Impurities in LWR fuel and structural materials

Nuclear fuel and fuel assembly structural materials contain various impurity elements. Many of these impurity elements are activated in the heavy neutron irradiation in a nuclear reactor and must be considered in the safety analysis of spent nuclear fuel handling, interim storage and final disposal. This report presents a literature survey on impurity elements found in UO2 fuel, Zry-2, Zry-4, Zr-2.5Nb, Inconel 718, Inconel X-750 and stainless steels SS 304 and SS 304L. The emphasis is on the fuel and zirconium alloys.

The largest impurity element found in fuel was iron and the second largest thorium. There was wide variation between different literature sources on what elements were reported and on the given concentrations. However, iron was found in all of the studied references and most agreed it to be one of the two largest impurity elements. Altogether 67 different impurity elements were
found in the literature sources for the UO2 fuel.

The largest impurity element in the zirconium alloys was either carbon or silicon. The number of impurity elements mentioned in the different literature sources were 24 (Zry-2), 74 (Zry-4) and 53 (Zr-2.5Nb). Cobalt was the largest impurity element for the inconels and nitrogen for SS 304. Only a few impurity elements were mentioned in the studied sources for these materials.

Maximum nitrogen concentration in the inconels and stainless steel was 1300 ppm. For all of the Zircaloys it was 80 ppm and for the fuel 200 ppm. The maximum concentration for chlorine was 20 ppm in Zry-2 and Zry-4 and 15 ppm in Zr-2.5Nb and the fuel. Chlorine was not mentioned in the references for Inconel and stainless steel. However, chlorine is not likely to be absent in these materials either and a more thorough literature survey concentrating on
Inconel and stainless steel would doubtlessly give estimates for it also.