Comparison of tokamak burn cycle options Page: 4 of 22
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algorithma. A near term estimate, based on current material costs and
fabrication techniques, is
CI - (mCu " $160) + (mNbTi " $460) + (M $30)
where mi is the mass in kg of the cable material (.opper, superconductor,
and steel). However, if tokamak reactors are commercialized we %ould
expect significant price reductions due to mass production and learning
experience. Future technology might provided a cost
CI ' (mC x $34) + ( a " $120) + (mas "bSn x $230)
where an advanced superconducting allay is included. We compute the
steel vacuum tank cost based on $24/kg.
All magnets are designed with adequate steel structure to survive
the life of the power plant. The total number of fusion cycles in the
reactor lifetime is based on a 40-y assumed lifetime and 80% availability
(1.0 " 109 a of operation). Our philosophy is that all burn cycles must
achieve this high availability to be of interest to a utility. We
attempt to calculate burn cycle requirements and system capital costs
needed to approach these goals. All costs are in 1983 dollars. An
accurate estimate of subsystem reliability, mean time to replace failed
components, and system availability is obviously not possible at present.
However, the data presented here provide a useful comparison of the
relative attractiveness of the various burn cycles.
3. THERMAL EFFECTS OF CYCLIC OPERATION - FATIGUE AND DISRUPTIONS
Our aim is to maximize first wall and limiter lifetime against
simultaneous failure modes. First, thermal fatigue is calculated, and we
find that cycle life generally decreases for thicker structures and coat-
ings. Next we study material loss from disruptions and show how compo-
nent cycle life increases with thicker structures and coatings. The com-
ponent dimension corresponding to the intersection of these life curves
is considered optimum for obtaining the longest cyclic life. Then the
minimum fusion burn length is found such that the total cyclic life is
not shorter than the expected component life against radiation damage.
We illustrate our lifetime analyses by reference to Fig. 3. The
thermal stress fatigue cycle lifetime, Nf. for first wall PCA is dia-
played for three different heat loads. As the tube wall gets thicker (a
increasing) thermal stress increases and Nf decreases dramatically.
Likewise, increases in W also severely reduce the fatigue life. We
note a lower limit to d, ue to primary stress from the coolant, is set
by permitting an upper tolerance of 5% radiation-induced creep strain at
the end of the tube life. The upper limit to P is reached when the plas-
ma side (outside) of the tube begins to exceed 500'C; above this ttmpera-
ture the structural qualities of PCA deter.orate. The significant factor
to us is that thicker tubes will withstand m:re damage from major disrup-
tions. Two curves in the figure show the number of fusion cycles of
operation before disruptions perforate a tube (assuming 70 um of eros' ,n
at the same spot each time) if the average frequency of disrupt one is
one out of a thousand (f-10-3) or one out of ten thousand (f-10- ) burn
periods. For a given probcbility of disruptions, f, and a given wall
load, WFW, there is an optimum thickness which gives the longest cyclic
lifetime against both thermal fatigue and disruptions. Now, for the
maximum Nf corresponding to the optimum 5 we would desire a tokamak burn
period. t_, sufficiently long that fatigue and disruptions are not more
limiting than radiation damage. This minimum burn len& :h is tf -
(Lrad/WnNf) - 100 a, where we allow 100 a between burns. Lrad is the
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Ehst, D.A.; Brooks, J.N.; Cha, Y.; Evans, K. Jr.; Hassanein, A.M.; Kim, S. et al. Comparison of tokamak burn cycle options, article, January 1, 1985; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc1059600/m1/4/: accessed April 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.