Krsko Nuclear Power Plant Research Articles

Dose Rate Assessment Around the PCFV Release Line During Severe Accident Conditions in Nuclear Power Plant Krsko

Abstract Passive containment filtered vent (PCFV) was installed in Nuclear Power Plant Krsko (NEK) in 2013 as part of the safety upgrade program. It is intended for severe accident consequences prevention and mitigation by ensuring the containment integrity. When the pressure in the containment reaches limiting value, containment atmosphere is released into the environment through the PCFV system exhaust line. But, before release in the environment, the containment atmosphere passes through five aerosol filters in the containment and one iodine filter in the auxiliary building (AB) to reduce isotopic activity. In this paper, dose rates around the exhaust line of the PCFV system resulting from radioactivity release in case of a severe accident were determined in a four-step methodology. The assumed severe accident scenario is a beyond design basis accident station blackout (SBO) in NEK, which was simulated using the MELCOR code. Its results were input for the radionuclide transport and removal and dose estimation (RADTRAD) radiological calculations to obtain the activities released in the containment. These activities were then transformed into the gamma source intensity and spectrum using the ORIGEN-S libraries. This form of the source term is required for Monte Carlo calculations which were performed using the MCNP6.2. Two Monte Carlo calculations were performed. One for which the radiation source was modeled to emanate from the containment atmosphere and the other from the PCFV duct fluid. The main reason for the calculation was to assess limiting dose rates around PCFV duct (radiation monitor location) during actuation after severe accident. That is why the model is simple and conservative. The other task was to demonstrate that this location is not suitable for longer personnel presence in case of equipment failure during the PCFV actuation. Due to conservative assumptions, predicted dose rates are the highest expected at that location for any severe accident scenario.

Radiation shielding analysis of the HI-STORM FW storage cask

This paper presents radiation shielding analysis of the HI-STORM FW storage cask designed by the Holtec International, Inc. for the Nuclear Power Plant Krsko Spent Fuel Dry Storage (SFDS) project. The first part of the analysis is addressing the problem of a particle source generation. For each spent fuel assembly (SFA) neutron and gamma source intensities and spectra were calculated using ORIGEN-S module of the SCALE 6.2.3 code package. The second part of the analysis is considering neutron and gamma dose rates calculation, for which hybrid shielding methodology based on Monte Carlo (MC) code MCNP6.2 and ADVANTG3.0.3 code was used. The dose rates for the reference case included the contributions of fuel neutron, fuel gamma, secondary gammas from (n, γ) reactions, as well as gammas from activated SFA hardware components due to Co-60 presence. Expectedly, the results have shown that the dose rates around the cask have nonuniform behavior due to a specific design of the cask. The heavy concrete body of the cask presents a good attenuator for all types of radiation, however, a considerable streaming through the top and bottom ventilation openings was noticed. It was also observed that an important role was played by the isotope Co-60 which emits two lines of high energy deep penetrating gammas. The convergence of dose rates and associated relative uncertainties during MC calculations have shown that significantly more histories were required for gamma calculations compared to neutron calculations. To investigate the impact of different SFA content in the casks on the dose rates, shielding analysis was made for 16 casks that will be loaded in the first campaign. For comparison, the calculations of a single HI-STORM FW cask were additionally performed using MAVRIC module of the SCALE 6.2.3 and the obtained differences in external dose rates of 20–30% are expected. The calculation cases showed the differences in casks doses due to different casks loading within loading plan criteria, reduction provided by heavy concrete in cask body (more important for gammas), importance of careful design of lower and upper casks openings to keep increase of dose rates, compared to closed configuration, within one order of magnitude, and compromise needed when describing details of intensity and spectrum of SFA sources. The increased calculation mesh density was used to show that selected discretization is appropriate, and the limited influence of storage pad thickness was demonstrated by performing calculation with thick concrete slab corresponding to Dry Storage Building (DSB) floor foundation thickness.

Evaluation of Impact of NEK Safety Upgrade Program Implementation on the Reduction of Total Core Damage Frequency

The nuclear accident in Japanese plants in 2011 has initiated a quick response from the countries utilizing a nuclear power. Krsko Nuclear Power Plant (NEK) was required by Slovenian Nuclear Safety Administration (SNSA) to perform consequential actions to reduce the risk of severe accidents and their consequences as low as feasible. NEK have analyzed the response to the severe accidents, and based on the results of this analysis, proposed the measures to be implemented in the shortest possible time. In response to SNSA request NEK has developed the Safety Upgrade Program (SUP) which contains a comprehensive set of measures for plant safety improvements. SUP Phase 1 has been implemented during the Outage 2013. This paper presents the work done on the evaluation of impact of plant modifications planned for implementation in Phases 2 and 3 of NEK Safety Upgrade Program on the quantitative figure of merit for the plant risk significance – total core damage frequency (CDF). A significance of the most valuable measures in terms of contribution to the CDF reduction was determined and a new fractional contribution of initiators categories was obtained. The quantification of total CDF was made by employment of RiskSpectrum PSA code and running a predefined set of analytical cases for all initiator categories covered by the scope of NEK PSA model. The basis for analysis was NEK’s most recent at-power PSA model which reflects the plant status with plant modernization implemented in SUP Phase 1. The analysis was done in two steps. In the first step plant modifications planned for SUP Phase 2 were addressed and in the second step plant modernization measures and upgrades planned in SUP Phase 3 were modeled in addition to those already modelled in the first step.

Analysis of the HI-TRAC VW Transfer Cask Dose Rates for Spent Fuel Assemblies Loaded in Nuclear Power Plant Krsko Storage Campaign One

Abstract In this paper, shielding analysis was performed to determine neutron and gamma dose rates around the transfer cask HI-TRAC VW (Holtec International Transfer Cask Variable Weight) loaded with spent fuel assemblies (SFA) from nuclear power plant (NPP) Krsko spent fuel dry storage (SFDS) campaign one. The HI-TRAC VW is a multilayered cylindrical vessel designed to accept a multipurpose canister (MPC) during loading, unloading, and transfer to dry storage building. The MPC can contain up to 37 spent fuel assemblies. The analysis was divided into two steps. The first step was the source term generation using origen-s module of the scale code package. The source was calculated based on the operating history of spent fuel assemblies currently located in the NPP Krsko spent fuel pool. The obtained particle intensities and source spectra of the SFA were used in the second step to calculate the dose rates around the transfer cask. A comprehensive hybrid shielding analysis included the calculation of dose rates resulting from fuel neutrons and gammas, neutron-induced gammas (n-γ reaction), and hardware activation gammas under normal conditions and during accident scenario. To obtain the dose rates within the acceptable uncertainties, forward weighted consistent adjoint driven importance sampling (FW-CADIS) variance reduction scheme, as implemented in advantg code, was adopted for accelerating final mcnp6.2 calculations. The dose rates around HI-TRAC VW cask were calculated using mcnp6.2 code for all 16 cask loadings belonging to campaign one in order to illustrate the impact of fuel assembly selection schemes proposed by company responsible for project realization - Holtec International (HI).

USE OF NUCLEAR POWER PLANT RELEASED TRITIUM AS A GROUNDWATER TRACER

Tritium presents natural radioactive isotope which can be used as tracer in hydrology. This paper presents usage of tritium in hydrogeological research of the Zagreb aquifer, which presents country`s strategic water reserves and is protected by the Republic of Croatia. In the research area higher tritium content in surface water and groundwater is the result of releasing coolant water from the Nuclear power plant Krsko. Tritium content was measured in 13 piezometers and one pumping well, in the wider area of the Petrusevec and Kosnica well fields. It has been shown that tritium from cooling water is much more pronounced than its natural seasonal variability, and it can be very useful as a tracer. Also, results showed that the intensity and delay of responses in aquifer depend on distance from the Sava River and piezometer depth. Furthermore, results suggest that variable hydrologic conditions can generate rapid change in the groundwater flow direction and affect tritium transport through the aquifer.

Validation of meteorological forecasts in fine spatial and temporal resolution produced as an input for dispersion models

In conditions of complex terrain, modelling of air pollutant dispersion still has a number of scientific challenges. Ideally, appropriate meteorological data should be available for modelling. Unfortunately, for many purposes, there is no time to carry out suitable measuring campaigns. Therefore the results of prognostic weather forecasts (NWP models) are being widely used. However, these models still have quite a few disadvantages when their results are used as input for dispersion models over complex terrain. This study presents the validation of the quality of the weather forecasts in the surroundings of the Nuclear Power Plant Krsko in Slovenia, an area with highly complex terrain and the resulting complex meteorological characteristics. The forecast is available for a horizontal resolution of 2 km and half hour temporal interval and seven days in advance. The predicted meteorological parameters are validated using the measured meteorological parameters.

Media Reporting of Nuclear Emergencies: The Effects of Transparent Communication in a Minor Nuclear Event

IN 2008, a nuclear event occurred at Krško nuclear power plant in Slovenia. Even though it was classified as level 0 on International Nuclear Event Scale, the transparency policy of the Slovenian nuclear safety authorities prompted it to notify the international community. This was the first time that the European Community Urgent Radiological Information Exchange (ECURIE) notification system was used outside the exercise framework. The event was reported in all major European. In this contribution, we report on a content analysis of media articles related to this event. The main research question was if a nuclear emergency generates significant media coverage, even in the case of a minor event and a transparent communication policy. The analysis included more than 200 articles from printed and spoken media in Slovenia and other countries. The analysis revealed a high‐intensity media coverage, emotional reactions and heated political discussion. The main media sources in countries with open political discussions on nuclear energy end up being the politicians, rather than resident experts.

Study of the bank filtered groundwater system of the Sava River at Zagreb (Croatia) using isotope analyses

Abstract Radioactive isotope tritium (3H) and stable isotopes of hydrogen (2H/1H) and oxygen (18O/16O) were measured during 2010 in the Sava River, precipitation and groundwater at 3 monitoring wells and 1 production well of the Petrusevec aquifer, close to the Sava River in the Zagreb area. Significant increase of 3H activity in the Sava River was observed in June, (200 ± 20) TU, and in groundwater of all wells with damped response (maximum 60 TU) and with delay of 3–5 months related to the Sava River. This increase was explained by release of tritiated water from the Krsko Nuclear Power Plant, 30 km upstream from Zagreb in the beginning of June 2010. Stable isotope analyses showed similar range of δ2H and δ18O values for the Sava River and groundwater samples with higher variations in surface water. Differences in monthly variations of δ18O values between particular monitoring wells, together with 3H values, indicated different infiltration times of surface water of the Sava River to different wells of ...

Estimation of Cancer Burden in Brežice Municipality, a Community Neighboring Krško Nuclear Power Plant in Slovenia

To evaluate cancer risk in Brezice municipality in the period 1984-2003 and compare it with the period 1970-1983, before Krsko nuclear power plant started operating in the vicinity. A descriptive geographical epidemiological study was performed to compare the cancer relative risks (RR) on the national, regional, and local level. We estimated RR for all cancers combined, the five most common cancer sites, and thyroid cancer and leukemias. Standardized incidence ratio (SIR) was used as RR indicator. If the number of cancer cases was small, raw SIRs were smoothed by Bayesian hierarchical model. The number of new cancer cases, as well as the estimated RR for all and individual cancer sites, increased in the period 1984-2003 in Brezice municipality and all over Slovenia. In the period 1984-2003, SIR for all cancers combined in Brezice was below the national and regional average, but RR for colorectal and cervical cancer was above this average. There were no evident clusters of districts with higher/lower RR within Brezice municipality in the period 1984-2003. RR of thyroid cancer and leukemias in Brezice was comparable with Slovenian average both in this period and before it. The obvious increase in cancer burden in Brezice municipality cannot be associated with Krsko nuclear power plant, but most likely with unhealthy lifestyle.

Determination of 55Fe in waste waters of the Krsko nuclear power plant measured simultaneously by liquid scintillation spectrometer (LSC) and X-ray spectrometer (XRS)

This paper presents comparison of two methods for the determination of 55Fe activity of waste waters discharged from the Krsko nuclear power plant (KNPP). Research was conducted on 12 composite samples of waste water collected in the waste monitor tank (WMT) during each month as well as on Analytics, Inc. cross-check sample. Results showed that the complicated and time-consuming method proposed by the Environmental Measurements Laboratory (EML) could be successfully replaced with a simple and fast based on the extraction of 55Fe from waste water by non-specific chelating agent ammonium-pyrrolidinedithiocarbamate (APDC) at pH 4 after separation from cobalt, and activity measurement by X-ray fluorescence spectroscopy (XRS). Results obtained by the XRS method were approximately 8.6% lower than those obtained by liquid scintillation spectrometer (LSC). The mean deviation of the XRS results from the activity of cross-check sample was 2.47%, which ensures that this method is accurate enough for environmental monitoring.

Characterization of neutron field in a NPP workplace

At the Krsko Nuclear Power Plant (NPP), albedo dosimeters are used for personal neutron dosimetry. Spectrometric measurements allow determination of reference dosimetric values of realistic neutron fields to be used for calibration of albedo dosimeters. The Laboratory for Neutron Metrology and Dosimetry from the Institute for Radiological Protection and Nuclear Safety (IRSN) was in charge of characterising neutron fields in the plant at two representative points with high neutron and gamma dose rate. Calibration of the dosimeters in the workplace used to be performed only by a spherical survey meter. Based on the reference dosimetric values, the Plant Dosimetry Laboratory has verified the response of albedo dosimeters.

An Experiment in Participative Environmental Decision Making

The paper presents a method of participatory decision-making involving the authorities at local level, experts and the public. It studies how to solve the problems caused by the heterogeneity of the parties taking part in the decision-making process. The method builds on mutual learning and consideration among the participants following a systematic approach led by a moderator. The method was tested at a workshop simulating a real-life situation. The test case studied at the workshop was the problem of identifying the best site for a low and intermediate level radioactive waste repository among three alternatives. The learning phase was the Krsko Nuclear Power Plant cooling problem. The results show that the participants were able to reach agreement on the prioritisation of the alternatives and to end up with suprisingly complex and fully reasonable decision models. We believe that this was achieved particularly due to the well defined procedure that guided the decision-making process, and the ability of all participants to see the influence of their views on the decision model.

Tritium Measurements in Slovenia - Chronology Till 2004

Almost all the analyses of tritium in Slovenia have been performed by the tritium laboratory at the Jozef Stefan Institute. Nearly 90 % of its measurements have been covered by two national programs, both approved by the Slovenian Nuclear Safety Administration: the radioactive monitoring program in the environs of Krsko Nuclear Power Plant (KNPP) and the program of global radioactive contamination monitoring in the environment. These programs include samples of groundwaters, surface waters, precipitation and drinking waters, as well as liquid and gaseous effluents from KNPP. Tritium was determined in some research projects and in hydrological studies of thermal waters, groundwater and coalmine waters. Tritium in the Karst region was mapped as well as the springs of entire territory of Slovenia. Around 5500 samples have been analyzed up to 2004.

Criticality Safety and Sensitivity Analyses of PWR Spent Nuclear Fuel Repository Facilities

Monte Carlo criticality safety and sensitivity calculations of pressurized water reactor (PWR) spent nuclear fuel repository facilities for the Slovenian nuclear power plant Krsko are presented. The MCNP4C code was deployed to model and assess the neutron multiplication parameters of pool-based storage and dry transport containers under various loading patterns and moderating conditions. To comply with standard safety requirements, fresh 4.25% enriched nuclear fuel was assumed. The impact of potential optimum moderation due to water steam or foam formation as well as of different interpretations, of neutron multiplication through varying the system boundary conditions was elaborated. The simulations indicate that in the case of compact (all rack locations filled with fresh fuel) single or “double tiering” loading, the supercriticality can occur under the conditions of enhanced neutron moderation, due to accidentally reduced density of cooling water. Under standard operational conditions the effective multiplication factor (keff) of pool-based storage facility remains below the specified safety limit of 0.95. The nuclear safety requirements are fulfilled even when the fuel elements are arranged at a minimal distance, which can be initiated, for example, by an earthquake. The dry container in its recommended loading scheme with 26 fuel elements represents a safe alternative for the repository of fresh fuel. Even in the case of complete water flooding, the keff remains below the specified safety level of 0.98. The criticality safety limit may however be exceeded with larger amounts of loaded fuel assemblies (i.e., 32). Additional Monte Carlo criticality safety analyses are scheduled to consider the “burnup credit” of PWR spent nuclear fuel, based on the ongoing calculation of typical burnup activities.

Granulometric and chemical composition of the Sava River sediments upstream and downstream of the Krsko nuclear power plant

Total concentrations of 13 elements (K, Ca, Ti, Cr, Mn, Fe, Cu, Zn, Rb, Sr, Y, Zr, Pb) in the size-fractionated Sava River sediments upstream and downstream of the Krsko nuclear power plant together with metal speciation within bulk sediment have been investigated. Trace metals generally increase with decreasing particle size, however, because of entrapment of organic matter in the 0.63–1 mm fraction, concentrations in the coarser sediment fraction are higher than expected. Exchangeable Pb, Zn, Cu, Mn, Cr and Fe are generally found to represent a negligible fraction of the total metal concentration of the bulk sediment. Seasonal variations of the Pb, Zn and Cu concentrations in the <0.5 mm fraction reflect decreased values during the spring period. Heavy metal concentrations in the 2003 waste water discharges from the Krsko nuclear power plant released into the Sava River were much lower than their maximum allowed values. Combined rubidium and organic matter normalization of the Zn, Pb and Cu concentrations, which was applied on the minus 0.063 mm fraction, indicated three potential sources of contaminants.

Decommissioning: a problem or a challenge?

With the ageing of nuclear facilities or the reduced interest in their further operation, a new set of problems, related to the decommissioning of these facilities, has come into forefront. In many cases it turns out that the preparations for decommissioning have come too late, and that financial resources for covering decommissioning activities have not been provided. To avoid such problems, future liailities should be thoroughly estimated in drawing up the decommissioning and waste management programme for each nuclear facility in time, and financial provisions for implementing such programme should be provided. In this paper a presentation of current decommissioning experience in Slovenia is given. The main problems and difficulties in decommissioning of the Zirovski Vrh Uranium Mine are exposed and the lesson learned from this case is presented. The preparation of the decommissioning programme for the Nuclear Power Plant Krsko is also described, and the situation at the TRIGA research reactor is briefly discussed.

Erosion behaviour of turbine rotor blades installed in the Krsko nuclear power plant

Turbines installed in nuclear power plants operate with wet steam. The flow of wet steam causes the erosion of the last stage rotor blades, and can cause a considerable damage to the condensing steam turbines. This paper reviews the results of many years of monitoring and researching of the behaviour of the erosion process and its mechanism for rotor blades of 664 MW turbine installed in Krsko nuclear power plant. On the basis of the results obtained and behaviour of the erosion process, the erosion rate for rotor blades material and a simplified model for their service life is estimated. The rotor blade erosion process was divided into three characteristic regions of erosion: the first incubation region showed no evident damage to the blade material, the second region with the maximum erosion rate and the third one with the lowest erosion. It was established that the rotor blades during their service life mostly operate in the third region with a lower rate of erosion. On the basis of the obtained behaviour of the rotor blades erosion process and a simplified model and calculation their service life of 290,000 h has been estimated, which is longer than it was expected.

Uncertainty associated with radioactive waste chacteristics

To clarify uncertainty in predictions of the quantity, radionuclide inventory and activity of waste from the Krsko nuclear power plant, and to illuminate its role in related policy‐making, we made a scenario analysis in order to find out the variation in waste characteristics if the plant operates five years shorter or longer than anticipated, or if it uses fuel of a higher enrichment (levels between 3 per cent and 5 per cent of U‐235). We used ORIGEN2 computer code for calculations connected to spent fuel, and developed a code for calculating low‐ and intermediate‐level waste. We present and interpret our results using language which can be understood by decision makers and the general public. We believe that the clarification of the issues gained through our analysis will contribute to more informed decision making and be effective in building confidence among professionals, the public and politicians in the process of identifying the most appropriate waste management options.

Ten years3H survey at the Krško Nuclear Power Plant

As part of the general off-site radioactivity monitoring at the Krsko Nuclear Power Plant, tritium is measured in the water of the river Sava at two locations (one upstream reference point and downstream point of total dispersion), the inlet and discharge of the essential plant service supply water, in four borehole waters, in tap water in Krsko and Bre zice, and in precipitations at four points. Since 1992 tritium has also been analysed in gaseous effluents in the exhaust stack. Tritium is electrolytically enriched and determined by a liquid scintillation technique. For liquid effluents an average normalized release of 17.6 TBq per GWe·y was obtained, while this value for gaseous effluents is 2.3 TBq per GWe·y.

14C Measurements at PWR-Type Nuclear Power Plants in Three Middle European Countries

Regular 14C sampling of discharged air began in 1988 at Paks Nuclear Power Plant (NPP), Hungary, and in 1991 at NPPs in Krsko, Slovenia and Bohunice, Slovakia. Monitoring of 14C discharges is carried out at all NPPs with similar differential samplers continuously collecting 14C in the form of 14CO2 and 14CnHm. The main results of airborne discharge monitoring are as follows: 14C activity concentration varied roughly within a factor of two around their mean values, 125 Bq m-3 and 90 Bq m-3 for Paks and for Krsko NPP, respectively. The pattern of discharge for Bohunice NPP is slightly different from that at the other two stations. At Bohunice, there has been a continuous increase in the discharge rate at power unit V1, starting with 70 Bq m-3 in 1991 and reaching a value of 190 Bq m-3 in 1995. The values for power unit V2 are 50 Bq m-3 and 82 Bq m-3. The average normalized yearly discharge rates are 0.887 (TBqGWe-1yr-1) for Paks, 0.815 (V1) and 0.500 (V2) for Bohunice, and 0.219 for Krsko. Most of the discharged 14C is in hydrocarbon form, 95% for Paks and Bohunice V2, but the CO2 fraction may reach 25% or 43% at Bohunice V1 and Krsko, respectively. At Bohunice V1, not only the discharge rate increased but the 14CO2 ratio to the total changed from 30% to 13%. The local radiological impact is estimated to be 1.5 μSv a-1 for Paks, 1.7 μSv a-1 for Bohunice, and 0.12 μSv a-1 for Krsko. The 14C excess in the environment has been measured at Paks NPP since 1989. Based on the monitoring data, the long-term average 14C excess from the Paks NPP was D14C=50% for hydrocarbons. Tree-ring analysis has shown a slight excess around Krsko NPP: D14C is equal to 199.9% for a tree at 1 km from the NPP compared with a “reference” one for which D14C was equal to 111.6% (in 1994).