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normal distribution plus a constant term as follows:


          Nu(r) = aler/2bU + a2                  (7.1)

where bU is the standard deviation of the uranium distribution and describes the width of the distribution from the maximum concentration. The parameters which are used to specify the coefficients a, and a2 are NU(O) and Nu(R), the uranium density at the center and at the reflector, respectively. Using the boundary conditions


          N u(r) In=0 :-- Nu(r)Ir=R    N u(R)    (7.2)


the coefficients are found to be


          NU(R) - NU(OO)N U        eR/2b    - NU (R)
     a1 = -               a2 -=

            -R2/2b 2               R2/2b 2
           e      -              e      U

                                                 (7.3)

The average uranium density is

     fR NU (r) rdr 2alb          /2b2

NU =r            -     U ( - e) + a2             (7.4)

     f rdr


        The radial hydrogen profile having two maxima as shown in Figure 16 can also be described by a symmetric normal distribution plus a constant: