Should a severe accident occur in a light water reactor (LWR), high-temperature steam and gas with fission products (FP) that generate decay heat will exit the degraded core to the reactor coolant system (RCS) piping. The mechanical strength of the RCS piping will deteriorate when the piping is heated far higher (close to 1,000degrees cent.) than the design maximum temperature (~ 360degrees cent.). Piping creep failure may then occur if the primary system pressure remains high. Since the location and timing of the piping failure greatly influence accident progression and FP transport, an accurate prediction method is necessary to evaluate the consequences of severe accidents. However, data concerning high-temperature creep deformation and related failure of piping has been very limited.
In the Wide Range Piping Integrity Demonstration (WIND) project, we performed high-temperature creep tests using specimens of various materials typical of those in RCS piping. Constitutive equations for creep strain and failure were then developed for each material. Pipe deformation and failure behavior at high temperature and pressure
(<about 1,000degrees cent. and about 15 MPa) was investigated further
by performing pipe integrity tests using a real RCS pipe (Fig. 1-7). Finally,
a method to predict the timing of piping failure was developed based on three-dimensional
elastoplastic creep analyses of pipe deformation using the developed creep strain
constitutive equations combined with creep failure constitutive equations. The
accuracy of the developed prediction method was verified by comparisons with the
results of the pipe integrity tests (Fig. 1-8).
The prediction method developed enables prediction of accident progression and evaluation of the consequences (Fig. 1-9) by incorporating this method into a severe accident analysis code. Now, we can accurately predict the time margin to piping failure in an accident. |
