Development of novel assays for ‘difficult-to-measure’ radionuclides in materials produced during nuclear decommissioning activities

During the nuclear reactor operational life and when it enters the decommissioning phase, large amounts of nuclear wastes are generated. All waste streams have to be characterized for their radioactive content to prove conformity with regulations related to unconditional or conditional release and final disposal. Gamma emitters such as 60Co or 137Cs, easy to measure radionuclides (ETM), can be quantified by a non-destructive analysis such as gamma-ray spectrometry. However, in the case of pure beta- and alpha- particle emitters, called difficult to measure radionuclides (DTM), a radiochemical separation is needed to obtain the required selectivity of the analysis method. Moreover, the radiotoxicity, mobility and half-lives of the radionuclides are important factors to consider in the waste disposal repositories from the safety point of view. Scaling factors are commonly applied to estimate the activity of DTM radionuclides based on the measurement of ETM radionuclides, but the disadvantage of this method is the large uncertainty on the estimated results.
This research focuses on analytical method development for several relevant DTM radionuclides, including 36Cl, 129I, 79Se, 151Sm, 147Pm and 41Ca. All these radionuclides are beta-particle emitting radionuclides or decay by electron capture (41Ca) and are considered in radiological safety assessments for waste management. 36Cl and 41Ca are neutron activation products in graphite and concrete respectively, while 79Se, 129I, 151Sm and 147Pm are fission products which can be present in exchange resins or in waters from the cooling system of the reactor.
The objectives of the project are: (1) development and optimization of sample decomposition techniques, (2) development of individual or sequential radiochemical separation and measurement methods and (3) application of automated radiochemical separation systems. Different type of samples such as graphite and concrete are considered in this project. For the sample decomposition, pyrolysis (RADDEC pyrolyser) and fusion (Katanax K3 automated fusion fluxer) techniques are investigated. Liquid scintillation counting (LSC) will be used to determine the activity concentration levels of all the selected radionuclides (considering beta interferences), inductively coupled plasma mass spectrometry (ICP-MS) will be used for chemical recovery quantification and tandem ICP-MS (ICP-MS/MS) will be investigated for quantifying the activity concentration of 36Cl and 151Sm.
The newly developed methodologies will reduce the uncertainties with respect to the inventory estimation for these DTM radionuclides, and thereby facilitate more accurate waste characterization and cost-effective waste management in nuclear decommissioning.