coolant and parasitic absorber present 
as cladding and structure. 
Changes in design that improve heat 
transfer, such as decrease in coolant- 
channel flow length, increase in size of 
channel, flux flattening, decrease in 
moderator-fuel ratio and subdivision of 
the fuel for increased heat-transfer sur- 
face, usually affect the neutron economy 
adversely. Consequently, a combina- 
tion of heat-transfer and critical ex- 
periments is desirable to achieve a relia- 
ble core design. 
Typical Program 
The duration of the critical experi- 
ments depends upon several factors in- 
cluding the novelty of the reactor de- 
sign, the importance of reliability in 
operation, the amount of pertinent data 
otherwise accumulated prior to startup 
and pressure to achieve early operation 
of the reactor. 
Exponential experiments. If time 
permits and if materials that make 
possible a reasonable simulation of the 
proposed design are available, a series 
of exponential experiments may first be 
initiated to explore nuclear character- 
istics of assemblies having a range of 
values for lattice spacing, clustering, 
fuel concentration, enrichment, etc. 
Exponential experiments involve a 
thermal-neutron sourcé plus an assem- 
bly containing only a fraction of the 
material required for a critical assembly 
with its self-sustaining chain reaction. 
Generally, an exponential program in- 
cludes only a measurement of buckling 
of a particular system although the 
availability of a high-intensity neutron 
source, such as a thermal column of a 
power reactor, makes possible the 
measurement of lattice parameters and 
neutron spectra. The main advantage 
of an exponential assembly is that the 
system is operated well below criticality 
and consequently requires no elaborate 
instrumentation, control and shielding 
systems. The disadvantage is that 
the accuracy of information observed is 
limited and results of measurements of 
nonuniform systems require extensive 
analysis and involve uncertainties in 
interpretation. 
Critical experiments. Once a ten- 
tative configuration and size for the 
core have been chosen either by calcu- 
lation or by exponential experiments, 
detailed knowledge of the nuclear char- 
acteristics of the proposed design is 
determined through a series of critical 
experiments. 
The initial phase of a typical program 
will include: 
® determining the cold, clean critical 
mass by step-wise addition of the 
selected materials 
® determining reflector savings from 
measurements of radial and axial flux 
distributions 
* evaluating the factors comprising 
k.» from observed fission, capture and 
flux patterns in a representative lattice 
cell 
*measuring the migration area 
(M? =7 +L?) by adding distributed 
nuclear poisons or making dimensional 
changes in the core. 
The program might then turn to an 
investigation of reactivity coefficients. 
Experimental evaluation of the tem- 
perature effects on reactivity generally 
involves a combination of actual heat- 
ing of the entire assembly over a limited 
range and heating of a sample of fuel 
to an appreciably higher temperature. 
Some indirect effects, such as density 
changes, can be simulated by substitu- 
tion of materials; for example, Styro- 
foam or organic materials can be used 
to mock up the reduced density of wa- 
ter at high temperature. The magni- 
tude of other reactivity effects occur- 
ring during operation of a power 
reactor, such as fuel burnup, fission- 
product production (particularly Sm?4° 
and Xe!5), and formation of vari- 
ous plutonium isotopes, is normally 
calculated. 
Next the critical experimenter’s at- 
tention may turn to evaluating control- 
rod_ effectiveness. The cold, clean 
critical is loaded with rods of the proper 
composition, number, size, shape and 
location to give the required reactivity 
margin. The rods are generally added 
individually or in symmetric groups 
and the system restored to criticality 
after each addition. The selection of 
rods is also influenced by considerations 
of heat removal so that the determina- 
tion of power-production patterns is an 
important part of this stage of the 
program. 
Zero-power experiments. The 
measurement of detailed flux-distribu- 
tion patterns for various rod configura- 
tions expected throughout the core life 
is carried out in the zero-power sys- 
tem. Since zero-power experiments 
may use assemblies of the actual fuel 
elements and control rods or perhaps 
the actual core itself (see p. 53), they 
are well suited for locating possible hot 
spots. Distributed neutron absorbers 
are introduced to permit the various 
rod positions anticipated in actual 
power operation, and differential cali- 
bration curves for the rods are obtained 
during the course of the flux-mapping 
program. Flux flattening by nonuni- 
form distribution of fuel can also be 
investigated at this time. 
Other important information that 
can be obtained from critica]-assembly 
measurements includes: 
® The interrelation between the loca- 
tion and strength of the startup source 
and the location and sensitivity of the 
instruments for control. 
ASSEMBLING an experimental core in 
Babcock & Wilcox's pool-type facility 
® Measurement of neutron and 
gamma radiation intensities to deter- 
mine the effectiveness and heating of 
the shield. 
* Neutron economy (conversion 
or breeding ratio). 
The extensive manipulation of the 
components in a critical assembly re- 
quires that the induced activity be kept 
low. Moreover the materials and de- 
sign generally preclude operation 
at very high temperatures. Conse- 
quently, critical experiments are not 
used for investigation of effects requir- 
ing high neutron fluxes and/or high 
temperatures. Thus a program of 
critical experiments gives no informa- 
tion on heat transfer, radiation damage 
or kinetic behavior at high power 
density. 
109 
