Research activity

I. The study of the impact of nuclear facilities on the environment

Uncontrolled tritium release from Paks NPP

Continuous monitoring of the observation wells around Paks NPP has been carried out since the starting of the NPP. Mostly tritium but sometimes fission and corrosion products were detected in the observation wells. To forecast their movement a groundwater flow model for the area was constructed. Visual MODFLOW was applied for this purpose. The groundwater flow in the Paks aquifer was modelled using values of Danube levels and rainfall heights from the period 1986-98 and a tritium distribution map was constructed. This way an estimation for the possible locations of the leakages, the total uncontrolled tritium release, the movement of the tritium in the future and its effect to the population was made.

Nuclear environmental protection

Understanding the characteristics of diffusion is essential in the assessment of radionuclide release through the backfill of waste repository. The diffusion behavior of different radionuclides in geological materials was studied. Diffusion coefficients (D) and time lag (tD) were measured, break-trough curves were plotted and mechanism governing radionuclide diffusion was studied, to interpret experimental data or to predict diffusion rates. Using the batch technique sorption experiments were carried out, as well. The sorption behavior including the change of distribution coefficients with decreasing the solution-to-solid ratio was investigated, sorption dynamic curves were plotted and distribution coefficients (Kd) were calculated.

Environmental isotope studies were carried out in sites that may be used in the future for disposal of high and low/medium level radioactive waste. Stable isotope ratios, tritium, 90Sr and 14C content, total beta activity of the water samples and activity concentrations of gamma emitters were measured in the region of Üveghuta and Udvari, in the region of Püspökszilágy or in the Aleurit Formation of Boda. From these environmental isotope data we calculated the residence times, mixing and origin of the groundwaters and the direction of the groundwater flow.

129I is one of the most dangerous radioactive isotopes produced by nuclear industry. Its concentration emission rate is limited in airborne releases and in liquid effluents as well as in every type of waste. Determination of 129I concentration by measuring its decay product 129Xe offers better sensitivity than other methods and the method can be applied in every liquid waste. 129I concentration have been measured by noble gas methods in the primary coolant of the Paks NPP.

Differential 14C samplers were developed to collect integrated samples for measuring 14C in chemical forms such as CO2, CH4 and other hydrocarbons in the stack air as well as in the environment. Two differential 14C samplers were installed in the ventilation stacks of the NPP, and five samplers were used in the vicinity of the Paks NPP. Experience gained from 10 years of monitoring 14C discharges from the Paks NPP shows that the normalized release rate is 0.8 TBqGWe-1y-1; this figure is about four times higher than the mean value of Western European PWR 14C discharges. The higher 14C production rate is due to the higher level of nitrogen impurities in the primary coolant. The global impact of 14C released to the environment is of importance because its collective effective dose equivalent commitment is far higher than the contribution from other global radioactive contaminants.

The continuous 85Kr sampling at Paks NPP began in 1995 with one month sampling period. The normalized 85Kr release of the plant is 0.3 TBqGWe-1y-1. The data show that a fraction of 10-5 of the total inventory of 85Kr is released at the nuclear power plant which is much lower than the predicted rate.

Inventory and characterisation of important radionuclides for safety storage and disposal of LLW/ILW produced at Paks NPP

For the characterisation of nuclide inventory in different radioactive waste originating from Paks NPP a significant number of measurements have been performed on spent ion-exchange resins and evaporator concentrates. Furthermore, in 1998 an intercomparison test has been performed among five Hungarian laboratories to qualify the analytical methods for activity measurements. By this way confidence has been gained in the reliability of these measurements. The characterisation of radionuclide inventory in the waste steam was performed in routine operation by correlation of hard to measure nuclides, to so called key nuclides, which could be measured easily. As a result, scaling factors reflecting the ratio of important - to - key nuclides were determined. The correlation was derived from statistical evaluation of activity measurement data. Further work should concentrate on improvements of knowledge about the correlation as a measure of quality assurance.

Fuel failure detection

Correlation studies had been carried out between concentration as well as isotope ratios of noble gases and fuel failures. From the defected fuel radioactive fission products or fissible materials might go into the coolant. In the case of microleaks only the volatile fission products can be transported through the cladding; in the case of macrofailures the fuel is in contact with the coolant and both fission products and fissible materials appear in the coolant. Different methods based on monitoring the activity concentrations of certain fission products (radioactive noble gases, iodines, etc.) in the coolant were developed.

Quality control activity

Quality control of graphite sealing used in the active zone of Paks NPP was carried out. Graphite pieces found in the primary coolant system were identified by δ13C measurements. We measured the 14C content of the graphite and calculated the residence time in the neutron flux based on the yield of the nuclear reactions 13C(n,γ)14C, 14N(n,p)14C and 17O(n,α)14C.

Development of radioanalytical methods

The LES's activities are focused on developing selective and sensitive radiochemical methods to separate and measure the activity concentrations of long-lived pure beta emitters (36CI, 14C, T, 129I, 99Tc) in low and intermediate level radioactive waste.

Qualification of headspace gases

The solid and solidified radioactive waste produced by the NPP is packaged and stored into steel drums. A sampling unit was developed to get sample from the gas formatted into these drums. Qualitative and quantitative analysis of the gas samples were carried out by mass spectrometry. The isotope ratio of carbon, hydrogen and oxygen in the gas phase were measured. These data inform us about the mechanism of gas formation and reaction-mechanism occurred during the period of storage. The 14C, tritium and helium content of the gas as well as the isotope ratio of the helium were also measured.

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