Procedures for the determination of stable elements in construction materials from the nuclear reactors at Risø National Laboratory

Methods for the accurate determination of stable isotopes of elements in construction materials with relevance to the work of the Danish Decommissioning have been developed.

Prior to the analysis the elements of interest must be released from the construction materials and this is done with several different digestion methods. For the analysis of aluminium, lead, graphite and steels the samples are digested with mineral acids and microwave heating at increased pressures in a sealed teflon vessel. The aluminium, lead and steel are fully dissolved after the digestion procedure whereas graphite is chemically inert to the acid treatment used, but the elements of interest are extracted from the graphite quite efficiently. Concrete is digested with open-vessel heating in a Modblock™ digesting unit in a two step procedure involving 40% HF followed by 32% HNO3. The heavy barite concrete is first treated as the concrete samples but a large residue of poorly soluble sulphates (mainly BaSO4) is left. The residue is fused with NaOH/Na2CO3 at 575°C and after some work up the product from the fusion is dissolved in dilute HNO3.

After the release of the elements from the materials, the samples are analysed by ICP-OES and ICP-MS multi-element analysis. In general the following elements are of interest to DD; Ag, Ba, Ca, Co, Eu, Fe, Li, Mo, Nb, Ni, Sm, Th and U. For graphite, steel, concrete and heavy concrete, analytical methods for the determination of all 13 elements have been developed (except Ca in steel). For aluminium and lead methods for the determination of Ag, Co, Li, Nb, Ni and U, as well as Ba in the lead have been developed.

When possible the methods have been verified against certified reference materials and calibration with standards additions and internal standard corrections have been used to correct for matrix effects most efficiently. The accuracy has also been checked with spikes when reference materials are not available. For the aluminium, lead, graphite and stainless steel, reference materials are available and used in the development. A Portland cement reference material is used in the development of the analytical methods for concrete material, whereas no reference material is available for the method development for the heavy concrete material. Whenever reference materials are used good agreement between measured and certified concentrations is observed.