results of using scale factor method to evaluate the content of difficult-to-measure radionuclides in operational solid RAdioactive Waste produced at ChNPP units before 1986 accident

 

O. V. Mykhailov¹, V. M. Bezmylov¹, A. K. Terzi²

 

¹ Institute for NPP safety problems of Ukraine’s NAS, Chornobyl, Ukraine

² GSP "Chernobyl NPP", Slavutych, Ukraine

 

During the normal operation of reactor installations, or during a NPP decommission, a large amount of radioactive waste (RAW) is generated [1]. The evaluation of radionuclide activities in RAW is required for its disposal in final repositories. Some of radionuclides are Easy-To-Measure (ETM) with using NDA (non-destructive assay) or DA (destructive assay). But most of radionuclides to be declared are alpha- and/or beta-emitters and Difficult-To-Measure (DTM), and need DA in a laboratory or application of a theoretical evaluation procedure. However, laboratory technologies available today are costly and take a long time. Similar difficulties hamper the full and large-scale characterization of RAW accumulated within the territory of Chernobyl NPP Exclusion Zone, including non-accidently operational solid RAW of ChNPP Units origin.

To solve the existing problem, numerous studies were conducted, which allowed establishing the availability of quantitative relationships between the activities of ETM and DTM radionuclides in different RAW materials and made the basis for scale factor (SF) application [2, 3].

The aim of this work is to evaluate a possibility to apply the SF for certifying DTM nuclides in solid RAW (SRW) accumulated over normal operation period of ChNPP Units in specially equipped near-surface repository for SRW (RSRW).

According to SF methodology, special investigation was conducted that included: sampling from each stream of waste materials (101 samples); determination of content of 24 radionuclides (ETM, DTM) to be certified in the laboratory; use of methods for statistical data analysis; identification of key nuclides (KN) and calculation of SF mean values, which make it possible, based on the results of gamma-spectrometry of KN content (¹³⁷Cs and ⁶⁰Co), to estimate alpha and beta-emitting DTM radionuclide activities in RAW; evaluation of difference between measurement and calculation data, including DTM nuclides, whose activity, according to laboratory measurements is less, than minimum detectable activity (MDA).

Based on data obtained, radionuclide vector was determined, which includes all SF values. For DTM nuclides, whose specific activity in SRW according to measurement data does not exceed MDA (14 nuclides), established SF values allow estimating the upper limit of possible specific activity of such radionuclides in the waste based on KN gamma spectrometry data. The results of realized work were submitted to the GSP “Chernobyl NPP” for using in design of algorithms for software of Radiation and Technology Control System.

 

1. Decommission of nuclear power installations / A. V. Nosovskyi, V. M. Vasylchenko, O. O. Klyuchnykov, Ya. V. Yashchenko; Edited by A. V. Nosovskyi. – K.: Tekhnika, 2005. – 288 p. (rus)

2. Development of methods to provide an inventory of radiologically relevant radionuclides: Analytical methods and correlation of data. Final report / M. Noe, W. Muller, R. Gens et al. / European Commission. Luxembourg, 1998. – 86 p.

3. Determination and use of scaling factors for waste characterization in NPP. IAEA Nuclear Energy Series NW-T-1.18. IAEA, Vienna, 2009. - 142 р.