Visual FoxPro - Voss Associates

The exposure rate of the turbine floor of a boiling water reactor
plant could be expected to be decreased to less than 1% at least
_________ after shutdown?
A) five minutes
B) one hour
C) twenty-four hours
D) thirty-six hours
E) forty-eight hours
The correct answer is: A
A good thumb rule for decay is that after seven half-lives,
activity is less than 1% of original activity (it is actually
1/(7)^2, or 1/128). Since the nuclide of concern on the turbine
floor of a BWR is N-16 (T1/2 = 7.1 sec; 6.1,7.1 MeV gammas), it
would take only 49.7 seconds to decay to less than 1%. N-16 is
produced from the (n,p) reaction on O-16 in the reactor coolant.