Selected aspects of materials behavior during severe accidents in nuclear reactors

Martin Steinbrück

Karlsruhe Institute of Technology, Institute for Applied Materials IAM-AWP, Germany

During a nuclear accident with loss of coolant the reactor core steadily heats up due to the release of residual decay heat and reduced heat transfer to the remaining steam. The temperature rise extends up to the point where stability limits of some materials in the core structure are reached and complex chemical reactions are involved.

Oxidation of zirconium alloy cladding material becomes significant from temperatures of about 1000°C causing mechanical degradation and loss of barrier (against release of fission products) effect. Furthermore, this reaction is the main source of hydrogen during a nuclear accident and is strongly exothermic, i.e. connected with release of heat comparable to and exceeding the residual nuclear power. The oxidation of zirconium alloys was already discussed during the HiTemp2011 conference [1] and will be only briefly summarized in this paper.

The various core component materials are thermodynamically unstable with respect to each other and chemical interactions occur which lead to the formation of liquid phases in LWR fuel rod bundles already at temperatures of ca. 1200°C, i.e. significantly below the melting temperatures of the involved materials. Initial degradation occurs in the control rods with Ag-In-Cd alloy and boron carbide, respectively, used as absorber materials. Whereas the low-temperature Ag-In-Cd alloy (used in most PWRs; melting temperature ca. 800°C) does not attack the enclosing stainless steel cladding, very rapid interactions between B₄C (used in BWRs, recent PWRs, and VVERs; melting temperature 2450°C) and stainless steel takes place at ca. 1250°C. The resulting absorber melt may attack adjacent fuel rods and is an additional source of hydrogen and heat by its rapid oxidation.

The paper discusses the materials interactions in the early phase of a severe nuclear accident, before large-scale melt pool formation and relocation into the lower plenum of the RPV take place, including some references to the accidents in Fukushima Daiichi.

[1] M. Steinbrück, High-temperature oxidation of zirconium alloys in various atmospheres, HiTemp 2011Conference, 20-22 September 2011, Boston, MA, USA.