Modeling of 14C Vertical Distribution in Bottom Sediments of the Ignalina Nuclear Power Plant Cooling Reservoir

The Ignalina Nuclear Power Plant (INPP) in Lithuania is a rare case when lake water is used instead of river or sea water for cooling. Lake Drūkšiai with water residence time of 3–4 year and undisturbed sediment layers is a unique system to assess the impact of a nuclear facility on the aquatic ecosystem with a sufficiently high temporal resolution. We constructed a model of radiocarbon cycling processes in lake ecosystem which evaluates the 14C specific activity vertical distribution in two organic sediment fractions: alkali-soluble and alkali-insoluble. Model calculations proved that during the first 15 years of operation since 1983, 14C annual aqueous releases from the INPP were in water dissolved inorganic carbon form and varied in the range of 2.4 ÷ 3.7 × 108 Bq/year. The results of the modeling of hypothetic scenarios also showed that there was the only one episode of elevated releases from the INPP in 2000–2001, which changed the interaction between the two organic sediment fractions for the period of 2000–2006. It was caused most probably by released chemicals from INPP but not by 14C contamination. Interaction processes between both sediment fractions recovered to its original state after 2006, indicating that the released additional chemical compounds lake ecosystem have been cleaned-up

Branduolinio kuro mikrostruktūros įtakos dalijimosi produktų smelkimuisi modelis

Dujinių branduolinio kuro (BK) dalijimosi produktų (DP) išsiskyrimui iš BK tablečių UO2 matricos žymią įtaka daro BK mikrostruktūros dariniai - granulės, DP ertmės granulėse ir tarp jų bei UO2 matricos defektai. Pastebėta, kad vidutinio ir didelio išdegimo BK ertmės pagreitina DP smelkimąsi, veikdamos kaip poringi kuro dariniai, lengvinantys DP išsiskyrimą. Siūloma atsižvelgti į BK tablečiu mikrostruktūros poringumą, moduliuojant DP difuzijos koeficientą periodine funkcija. Tokia radialioji difuzijos koeficiento priklausomybė siejama su BK dariniais- tarpgranulinėmis ertmėmis ir granulių kraštais, egzistuojančiais šviežiame ir panaudotame BK. Skaitmeninis DP BK tabletėje difuzijos lygties sprendimas parodė, kad DP iš BK matricos smelkiasi greičiau lyginant su atveju,kai į kuro mikrostruktūrą neatsižvelgiama. Tai siejama su greitesne DP difuzija BK tarpgranulinėse ertmėse, kai BK išdegimas yra vidutinis ir didelis (> 30 MWd=kg U). Rasta, kad DP koncentracijos radialusis pasiskirstymas BK tabletėje yra kokybiškai panašus į pateikiamus literatūroje. Modeliuojant dalijimosi produktų smelkimąsi į branduolinį kurą, išaiškinta, kad moduliuota difuzija atitinka įprasto difuzijos koeficiento temperatūrinės priklausomybės nukrypimą nuo aktyvacinio dėsnioA model of the gaseous fission product release from the nuclear fuel pellet is proposed. The presence of fission product bubbles in the fuel matrix is simulated as periodical modulation of the diffusion coefficient by chain of Gaussian functions. The numerical solution of a diffusion equation shows enhanced fission product release at larger bubble concentrations. It is related to the facilitated diffusion of gaseous fission products in grain boundary bubbles of medium and higher burnup fuel (>30 MWd=kg U). The calculated profile of the fission product concentration distribution is similar to that obtained in the experimental studies. It is demonstrated that the influence of the developed fuel microstructure on the gaseous fission product release can be equivalently taken into account by modifying the Arrhenius behaviour of the diffusion coefficientFizikos institutasVytauto Didžiojo universiteta

Modelling the influence of nuclear fuel microstructure on fission product release

Šiame darbe siūloma atsižvelgti į branduolinio kuro (BK) tablečių polikristalinę mikrostruktūrą, moduliuojant UO2 matricos difuzijos koeficiento išraišką harmonine funkcija su periodu, artimu BK granulės dydžiui. Skaitmeninis lygties sprendimas rodo, kad dalijimosi produktai (DP) iš BK matricos smelkiasi lėčiau lyginant su atveju, kai į kuro mikrostruktūrą neatsižvelgiame. Siejame tai su DP pagavimu BK tarpgranulinėse ertmėse. Nustatytas DP koncentracijos radialinis pasiskirstymas BK tabletėje yra kokybiškai panašus į pateikiamus literatūrojeThe gaseous fission product release model including modulation of the diffusion coefficient by harmonic function with a period matching the fuel grain size has been considered. Numerical solution of diffusion equation shows reduced diffusion rate. It is related to the trapping of fission products in inter-granular bubbles of the fuel. The calculated profile of fission product concentration distribution is similar to the one obtained in the experimental studiesFizikos institutasVytauto Didžiojo universiteta

Assessment of radioactive contamination in primary circuit of WWER-440 type reactors by computer code OSCAR for the decommissioning case

International audienceThe article presents the results of modeling of the contamination of primary circuit of WWER-440 type reactor.The modeling has been performed using the OSCAR computer code (Nuclear research Centre in Cadarache, CEA,France), version V1.3 for decommissioning of nuclear power plants, taking into account the peculiarities of theWWER-440 reactor primary circuit, such as geometry, volumes, operational regime, materials. The results havebeen analyzed in the light of the scaling factor approach with the aim to demonstrate the modeling capabilitiesto identify the possibility of scaling factor application and to reproduce the activity correlation between difficultto measure and key nuclides. Calculated scaling factors for corrosion products are comparable with the ones ofother nuclear power plants. Calculation results of surface activity contamination confirmed the applicability of $^{60}$Co as a key radionuclid

Distribution of radiocarbon in sediments of the cooling pond of RBMK type Ignalina Nuclear Power Plant in Lithuania.

The vertical distribution of radiocarbon (14C) was examined in the bottom sediment core, taken from Lake Drūkšiai, which has served as a cooling pond since 1983 for the 26 years of the Ignalina Nuclear Power Plant (INPP) operation using two RBMK-1500 reactors (Russian acronym for"Channelized Large Power Reactor"). 14C specific activity was measured in alkali-soluble and -insoluble fractions of the sediment layers. Complementary measurements of the 210Pb and 137Cs activity of the samples provided the possibility to evaluate the date of every layer formation, covering the 1947-2013 period. In addition, 14C distribution was examined in the scales of pelagic fish caught between 1980 and 2012. Our measurements reveal that, during the period 1947-1999, the radiocarbon specific activity in both fractions exhibits a parallel course with a difference of 5 ± 1 pMC (percent of modern carbon) being higher in alkali-soluble fraction, although 14C specific activity in both fractions increased by 11.4-13.6 pMC during the first 15 years of plant operation. However, during the 2000-2009 period, other than previously seen, a dissolved inorganic carbon (DIC) → aquatic primary producers → sediments 14C incorporation pattern occurred, as the radiocarbon specific activity difference between alkali-soluble and -insoluble fractions reached 94, 25, and 20 pMC in 2000, 2006, and 2008, respectively. Measurements in different sediment fractions allowed us to identify the unexpected organic nature of 14C contained in liquid effluences from the INPP in 2000-2009. The discrepancy between 14C specific activity in fish scales samples and DIC after 2000 also confirmed the possibility of organic 14C contamination. Possible reasons for this phenomenon might be industrial processes introduced at the INPP, such as the start of operation of the cementation facility for spent ion exchange resins, decontamination procedures, and various maintenance activities of reactor aging systems and equipment

What cooling pond sediments can reveal about 14C in nuclear power plant liquid effluents: Case study Lake Drūkšiai, Ignalina nuclear power plant cooling pond.

The vertical distribution of radiocarbon (14C) was examined in two bottom sediment cores, taken from Lake Drūkšiai which had been used as a cooling pond for the Ignalina nuclear power plant (INPP) with two RBMK type reactors. The aim of this work was to reconstruct 14C amounts in the lake ecosystem during an 8-year period after the INPP was closed, as any official monitoring of 14C in liquid releases from the INPP was not performed. The possibility of comparing the variation of the 14C specific activity in the corresponding layers of the same period of 3 different cores (one taken in 2013 and two in 2019) revealed the variability of the determined values of liquid radiocarbon discharges from the INPP into the lake. Cores taken in 2019 showed a permament14C release rate of 0.76±0.06 GBq/y all eight years after the closure of the INPP. The 14C release rate established from radiocarbon measurements in both cores did not differ by more than 0.8 GBq/y. However, including data from the core taken several years ago, the estimated radiocarbon release rate values varied within 1.3 GBq/y

Dating lake sediments by ²¹⁰Pb and ¹³⁷Cs and CRS sedimentation rates.

Dating lake sediments by 210Pb and 137Cs and CRS sedimentation rates.</p

Rapid analysis method for the determination of 14C specific activity in irradiated graphite.

14C is one of the limiting radionuclides used in the categorization of radioactive graphite waste; this categorization is crucial in selecting the appropriate graphite treatment/disposal method. We propose a rapid analysis method for 14C specific activity determination in small graphite samples in the 1-100 μg range. The method applies an oxidation procedure to the sample, which extracts 14C from the different carbonaceous matrices in a controlled manner. Because this method enables fast online measurement and 14C specific activity evaluation, it can be especially useful for characterizing 14C in irradiated graphite when dismantling graphite moderator and reflector parts, or when sorting radioactive graphite waste from decommissioned nuclear power plants. The proposed rapid method is based on graphite combustion and the subsequent measurement of both CO2 and 14C, using a commercial elemental analyser and the semiconductor detector, respectively. The method was verified using the liquid scintillation counting (LSC) technique. The uncertainty of this rapid method is within the acceptable range for radioactive waste characterization purposes. The 14C specific activity determination procedure proposed in this study takes approximately ten minutes, comparing favorably to the more complicated and time consuming LSC method. This method can be potentially used to radiologically characterize radioactive waste or used in biomedical applications when dealing with the specific activity determination of 14C in the sample

CO₂ flow intensity signal measured by TCD (black line) and cps spectra of samples No. 3 (green), No. 6 (blue) and background (red) as determined with the semiconductor detector.

<p>In total, three combustion cycles (400 s each) were used to completely combust the non-ground graphite sample.</p

An example spectrum of ¹⁴C measured using semiconductor detectors.

<p>The green color denotes the spectrum of sample No. 3 and the black color denotes a typical background spectrum. Both spectra were collected over 100 s time intervals.</p