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Nuclear Reactor Digitization Project

Raymond L. Murray Reactor Project Notebook

COMMITTEE REPORT

Internal Design of Reactor

The program to date has consisted of determining best values of critical
numbers; these should allow the design to proceed with less fear of serious error.

Auxiliary information to the above includes:

Flux: Two-group reactor theory
it were a sphere. Since this method lumps all fast neutrons into a single

"group", the predicted critical U

235 mess of 0.96 kg (compared with the
value of .87kg) is rather good. The thermal flux at the edge of the core computed

was 3.75 x 10

11 neutrons/cm2/sec as compared with tho 11.
The theoretical attenuation in the graphite of the reflector and thermal column was

The γ-flux may be estimated from the 10 kw power level, the number of γ's
per fission and the area of the container. A figure of

5.4 x 1011/cm2/sec of 2 MoV

Tolerances: The present accepted levels are: slow neutrons 1500/cm2/sec; 2 MoV γ
rays 1050/cm

2/sec. Those correspond to a 0.1 r/8 hour day.

Attenuation of γ rays: Inverse-square spreading exponential attenuation is
assumed, with absorption coefficients (μ in o

-μx, where x

Attenuation of slow and fast neutrons: Data from various sources diff or widely.
All predictions, however give safety with

Thickness of concrete shield: A tentative choice of a 5 ft diameter central cavity
filled with graphite

overall dimension of 17 feet, leaves a shield thickness of 6 feet. This is proposed
as fixed unless safety is questionable, which is not the case. From the standpoint

of γ rays, the effects of reflector graphite, shield concrete (ordinary type) and

distance, reduce the flux from 5.4 x 1011 to less than 1 γ/cm2/sec. From another
viewpoint the necessary thickness to reach tolerance levels is approximately 4 ft,

Thickness of reflector: In order to satisfy two requirements (a) a low critical
U

235 mass and (b) a maximum thermal flux at the core surface,
the largest practical thickness is used. This seems to be

20". The rate of change
of critical radius and the rate of change of the ratio of wall flux to central flux

were computed by one-group theory. With this thickness, the critical volume differs

Thermal column length: To achieve a length compatible with the Los Alamos reactor,
it is indicated that the end should be located near the

side

shield. If the Cd shield at the end is used, a built-in Pb block shield is needed

to stop the γ's from Cd; if Boron instead were the neutron shield, the Pb could be

eliminated. A

7 foot thermal column can easily be obtained in the space.

Load shield outside the reflector: If the thermal neutron flux from the end of the
column is to be free of reactor γ's a lead

shield must be inserted, presumably next to the reflector. One question that must

be decided is - What is γ free? A slab 2" thick will cut the flux to ≃ 1400/cm

2/
sec, slightly above health tolerance; a

4" slab will cut it to ≃ 110 or 1/60
tolerance. It is found that little thermal neutron absorption is encountered in

either case. Even in 4" of Pb the flux is reduced by only 6%. (The alternative, Bi,

Problems yet to be looked into further by means of calculations are listed:

adequate)

of U

235 per port, comparing reactor cores with and without reflector, and
5 grams, on the basis of a fractional solid angle for escape. A

guess would