RESEARCH 
REACTORS 
Selection and Operation 
By WILLIAM M. BREAZEALE* 
The Babcock & Wilcox Company, Lynchburg, Virginia 
A RESEARCH REACTOR Is one designed 
primarily as a source of neutrons and 
gamma rays. Although a great num- 
ber of designs have been proposed, all 
but a very few of the research reactors 
built so far in this country fall into one 
of three classes, the water boiler, the 
open-pool, and the tank type. The 
principal exceptions are the large graph- 
ite piles at Oak Ridge and Brookhaven 
and the graphite pile (CP-3) at Ar- 
gonne. It is unlikely that any more 
of this type will be built. 
Water Boiler 
Water boiler (1) is the designation 
given to the small homogeneous reac- 
tors employing a light-water solution 
of uranyl sulphate or uranyl nitrate 
contained in a stainless-steel sphere or 
cylinder. Cooling is accomplished by 
immersing stainless-steel coils carrying 
cooling water in the solution. The re- 
actor operates at atmospheric pressure. 
The water is dissociated rather rapidly, 
about 24 ft? of H, and O2 (STP) being 
formed per kw-hr. This gas evolution 
limits the operating power to ~50 kw. 
The chief advantages of this design 
are its low cost, small critical mass, and 
large negative temperature coefficient. 
The fact that the moderator and fuel 
are intimately mixed insures that there 
is no time lag in moderator expansion 
with increase in fuel temperature. The 
AEC has revealed plans for power- 
transient tests (2) on homogeneous re- 
actors but no data from these tests has 
yet been published. 
The chief disadvantages of the design 
are the problems associated with han- 
dling intensely radioactive and corro- 
*On leave from Pennsylvania State 
University. 
70. 
sive liquids, the fact that the power 
level is limited to a moderate value, the 
danger of precipitation of the fuel in 
the form of insoluble uranium peroxide 
if the solution temperature is not care- 
fully watched, and the necessity for re- 
combining the H, and Oy» and storing 
the radioactive fission product gases 
until they decay sufficiently to permit 
discharge into the atmosphere. In the 
North Carolina State College design a 
virtue has been made of the last draw- 
back by employing the chamber con- 
taining the radioactive gases as a neu- 
tron-free gamma-irradiation facility. 
Open Pool 
Open-pool reactor (3) in its simplest 
form is an heterogeneous assembly of 
MTR-type fuel elements suspended in 
a pool of light water. The pool water 
serves as a coolant, moderator and 
shield. If power levels are restricted 
to 200 kw, convection cooling is satis- 
factory; above this level forced circula- 
tion is indicated. The upper level of 
open-pool operationis2—5 Mw. Above 
this range the pool becomes so deep 
that handling of the fuel elements and 
samples becomes somewhat awkward. 
Also at these levels certain second-order 
effects begin to be noticeable. Chief of 
these is the Na activity. This activ- 
ity arises from the production of the 
Na” recoils in the aluminum by the 
fast-neutron reaction Al?7(n,a) Na?*4 
and by activation of the residual Na** 
in the water. It becomes a serious 
problem as the operating power level 
approaches five megawatts. Consider- 
able care is also required in returning 
the cooling water to the pool to insure 
that the N!® activity (7-sec half-life) 
has decayed sufficiently before it dif- 
fuses to the surface. 
The advantages of the open-pool de- 
sign include low cost, flexibility, and 
the fact that the fission products are 
retained in the fuel elements eliminating 
the necessity for radioactive gas han- 
dling and recombining equipment. 
The penalty paid for the extreme flexi- 
bility takes the form of a larger critical 
mass. A working loading will be two 
or three times that of the water boiler. 
As far as ultimate safety is concerned, 
experiments (4) have indicated that if 
the reactor is left on a rising period, 
corresponding to 2% or less in excess 
reactivity, the moderator will simply 
boil and limit the power rise to a level 
or 1 or 2 Mw. Further investigation 
of the transient response of heterogene- 
ous lattices is now being carried out 
under AEC sponsorship, and it is pos- 
sible that the results will modify the 
2% figure. |See p. R5, this issue.] 
Tank Type 
Tank-type reactors (6, 6) are re- 
quired when power level exceeds a few 
megawatts. They have a closed cool- 
ing circuit and aluminum-uranium-al- 
loy fuel plates. The entire primary 
cooling circuit is shielded. Because 
there is no free surface it is now feasible 
to use heavy water as coolant and 
moderator. Power levels for the tank 
type vary from 1 Mw for the CP-5 to 
175 Mw for the ETR now being con- 
structed at the National Reactor Test- 
ing Station.* 
The heavy-water type has a larger 
* Mention should be made of the “‘ Argo- 
naut’’ a low power, tank-type research 
reactor which is being designed and built 
by the Argonne National Laboratory. 
This design employs graphite as reflector, 
light water as a moderator and coolant, 
and is intended for intermittent service at 
power levels up to 10 kw. 
