A book edited by Jörg (“yerg”) Starflinger and Thomas Schulenberg discusses how a “high performance” design named for this substance may be destabilized by density waves. This substance partly lends its name to the VVER (“V-V-E-R”) design, which uses assemblies made of a zirconium alloy and exploits a negative void coefficient. Once-through cycles can be used in a class of Generation IV (“four”) designs in which this (-5[1])substance is kept supercritical. (-5[1])This substance moves through a primary loop after being raised to about 150 bars in a popular design (10[1])named for it being “pressurized.” Ahead of graphite, (10[1])this is the most popular substance (10[1]-5[1])used to slow down neutrons (10[1]-5[1])in (10[1])fission reactors. (-5[5])For 10 points, (10[1])what liquid (10[1]-5[1])is both (10[2]-5[1])the most common (10[1])nuclear moderator (10[1])and the most common (-5[1])nuclear coolant, being more popular than its deuterated (10[1])“heavy” (0[1])variant? (10[2])■END■ (10[9])