[en] The moderator temperature coefficient of reactivity (MTC) is an important parameter in safety analyses for thermal reactors. A positive MTC can exacerbate the severity of heatup transients, while a negative MTC can worsen the severity of cooldown transients. In particular, a strongly negative MTC is the major determinate of the severity of the steamline-break accident for pressurized water reactors (PWRs). Conversely, positive and negative MTCs can mitigate the severity of cooldown and heatup transients, respectively. Consequently, the accurate measurement and prediction of MTCs is an important factor in demonstrating that power reactors can be operated safely. ANS 19.11, the standard for ''Calculation and Measurement of the Moderator Temperature Coefficient of Reactivity in Water-Moderated Power Reactors,'' recently has been approved as a national standard by the American National Standards institute (ANSI) and the American Nuclear Society (ANS). At present, the scope of the standard is limited to PWRs, because that is the only type of power reactor currently sited in the US for which measurement of the MTC is required. The standard addresses the calculation and measurement of the MTC, and it also addresses the calculation of the ITC