FFR_chap11.txt

Parr 11

MOLTEN-SALT REACTORS

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H. G. MacPrERsoN, Editor
Oak Ridge National Laboratory

Introduction

Chemical Aspects of Molten-Fluoride-Salt Reactor Fuels
Construection Materials for Molten-Salt Reactors

Nuclear Aspects of Molten-Salt Reactors

Equipment for Molten-Salt Reactor Heat-Transfer Systems

Aireraft Reactor Experiment

Conceptual Design of a Power Reactor

CONTRIBUTORS

L. G. ALEXANDER
J. W. ALLEN

E. S. BeTTIs

F. F. BLANKENsHIP
W. F. Boupreav
E. J. BruepIiNg
W. G. Coss

W. H. Cook

D. R. Cunko

J. H. DEVANN

D. A. Dougras
W. K. ErGEN

W. R. GRrIMES

H. INnovyr

D. H. JaANsEN

G. W, KriLHovtz
B. W. Kinvon
M. E. Lackey

H. G. MacPHERSON
W. D. Man~Ly

L. A. ManN

W. B. McDoraLp
H. J. METz

P. PATRIARCA

H. F. PopPPENDIEK
J. T. ROBERTS

M. T. RoBINSON
T. K. Rocur

H. W. SavacE

G. M. SLAUGHTER
E. Storro

A. TaBoapa

G. M. TonLsonN

F. C. VONDERLAGE
G. D. WHITMAN

J. ZASLER
PREFACE

The Oak Ridge National Laboratory, under the sponsorship of the
U. 5. Atomic Energy Commission, has engaged in research on molten
salts as materials for use in high-temperature reactors for a number of
years. The technology developed by this work was incorporated in the
Aircraft Reactor Ixperiment and made available for purposes of civilian
application. This earlier technology and the new information found in the
civillan power reactor effort is summarized in this part.

So many present and former members of the Laboratory staff have
contributed directly or indirectly to the molten salt work that it should be
regarded as o contribution from the entire Laboratory. The technical
direction of the work was provided by A. M. Weinberg, R. C. Briant,
W. H. Jordan, and S. J. Cromer. In addition to the contributors listed for
the various chapters, the editor would like to acknowledge the efforts of
the following people who are currently engaged in the work reported:
R. G. Affel, J. C. Amos, C. J. Barton, C. C. Beusman, W. E. Browning,
N. Cantor, D. O. Campbell, G. I. Cathers, B. H. Clampitt, J. A. Conlin,
ALl H. Cooper, J. L. Crowley, J. Y. Estabrook, H. A. Friedman, P. A.
Gnadt, A. G. Grindell, H. W. Hoffman, H. Insley, S. Langer, R. E. Mac-
Pherson, R. E. Moore, . J. Nessle, R. I'. Newton, W. R. Osborn, 1. E.
Romie, C. I'. Sales, J. II. Shaffer, G. P. Smith, N. V. Smith, P. G. Smith,
W. L. Snapp, W. K. Stair, R. A. Strehlow, C. D. Susano, R. E. Thoma,
D. B. Trauger, J. J. Tudor, W. T. Ward, GG. M. Watson, J. C. White, and
H. C. Young.

The technical reviews at Argonne National Laboratory and Westing-
house Electric Corporation aided in achieving clarity.

The editor and contributors of this part wish to express their apprecia-
tion to A. W. Savolainen for her assistance in preparing the text in its
final form.

Oak Ridge, Tennessee H. G. MacPherson, Editor
June 1958
CHAPTER 11
INTRODUCTION*

The potential utility of a fluid-fueled reactor that can operate at a high
temperature but with a low-pressure system has been recognized for a
long time. Some years ago, R. C. Briant of the Oak Ridge National Lab-
oratory suggested the use of the molten mixture of UI'y and Thly, together
with the fluorides of the alkali metals and beryllium or zirconium, as the
fluid fuel. Laboratory work with such mixtures led to the operation, in
1954, of an experimental reactor, which was designated the Aireraft Reactor
Experiment (ARI).

Fluoride-salt mixtures suitable for use in power reactors have melting
points in the temperature range 850 to 950°F and are sufficiently compatible
with certain nickel-base alloys to assure long life for reactor components at
temperatures up to 1300°. Thus the natural, optimum operating tem-
perature for a molten-salt-fueled reactor 1s such that the molten salt is a
suitable heat source for a modern steam power plant, The principal
advantages of the molten-salt system, other than high temperature, in
comparison with one or more of the other fluid-fuel systems are (1) low-
pressure operation, (2) stability of the liquid under radiation, (3) high
solubility of uranium and thorium (as fluorides) 1 molten-salt mixtures,
and (4) resistance to corrosion of the structural materials that does not
depend on oxide or other film formation.

The molten-salt system has the usual benefits attributed to fluid-fuel
aystems.  The principal advantages over solid-fuel-element systems are
(1) a high negative temperature cocfficient of reactivity, (2) a lack of radia-
tion damage that can limit fuel burnup, (3) the possibility of continuous
fission-product removal, (4) the avoldance of the expense of fabricating
new fuel elements, and (5) the possibility of adding makeup fuel as needed,
which precludes the need for providing excess reactivity. The high negative
temperature coeflicient and the lack of excess reactivity make possible a
reactor, without control rods, which automatically adjusts its power in re-
sponse to changes of the electrical load. The lack of excess reactivity also
leads to a reactor that is not endangered by nuelear power excursions.

One of the attractive features of the molten-salt system is the variety of
reactor types that can be considered to cover a range of applications. The
present state of the technology suggests that homogeneous reactors which
use 2 molten salt composed of Bel's and either LiTF or Nal?, with Ul for
fuel and ThEy for a fertile material, are most suitable for early construction.

*By H. G. MacPherson.

567
568 INTRODUCTION [cHAP. 11

These reactors can be either one or two region and, depending on the size
of the reactor core and the thormum fluoride concentration, can cover a
wide range of fuel inventories, breeding ratios, and fuel reprocessing sched-
ules. The chief virtues of this elass of molten-salt reactor are that the design
is based on a well-developed technology and that the use of a simple fuel
cvele contributes to reduced costs,

With further developnient, the same base salt, that is, the mixture of
Bel, and Li7I°, can be combined with a graphite moderator in 2 hetero-
geneous arrangement to provide a self-contained Th—U233 system with a
breeding ratio of one. The chief advantage of the molten-sult system over
other liquid systems in pursuing this objective is that it is the only system
in which a soluble thorium compound can be used, and thus the problem
of slurry handling is avoided. The possibility of placing thorium in the
core obviates the necessity of using graphite as a core-shell material.

Plutonium is being investigated as an alternate fuel for the molten-salt
reactor. Although it 18 too early to describe a plutonium-fueled reactor in
detail, it is highly probable that a suitable Pulis-fueled reactor can be
constructed and operated.

The high melting temperature of the fluoride salts 15 the principal dif-
ficulty i their use. Steps must be taken to preheat equipment and to keep
the equipment above the melting point of the salt at all times. In addition,
there is more parasitic neutron capture in the salts of the molten-salt
reactor than there is in the heavy water of the heavy-water-moderated
reactors, and thus the breeding ratios are lower. The poorer moderating
ability of the salts requires larger critical masses for molten-salt reactors
than for the aqueous systems. Finally, the molten-salt reactor shares with
all fluid-fuel reactors the problems of certain containment of the fuel, the
reliability of components, and the necessity for techniques of making
repairs remotely. The low pressure of the molten-salt fuel system should
be beneficial with regard to these engineering problems, but to evaluate
them properly will require operating experience with experimental reactors.