Clad failure codes

The differences seen in timing of key events such as clad failure, core plate failure, lower head penetration failure, etc., in these machine-dependency and time-step studies vary by much smaller times (on the order of 10-100s) than the timestep-variation results presented by BNL to the Peer Review for their Peach Bottom station blackout analysis (which often varied by 1,000-10,000s). A large part of this reduction in numeric sensitivity probably represents the significant efforts of the code developers since the Peer Review in identifying and eliminating numeric sensitivities in MELCOR. Unfortunately, we have no comparable results of time-step studies for the more recent BNL Oconee analyses. 

Some examples of the validation work on selected PHEBUS SFD tests illustrate the current ability of the code to predict cladding oxidation, dissolution of UO2 and Zr02 by molten Zr, failure of oxidized cladding and finally relocation of molten materials. 

In LWRs, buckling collapse and creep rupture are not included in the design failure modes, because experimental verifications have shown that these failure modes are not limiting as long as the plastic deformation of the fuel rod is less than 1.0%. The core pressure and temperature of the Super LWR are much higher than those in LWRs, so these failure modes need to be included in the design failure modes. The evaluations of stresses on the cladding are based on ASME Boiler and Pressure Vessel Code Section III as adopted in BWRs for simplified evaluations. 

No mechanical failure of cladding AS ME B PV code III No plastic strain... 

Loss-of-coolant-flow accidents are those in which a reduction in the rate of heat removal as a result of pump power failure will result in a temperature rise of the fuel and cladding if there is not a corresponding decrease in energy input. In fact, if allowed to proceed, voiding could occur. Such an analysis up to the point of voiding is normally carried out by a computer code such as AIROS 17) which calculates the point neutron kinetics effects on the reactor power and relates the power to fuel and coolant temperatures. 

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