Reactivity feedback from irradiated pin failure in unprotected slow TOP accidents in LMFBR's

The present work is an outgrowth of studies made in support of CRBR licensing, but the conclusions drawn should be generally applicable to oxide-fueled LMFBR's. The accident under consideration is a 10 cents/s unprotected TOP (transient overpower), for which a series of PLUTO2/SAS4A calculations has been performed using a higher power CRBR EOC3 fuel pin which had 275 days irradiation. The assumption was made in the licensing work that a short pin failure will occur at the axial midplane, maximizing the positive fuel motion reactivity effect, as it was felt that a less conservative assumption could not be conclusively justified. This assumption is also made in the present case.