Yankee Reactor Pressure Vessel Surveillance: Notch Ductility Performance of Vessel Steel and Maximum Service Fluence Determined From Exposure During Cores II, III, and IV

Charpy V-notch specimens representative of one of the several heats of A302-B steel forming the Yankee reactor pressure vessel, and irradiated as part of the Yankee surveillance program, have been tested by the Naval Research Laboratory. Specimens of this particular heat, irradiated in near-core (accelerated) as well as in vessel wall locations, showed more embrittlement than did specimens of a reference steel heat of the same nominal A 302-B composition irradiated simultaneously in the same surveillance capsules. Those specimens from both the Yankee vessel heat and the reference heat irradiated at the vessel wall location depicted a higher damage rate than that for the accelerated location. The cause of this difference in embrittlement response could not be attributed to an effect of cyclic, service irradiation temperatures, but could be traced to a qualitative relationship of thermal to fast (>1 Mev) neutron fluxes. This ratio was in excess of about 9:1 at the vessel wall location versus a ratio less than about 9:1 for the accelerated location. The computation of a maximum service fluence of 1.46 × 1019 n/cm2 >0.5 Mev was made possible by establishment of the neutron spectrum at the reactor vessel wall using computer calculations. The maximum fluence derived by this technique compared favorably with another value given by an independently-developed calculated neutron spectrum. The NRL computed service fluence in concert with the embrittlement data projects a maximum transition temperature increase of 265 deg F, a level of embrittlement considered acceptable for the Yankee reactor vessel after thirty fuel cycles of operation at 600 MW(t).