The Heavy-Section Steel Irradiation (HSSI) Program had previously irradiated five reactor pressure vessel (RPV) steels/welds at fast neutron fluxes of about 4 to 8 x 10<
sup>
11<
/sup>
n/cm<
sup>
2<
/sup>
/s (>
1 MeV) to fluences from 0.5 to 3.4 10<
sup>
19<
/sup>
n/cm<
sup>
2<
/sup>
and at 288 �C. The unirradiated fracture toughness tests were performed by Oak Ridge National Laboratory with 12.7-mm and 25.4-mm thick (0.5T and 1T) compact specimens, while the HSSI Program provided tensile and 5 x 10-mm three-point bend specimens to SCK-CEN for irradiation in the in-pile section of the Belgian Reactor BR2 at fluxes >
10<
sup>
13<
/sup>
n/cm<
sup>
2<
/sup>
/s and subsequent testing by SCK-CEN. The BR2 irradiations were conducted at about 2 and 4 x 10<
sup>
13<
/sup>
n/cm<
sup>
2<
/sup>
/s with irradiation temperature between 295 �C and 300 �C (water temperature), and to fluences between 6 and 10 x 10<
sup>
19<
/sup>
n/cm<
sup>
2<
/sup>
. The irradiation-induced shifts of the Master Curve reference temperatures, ?T<
sub>
0<
/sub>
, for most of the materials deviated from the embrittlement correlations much more than expected, motivating the testing of 5 x 10-mm three-point bend specimens of all five materials in the unirradiated condition to eliminate specimen size and geometry as a variable. Tests of the unirradiated small bend specimens resulted in Master Curve reference temperatures, T<
sub>
0<
/sub>
, 25 �C to 53 �C lower than those from the larger compact specimens, meaning that the irradiation-induced reference temperature shifts, ?T<
sub>
0<
/sub>
, were larger than the initial measurements, resulting in much improved agreement between the measured and predicted fracture toughness shifts.