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Nuclear Reactor Digitization Project

Raymond L. Murray Reactor Project Notebook

The position of the window relative to the thermal column and nearest
exposure port is shown below:

To obtain the order of magnitude of radiation at the window scattered
from the thermal beam, a calculation of the flux at the point P (see figure

The flux at this point due to the scattering from a volume dσdχ is
given by

The above equation can be written as

Assuming that the increase in the φs contribution on the left of the
center line in beam will nearly be compensated by decrease on the right, we

may

The average number of atoms will be

Assuming a 4' x 4' beam with a flux of 107 m/cm2sec and Ro = 19' we have
at a point 7' from the edge of window a flux of

The above value is based upon calculations assuming a collimated bean.
This will not be exactly true. For a bean which is not collimated we can

write

The correct value will be somewhere between the two. Assuming the
larger value to be correct and a tolerance of 1000 n/sec cm

2 for thermal
neutrons, the window thickness should be given by

At the experimental ports when open there will be a collimated flux of
2 x l0

8 n/cm2sec for fast neutrons and 1.2 x 107 n/cm2sec. slow neutrons.
Substituting in the first equation with A = 180 cm

2 and Σs = 5.4 x 10-3

Since H20 is comparable to Portland cement in attenuation of fast
neutrons, a 6" window will reduce the fast neutron component below tolerance.

The slow neutron flux at the window will be given by

To be completely safe it is believed a window of 6 inches should be
used with Borax added to enhance slow neutron absorption as well as some