Answer
1.5
Work Step by Step
After each fission, the number of neutrons increases by a factor of 1.0004. The time between fissions is 1 ms, so in 1 second (i.e., 1000 milliseconds), the number of neutrons increases by a factor of $(1.0004)^{1000} =1.5 \times10^0$.
The reaction rate is proportional to the number of neutrons present, so it goes up by the same factor.
