Nuclear Instruments and Methods in Physics Research A271 (1988) 65-70 North-Holland, Amsterdam
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NEUTROh4-LEJkN FUSION REACTOR STUDIES FOR THERMAL PLASMAS W. KERNRICHLER and M. HEINDLER Alternate Energy Physics Program, Institute for Theoretical Physics, Grati University of Technology, Graz, Austria
The attractiveness of various advanced fusion fuel cycles for use in commercial fusion reactors is investigated . A plasma and reactor simulation code and a nuclear data library were developed for that purpose. Under the basic assumption of a two-component plasma (thermalized bulk plasma and suprathemal reaction products), lithium and boron were found not to qualdy as fuel components because of their poor power performance. For (semi-)catalyzed D and for D-3 He, a reasonably large window was identified in parameter space for which the overall energy balance is positive. Various "attra^tiveness indicators" (nr utroa load, tritium, etc.) are evaluated and compared to the corresponding values for D-T fueled reactors .
1. Introduction
Within the Alternate Energy Physics Program at the Graz University of Texhnology, one of the primari objectives is th.; asse~asment of the potentialities o¬ advanced fusion fuels. 'Toward this goal a computer code was developed that calculates, for a given set of input parameters, the various plasma parameters, the reaction and production rates of interest, and the overall energetics of thc- fusion -reactor. The plasma constituents are taken to be thermalized (stationary background plasma) cr suprathermaf, (injected ions and fusion-g(nerated ions). The co i~'inement scheme is represented by appropriate loss, tines and scaling laws, the reactor scheme by various energy transfer and transformation efficiencies. Most importantly, the code offers high flexibility with respect to the choice of the fuel mix.: Both front end impositions (fuel injection ratios) and back end impositions (fuel recycling ratios) can be dealt with . The details of the code nave been described in refs. [1,2]. In the present context, D-3 He fuel cycles are emphasized . This recognizes the increasing interest that this advanced fuel has enjoyed since new options for the suppler of the scarce fuel component 3 He were brought to the attention of the fusion community [3]. Therefore, D_3 He is investigated ïn particular with respect to its prospect as a radiologically benign, energetically attractive advanced fusion fuel . In order to explore these z_ potentialities, a large range of D- 3He density mixes is considered, ranging from that in D-D fusion cycles without external supply ( 3 He-catalyzed D-I7 fusion) to 3 He-rich mixtures with appreciable 3 He supply requiremc;ats . In contrast to other research groups, vice neither strive to resolve stability problems nor attempt to contribute to the interpretation of present or previous experiments - such as migma [4] - that are limited to very 0168-9002/88 //$03 .50 O Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division,)
low plasma densities (as compared to reactor-relevant ones). We rather choose to assume that °a (quasi-) stationary plasma can be confined at the desired reactorrelevant conditions. Likewise, we do not intend to make evaluations of various ideas regarding 3 He supply concept a. We rather choose to assume thm 3 He is a very "expensive" but nevertheless available commodity. In summarizing, let us emphasize that the objective of our fuel assessment study is to find out what such a r--actor would look like, mainly from an energetic and radiological point of view, if it were possible to build it . In addition to D- 3 He fueled reactors, we have also considered D-6 Li and proton-based alternatives such as p- 11 B and p-6 Li . For boron, we refer to ref. [5], and for lithium a few discouraging re-suits will be given here . 2. The calculationai procedurfc Our analysis is based on a physics model of a burning fusion plasma . It has to comply with those elements of information that can be anticipated for reactors that would eventually burn advanced fuels under economically accf ;ptable conditions. Here we have put much emphasis on a proper description of the interaction between suprathermal ions (injected ions and fusion products) and the background plasma . To this end vie have introduced into the calcu látron of ihe energy distrilüut.1Vri ILIDC-Lflál11 Of the INS d _ u population the effeef: of large angle scattering events :fnd the possibility for fusion events and leakage during the slowing down process (in the following referred to as "fast fusion" and "fast loss", respectively) . Within the framework of a modified continuous slowing down approach, the distribution function of suprathermal ions could be represented as a functional of energy transfer rates, fusion rates, and loss rates (see also ref. [1]). Our 1. NUCL.EONICS/ENERGETICS
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anaysis here reveals explicitly the extent to which nu%-,lear elastic scattering, fast fusion, and fast loss affect the fraction of injection or fusion energy transferred to the plasma electrons. This has a direct impact on the ion-to-electron temperature ratio and thus influences crucially the power balance of the plasma. An extension to a discontinuous description of the slowingdown process is being undertaken (but not reported here) in order to get detailed information about the effect of "knock-on" ions on the various reaction rates. Furthermore, the net increase of fusion reaction rates as a result of fast fusion was investigated, in typical fuel mixes, for each fusion reaction channel . It was found that side reactions are particularly subject to important reactivity increases that may exceed an order of magnitude. This is of major importance for the evaluation of the amount of neutrons, radionuclides, and prompt gamma radiation associated with the fuel of interest. In contrast, the influence of suprathermal fusion on the main reactions occurring in a D- 3He plasma - i.e., D(d, n)3He, D(d, p)T, T(d, n)4 He, and 3 He(d, p)4He is very small; therefore the overall fusion power density is not affected . It is recognized here that the confinement concept has a direct influence on these kinetic plasma equations . With the basic assumptions of stationary operation and homogeneous background plasma, these effects are here taken to be described by appropriate particle and energy confinement times. Arbitrary scaling of these confinement times with energy (temperature) and density can be selected; here we have chosen, for the sake of a more convenient interpretation, to set all confinement times constant with respect to energy and density variations . This flexibility permits both general review studies yielding "typical" results for families of reactor concepts, such as field reversed configurations, and also a very specific analysis of a particular confinement scheme for which the respective data are known from more detailed calculations. 3. Nuclear database The nuclear database for all calculations is our own Nuclear Data Library DATLIB [6] . It is presented in detail in a paper by Feldbacher published in these Proceedings [71 . As an example, we show in figs. ía and 1b cross sections for fusion and for gamma-producing reactions occurring in D-3 He plasmas . 4. B,Qactor design and performance pa meters Dur previous studies on advanced fusion. fuels [1,5,8,9) suggest that a commercially viable advanced fusion reactor may well display and/or require char-
Table 1 Reference plasma and reactor parameters Magnetic field strength Magnetic field utilization Particle confinement time (fast and thermal particles) Faw1 recycling efficiency Mean energy of leaking thermalized particles in units of ion temperature Synchrotron calculation plasma diameter first wall reflectivity Fra lion of heating power deposited in the ion population Energy transformation efficiencies (thermal to electrical, electrical to heating power, etc.) Fraction of power output consumed in situ Energy multiplication of 2.4 and of 14.1 MeV neutrons in blanket, respectively
10T 0.20 2.O s 0.99
0.045 2.0 m 0.95 1.0 0.35 0.10 1.3; 4.0
acteristics close to those summarized in table 1 . We have shown that only large deviations from these parameters may change the main conclusions of our fuel evaluation . But changes in essential parameters, e.g. magnetic i1eld strength and the ß-value, will of course influence ~ he figures given here (the fusion power density is proportional to p2B4, higher ß-values will reduce synchrotron radiation, etc.). . We have made two essential assumptions in domains in which no educated guess seems possible at this point: we have assur7ed zero tacru3al conductivity and zero impurity concentration, thus producing the best possible results . Allowing for thermal conductivity and impurity concentrations, however, will seriously influence the plasma performance. Energy confinement times for heat conductivity of the order of 10 s or impurity concentrations of the order of 4% of a Z = 6 material will prevent ignited operation of a D-3 He plasma. Therefore, it seems to be a key task for a D-3 He reactor design to reduce thermal conductivity to a very low minimum and to have an excellent impurity control . 5. Evaluation of D-3He mixes as clean fuel candidates As a first step we have calculated all terms of the plasma particle and power balance (charged particles, neutrons, soft and hard electromagnetic radiation) and have tracked the various energy flows through the reactor in order to get the net electric power output of the power plant. This defines those combinations of 3 He-to-D density mix ratios and ion temperatures for which ignition is possible. Fig . 2 shows this "operational window" . In
W. Kembichler, M. Heindler / Neutron-lean fusion reactor .audies
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0
E
T(P .Y)a
10
Projectile energy, E p [keVj
Fig. 1 . (a) and (b) Selection of cross sections of relevance in the present study from. our nuclear crass section library DATLIB [6]. this figure we have used the injection rate ratio rather than the plasma density ratio because it seems to us that the former is the quantity that is directly accessible to the operator rather than the associated density mix. For 3He concentrations of the order of those encountered in 3 He-catalyzed D-cycles (no external 3 1-íe supply) and beyond, the injection rate ratio is found to be almost identical to the density ratio; this is due to the fact that advanced fuels achieve a low burn-up fraction in plasmas with confinement times cf the crder of a few seconds . An analogous plot is shown in fig. 2 for the domain in ~,hich the overall energy balance is positive, which
definitely is to be considered a minimum requirement for a power reactor . The area between these two curves, i.e.. the domain for operation without ignition and yet with net power output, is found to be surprisingly small. This reflects our previous finding [5] that, roughly speaking, advanced fuels are either ignited or they disqualify as fuel for a power reactor. Subignited operation therefore cannot be considered an alternative for poorly performing fuels but rather an option for stable operation of an otherwise ignitable fuel. This refers in particular to those confinement concepts in which a continuous injection of highly energetic fuel ions is an essential element of the concept (e.g. migma, rcf. [4]). :. NUCLEONIC"i' /ENERGETICS
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1/512 0 5.
10 .
50 .
100 .
500 .
Ion temperature, T i [keV] Fig . 2. Illustration of the combinations of 3 He-to-D injection rate ratios and of ion temperatures that give access to positive net electrical energy output and for which the plasma can ignite, respectively . The dotted line separates the domain with external 3 He supply requirement (upper region) from that domain which requires exclusively deuterium from external sources dower region) . The curves displayed in fig. 2 are calculated for the case that tritium is not recycled from the plasma exhaust . The effect of tritium recycling ("tritium-catalyzed operation") turns out to be marginal in domains with external 3 He supply, i .e ., above the dotted curve (this curve indicates to what extent 3 He can be recovered from the plasma exhaust and reinjected into the plasma, thus defining 3 He-catalyzed operation). For the domain of 3 He self-sufficiency, tritium recycling extends the domain of ignition slightly to 3 He-poorer mixes while the domain of net power output is extended even to the case of complete absence of 3 He recycling . Now that we have identified the combination of 3 He-to-D mix ratios and ion temperatures that is of interest for power application, we proceed to other indicators that measure how clean advanced fuels can be expectwd to be or , in relative terms, how much cleaner thoy couid be than "conventional" D-T fuel . We focus here on the extent to which fusion energy appears as kinetic power of neatrons, and we investigate to what extent radionuclides make the fuel radioactive . It is common procedure to express these indicators of "dirtiness" in terms of the fusion power, i .e ., neutronization, pnipfu specific !tritium inventory, `4T/pfu
where A T is the activity of tritium in the plasma (this quantity A T is directly proportional to the quantity of
tritium to be handled externally when the plasma exhaust is to be processed) . This procedure is appropriate only as long as the fusion power is also a measure of the electrical power that the reactor is able to supply to the electric grid . It is clear that this is not always true, particularly in the case of subignited plasma operation. Therefore, a comparison between advanced fuels and D-T would generally be biased in favor of the advanced fuel alternative. In figs . 3 and 4 we therefore refer the neutronization and the plasma radioactivity, respectively, to the net electric power output of the reactor. This makes it possible to engage in a meaningful comparison between D_3 He fuel options and a reactor burning a 1 :1 D-T mix under otherwise identical conditions. It is evident that, for decreasing 3 He-to-D mix ratios, the neutronization and the specific radioactivity tend asymptotically to infinity (case without tritium recycling), because the net power output becomes zero with decreasing 3 He concentrations in the plasma . Also the effect of tritium recycling becomes evident : in the density mix domain that is accessible without external 3 He sources, tritium recycling is beneficial both with respect to the neutron and the tritium load per unit electric power supply . In this domain, however, the improvement of the advanced fuel over D-T is very modest .
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W. Kernbichler, M. Heindler / Neutron-lean fusion reactor studies
Table 2 D-6 Li fuel cycles: maximum. reactor Q-value Q mom`, ion tem perature at which Q'a" is obtained, and maximum fusion power density Pfua' accessible for three 6 Li-to-D ion density ratios (0.0, 0.1, 0.3), with and without recycling of tritium and 3 He QMax Ti(QMax) Fuel cycle pfu [keV] [MW/ms l
0 .5 U u
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3He to D injection rate ratio Fig. 4. Minimum tritium inventory per unit net power output as a function of the 3He-to-D injection rate ratio, with (dotted) and without (solid line) tritium recycling from the plasma exhaust . The corresponding value for a 1 :1 mix of D-T is shown for comparison. Thus the operational domain of interest is definitely characterized by a considerable external 3Hc supply requirement . Both the neutronization and the tritium load decrease sharply with increasing 3He supply. In this domain there is no incentive to recycle tritium, however. In this 3 He-to-D density ratio domain the accessible power density is relatively intensitive to the fuel mix ratio and hardly affixted by the extent to which tritium is recycled. This minor improvement of the energy balance certainly does not compensate for the important decline in "cleanness" associated with tritium recycling . It is to be noted that fig,. 3 and 4 give "minimum" neutron and tritium load values. For each 3He-to-D mix ratio, the minimization her,e refers to the choice of that plasma temperature that pertains to the lowest accessible value. This implies that the higher degree of cleanness that can be obtained with increasing 3 He input is also associated with increasing operating temperature requirements (of the order of 30 keV for low and of 50 keV for high 3 He-to-D density ratios, respectively) .
D-D Dp_ 6Li(0.1) D-6 Li(O.3)
0.909 0.335 0.206
40 50 80
D-D(T; He) D-6Li(0.1 Z T; He) D-6Li(0.3; T; He)
5.18 3.63 2.05
10.0 1.959 0.348
40 60 80
6.90 5 .39 3.58
Table 3 P-6Li fuel cycles: maximum reactor ¢value, corresponding ion temperature, and maximum fusion power density accessible for four 6Li-to-P ion density ratios (0.1 to 1 .0) Q um Fuel T (Q `) Pt cycle [keV] [MW/m3 ] With Without synch synch P- 6Lí(0.1) 0.123 0.129 300 0.067 P-6Lí(0.3) 0 .125 0.136 500 0.124 P-6 Lí(0.5) 0.126 0.139 600 0.147 P-6Li(1.0) 0.127 0.141 650 0.172 Table 4 P-6Li plasma (6 Li-to-P density ratio of 0.3): neutron power, power of gamma radiation, tritium and 70e activity in the plasma, per unit fusion power at different : ion temperatures T [keV]
P.,/Pf .
P,/Pi .
200 500 1000
2.15X10 -3 1.19X1()-2 1.87x10 -2
1.31x10" 8 4.08x10 -8 5.17x10 -8
AT/Pfu [Ci/MW] 1 .2x10 -5 5.0X10-5 7 .8x10 -5
A 7./Pf. [Ci/MW] 0.088 0.27 0.44
4 and 5 for D- 3 He. We restr. ,ót ourselves here to the presentation of a few results that show that poor power amplification performance disqualifies 6Li as a fuel component. This is particularly disturbing because of 6Li's very interesting features regarding specific neutron and gamma radiation power load and tritium inventory, hampered oniy by the important '?Be radioactivity . Tables 2-4 summarize our findings. Note that the figure in brackets gives the 6 Li-to-D and 6 Li-to-P density ratios in the plasma, respectively .
6. Analysis 43ff 6í.i-based fuel cycles
7 . Conclusion
We hava: investigated the potentiality of 6Li-based fuels using the same procedures as reported in sections
For D_3 He fuel cycles it was shown that reasonable values for the net electrical reactor output per unit 1. NUCLEONICS/ENERGETICS
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plasma volume can be achieved for contributions of 'He to the injection current above = 5%. In particular, the external supply of 3He above that which is generated in the plasma and the extent to which tritium is recycled were found to have little effect on the fusion power density. With respect to the neutronization and the tritium handling requirement it was found that the "cleanness" of the fuel increases monotonically with the amount of external 3 He supply. For D- 6 Li fuel cycles it was found that the fuel performs better the less 6Li is added to the deuterium plasma. The topping of D-fuels with 6 Li . is clearly counterproductive [10]. For p-6Li, similar discouraging results are reported, and for p- 11 B the energetic shortcomings were shown in a previous paper [5] . Therefore, D- 3 He, with 3 He content corresponding to 3He-catalyzed D and beyond, seems to be the only advanced fuel of interest for a plasma with reactor-relevant densities . In this case, the specific neutronization (fraction of neutron power carried by neutrons) can be reduced from 0.8 (D-T, 1 :1 mix) to about 0.25 for ßl.-1 _catalyzed D and to a few percent or less for 3Heto-D ratios of unity or higher. Recycling of tritium does not do much to improve the attainable power density, but has an adverse effect on the neutronization and hardens the neutron spectrum, Therefore, it may be advantageous to have tritium decay to 3He before it is reinjected into the plasma. This would also help bring down the tritium inventory in the plasma by another order of magnitude and accordingly reduce the tritium handling requirement at the expense o¬ tritium storage requirements. In a weapons proliferation context this may be considered to be a serious disadvantage since the amount of tritium involved is quite large: for 3 He-
catalyzed D the net tritium production rate would be of the order of 50 kg tritium per year for a 1 GW reactor.
References [1] W. Kernbichler, R. Feldbacher, M. Heindler, A. Nassri and K. Schdpf, in: Plasma Physics and Controlled Fusion Research (Proc. 11th Int. Conf., Kyoto, Japan, 1986) IAEA-CN-47/H-III-9, (IAEA, Vienna, 1988) p. 373. [2] W. Kernbichler, Doctoral Thesis, Graz University of Technology (1987). [3] L.J. Wittenberg, J.F. Santarius and G.L. Kulcinski, Fusion Technol . 10 (1986) 167 . [4] C. Powell, J. Nering, B.C. Maglich and A. Wilmerding, (Int. Symp. on Feasibility of Aneutronic Power, Princeton, NJ, 1987) Nucl. Instr. and Meth. A271 (1988) 41. [5] W. Kernbichler, R. Feldbacher and M. Heindler, in: Plasma Physics and Controlled Nuclear Fusion Research (Proc. 10th Int . Conf., London, 1984) IAEA-CN-44/I-I-6, Vol. 3 (IAEA, Vienna, 1985) p. 429. [6] R. Feldbacher, The AEP Barnbook DATLIB, nuclear reaction cross sections and reactivity parameter library and files, INDC(AUS)-12/G (IAEA, Vienna, 1987). [7] R. Feldbacher and IVI. Heindler, ref. [4], p. 55 [8] W. Kernbichler and M. Heindler, in: Alternative Energy Sources VII (Pros. Int. Conf. Miami Beach, Florida, 1985) ed. T.N. Veziroglu (Hemisphere, Washington, 1987) p. 393. [9] W. Kembichler and M. Heindler, Emerging Nuclear Energy Systems (Pros. 4th Int. Conf. Madrid, Spain, 1986) eds. G. Velarde and E. Minguez (World Scientific, Singapore 1987) p. 430 . [10] V.T. Voronchev, V.I. Kukulin and V .M. Krasnopolskij, Nuei. Fusion 24 (1984) 1117.