Statistical analysis of fuel cycle data from Swedish Pressurized Water Reactors and the impact of simplifying assumptions on simulated nuclide inventories

Abstract

When analyzing and assessing properties of spent nuclear fuel (SNF) such as radionuclide inventories, the power history of the fuel during its time spent inside the reactor core plays an important role. This information can be very useful in the field of nuclear safeguards wherein a safeguards inspector can use it to verify the fuel properties such as burnup, initial enrichment and cooling time (or collectively termed as the “BIC” set of variables). However, such information may often be unavailable to the safeguards inspector or the level of detail in the available information may be lacking. Therefore, when analyzing SNF for various purposes (such as for safety, safeguards and back-end purposes), the power history of the fuel is most often disregarded altogether and the inspectors only look at the fuel BIC. If the power history-level information is considered, it is not uncommon to make simplifying assumptions about how the fuel is burned in the reactor. In this work, we perform an exploratory analysis of fuel cycle data from two PWR units of the Ringhals nuclear power plant in Sweden. The said analysis describes the variation in the number of cycles, cycle lengths, downtimes et cetera in order to develop a simplified yet representative model of irradiation that may be used to construct synthetic data libraries. Furthermore, we look into impact of changes in the power history on the nuclide inventories of key gamma emitters and isotopes responsible for decay heat in the SNF of the fuel in three different irradiation scenarios. Our results show that in most cases with fuels that are considerably long-cooled, it is acceptable and even preferred to use a simplified power history over an idealized or a representative irradiation model obtained from exploratory analysis of the fuel cycle data. However, for short-cooled fuels, using a simplified or even an idealized history is less preferable over modeling the detailed power history of the fuel due to the presence of short-lived nuclides in the SNF which are more sensitive to variations in the power history.