A Chemical Perspective on the Tellurium Source Term in the Context of Severe Nuclear Power Plant Accidents

Sammanfattning: In the event of a severe nuclear accident, one of the biggest concerns is the release of radioactive nuclides. Typically, the most volatile is likely to be released. One that is considered among the most volatile is tellurium. Additionally, health implications towards the public by tellurium can be by itself or the decay product iodine. Several aspects concerning the volatility under mainly oxidizing (e.g., air ingress) and inert (injection of inert gas preventing hydrogen explosion) conditions of the tellurium source term from a chemical perspective have been explored. These aspects are the consequence of seawater as an emergency cooling medium, the effect of cesium iodide on the transport on and interaction in the reactor coolant system (RCS), and the effect of surfaces located inside a boiling water reactor (BWR). Finally, the effectiveness of the containment spray system (CSS) using different solutions was investigated. Under oxidizing conditions, tellurium was noticeably affected by all aspects. The seawater experiments showed that tellurium was enhanced by sodium chloride. From the RCS experiments, the addition of cesium iodide decreased the transported amount of tellurium and a new tellurium species was observed, but uncertain if it was correlated to cesium. Regarding the BWR surfaces, only zinc was removed from the surface by the tellurium when humidity was high. However, neither if this enhanced the volatility or if a new compound formed were clear. Finally, high removal efficiency was observed of the spray system under all conditions. The investigation of tellurium under inert conditions had less of the investigated aspect affecting the volatility. The seawater experiment showed no change to the volatilization of tellurium. However, in the RCS the addition of cesium iodide resulted in both an increase in transported tellurium and new tellurium species, potentially correlated to cesium. None of the BWR surfaces had any effect on tellurium. Finally, the effectiveness of the CSS was lower than under oxidizing conditions but still above 50%. However, the addition of cesium iodide considerably enhanced the removal. Therefore, indicating a change of the tellurium speciation. From this work, it is clear that the chemistry concerning the volatility of tellurium still needs further attention. Under an air ingress scenario, using seawater would enhance the release of tellurium. Opposed to the cesium iodide, that would reduce it. In the containment, tellurium would interact with surfaces made out of zinc. For a scenario when inert gas has been injected, the use of seawater would have less impact if any and the presence of cesium iodide would enhance the transport of tellurium. No surface in the BWR containment would interfere. However, under both scenarios, an intact containment and a functional CSS would effectively trap the tellurium in the sump.