2nd Petrus-OPERA Conference on Radioactive Waste Management and Geological Disposal
16
Radionuclide transport model of the near field and far field of geological
repository
D. Barátová, V. Nečas
Slovak University of Technology in Bratislava,
Institute of Nuclear and Physical Engineering, Bratislava, Slovakia
Abstract
The results on the analysis of radionuclide release from the geosphere of hypothetical repository in crystalline rocks are presented. Radionuclides with poor retentive properties (C-14, I-129, Cl-36, Cs-135), represent the largest contribution to the total release rate from the geosphere. The release rates of radionuclides with low solubility limits can be significantly reduced. The presence of stable isotopes of the same element has the effect of reducing the solubility of safety relevant radionuclides. The calculations of release rates were carried out for one disposal container using the simulation software GoldSim.
Introduction
In Slovakia, a preferred alternative of long-term spent fuel management is a direct disposal of spent nuclear fuel which will be disposed together with radioactive waste which is not suitable for the National Radioactive Waste Repository in Mochovce (near-surface repository).
Within the deep geological repository development program in the Slovak Republic between years 1996 and 2001 were in a gradual site selection process selected five reconnaissance localities in the environment of crystalline and sedimentary rocks. Since 2010, company JAVYS, a.s. has become the implementer of deep geological disposal in the Slovak Republic and the deep geological repository program was renewed [1].
Disposal system
Spent fuel (from VVER-440 reactors) is considered to be disposed in containers made of stainless steel (inner part) and carbon steel (outer part). The disposal capacity of one container is 7 fuel assemblies [2]. Within this assessment the calculations were performed for the spent fuel with an initial average enrichment of 4.87 % of U-235 and burnup 60 MWd/kgU .The analysis was carried out for one disposal container (7 fuel assemblies). The storage time before the final disposal is 60 years. Disposal container is surrounded by a bentonite buffer with a wall thickness of 300 mm.
In Slovakia, there has not been selected a final locality for the geological disposal facility. Due to this fact, the assessment of long-term safety was performed for a hypothetical geological repository located in crystalline rocks and by using also the international research achievements (Czech Republic, Switzerland, Sweden, Japan). Transport pathways are in crystalline rocks represented by individual fractures.
Conceptual model
Spent fuel is a complex and heterogeneous system and therefore was within the model conceptually divided into the structural material, UO2 matrix and instant release fraction. Instant release fraction is a fraction of inventory which is after water contact released rapidly, in the term of long-term safety instantaneously. Then the long-term release occurs congruently with the degradation of the fuel matrix and structural material. After the disposal canister fails due to normal evolution processes (1000 years) and water comes into contact with the source term (fuel and structural material), released radionuclides start to migrate through the bentonite buffer, excavation disturbed zone (EDZ) and crystalline host rock. Concentrations of radionuclides in the void volume of the disposal container and in the bentonite buffer are limited by the solubility of each chemical element. The solubility limit is partitioned between stable and radioactive isotopes of the elements. Since it is considered that the buffer is fully water-saturated, nuclides migrate through the bentonite buffer by radial diffusion and are retarded by sorption on the buffer material.
Host rock is modelled like a fractured zone where each transport pathway has a different transmissivity. The variability in transmissivity of individual transport pathways is represented by using a log-normal distribution [3], [4]. The transport pathways have a length of 100 m and flow into the major water-conducting fault whose length is 300 m. In the individual fractures of the host rock as well as in the major-conducting fault, the
2 one-dimens into the adj assumed in groundwate pathways. Results The release assemblies software G transport wi Transport o release rate host rock m activation a the total re properties ( total release UO2 matrix. originated fr sorption is relatively hig the most si dependent o Nuclides pr limits. The e Tc, Pd and limitation is Ra-226 from The presen of safety re Se-79 are a 2nd Petrus-OPE sional advect jacent rock a n the concept er flow. No i e rates from were calcul oldSim whic ithin a compl Figure 1: of nuclides th es of many n matrix and th nd fission pr elease rate high solubilit e rate for a lo . It was assu rom UO2 ma considered gh in compa ignificant in on the retent recipitate wh elements wh d Sn. Eleme showed in F m the near fie ce of radioa elevant radio and are not a
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hat the relea that the re e porewater container ar onite buffer. the calculat s the effect o n the isotope al Disposal diffusion of ces of the h ction perpend the individu r containing out using th ynamically m he aquifer. considerably ntonite as we sults it can b he largest co ave very po se nuclides d he structural n organic for o solubility li nd its specif ase rate of th lease rates r exceeds th re U, Pu, Np The effect tions, the rel of reducing t es (stable an
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Conclusion
The near fie that many o results it ca represent th to have high release rate porewater e controlled b the same el Acknowled This project References [1] National and Rad SR. Janu [2] Radioact in a Hyp [3] Japan N for HLW [4] Japan At Fuel in J [5] GOLDSI USA, 20 2nd Petrus-OPE : On the left he right - the n
eld and far f of nuclides an be conclu he largest co h solubility lim e for a long p exceeds thei by solubility a lement has t dgments t has been su s Nuclear Fun dioactive Was uary 2014. tive Waste R othetical Loc uclear Cycle W Disposal in tomic Energy Japan: First P IM Technolog 014. ERA Conferen t - the effect e effect of s field model o are effective uded that ac ontribution to mits and low period of time r elemental s are reduced he effect of r upported by nd for Decom
ste: The Stra Repository Au cality. Czech e Developme Japan. Japa y Agency (JA Progress Re gy Group LL nce on Radioa t of solubilit haring solu t of hypothetic ely sorbed o ctivation and
the total rel w distribution e. Nuclides p solubility limi in relation to reducing the the Slovak G mmissioning
ategy for the uthority (RAW Republic: R ent Institute. H an: JNC, Apri AEA). Prelim port on Direc LC. GoldSim active Waste M 18 ty limitation bility betwe he near field cal geologica on the bento fission prod ease rate fro coefficients precipitate w its. The relea o their invent solubility of Grant Agency of the Nucle Final Stage WRA). Upda RAWRA, 201 H12: Project il 2000. minary Assess ct Disposal. J Contaminan Management a on the relea een isotopes d. al repository onite buffer a ducts like C-om the geos and that is w hen the conc ase rates of tories and ha nuclides. y for Science ar Installatio of Peaceful ated Referenc 2. t to Establish sment of Ge Japan: JAEA nt Transport M and Geologica ase rate of R s on the rele was develop and host roc 14, Cl-36, I-phere. Cl-36 why these nu centration of radionuclide alf-lives. The e through gra ns and for H Utilization of ce Project of h the Scientifi ological Disp A, December Module, Use al Disposal Ra-226 from ease rate of
ped. The res ck matrix. Ba -129, Se-79 6 and I-129 a uclides domin f released nu es which con e presence o ant VEGA 1/0 Handling of S f the Nuclear f Geological fic and Techn posal System r 2015. er`s Guide. W m the near Se-79 from sults showed ased on the and Cs-135 are assumed nate the total uclides in the centration is f isotopes of 0796/13. pent Fuel r Energy in Repository nical Basis m for Spent Washington, d e 5 d l e s f
