An Exploration of the Contamination of AISI 304L Stainless Steel when Corroded in Nitric Acid Based Spent Nuclear Fuel Reprocessing Dissolver Simulant

Nuclear fuel reprocessing plants require effective decommissioning technologies in order to reduce the hazardous inventory prior to dismantling and minimise the waste volume generated at the end of facility life. As fuel is dissolved in nitric acid during the reprocessing process, corrosion of the dissolver, which is primarily made from AISI 304L and Nitric Acid Grade (NAG) 18/10L [1], occurs. During this corrosion of stainless steel that makes up components of nuclear reprocessing facilities, radioisotopes originating in the dissolved spent fuels become entrenched within the protective Cr-rich oxide that forms on the surface of the steel. This contamination poses a significant hazard during decommissioning due to the high radiation levels and limited accessibility of some components. An effective decontamination strategy would mean safer decommissioning, significant financial savings per cubic meter of waste, and could allow for recycling and reuse of some components [2] which will mean lower waste volumes will require permanent disposal.<br/>To create an effective decommissioning strategy, an accurate picture of both the chemical and radiological contamination of the stainless-steel vessel and pipework must be known. Access is limited by the high radiation field inside process cells and vessels [3], meaning the condition of inner surfaces is not directly known. This has created a need to simulate environments by deliberately contaminating stainless steels under conditions as similar as possible to those found on plant so the resulting surfaces can be characterised. In this series of work, this simulated contamination has been achieved through two methods – boiling and electrochemistry. The boiling tests involved both submerging and suspended AISI 304L coupons above 105 °C 3 M nitric-acid based dissolver simulant for 10 to 110 days, whereas the electrochemical test involved using an AISI 304L coupon as the working electrode in a 3-electrode cell and holding the system at 80 °C and an elevated potential for ≈2.5 days. Simulant reprocessing liquor was supplied by the UK National Nuclear Laboratory. The coupons generated during this work were first analysed using Scanning Electron Microscopy and Raman spectroscopy in Lancaster University’s UTGARD laboratory. Following this, they were taken to the Materials Research Facility (MRF) in Culham to use the Focussed Ion Beam (FIB) to make atom probe specimens which were analysed at the University of Oxford using their Nuclear Materials Atom Probe (NuMAP) facility.<br/>From the six samples taken to the MRF, 24 usable atom probe tomography tips were generated, of which 19 successfully generated data. Analysis of all 19 data sets is still ongoing, however preliminary analysis shows the presence of heavy metal contamination within the oxide layer of the coupons submerged during the boiling tests, including V, Mo, Rb, Cs, Gd, Ce, Tc, and Nd. These contaminants all appear to be engrained within the oxide layers, and do not penetrate past the native nickel enriched region close to the metal-oxide interface. These results are significant, in particular the presence of V and Ce which are common surrogates for the highly radioactive Np and Pu respectively. The entrenchment of these radioactive contaminants could be responsible for the high contamination levels of the facility, despite previous attempts at decontamination.<br/><br/>References<br/>1. R. D. Shaw, “Corrosion prevention and control at the Sellafield nuclear fuel reprocessing plant,” International Atomic Energy Agency, Austria, February 1993;<br/>2. NDA, Innovate UK, Magnox Ltd, Sellafield Ltd, “Sort and Segregate Nuclear Waste: specification,” UK government, July 2020<br/>3. F. Berkhout, Fuel reprocessing at THORP: profitability and public liabilities, London, 1992