Some Engineering Design Considerations for a Thermonuclear Power Plant. Page: 1 of 6
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Some Engineering Design Considerations for a Thermonuclear Power Plantt
P. Bonanos
Plasma Physics Lab :atory
Princeton University
Princeton, New JerseyIntroduction
The achievement of an operating controlled thermo-
nuclear power plant will require the solution of a
variety of unique scientific and engineering problems.
This survey describes the nature of the plasma power
source in an engineering sense, lists some of the known
problems requiring study and illustrates several
specific details in the design of toroidal fusion
reactors for which the author and fellow workers are
developing solutions.
The Power Source - Engineering Characteristics
The fusion reactions of highest cross-section that
yield high energy are listed in Table I.
Table I
(1) D + D -- He(O.82eV) + n(2.45MeV)
(2) D + D -- T(1.WMeV) + p(3.03MeV)
(3) D + Hel- He(3.67MeV) + p(14.67MeV)
(4) D + T- He(3.52MeV) + n(14.06MeV)'(5) T + T - He" + 2n
(11.32MeV)
(6) 3D - He4 + n + p 21.6MeV per D-D fusion
The sixth reaction does not mean a three body
collision, but rather the simultaneous occurrence of
the first four reactions with the fuel mixture such
that they have equal rates and may be called a
catalyzed D reaction.' In all cases however, the
reaction products are energetic charged particles and
neutrons. -
The reaction rate for a two ion species plasma is
given by
R - nin2 <:Iv>
where n1, n2 are the respective ion densities and <av>
is the product of the reaction cross-section and
relative velocity averaged over the velocity range.
For Maxwellian energy distributions of temperature T
the plasma components may be considered gases having
partial pressures njkT. The gas is confined~by a mag-
netic field of pressure B2/2% . The pressure ratio is
defined as
6, , nikT/(B2/21 )
and for constant 8 the reaction rate is then
R - 0102 B4<Ov>
(2kP )2 2
The reaction parameter <av>/T2 is shown in Fig. 1.
Clearly the D-T reaction dominates the useful tempera-
ture range by some two orders of magnitude.
The plasma -ill also emit photons. Bremsstrahlung
or soft x-rays result from the free-free electron
transitions "nd synchrotron or cyclotron radiation from:
the magnetically confined gyratory electrons.
Detailed studies of fuel mixtures and reactivities
are available.2'3 It will suffice here to simply
AThis work was supported by USAEC Contract
AT(30-1)-1238MASTER
characterize the D-T and D-D reactions as having high
and low power densities respectively and illustrate
their magnitude with two reference designs in Fig. 2.
A toroidal plasma is assumed.
General Reactor Arrangement
A schematic reactor configuration appears in
Fig. 3. The plasma is confined within a vacuum chamber
which is surrounded by a "blanket". The blanket is a
thermal convertor, moderating and absorbing the neutron
energy as heat and delivering it to an external thermo-
dynamic power cycle. It must also shield the magnet
which is wound using superconductors, and minimize
neutronic and gamma heat deposition in the low temp-
erature zone. For a D-T reactor it must breed tritium,
since this is not available naturally.
The Blanket - Engineering Requirements
For fuel cycles consuming tritium it must be bred
using the Li5(n,t) and Li7(n,tu) reactions whose cross-
sections are shown in Fig. 4. Lithium is an excellent
self-moderator and, when enriched in Li, is a
slightly better breeder. More significant perhaps is
the reduced leakage flux, as will be demonstrated later.
It is chemically active however, and if used at high
temperature requires refractory metals for containment.
Breeding ratios in fusion reactors need not be high
and doubling times of a few years are easily achieved.
Thus the arrangement of the lithium layers within the
blanket may not be critical nor is the consequent
pumping power associated with moving liquid metals in
magnetic fields.
The most severe blanket problem is one of radia -
tion damage metallurgy. The first surface (facing the
plasma) of the blanket sees an incident 14.1MeV neutron
flux of some 10s sec1 cC1. Every atom in Nb first
wall is displaced nearly every day. Further,- the
prospects for the experimental evaluation of new mate-
rials exposed to this intense flux are nil, short of
construction of a prototype fusion reactor. Robinson'
of Oak Ridge uses an analytical uc-el to extrapolate
softer spectrum damage to the harder flux and estimates
a four-fold increase for Nb. Other models yield
14MeV/lMeV damage production ratios of from 2.4 to 7.8.
Some simulation experiments have been proposed:
(a) neutron irradiation of boron doped samples to in-
crease helium production, (b) bombardment by heavy
ions to accelerate atomic displacements an-, (c) ir-
radiation of normal and doped samples by high energy
electron beams. None of these completely satisfies
fusion reactor conditions but, in total, they may
form a basis for good first trials.
There are other desirable qualities for blanket
structural materials. At the first surface the vapor
pressure must be low as must be the sputtering yield
to minimize production of heavy neutrals which con-
taminate and cool the plasma by charge exchange. and
Bremsstrahlung. The activation and afterheat should
be lo. Elevated temperature engineering properties
such as strength, ductility, and thermal conductivity
must have reasonable values. Ideally, neutron absorb-
tion cross-sections should be low; (n,2n) cross-
sections high. The material should be easy to fabri-
cate i.e., weldable, and be inexpensive. Thu presentDISTRIBUTIO( 8f THIS DOCUMENT IS UNITED
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Bonanos, P. Some Engineering Design Considerations for a Thermonuclear Power Plant., report, October 31, 1972; New Jersey. (https://digital.library.unt.edu/ark:/67531/metadc1026026/m1/1/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.