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Contribution of external neutron sources in excess neutron generation for SCNES
Progress in Nuclear Energy. Vol. 32, No. 314. ~3. 697-705, 1998 0 1997 Published by Elswier Science Ltd Printed in Great Britain 0149-1970/98 $19.00 + 0.00
PII: so149-1970(97)00082-6
CONTRIBUTION
OF EXTERNAL NEUTRON SOURCES
IN EXCESS NEUTRON GENERATION
M.SAITO, ACHMELEV’,
V.ARTISYUK,
FOR SCNES
M.SUZUKI, Yu.KOROVIN**, Y.FUJII-E”’
Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology 2-12-l o-okayama, Meguro-ku, Tokyo, 152 Japan **Obninsk Institute of Nuclear Power Engineering 2, Studgorodok, Obninsk, Kaluga region, Russia
ABSTRACT Self-Consistent Nuclear Energy System simultaneously meets four requirements: energy generation, fuel breeding, burning of radionuclides and system safety. The key element of the system is excess neutron generation. Analysis of various neutron sources is done with respect to transmutation requirements. Impact of neutron source on energy system performance is analyzed in terms of excess neutron cost. Special emphasis is made on Fusion Neutron Source. 0 1997 Published by Elsevier Science Ltd INTRODUCTION The present stage of nuclear energy development is associated with slowing down of the nuclear power plants deployment mainly because of Three-Mile-Island accident, Chernobyl catastrophe and problems of radiowastes disposal. First stage in nuclear energy development dealing with demonstration of possibility to utilize nuclear energy has been completed. To summarize the experience gained on previously and tendency become clear, new phylosophical basis for the nuclear energy is needed to be developed. It is obvious that public opinion will be much more influential in decision making in near future and new approach should be addressed to the broad public community. These guidelines were implemented in Self-Consistent Nuclear Energy System (SCNES) approach (Fujii-e et a1.,1992a): energy generation based on fission energy recovery with maximum efficiency, fuel breeding to provide inexhaustible fuel supply, burning of radionuclides produced in the system, system safety relied on simple logic which is to be intelligible both to experts and public.
Present address: *Moscow Engineering and Physics Institute, 31, Kashirskoe Shosse, Moscow Russia **‘Atomic Energy Commission, Japan, Kasumigaseki, Chioda-ku, Tokyo 100
697
M. Saito et al.
698
Excess neutron generation is the key element of SCNES (Fujii-e et a1.,1992b). Advanced metal fuel fast reactors (FR) chosen as a possible candidate for SCNES demonstrate high excess neutron potential per fission, that is high enough to exceed neutron requirements for the transmutation of FP provided their isotope separation (Akatsuka et a1.,1994). Because of uncertainties in separation process influence on the environment (both radiological and chemical) we focus on consideration of SCNES with enhanced neutron generation requirements provided by external neutron sources (NS). Since neutron generation is inevitably associated with energy production (both to support inherent neutron producing processes in NS and energy release in NS itself) special emphasis is placed to find NS characterized by low energy burden on the SCNES. The efficiency of external NS would influence both economics and SCNES composition depending on energy needed for its operation. The paper presented deals with analysis of contribution of various NS in excess neutron generation.
EVALUATION FUNCTION OF NEUTRON SOURCE IN SCNES The analysis performed is based on the methodology of excess neutron cost developed previously (Chmelev et alJ996). Here we offer the main lines of the methodology. ess Neu&on_G& In our approach we take into account nuclear processes involved in NS operation rather than single installations. From this view-point NS can be determined as a combination of two types of the processes. First type is the fission process associated with NS for example to feed accelerator in case of AcceleratorDriven Core (ADC) or to support plasma performance in Fusion Neutron Source (FNS) with negative energy balance. The second type includes all possible reactions inherent in neutron production which are distinguished from ordinary fission such as spallation, fusion, stripping. Excess neutron generation relates to transmutation requirements of fission reactions destined to net power production. It excludes neutron consumption as a result of NS operation: both fuel self-sustaining and elimination of radiowastes accumulated in NS, if any. The basic criterion of SCNES efficiency which is used in our analysis is utilization factor for energy production (UF):
where E,,, - total thermal power generated in the nuclear energy system; q,,, - conversion efficiency of thermal energy to electricity averaged over the system (for one based on FR only - q,,,=0.4); EC - electrical power consumed in the system itself. For SCNES involving NS this turns to
where 6 - stands for the system configuration - ratio of power associated with NS to net one:
e,
6=
N/” +erNNS ej N;”
Excess neutron generation
699
for SCNES
where ei - energy release in NS per one inherent reaction (in the target of ADC, fission energy for FR, fusion energy for FNS, etc. ); er - fission energy (MeV/f);
N,‘- reaction rates associated
production (in NS and net one). Neutron balance in SCNES is achieved when neutron consumption equation below) are covered by neutron excess generation (right side): n:,ElN;el +n,yN/NS = N”(SNs
_,,P
-nF
requirements
-,,i”” -n,Ns>
where SNs - number of neutrons generated per one inherent reaction in NS; n: NS per one inherent reaction, if any; ny - neutron requirements
(left side of the
(4) - breeding requirements
to transmute radionuclides
FP; ncNs- parasitic capture in NS; n? - useless neutron leakage from NS; n,y,
with energy
in
different from
n$ neutron requirements
to
back up fission processes dedicated to NS and net power production. To simplify Eq.(4) fuel selfsustaining in SCNES is assumed. In a view of the equation above we readily find that S is determined by transmutation requirements of net energy production and NS characteristics:
ENC incorporates
specific features of NS:
Denumerator stands for excess neutron generation in NS and numerator - for energy inevitably accompanying this process. ENC ( excess neutron cost) is a useful combination of NS characteristics that enables one to estimate the influence of particular NS on SCNES as a whole. ENC estimations for various NS are presented below. ENC for As it was pointed out previously, the neutron generation potential of an advanced metal fuel FR is sufficient to satisfy SCNES requirements on condition of FP isotope separation. In general, they may be referred to as a NS. Being energy producer also, they need not an additional energy support, so N,‘” =O. Fission reaction is the only process inherent in neutron generation (ey
= e, ). Taking into account neutron
transport in a reactor, we may write ENC as follows et
ENCFH = VJ -(BR+l+
L)_ l+a
l+Cl l+e
(7) -n< -n,
-fl,,
here v, -neutron yield per one fission, BR - breeding ratio, a - capture to fission ratio of fissile luclides, - fraction of fertile nuclei fission.
E
7ocl
M. Saito er al.
The inherent process to produce neutrons in ADC is interaction between high energy charged particle and special target often referred to as a spallation target. High energy particle (e.g. proton) is generated by electricity so does not need a special breeding option in NS (np =0). Significant neutron yield from the target per one proton (e.g. fs=47 n/p for lead target irradiated by proton beam with energy e, =1.6 GeV) allows to consider ADC as a NS. Inevitable energy to be operated with is the thermal energy to initiate incident proton: e, = e,, / (17,~ ) , here 17,- conversion efficiency of electricity to beam energy. These NY” energy requirements are covered by energy release in the target ( eiN’+ e f - NNS - inherent energy appeared in the target due to ionization loss and fissioning) and supporting fissions wherever they occur: in the reactor coupled with ADC or in the blanket surrounding target: N,”
NNS=
1
N;“”
e
__p_-ei]--
&A~
(8)
NNS
It should be mentioned that ionization loss deposits an appreciable energy, approximately 40% of the initial proton energy, while target fissioning depends on target composition being relatively small in the lead (30 MeV per one 1.6 GeV proton). Some surplus in neutron generation is expected to be achieved in U-Bi target (60 against 47 n/p) due to uranium fissioning that produces approximately 1.7 GeV energy per one proton significantly reducing the number of supporting fissions (Artisyuk et al. 1996). Ignoring transmutation requirements of spallation products in the target (ny =0) we conclude that eF /
ENCADC=
h vth)
sNS-nc -n,
(9)
-np
Nr" F
To clarify the amount of energy to be operated with we refer to DT fusion reaction as an example of FNS. Neutron multiplication in the blanket is absolutely necessary to provide radiowaste incineration and tritium breeding simultaneously. Neutron generation in FNS is contributed with both fusion neutrons and ones produced in the multiplier surrounding fusion region. Either non-fissionable multiplier (Be, Pb, etc) or fissionable one (U, Th) can be used in FNS. The total number of neutrons escaped from the multiplier (s”“> is treated as a source strength. It is influenced by competition between neutron producing reaction (fission, (n,xn), etc) and neutron capture. It should be stressed that multiplier makes a sense only if it provides an additional contribution to primary fusion neutrons (s”“>l). We consider FNS operated in two options with respect to energy balance. Taking NfNS=0 for the case of exothermal process with non-fissionable multiplier and neglecting niN’we have
ENCFNs =
(10)
e&v
SNs-nz -n,
-n,
where ef”, - energy release in FNS triggered by one fusion; nF - neutron requirements to breed tritium, normalized to one fusion. In case of endothermal process, FNS requires supporting fissions, therefore ENC is
Excess neutron generation for SCNES
701
ENCFNs =
(11)
It is to be noted that FNS with fissionable multiplier may be also presented by this formula. In our analysis plasma performance is discussed in terms of traditionally quotated Q -\,alue that is specified as ratio of energy produced by fusion reaction to one consumed in plasma to maintain its temperature. Q -value determines the number of supporting fissions dedicated to FNS in endothxmal case:
g(Q)
=
F[&
(12)
- l] ,h
(1
where q, -efficiency of plasma heating by injection. For both cases considered
above efU, is estimated as
efis=en +e,
+e,, -qmu”
where e, , e, -energy of particles (neutron and alpha) produced in DT fusion reaction;
en,” -energy release
as a result of neutron capture reactions in the blanket per one fusion; qmu”- energy consumed endothermal threshold reactions per one fusion.
in neutron
Stripping reaction provides neutron production through the deuteron disintegration. The most striking feature of this reaction is the peak in neutron energy distribution which is about half of the incident deuteron energy. Stripping reaction is the dominant one at deuteron energies higher than 15 MeV (below this energy a compound nucleus is formed and neutrons are emitted isotropically with the energy distribution given by classical evaporating model). We refer to two types of NS based on this process. The first one is D-Li source broadly discussed now as a neutron source for fusion materials irradiation test facility (Lawrence et a1.,1989). Deuterons with energies 35-40 MeV are used to generate a fusion-like spectrum from the thick-target neutron yield of the Li(d,n) stripping reaction. Tte spectrum obtained peaks at 14 MeV, each deuteron producing 0.06 neutrons (at low deuteron energies neutron yield is less). Most of the initial deuteron energy is deposited in a lithium jet as an ionization loss ( e,hii = e,, ). All the equations to estimate ENC and number of supporting fissions are exactly the same as implied in ADC analysis, except no fissions occur ( Niilrg =O). Deuteron accumulator is another NS considered here. It evades deuteron stopping without neutron production (Ado, et a1.,1991). The small atomic number target (e.g. deuterium gas) is placed in a vacuum chamber of deuteron storage ring in such a way that deuteron of constant energy circulates I.hrough the target as many times as it necessary until a neutron producing reaction occurs. Ionization loss is compensated by a high-frequency electric field that requires an appreciable amount of energy. Totally, inevitable energy associated with one deuteron is
e,”
-‘(~+“) r,,
<,
(14) ‘7,
102
M. Saito et al.
where 17,=0.95-is efficiency of a storage ring; ei =OS MeV -ionization loss per cycle, N -number of cycles before neutron producing reaction occurs (500 for deuterium target). Since each deuteron before disintegration deposits ei N, the number of fissions supporting accelerator is derived from
NfNS le, -=_ t NNS ef %%h
1
+e,N(-
- 91 %
%h
Of cause for the both cases of stripping based NS, in ENC estimations should be included.
useless leakage and parasitic capture
The basic phenomenon responsible for muon-catalyzed fusion (MCF) is substitution of electron in atom orbit of deuterium or tritium with negative muon which mass is 207 times of electron. Atoms of deuterium and tritium are appeared to be close to each other to the extent that probability to react is high enough: each muon during its life-time initiates about 100-150 fusion reactions. Muons are produced through decay of negative pions originated in a thin target of small atomic number material (C, Be) irradiated by high energy deuterons. Like plasma based FNS, MCF needs tritium breeding, so neutron multiplication is necessary. The source strength may be written as follows: *N
SNs =K*N. P
R
$1.
*x
=
here Nn_ -number of n- produced per one incident deuteron (0.13-0.16 for ed =1.5 GeV); N,d. -number of CL- stopped in DT mixture (0.053-0.174 per one n-); X, - number of fusion reactions catalyzed by one p(X,=100); K-total number of neutrons escaped from multiplier per one 14 MeV neutron (K=1.8 for lead). It should be noted that about 40% of deuteron beam is carried out of the target without interactions. It may be used to produce neutrons in the spallation target in the same manner as in ADC. This reduces the number of supporting fissions in accordance with: 07)
where r,0 - is the fraction of beam unscattered in the pion producing target. Total neutron strength turns in SNs = S,: + ~/IS,?, the last term standing for contribution
of neutrons
produced in the spallation target ( S,TF = 52 for lead target and ed =1.5 GeV).
BREAK-EVEN
TRANSMUTATION
REQUIREMENTS
At least the supporting fissions should be covered with neutron production to approve the feasibility of neutron sources based on the reactions above. Herein we compare the methods for neutron production in terms of break-even transmutation requirements PI,: allowed, below which NS could be established. The upper bound for transmutation
requirements
(neglecting useless neutron consumption
and leakage) are
Excess neutron generation for SCNES
703
Break-even transmutation requirements are shown in Table 1. For all the 14 MeV NS (stripping, MCF, FNS) multiplication in lead is assumed. To illustrate plasma performance in FNS we refer to Q=l, more detailed analysis being presented in the next section. The results show that regardless of real neutron requirements, ADC and FNS to be more effective than stripping based NS and MCF. The last one is characterized by small neutron fraction resulted from fusion reaction compared to neutron prod.uction in a spallation target. It is to be stressed that only general neutron producing properties are under study. For the specific objectives like transmutation of FP through (n,2n) reaction, stripping and MCF may offer some advantages, but this issue is beyond the framework of this paper. Table 1 Break-even transmutation NS type: option:
Stripping based NS
ADC Pb-target U-Bi-target
1.0
requirements
1.3
D-Li
Deuteron accumulator
DT-fusion only
0.1
0.47
0.028
DT-fusion t spallation
DT-plasma Q=l
0.5
1.4
GENERAL ANALYSIS OF FNS In general,
it appears
that three main issues determine
multiplier properties (s”” ), energy supply for plasma performance
the excess
neutron
generation
(Q ) and requirements
in FNS:
of SCNES ( nlF ).
Interrelation between them outlines framework of FNS as a neutron source in SCNES and highlights range of appropriate boundary S,“” values that have to cover at least the internal needs of FNS itself: (19)
Concealed in Fig.1 are some specific features concerning FNS. For quantitative analysis we assume r~”=0.8 (Kammash, 1977). According to coolant temperatures (inlet and exit are 250 and 310 “C , respectively
(Gohar,
1995)) we refer to q,,, =0.25. Tritium
breeding
requirement
is nr :=1.05. For
estimation of inherent energy release we use e, =14.1 MeV, e, =3.5 MeV, qm”’=6.8 MeV (energy threshold of (n,2n) reaction in Pb), e,,, =8.62 MeV. Regardless of SCNES option, at Q -value corresponding to zero energy balance in FNS (Q, = l/ (v~,~I,,,)= 5 ) the last term in Eq.19 is zero that means no fissions are required to support plasma heating.
For exothermal
option (Q > Q0 ) 5, ”
is restricted
only by tritium breeding.
Takng
overall
reduction in neutron source strength due to parasitic capture and useless leakage as 15%, we may conclude that for exothermal FNS SF = 1.23. For endothermal
case (Q < Q0 ), SF
is determined
mainly by supporting energy, i.e. by Q -value and
n,y .For SCNES composed of FR only, we take metal U-Pu fueled reactor as a reference one. According to
M. Saito et al.
704
(Akatsuka et a1.,1994) in such a FR the neutron excess achievable per one fission is 0.42 n/f and transmutation requirements 1.12 n/f. NS in FR based SCNES is expected to cover the lack of neutrons amounted to 0.7 n/f. Assuming fuel self-sustaining for the systems composed of both FR and TR, neutron excess available for transmutation in FR is drastically decreases because of necessity to breed fuel for TR. It leads to neutron requirements as high as 1.12 n/f to be covered with NS. Lowest solid line in Fig.1 stands for limits of FNS application in SCNES composed of FR only and uppermost solid line - in multicomponent TR+FR system. Because ADC resembles endothermal FNS in having an external energy supply, we pointed out the region of competitiveness
of ADC (dotted line in Fig.1.). Appropriated
SF specifying
this region was obtained by
making equal ENC of ADC and FNS. We offer Pb as a meaningful example of multiplier (maximum sNs=1.8). It is obvious to conclude that FS with Pb multiplier could bring both energy systems above to Self-Consistent option, ADC being no competitive with FNS even at small Q -values close to unity which was announced to be achievable in experimental device JET (Bertoliny, 1995). Needless to say about ITER project in which plasma may be considered as a full ignited one (injected power is much less than that provided by a -particle heating), the resulting Q -value being as high as 15 (Rebut, 1995). To demonstrate the impact of ENC on the nuclear energy system configuration and utilization factor we refer to SCNES based on FR only in FP non-separation option (lz,y=nr =0.7 n/f). Since in nonseparation option transmutation requirements for FR exceed this value, FR can not be considered as a neutron source and appropriated ENC is negative. In ENC estimations we assumed the following parameters: y, =2.87, BR=l, a =0.13, E =0.15, n/=0.1, n, =0.2. Utilization factor as a function of ENC is shown in Fig.2. For ADC we assume both qa and r~,, to be 0.4. Neglecting binding energy at starting point (ENC=O) we approach the UF amounted to 0.4 (thermodynamic efficiency of FR). FNS demonstrates a clear advantage (if zero energy balance is achieved) over traditional ADC with lead target. ADC with U-Bi target or FNS with negative energy balance give intermediate numbers between ones shown in Fig.2. FNS being introduced in nuclear energy system could provide an effective neutron generation without significant decrease in efficiency of energy generation.
1.8
Neutron multiplication -_-----------------_in lead Requirements for FP transmutation ADC competitiveness
I 1000
Fig.1 Boundary neutron source strength as a function of plasma performance.
. . .
.
I
2oc ENC (MeVh)
Fig.2 Utilization factor as a function of ENC.
Summary of ENC evaluations along with utilization factor (UF), system configuration (6) as well as (I-UF)/UF - ratio of thermal wastes to net energy production in SCNES are presented in Table 2. SCNES based upon FR was chosen as a reference. For FNS we emphasized ratio of energy initiated by fusion reactions to the total thermal energy produced in the system (IY~,.~ l/Z,/, ). It is worth while mentioning that this
Excess neutron generation for SCNES
705
reactions to the total thermal energy produced in the system (_& / E,,, ). It is worth while mentioning that this ratio is not so high - about 12%. Fissions to support plasma heating significantly increase energy production associated with FNS - 62% for JET-like FNS. For SCNES with traditional ADC as a neutron source, utilization factor decreases to 7%. Table 2 Impact of External Neutron Source on SCNES ADC & ENC (MeVin) System configuration (6 ) Utilization factor (UF) (l-UF)/UF & t Eth
1330 4.7 0.07 13.3
FNS (U-Bi)* 650 2.3 0.12 7.0
FNS (JETQ =l) 462 1.6 0.15 5.6 0.12 (0.62)
(ITER,Q =1.5) =ZZZ=== 40 0.14. 0.37’ 1.7 O.lZ!
* No account of Pu produced
CONCLUSIONS Analyses of various neutron sources involved in SCNES were performed with respect to break -even transmutation requirements . NS impact on SCNES characteristics was done in terms of Excess Neutron Cost. Fusion Neutron Source, even based on the present-day plasma technology, shows advantage over other possible NS.
REFERENCES Ado Yu. et al(1991) Neutron Production by Deuteron-Deuterium Interaction in a Gas Target. K.erntechnik 56.191 Akatsuka H. et al(1995) Scientific Feasibility of Incineration in SCNES. WSvmD. Global . . Evironment Elsevier, Oxford. Artisyuk V. et a1.(1995) Potential of Accelerator-Driven Cores in SCNES.m.106 Bertoliny E. (1995) Impact of JET Results and Engineering Development on Definition of ITER Design Concept. Fus.Eng.andDes.27.27 Chmelev A.N. et a1.(1996) A Possible Contribution of Various Neutron Sources in Exce:,s Neutron Generation in SCNESProc. Int. Conf a NucvObninsk, Russia, 24-28 June Fujii-e Y. et al(1992a) An Approach to Self-Consistent Nuclear Energy System.-. ANS.66.342 Fujii-e Y. et a1.(1992b) An Approach to Self-Consistent Nuclear Energy System (Potential of Fast Reactors). Proc. Int. Conf oJ1 Safetv of Ad anced Nuclear Power Plants v.2, 11.3- 1 Gohar Y. et al. (1995) ITER Blanket Design. A52 Kammash T. (1973) CReactorn Arbor Science PublInc. Lawrence P. et a1.(1989) A High-Flux Accelerator-Based Neutrom Source for Fusion Technology and Material Testing.J.of Fw.201 Rebut P.-H.(1995) ITER: The First Experimental Fusion Reactor.Des.27.3
PII: so149-1970(97)00082-6
CONTRIBUTION
OF EXTERNAL NEUTRON SOURCES
IN EXCESS NEUTRON GENERATION
M.SAITO, ACHMELEV’,
V.ARTISYUK,
FOR SCNES
M.SUZUKI, Yu.KOROVIN**, Y.FUJII-E”’
Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology 2-12-l o-okayama, Meguro-ku, Tokyo, 152 Japan **Obninsk Institute of Nuclear Power Engineering 2, Studgorodok, Obninsk, Kaluga region, Russia
ABSTRACT Self-Consistent Nuclear Energy System simultaneously meets four requirements: energy generation, fuel breeding, burning of radionuclides and system safety. The key element of the system is excess neutron generation. Analysis of various neutron sources is done with respect to transmutation requirements. Impact of neutron source on energy system performance is analyzed in terms of excess neutron cost. Special emphasis is made on Fusion Neutron Source. 0 1997 Published by Elsevier Science Ltd INTRODUCTION The present stage of nuclear energy development is associated with slowing down of the nuclear power plants deployment mainly because of Three-Mile-Island accident, Chernobyl catastrophe and problems of radiowastes disposal. First stage in nuclear energy development dealing with demonstration of possibility to utilize nuclear energy has been completed. To summarize the experience gained on previously and tendency become clear, new phylosophical basis for the nuclear energy is needed to be developed. It is obvious that public opinion will be much more influential in decision making in near future and new approach should be addressed to the broad public community. These guidelines were implemented in Self-Consistent Nuclear Energy System (SCNES) approach (Fujii-e et a1.,1992a): energy generation based on fission energy recovery with maximum efficiency, fuel breeding to provide inexhaustible fuel supply, burning of radionuclides produced in the system, system safety relied on simple logic which is to be intelligible both to experts and public.
Present address: *Moscow Engineering and Physics Institute, 31, Kashirskoe Shosse, Moscow Russia **‘Atomic Energy Commission, Japan, Kasumigaseki, Chioda-ku, Tokyo 100
697
M. Saito et al.
698
Excess neutron generation is the key element of SCNES (Fujii-e et a1.,1992b). Advanced metal fuel fast reactors (FR) chosen as a possible candidate for SCNES demonstrate high excess neutron potential per fission, that is high enough to exceed neutron requirements for the transmutation of FP provided their isotope separation (Akatsuka et a1.,1994). Because of uncertainties in separation process influence on the environment (both radiological and chemical) we focus on consideration of SCNES with enhanced neutron generation requirements provided by external neutron sources (NS). Since neutron generation is inevitably associated with energy production (both to support inherent neutron producing processes in NS and energy release in NS itself) special emphasis is placed to find NS characterized by low energy burden on the SCNES. The efficiency of external NS would influence both economics and SCNES composition depending on energy needed for its operation. The paper presented deals with analysis of contribution of various NS in excess neutron generation.
EVALUATION FUNCTION OF NEUTRON SOURCE IN SCNES The analysis performed is based on the methodology of excess neutron cost developed previously (Chmelev et alJ996). Here we offer the main lines of the methodology. ess Neu&on_G& In our approach we take into account nuclear processes involved in NS operation rather than single installations. From this view-point NS can be determined as a combination of two types of the processes. First type is the fission process associated with NS for example to feed accelerator in case of AcceleratorDriven Core (ADC) or to support plasma performance in Fusion Neutron Source (FNS) with negative energy balance. The second type includes all possible reactions inherent in neutron production which are distinguished from ordinary fission such as spallation, fusion, stripping. Excess neutron generation relates to transmutation requirements of fission reactions destined to net power production. It excludes neutron consumption as a result of NS operation: both fuel self-sustaining and elimination of radiowastes accumulated in NS, if any. The basic criterion of SCNES efficiency which is used in our analysis is utilization factor for energy production (UF):
where E,,, - total thermal power generated in the nuclear energy system; q,,, - conversion efficiency of thermal energy to electricity averaged over the system (for one based on FR only - q,,,=0.4); EC - electrical power consumed in the system itself. For SCNES involving NS this turns to
where 6 - stands for the system configuration - ratio of power associated with NS to net one:
e,
6=
N/” +erNNS ej N;”
Excess neutron generation
699
for SCNES
where ei - energy release in NS per one inherent reaction (in the target of ADC, fission energy for FR, fusion energy for FNS, etc. ); er - fission energy (MeV/f);
N,‘- reaction rates associated
production (in NS and net one). Neutron balance in SCNES is achieved when neutron consumption equation below) are covered by neutron excess generation (right side): n:,ElN;el +n,yN/NS = N”(SNs
_,,P
-nF
requirements
-,,i”” -n,Ns>
where SNs - number of neutrons generated per one inherent reaction in NS; n: NS per one inherent reaction, if any; ny - neutron requirements
(left side of the
(4) - breeding requirements
to transmute radionuclides
FP; ncNs- parasitic capture in NS; n? - useless neutron leakage from NS; n,y,
with energy
in
different from
n$ neutron requirements
to
back up fission processes dedicated to NS and net power production. To simplify Eq.(4) fuel selfsustaining in SCNES is assumed. In a view of the equation above we readily find that S is determined by transmutation requirements of net energy production and NS characteristics:
ENC incorporates
specific features of NS:
Denumerator stands for excess neutron generation in NS and numerator - for energy inevitably accompanying this process. ENC ( excess neutron cost) is a useful combination of NS characteristics that enables one to estimate the influence of particular NS on SCNES as a whole. ENC estimations for various NS are presented below. ENC for As it was pointed out previously, the neutron generation potential of an advanced metal fuel FR is sufficient to satisfy SCNES requirements on condition of FP isotope separation. In general, they may be referred to as a NS. Being energy producer also, they need not an additional energy support, so N,‘” =O. Fission reaction is the only process inherent in neutron generation (ey
= e, ). Taking into account neutron
transport in a reactor, we may write ENC as follows et
ENCFH = VJ -(BR+l+
L)_ l+a
l+Cl l+e
(7) -n< -n,
-fl,,
here v, -neutron yield per one fission, BR - breeding ratio, a - capture to fission ratio of fissile luclides, - fraction of fertile nuclei fission.
E
7ocl
M. Saito er al.
The inherent process to produce neutrons in ADC is interaction between high energy charged particle and special target often referred to as a spallation target. High energy particle (e.g. proton) is generated by electricity so does not need a special breeding option in NS (np =0). Significant neutron yield from the target per one proton (e.g. fs=47 n/p for lead target irradiated by proton beam with energy e, =1.6 GeV) allows to consider ADC as a NS. Inevitable energy to be operated with is the thermal energy to initiate incident proton: e, = e,, / (17,~ ) , here 17,- conversion efficiency of electricity to beam energy. These NY” energy requirements are covered by energy release in the target ( eiN’+ e f - NNS - inherent energy appeared in the target due to ionization loss and fissioning) and supporting fissions wherever they occur: in the reactor coupled with ADC or in the blanket surrounding target: N,”
NNS=
1
N;“”
e
__p_-ei]--
&A~
(8)
NNS
It should be mentioned that ionization loss deposits an appreciable energy, approximately 40% of the initial proton energy, while target fissioning depends on target composition being relatively small in the lead (30 MeV per one 1.6 GeV proton). Some surplus in neutron generation is expected to be achieved in U-Bi target (60 against 47 n/p) due to uranium fissioning that produces approximately 1.7 GeV energy per one proton significantly reducing the number of supporting fissions (Artisyuk et al. 1996). Ignoring transmutation requirements of spallation products in the target (ny =0) we conclude that eF /
ENCADC=
h vth)
sNS-nc -n,
(9)
-np
Nr" F
To clarify the amount of energy to be operated with we refer to DT fusion reaction as an example of FNS. Neutron multiplication in the blanket is absolutely necessary to provide radiowaste incineration and tritium breeding simultaneously. Neutron generation in FNS is contributed with both fusion neutrons and ones produced in the multiplier surrounding fusion region. Either non-fissionable multiplier (Be, Pb, etc) or fissionable one (U, Th) can be used in FNS. The total number of neutrons escaped from the multiplier (s”“> is treated as a source strength. It is influenced by competition between neutron producing reaction (fission, (n,xn), etc) and neutron capture. It should be stressed that multiplier makes a sense only if it provides an additional contribution to primary fusion neutrons (s”“>l). We consider FNS operated in two options with respect to energy balance. Taking NfNS=0 for the case of exothermal process with non-fissionable multiplier and neglecting niN’we have
ENCFNs =
(10)
e&v
SNs-nz -n,
-n,
where ef”, - energy release in FNS triggered by one fusion; nF - neutron requirements to breed tritium, normalized to one fusion. In case of endothermal process, FNS requires supporting fissions, therefore ENC is
Excess neutron generation for SCNES
701
ENCFNs =
(11)
It is to be noted that FNS with fissionable multiplier may be also presented by this formula. In our analysis plasma performance is discussed in terms of traditionally quotated Q -\,alue that is specified as ratio of energy produced by fusion reaction to one consumed in plasma to maintain its temperature. Q -value determines the number of supporting fissions dedicated to FNS in endothxmal case:
g(Q)
=
F[&
(12)
- l] ,h
(1
where q, -efficiency of plasma heating by injection. For both cases considered
above efU, is estimated as
efis=en +e,
+e,, -qmu”
where e, , e, -energy of particles (neutron and alpha) produced in DT fusion reaction;
en,” -energy release
as a result of neutron capture reactions in the blanket per one fusion; qmu”- energy consumed endothermal threshold reactions per one fusion.
in neutron
Stripping reaction provides neutron production through the deuteron disintegration. The most striking feature of this reaction is the peak in neutron energy distribution which is about half of the incident deuteron energy. Stripping reaction is the dominant one at deuteron energies higher than 15 MeV (below this energy a compound nucleus is formed and neutrons are emitted isotropically with the energy distribution given by classical evaporating model). We refer to two types of NS based on this process. The first one is D-Li source broadly discussed now as a neutron source for fusion materials irradiation test facility (Lawrence et a1.,1989). Deuterons with energies 35-40 MeV are used to generate a fusion-like spectrum from the thick-target neutron yield of the Li(d,n) stripping reaction. Tte spectrum obtained peaks at 14 MeV, each deuteron producing 0.06 neutrons (at low deuteron energies neutron yield is less). Most of the initial deuteron energy is deposited in a lithium jet as an ionization loss ( e,hii = e,, ). All the equations to estimate ENC and number of supporting fissions are exactly the same as implied in ADC analysis, except no fissions occur ( Niilrg =O). Deuteron accumulator is another NS considered here. It evades deuteron stopping without neutron production (Ado, et a1.,1991). The small atomic number target (e.g. deuterium gas) is placed in a vacuum chamber of deuteron storage ring in such a way that deuteron of constant energy circulates I.hrough the target as many times as it necessary until a neutron producing reaction occurs. Ionization loss is compensated by a high-frequency electric field that requires an appreciable amount of energy. Totally, inevitable energy associated with one deuteron is
e,”
-‘(~+“) r,,
<,
(14) ‘7,
102
M. Saito et al.
where 17,=0.95-is efficiency of a storage ring; ei =OS MeV -ionization loss per cycle, N -number of cycles before neutron producing reaction occurs (500 for deuterium target). Since each deuteron before disintegration deposits ei N, the number of fissions supporting accelerator is derived from
NfNS le, -=_ t NNS ef %%h
1
+e,N(-
- 91 %
%h
Of cause for the both cases of stripping based NS, in ENC estimations should be included.
useless leakage and parasitic capture
The basic phenomenon responsible for muon-catalyzed fusion (MCF) is substitution of electron in atom orbit of deuterium or tritium with negative muon which mass is 207 times of electron. Atoms of deuterium and tritium are appeared to be close to each other to the extent that probability to react is high enough: each muon during its life-time initiates about 100-150 fusion reactions. Muons are produced through decay of negative pions originated in a thin target of small atomic number material (C, Be) irradiated by high energy deuterons. Like plasma based FNS, MCF needs tritium breeding, so neutron multiplication is necessary. The source strength may be written as follows: *N
SNs =K*N. P
R
$1.
*x
=
here Nn_ -number of n- produced per one incident deuteron (0.13-0.16 for ed =1.5 GeV); N,d. -number of CL- stopped in DT mixture (0.053-0.174 per one n-); X, - number of fusion reactions catalyzed by one p(X,=100); K-total number of neutrons escaped from multiplier per one 14 MeV neutron (K=1.8 for lead). It should be noted that about 40% of deuteron beam is carried out of the target without interactions. It may be used to produce neutrons in the spallation target in the same manner as in ADC. This reduces the number of supporting fissions in accordance with: 07)
where r,0 - is the fraction of beam unscattered in the pion producing target. Total neutron strength turns in SNs = S,: + ~/IS,?, the last term standing for contribution
of neutrons
produced in the spallation target ( S,TF = 52 for lead target and ed =1.5 GeV).
BREAK-EVEN
TRANSMUTATION
REQUIREMENTS
At least the supporting fissions should be covered with neutron production to approve the feasibility of neutron sources based on the reactions above. Herein we compare the methods for neutron production in terms of break-even transmutation requirements PI,: allowed, below which NS could be established. The upper bound for transmutation
requirements
(neglecting useless neutron consumption
and leakage) are
Excess neutron generation for SCNES
703
Break-even transmutation requirements are shown in Table 1. For all the 14 MeV NS (stripping, MCF, FNS) multiplication in lead is assumed. To illustrate plasma performance in FNS we refer to Q=l, more detailed analysis being presented in the next section. The results show that regardless of real neutron requirements, ADC and FNS to be more effective than stripping based NS and MCF. The last one is characterized by small neutron fraction resulted from fusion reaction compared to neutron prod.uction in a spallation target. It is to be stressed that only general neutron producing properties are under study. For the specific objectives like transmutation of FP through (n,2n) reaction, stripping and MCF may offer some advantages, but this issue is beyond the framework of this paper. Table 1 Break-even transmutation NS type: option:
Stripping based NS
ADC Pb-target U-Bi-target
1.0
requirements
1.3
D-Li
Deuteron accumulator
DT-fusion only
0.1
0.47
0.028
DT-fusion t spallation
DT-plasma Q=l
0.5
1.4
GENERAL ANALYSIS OF FNS In general,
it appears
that three main issues determine
multiplier properties (s”” ), energy supply for plasma performance
the excess
neutron
generation
(Q ) and requirements
in FNS:
of SCNES ( nlF ).
Interrelation between them outlines framework of FNS as a neutron source in SCNES and highlights range of appropriate boundary S,“” values that have to cover at least the internal needs of FNS itself: (19)
Concealed in Fig.1 are some specific features concerning FNS. For quantitative analysis we assume r~”=0.8 (Kammash, 1977). According to coolant temperatures (inlet and exit are 250 and 310 “C , respectively
(Gohar,
1995)) we refer to q,,, =0.25. Tritium
breeding
requirement
is nr :=1.05. For
estimation of inherent energy release we use e, =14.1 MeV, e, =3.5 MeV, qm”’=6.8 MeV (energy threshold of (n,2n) reaction in Pb), e,,, =8.62 MeV. Regardless of SCNES option, at Q -value corresponding to zero energy balance in FNS (Q, = l/ (v~,~I,,,)= 5 ) the last term in Eq.19 is zero that means no fissions are required to support plasma heating.
For exothermal
option (Q > Q0 ) 5, ”
is restricted
only by tritium breeding.
Takng
overall
reduction in neutron source strength due to parasitic capture and useless leakage as 15%, we may conclude that for exothermal FNS SF = 1.23. For endothermal
case (Q < Q0 ), SF
is determined
mainly by supporting energy, i.e. by Q -value and
n,y .For SCNES composed of FR only, we take metal U-Pu fueled reactor as a reference one. According to
M. Saito et al.
704
(Akatsuka et a1.,1994) in such a FR the neutron excess achievable per one fission is 0.42 n/f and transmutation requirements 1.12 n/f. NS in FR based SCNES is expected to cover the lack of neutrons amounted to 0.7 n/f. Assuming fuel self-sustaining for the systems composed of both FR and TR, neutron excess available for transmutation in FR is drastically decreases because of necessity to breed fuel for TR. It leads to neutron requirements as high as 1.12 n/f to be covered with NS. Lowest solid line in Fig.1 stands for limits of FNS application in SCNES composed of FR only and uppermost solid line - in multicomponent TR+FR system. Because ADC resembles endothermal FNS in having an external energy supply, we pointed out the region of competitiveness
of ADC (dotted line in Fig.1.). Appropriated
SF specifying
this region was obtained by
making equal ENC of ADC and FNS. We offer Pb as a meaningful example of multiplier (maximum sNs=1.8). It is obvious to conclude that FS with Pb multiplier could bring both energy systems above to Self-Consistent option, ADC being no competitive with FNS even at small Q -values close to unity which was announced to be achievable in experimental device JET (Bertoliny, 1995). Needless to say about ITER project in which plasma may be considered as a full ignited one (injected power is much less than that provided by a -particle heating), the resulting Q -value being as high as 15 (Rebut, 1995). To demonstrate the impact of ENC on the nuclear energy system configuration and utilization factor we refer to SCNES based on FR only in FP non-separation option (lz,y=nr =0.7 n/f). Since in nonseparation option transmutation requirements for FR exceed this value, FR can not be considered as a neutron source and appropriated ENC is negative. In ENC estimations we assumed the following parameters: y, =2.87, BR=l, a =0.13, E =0.15, n/=0.1, n, =0.2. Utilization factor as a function of ENC is shown in Fig.2. For ADC we assume both qa and r~,, to be 0.4. Neglecting binding energy at starting point (ENC=O) we approach the UF amounted to 0.4 (thermodynamic efficiency of FR). FNS demonstrates a clear advantage (if zero energy balance is achieved) over traditional ADC with lead target. ADC with U-Bi target or FNS with negative energy balance give intermediate numbers between ones shown in Fig.2. FNS being introduced in nuclear energy system could provide an effective neutron generation without significant decrease in efficiency of energy generation.
1.8
Neutron multiplication -_-----------------_in lead Requirements for FP transmutation ADC competitiveness
I 1000
Fig.1 Boundary neutron source strength as a function of plasma performance.
. . .
.
I
2oc ENC (MeVh)
Fig.2 Utilization factor as a function of ENC.
Summary of ENC evaluations along with utilization factor (UF), system configuration (6) as well as (I-UF)/UF - ratio of thermal wastes to net energy production in SCNES are presented in Table 2. SCNES based upon FR was chosen as a reference. For FNS we emphasized ratio of energy initiated by fusion reactions to the total thermal energy produced in the system (IY~,.~ l/Z,/, ). It is worth while mentioning that this
Excess neutron generation for SCNES
705
reactions to the total thermal energy produced in the system (_& / E,,, ). It is worth while mentioning that this ratio is not so high - about 12%. Fissions to support plasma heating significantly increase energy production associated with FNS - 62% for JET-like FNS. For SCNES with traditional ADC as a neutron source, utilization factor decreases to 7%. Table 2 Impact of External Neutron Source on SCNES ADC & ENC (MeVin) System configuration (6 ) Utilization factor (UF) (l-UF)/UF & t Eth
1330 4.7 0.07 13.3
FNS (U-Bi)* 650 2.3 0.12 7.0
FNS (JETQ =l) 462 1.6 0.15 5.6 0.12 (0.62)
(ITER,Q =1.5) =ZZZ=== 40 0.14. 0.37’ 1.7 O.lZ!
* No account of Pu produced
CONCLUSIONS Analyses of various neutron sources involved in SCNES were performed with respect to break -even transmutation requirements . NS impact on SCNES characteristics was done in terms of Excess Neutron Cost. Fusion Neutron Source, even based on the present-day plasma technology, shows advantage over other possible NS.
REFERENCES Ado Yu. et al(1991) Neutron Production by Deuteron-Deuterium Interaction in a Gas Target. K.erntechnik 56.191 Akatsuka H. et al(1995) Scientific Feasibility of Incineration in SCNES. WSvmD. Global . . Evironment Elsevier, Oxford. Artisyuk V. et a1.(1995) Potential of Accelerator-Driven Cores in SCNES.m.106 Bertoliny E. (1995) Impact of JET Results and Engineering Development on Definition of ITER Design Concept. Fus.Eng.andDes.27.27 Chmelev A.N. et a1.(1996) A Possible Contribution of Various Neutron Sources in Exce:,s Neutron Generation in SCNESProc. Int. Conf a NucvObninsk, Russia, 24-28 June Fujii-e Y. et al(1992a) An Approach to Self-Consistent Nuclear Energy System.-. ANS.66.342 Fujii-e Y. et a1.(1992b) An Approach to Self-Consistent Nuclear Energy System (Potential of Fast Reactors). Proc. Int. Conf oJ1 Safetv of Ad anced Nuclear Power Plants v.2, 11.3- 1 Gohar Y. et al. (1995) ITER Blanket Design. A52 Kammash T. (1973) CReactorn Arbor Science PublInc. Lawrence P. et a1.(1989) A High-Flux Accelerator-Based Neutrom Source for Fusion Technology and Material Testing.J.of Fw.201 Rebut P.-H.(1995) ITER: The First Experimental Fusion Reactor.Des.27.3