I'd like to know how others are handling decay heat projections. Our Ops procedures here at Cook were recently changed to link containment closure time requirements to RCS time-to-boil projections. Our operators are asking for more realistic, less conservative (.i.e., longer) times.
First, the source term. We are still using the old Branch Technical Position paper ASB 9-2 that was part of earlier NUREG-0800 revisions. The latest NUREG-0800 now references ANSI/ANS-5.1-2005 for calculating decay heat. We would start using that; however, we cannot find a defensible value for R ("atoms of U-239 produced per second per fission per second evaluated for the reactor composition at the time of shutdown; the value of R shall be supplied and justified by the user") for the heavy element contribution in section 4. The examples in the appendices use R=0.6. What are you using for decay heat calcs?
Second, the sink term. We have no heat transfer modeling of our vessel and RCS. Rather, we make some very conservative assumptions about vessel internal volumes and then credit only the water volume within the vessel for cases when the cavity is not flooded. Now, we know that there is metal and air and concrete as well as natural circulation within the RCS that will act as the decay heat sink. We just can't model all that. How do you model the decay heat sink for time-to-boil calcs?
Thanks.
David Goff Reactor Engineer Donald C. Cook Nuclear Plant 269-465-5901 x1465
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