The coolant chemistry during the X-726 fuel irradiation is described. This was the first fuel irradiation in HB-40. The fouling potential remained <= 0.2 ug.cm{sup -2}.h{sup -1} throughout the test and other impurities were acceptably low. The high boiler content increased from 2 to 12% during the irradiation. No problems were encountered during the changeover from Santowax OM to HB-40. (auth)