FISSION-PRODUCT RELEASES AT SEVERE LIGHT-WATER REACTOR ACCIDENT CONDITIONS - ORNL CEA MEASUREMENTS VERSUS CALCULATIONS/

During the 1970s, reactor safety authorities developed increasing inte rest in methods for accurately predicting the extent of hazards associ ated with severe accidents in light water reactors (LWRs). In response to these concerns, out-of-pile experimental projects were initiated b y the U.S. Nuclear Regulatory Commission and the French Nuclear Protec tion and Safety Institute, at Oak Ridge National Laboratory (ORNL) and the Commissariat a l'Energie Atomique (CEA), respectively. Both exper imental efforts were designed for source term characterization of the fission products (FPs) released from LWR fuel samples under test condi tions representative of severe accidents, i.e., in oxidizing or reduci ng atmospheres at temperatures up to 2700K (at ORNL) and 2570K (at CEA ). The experimental devices, procedures, and parameters are described. The combined database of available results is summarized and related to experimental conditions. Using Booth diffusion theory, diffusion co efficients of the FPs were calculated, and their evolution with temper atures in the 1070 to Z700 K range were plotted. The results show the good agreement between the independently determined ORNL and CEA FP di ffusion coefficient values. By plotting the data in Arrhenius fashion, if has been possible to do the following: 1. quantify the thermal act ivation coefficients for both volatile and low volatile FPs 2. identif y classes of FPs whose release behavior does not follow a purely diffu sional mechanism, but rather depends on chemical interactions with the environment; i.e., they exhibit mixed diffusional and transport-depen dent mechanisms. A model supported by these experimental results and b ased on the CORSOR-Booth code is proposed and compared with CORSOR cal culations. To facilitate the comparison, a model based on FP diffusion mechanisms that was developed at ORNL and adapted with CEA experiment al data is proposed. This CEA model, as well as CORSOR-Booth calculati ons are compared with the ORNL experimental data in a blind test. Addi tional experimental work is in progress to determine the releases of n onvolatile FPs and transuranics at temperatures up to fuel melting as well as FP deposition mechanisms under both oxidizing and reducing con ditions. An effort is made to quantify the dependence of the temperatu re transient delta T/delta t as well as the oxygen potential effects o n the release kinetics. Those data will allow further verification and extension of the field of application for our model.