Methodology and conclusions of activation calculations of WWER-440 type nuclear power plants

Highlights

• Activation calculation of two WWER-440 type nuclear power plants.

• Detailed description of the applied activation calculation methodology.

• Graphical results for total activity and waste index categorization.

• General conclusions for activation applicable in the case of PWR reactors.

Abstract

Activation calculations for two nuclear power plants of WWER-440 type have been performed by the authors in order to assist the decommissioning planning by assessing the radioactive inventory present at the time of and at different times after the final shutdown. According to related international literature and studies performed earlier by the authors, considering the activity more than 99% of this inventory is concentrated in the materials directly surrounding the reactor core, where the predominant evolution of radionuclides is generated by neutron induced nuclear reactions. In order to obtain the highest possible accuracy in modelling, three-dimensional Monte Carlo neutron transport calculations were performed. Besides the methods and models applied to these analyses, the paper also summarizes the results that can be generally applied to such nuclear power plant types. At the time of shutdown, the total activity of the stainless steel components is about 6 × 10¹⁶ Bq and 1.3 × 10¹⁷ Bq for the two NPPs considered. The biological shielding concrete constitutes approximately 7 × 10¹³ Bq and 1.1 × 10¹⁴ Bq.

Introduction

Decommissioning of nuclear power plants constitutes vital part of their lifecycle. Therefore, in order to ensure the implementation of a safe, environmentally and economically efficient decommissioning procedure, planning is one of the most important preparatory tasks. Having a well-elaborated decommissioning plan enables a reasonable estimation of the amount and radiological characteristics of the radioactive waste born during the operational life of a nuclear power plant. Based on international literature and previous experience of the authors, in terms of the activity the vast majority (around 99%) of the radioactive waste produced during the operational life of a power plant, other than spent fuel is the product of neutron induced activation. The effect of neutron activation is the most significant for the components of the reactor assembly and biological shielding structures. The remaining radioactive waste is the product of surface contamination of different appliances and structures constituting parts of the technological systems of the power plant. By determining an estimate for the quantitative and radiological characteristics of the radioactive waste (i.e. mass, volume, specific activity by isotope, total activity and waste category index), the repository requirements of a nuclear power plant can be assessed in advance.

Various reports, conference papers and journal articles study the topic in question. The Technical Report No. 389 of the International Atomic Energy Agency (1998) summarizes the most important general aspects to account for in activation analysis and enumerates relevant examples for activation analysis of several reactors which have already been shut down. Love et al. (1995) characterize a representative of commercial-sized pressurized water reactors by MCNP calculations. Woollam (2006) and Westall and Tawton (2012) use measurement results and activation calculations to describe the inventory of Magnox reactors. On the other hand, CANDU waste characteristics are determined for example by Cho et al. (2011). Regarding WWER-440 reactors, detailed waste characterization has only been performed for the Loviisa NPP (Antilla et al., 1989).

In this paper, a methodology (see Fig. 1) for the quantitative and qualitative assessment of neutron induced activation is presented and applied for WWER-440 reactor types. The method makes use of input data sets which are primarily based on the available geometric and material composition data, the operational history and the characteristic fuel load pattern practices of the reactors.

Monte Carlo based neutron transport calculations were performed in order to determine the transmutation speed in different material regions in and around the reactor, i.e. the reaction rates of the relevant and significant neutron induced nuclear reactions for isotopes found in steel components and the biological shielding. Measured reaction rate data of activation foils irradiated along with surveillance specimens were used to validate the developed model.

Due to the inhomogeneous spatial distribution of the neutron flux in the different components, hypothetical vertical and horizontal subdivision of the components was performed. Volume averaged reaction rate values were calculated by the MCNPX 2.7.0 code for every considered reaction in all of the steel and concrete regions. Based on the obtained reaction rate values, total and specific activity values were calculated and waste categories were determined for each relevant isotope and for the sum of isotopes considered for each region by the application of a self-developed computer code.

In Section 4, the paper shows general results and conclusions obtained from two projects. One of them was aimed at assessing the radioactive inventory produced by neutron activation in the Armenian Nuclear Power Plant (Czifrus et al., 2013), while the other had the goal of determining the characteristics of the activation type radioactive waste generated in the Paks Nuclear Power Plant (Fehér et al., 2005). Both projects were carried out by the authors at the Institute of Nuclear Techniques of the Budapest University of Technology and Economics.

At the end of the paper, the main conclusions, which can be drawn from such activation calculations, are given. A summary of the essential factors contributing to calculation uncertainties and qualitative information about the generated waste as volume, amount and classification, are also discussed. The results are compared with those obtained by authors of some similar papers.