indicate the scope of each category. 
Engineering. Calculations in this 
category involve engineering problems 
that do not involve nuclear consider- 
ations such as heat conduction, stress 
analysis, and flow problems. Although 
the most important constraints on re- 
actor design are probably in engineer- 
ing rather than in physics, relatively 
few codes have been written in this 
category. For example, even though 
heat limitations are of vital importance 
for any reactor used as a heat or power 
source, very little effort has been ex- 
pended on codes for these problems. 
Physics. Included is any calculation 
arising in physics, and particularly in 
nuclear physics that is related to reac- 
tors. In particular, most of the pre- 
liminary calculations of parameters that 
are needed in the other categories are 
placed here (e.g., the evaluation of reso- 
nance absorption integrals to provide 
effective multigroup cross sections). 
Shielding. These calculations are 
concerned with the shielding of person- 
nel and equipment from neutrons and 
gamma rays. Although adequate but 
not excessive shielding is needed for 
stationary reactors, most shielding 
codes seem to have arisen in connection 
with mobile reactors, where weight is 
of paramount importance. These 
shielding calculations fall roughly into 
one of three mathematical categories: 
Monte Carlo, numerical quadrature, 
and numerical solution of an approxi- 
mation to the Boltzmann equation. 
Short-term operability. This cate- 
gory includes kinetic calculations, tem- 
perature-coefficient calculations, and 
any similar ‘‘short-term’’ calculations 
pertaining to reactor operation. These 
codes help determine the safety, stabil- 
ity, and design parameters of reactors. 
Most of them investigate the kinetic 
response of reactors to step or ramp 
changes in reactivity. The reactor is 
usually considered to respond to 
changes as a unit, with time as the only 
independent variable. The resulting 
equations are ordinary nonlinear, dif- 
ferential equations. In solving these 
types of problems on analog computers, 
it has been helpful to observe immedi- 
ately the changes brought about by 
changes in the input parameters. This 
fact suggests that digital computers 
with immediately observable outputs, 
such as the cathode-ray-tube outputs 
of the ORACLE and IBM 704, could 
be profitably used in a similar manner. 
Long-term operability. This cate- 
32 
gory involves calculations of fission- 
product buildup, burnup of fuel and 
poisons, breeding or conversion ratios as 
a function of exposure, and core-loading 
life. A series of criticality calculations 
with appropriate intermediate cal- 
culations can be used for ‘‘long-term”’ 
codes. 
Diffusion and age-diffusion. This 
is the largest category of reactor codes 
and has as its starting point the diffusion 
and age-diffusion equations, with vari- 
ous alterations. The neutron fluxes 
and the critical mass or radius are what 
is desired. The age-diffusion equation 
can be derived from the transport equa- 
tion by expanding the angular fluxes in 
spherical harmonics. The age-diffu- 
sion equation results from retaining 
only the first two terms in this expan- 
sion. Because of the similarity to re- 
taining the first two terms in a Legendre 
series expansion, this is called a P; ap- 
proximation. P3, Ps, and higher ap- 
proximations can be obtained similarly. 
Transport approximations. These 
codes start with the Boltzmann trans- 
port equation and solve for the neutron 
Nuclear Code Fraternities 
The Nuclear Codes Group, in exist- 
ence since 1956, provides for inter- 
change of information about nuclear 
codes on a nation-wide basis. At present 
the group has members from over forty 
installations, including nearly all com- 
panies and government installations in- 
terested in reactor design. Roughly half 
of these installations own, rent, or have 
ready access to large machines, and 
roughly half have medium machines 
available. 
The group publishes a quarterly News- 
letter containing information on _ the 
status of reactor codes at various in- 
stallations. Specific information on a 
given program can be obtained directly 
from the originator or from the local 
representative of the computer manufac- 
turer concerned. In addition general 
meetings are held semiannually concur- 
rently with those of the American 
Nuclear Society. Inquiries should be 
directed to the chairman of the group, 
R. J. Graydon, Atomics International, 
Box 309, Canoga Park, Calif. Specific 
requests about the Newsletter should be 
sent to S. Schechter, New York Univ. 
AEC Computing Facility, 25 Waverly 
Place, New York 3, N. Y. 
Two other organizations, Share and 
Use, made up respectively of users of 
the IBM’s 704 and Remington Rand’s 
1103, have also given nuclear codes 
consideration. Share, the larger organi- 
zation, contains about 70 installation 
members, of whom about 10 are con- 
cerned primarily with nuclear energy. 
fluxes and some critical quantity. The 
Boltzmann equation is a complicated 
integral-differential equation expressing 
the conservation of neutrons. The 
primary unknown function, the angular 
distribution of neutrons, can be a func- 
tion of three space variables, three 
velocity variables, and one time vari- 
able. The physical or mathematical 
problem is completed when certain con- 
stants, primarily cross sections, and 
boundary conditions are prescribed. 
Others. This category includes re- 
actor chemistry, biology, mathematics, 
and codes that do not fit into the other 
categories. 
Even with this classification system, 
it is very difficult to discuss compre- 
hensively the nuclear codes that have 
been prepared. Among the contribut- 
ing difficulties are (a) the security 
classification, in most cases unneces- 
sary, of the codes and descriptions of 
the codes; (b) the lack of published and 
distributed write-ups of codes; (c) the 
inadequate interchange of information 
which has existed between workers 
preparing codes; (d) the newness and 
unsystematic development of the field; 
and (e) the almost continuous changes 
undergone by even the more successful 
codes. 
However the bibliographies (1-3) 
of the nuclear codes, in addition to 
providing specific information about 
each of the available codes, do give a 
good feeling,for the magnitude and 
distribution of the effort so far. The 
total number of codes listed in these 
sources comes to about 140. The 
breakdown by category is as follows: 
Engineering 8 
Physics 36 
Shielding 10 
Short-Term Operability 7 
Long-Term Operability 8 
Diffusion and Age-Diffusion 61 
Transport Approximations 10 
Even a casual glance at the dis- 
tribution of codes shows that the 
greatest amounts of effort and time 
have been spent on solving the neutron- 
transport problem. More than half 
of all the codes are listed under the 
diffusion and age-diffusion category 
or under transport approximations. 
Within this family the various multi- 
group codes represent a particularly 
large investment of time and money. 
The two-dimensional code CURE, for 
example, involved over 8,000 man- 
hours and 70 machine-hours on the 
