The detonation reaction rate in mus(-1) is derived from Size Effect data us
ing the relation - DUs(partial derivativeU(s)/partial derivativey)(-1), whe
re y = 1/R-o, where U-s is the detonation velocity for a ratestick of radiu
s R-o and D is the infinite-radius detonation velocity. These rates are gen
erally not constant with radius and have pressure exponents ranging from <
-5 to >5. JWL++, a simple Reactive Flow code, is run with one rate constant
on many samples to compare its rates. JWL++'s pressure exponents vary from
about 0.5 to 2.5, and failure occurs outside this range. There are three c
lasses of explosives: (1) those for which the pressure exponent is between
1 and 2 and the rate is nearly constant (e.g. porous urea nitrate): (2) hig
her pressure explosives with a concave-down shape and large positive pressu
re exponents (dense TNT); and (3) explosives with negative pressure exponen
ts and concave-up shapes (porous PETN). JWL++ fits only the first class wel
l and has the most trouble with class 3. The pressure exponent in JWL++ is
shown to be set by the shape of the Size Effect curve - a condition that ar
ises in order to keep a constant reaction rate for all radii. Some explosiv
es have too much bend to be modeled with one rate constant, e.g. Comp. B ne
ar failure. A study with creamed TNT shows that the rate constant need not
be changed to account for containment. These results may well be pertinent
to a larger consideration of the behavior of Reactive Flow models.