|Abstract. The concept of a hybrid fission-fusion reactor initiated by a laser is proposed. Fusion reactions are initiated by power beams transported from a laser driver system into a thermonuclear chamber surrounded by a twin-zone sub-critical multiplier blanket. It is important to note that the proposed system operates in an essentially unsteady-state mode. The neutronics and power parameters of the fission-fusion reactor system are evaluated.|
Several concepts have been developed recently for laser-driven inertial
confinement fusion (ICF) power reactors with blankets containing fissile fuel.
Such systems could reduce the power requirements for the laser beam,
increase the net driver efficiency, and provide the high safety of a hybrid
fission-fusion reactor. As previously shown , extended electric power
production might be obtained with an installation having the following
operational parameters: radiation energy transported to the target by two
lasers is 200-300 kJ/pulse, pulse frequency is 1 Hz, thermal power is 35-40
MW, blanket multiplication factor is 0.9-0.95, electric power needed for
driver is 10 MW, efficiency of laser is 2-3%, output electric power is 5 MW.
Even more efficient energy production could be achieved by using a coupled fissile blanket system . The proposed fission-fusion reactor concept is based on the principle of the cascade multiplication of neutrons from a micro-pellet fusion burn initiated by a laser beam. The idea of cascade multiplication is to obtain the maximal fission energy per single thermonuclear source neutron in a coupled sub-critical uranium blanket system. This approach has been reasonably offered for a long time with reference to a sectioned fission reactor and has been achieved in various coupled reactor systems, such as twin-core pulsed reactors [3,4], systems of coupled fission reactors and sub-critical assemblies , etc. In addition, similar installations could be used as the energy sources for pumping of laser drivers used for ICF .
The proposed fission-fusion reactor consists of a thermonuclear chamber, a
nuclear pumped laser driver (NPLD), and two-cascade sub-critical blanket
system (see Fig.1). Two power beams (transported from a laser driver system
into the thermonuclear chamber) initiate the fusion burn. A general lay-out
for a compact high-power NPLD is shown in Fig.2. The NPLD consists of a
thermal sub-critical laser module controlled by the neutron flux from a fast
burst nuclear reactor. The laser module is designed as a cylindrical
structure with a longitudinal cavity for the core of the fast burst reactor
and the reactivity modulator. A periodic-pulsed IBR type reactor employing a
liquid metal coolant is used as a neutron source. The pumped section of the
laser module is filled with a laser active medium containing a fissile
material and includes elements of the neutron moderator. This section is
surrounded on all sides by a neutron reflector; the flanks are manufactured
of an optically transparent material and provide for the input and output of
the laser beam. The proposed NPLD concept has been described in detail in
Ref.7. A prototype of the coupled blanket system for a hybrid reactor which
multiplies the neutrons produced from a fusion burn by ~100 is shown in Fig.3.
The internal blanket (1) is a sub-critical fast core utilizing the fuel pins
of a BN600 reactor (uranium dioxide fuel enriched by 26 %) . The core is
cylindrical (height 130 cm, and diameter - 138 cm) with an internal axial
cavity (diam. 48 cm), which is designed to contain the thermonuclear reactor
chamber and the "first wall" (5), as well as a deuterium-tritium (D-T) target,
the laser beam, etc. In order to reduce the coupling coefficient of the
thermal blanket to the fast blanket, the fast blanket is surrounded with a
0.5-cm thick boron carbide (natural enrichment) coating (6).
To remove the heat released as a result of nuclear reactions, the fast
blanket?s core is supplied with a liquid metal (sodium) cooling system.
The external blanket (2) is a sub-critical heavy-water reactor with a thermal
neutron spectrum containing fuel pins consisting of aluminum pipes
(0.0838-cm thick; 1206-cm long; external diameter - 1.1506 cm) filled with
uranium of natural enrichment (~0.7 %). The pins are placed in a triangular
lattice (step - 3.806 cm) inside a cylindrical steel tank (4) filled with
heavy water. The external radius of a tank is 425 cm, internal radius -
300 cm, the thickness of the steel wall - 1 cm. The top and bottom of the
system are closed with a 3-cm thick beryllium neutron reflector (3).
The operation mode of system is as follows. The released energy ENPLD of the
laser module controlled by a pulse reactor is converted into a laser beam
with efficiency ~2 to 3%, which (energy of Elaser) initiates a fusion burn
of a D-T target having an energy Eburn in a thermonuclear chamber supplied with
a blanket system/amplifier of burn neutrons.
Figure 1. Schematic of a hybrid fission-fusion reactor.
1-NPLD, 2-target chamber, 3-first blanket cascade,
4-second blanket cascade, 5-steam-generators and turbine unit.
Figure 2. The NPLD scheme:
1-laser module, 2-pulse reactor core, 3-reactivity modulator,
4-neutron reflector, 5-optical windows (units of measure - cm).
In order to compensate for tritium burn-up, the thermal blanket is equipped with a tritium-breeding chamber containing Li. Tritium is bred by emitted neutrons as a result of the reactions Li6 + n = He4 + H3 and Li7 + n = He4 + H3 + n'. The number of T-chambers is determined by the maximal efficiency of the closed tritium cycle.
Figure 3. The scheme of the coupled system consisting of fast and thermal blankets (half of total height, units of measure - cm).
The laser driver consists of a fast reactor and thermal laser module with a
two- cascade blanket, which are coupled into a reactor system operated in
pulse- periodical mode. The two-point kinetic model to describe the neutron
transient processes in such system is as follows :
Here nj is the intensity of fissions in j-th component of coupled
system (blanket or NPLD); kjj and lj are multiplication
factor and the average lifetime of prompt neutrons respectively in the j-th
component; kij is a coupling coefficient; bji,
lij, Cji are parameters of
the delayed neutrons; Dj is the number of delayed neutron groups;
Sj is the distribution function of the first fissions produced
by neutrons from a thermonuclear source target in j-th blanket (or neutron
source function in a pulse-periodical reactor of the NPLD). Being
supplemented with the periodicity conditions (tp -
duration of a period), the model (1) gives a complete description of the
time-dependent fission processes in the coupled system in conformity with
the point kinetic model.
In order to evaluate the power parameters of the blanket system, the formulas
for total released energy
Integration (1) according to the conditions of periodicity gives the formula:
If M = (E1 + E2)/e1I -
the net fission multiplication in the system, then the formula for M is:
Figure 4. Multiplication factor of the system versus k12.
Figure 5. k21 as function of k12.
Figure 6. M1 as function of k12.
Figure 7. Transient neutron processes in a blankets.
Thus, the neutronics analysis of the blankets shows that for the system with
a net multiplication M=100, it is necessary to provide the following
parameters: k11=k22 ~ 0.94-0.95; k21 ~ 0.2-0.3;
k12 ~ 0-0.002. In this cases it may be expected that
Keff ~ 0.95-0.96; M1 ~ 15-20. The neutron physical
characteristics of the pilot blanket system shown in Fig.3 are obtained with
Monte Carlo method using MMKFK-2 code : e1=0.5;
k11=k22 ~ 0.941±0.002;
k21=0.245±0.002; k12= (1.65 ± 0.12) · 10-3;
l1 =4·10-7 sec; l2 =3.5·10-4 sec.
It is clear that these computed parameters correspond to the requirements
outlined above. The operation mode of the coupled blanket system is
analogous to that of a pulse periodic IBR-type  reactor, and it is easy
to derive the algebraical formulas to estimate the NPLD characteristics
using expression (1). The theoretical analysis of NPLD neutronics and power
parameters is carried out in Ref.7. The computed value of the multiplication
factor for the laser module is 0.9. The NPLD system proposed was shown to be
capable of providing the pumping energy ENPLD ~8 MJ and output laser energy
Elaser ~ 160 kJ with a repetition rate 1 Hz. The values of power
parameters for the blankets are E1=10 MJ, E2=41.7 MJ.
For that case, the required net neutron output from a micro-pellet is
~3·1015 neutrons per pulse. Such neutron output could be
provided by using two laser drivers with Elaser ~300 kJ/pulse .
In this case, the energy released in a fusion reaction is equal to the
input laser energy (break-even of the coefficient of energy amplification
for a D-T target is ~1). The output electrical power is ~22 MW.
Figure 8. The computed pulses in a blankets.
Figure 9. The computed pulses compared with algebraically calculated ones.
Transient neutron processes in the fast and thermal blankets computed with
the POKER code  are shown in Fig.7 (for the first ten pulses, the
frequency is 1 Hz). The blanket powers distributed in time are shown in
Fig.8. The initial thermonuclear neutron?s pulse, having a rectangular time
shape with the duration 10-9 sec and net output of 3·1015
neutrons, was used in the computations. It is obvious (see Fig.9) that the
data calculated using an algebraical expression are in good agreement with
the detailed numerical simulation.
Note that for the case mentioned above, the multiplication (in terms of
energy) will be e1Efis/EfusM
~ 500 - 700 (Efis is an energy of the unit act of fission of the
uranium nucleus; Efus is an energy of unit act of fusion).
Thus high values of energy multiplication make it possible to use other
high-power lasers like NOVA  as a potential ICF driver. Unfortunately,
a significant part of output electric power produced (about 10 MW with a
laser efficiency of ~2%) will be used then for production of the laser beam,
and the problem of power supply for the "first laser flash" must be solved.
Thus, a hybrid fission-fusion reactor concept is proposed. To demonstrate
the feasibility of a two-cascade amplifier for thermonuclear neutrons,
operated in pulse-periodical mode, an analysis of neutron parameters of a
system consisting of two blankets and two NPLDs has been carried out.
The requirement parameters for a reactor with amplification M=100 have been
determined. The analytical formulas to estimate the power characteristics of
the fission-fusion reactor have been derived, and comparative analysis of
analytical and numerical simulation results has been performed. The formulas
are shown to permit the evaluation of the basic power characteristics of
coupled blanket system with sufficient precision.
In summary, it should be noted that proposed fission-fusion reactor has a
high level of inherent safety: the blanket system is always deeply
subcritical, the computed total neutron emission from the system does not
exceed 8%, and it is not necessary to provide the complex control and
shielding systems. Further, the heavy-water thermal blanket makes it
possible to use uranium of natural enrichment. Such blanket systems could be
used as neutron amplifier for any type of hybrid installation (e.g., for
cascade amplification of neutrons produced by high-energy charged particles
from a target in an electronuclear unit ).