My leakage fration too high in result

Hi guys,
I am running a pwr pincell with reflective B.C. the running mode is k-eigenvalue calculation. and I got the result follow:

 ============================>     RESULTS     <============================

 k-effective (Collision)     = 1.00843 +/- 0.00033
 k-effective (Track-length)  = 1.00843 +/- 0.00033
 k-effective (Absorption)    = 1.00898 +/- 0.00027
 Combined k-effective        = 1.00877 +/- 0.00023
 Leakage Fraction            = 0.73030 +/- 0.00030

because of the leakage fraction is too high to unbelievable, I think it is non-physical result, but I do not know how to solve it.
I need some help from you guys!
Thank you in advance~

Best regard!

The only thing I can think of is if you are applying vacuum boundary conditions in the z direction. For particles to leak, there must be a vacuum boundary somewhere.

Thank you Paul!
Thank you for your effort!
You clear my doubt.
(I have much to learn, Than you!)

Best regards!