalf of Gesellschaft fürKarlsruhe show that by appropriate technical measures applied inaddition the filling of bitumen products with a specific activi-ty of up to 1 Ci/l would not give rise to radio ic problemsin the prototype cavity.Major differences are not found in the maximum temperatures ofthe prototype filled either instantaneously or step by step.However, with different dimensions of the cavity or differentwaste properties the differences might weIl be on larger scale.In the previous studies the ~-emitters have not been taken intoaccount which are pr~sent as impurities in the wastes~ Studiesto this effect are under way.5. CONCLUSIONSThe following conclusions can be drawn from the results ofinvestigations:Disposal of medium level bitumen products generated now andover the next 3 to 4 years at the Karlsruhe Nuclear ResearchCenter with a medium specific activity of about 0.1 Ci/l(maximum value 0.3 Ci/I) and an age of fixed radionuclidesof about 0.5 year would not cause radiolytic and heat pro-blems in the prototype cavWhen bitumen products shall be disposed of in non-ventilatedstorage spaces, an upper limit must be set for the specificactivity in the waste products because of radiolytic gas for-mation and heat generation. This upper limit would be belowthe value of 1 Ci/l which under the aspect of radiationresistance of the bitumen fixing material would still be14admissible, since radiolysis and heat problems arise beforethe bitumen is damaged by radiation.Disposal in subsoil storage spaces (in this case in theprototype cavity) of bitumen products having specific ac-tivities of up to 1 Ci/l is possible without hazard to thesafety by radiolytic gas or heat generation, provided thatthe following measures are taken:a) Ensuring the exchange of air, which reduces the radioly-tic gas concentration in the storage spaces, or fillingthe storage space with cover gas (e.g. CO 2 , N2).b) Construction of slim cavities (diameter« height) forbetter radial heat dissipation to the surrounding saltor increase of effective heat conductivity of the cavitycontent, e.g. by providing media with a good thermalconductivity. Another possibility is offered by theinterim storage of waste products prior to filling theminto the prototype cavity.5REFERENCES/1/ W. Comper, unpublished re ts/2/ M. E. Meek, B.F. Rider, NEDO-12154 (1972)/3/ C.M. Lederer, I.M. H~~lander, S. Perlman,Table of Isotopes, 6 ed. John Wiley and Sons,New York (1967)/4/ BNL - 325 (1972)/5/ W. Seelmann-Eggebert, G. Pfennig, H. Münzel,Table of Nuclides (1974)/6/ W. Kluger in H. Krause,KFK 1830 (1973)/7/ E. Korthaus, unpublished results/8/ H. Schmidt,Thesis, Rheinisch-Westfälische Technische HochschuleAachen (1971)/9/ E. Korthaus, unpublished results/10/ Engler-Bunte-Institut der Universität Karlsruhe"Gutachtliche Stellungnahme zu gastechnischen Fragenbei der Lagerung von radioaktiven Abfällen in derPrototyp-Kavernenanlage im Salzbergwerk Asse",unpublished results

Radiolysis and temperature effects in case of underground storage of bitumen

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Radiolysis and temperature effects in case of underground storage of bitumen

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