Hybrid Fission-Fusion Reactor Initiated by a Laser


A.P. Barzilov, A.V. Gulevich, O.F. Kukharchuk and A.V. Zrodnikov
Institute of Physics & Power Engineering
Obninsk 249020 RUSSIA

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.

Keywords: Inertial confinement fusion, hybrid reactor, laser initiation, coupled blankets, cascade multiplication of neutrons, pulse-periodic mode.

Introduction

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 [1], 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 [2]. 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 [5], etc. In addition, similar installations could be used as the energy sources for pumping of laser drivers used for ICF [6].


Fission-fusion reactor scheme

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 %) [8]. 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.


Schematic of a hybrid fission-fusion reactor.

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.


The NPLD scheme

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).


It is important to note that the proposed system operates in essentially a unsteady-state mode. The blanket coolant system of both cascades absorbs a net released energy during a period (E1 and E2), and then traditional steam-generator and turbine systems convert thermal energy into the output electric power Eel = (E1 + E2 + Eburn) n with efficiency n ~42%. Low-potential thermal energy Eth is utilized. It should be noted that the energy Eburn of high-temperature charged particles may be transformed into Eel using direct conversion methods. In addition, the energy released in the laser driver could also be transformed into the electrical energy.

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.


The scheme of the coupled system

Figure 3. The scheme of the coupled system consisting of fast and thermal blankets (half of total height, units of measure - cm).


Analysis of neutronics and power parameters

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 [9]:

Equation 1

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

Formula for total released energy

during a period in j-th blanket must be obtained. For simplicity, assume that the source Sj is as Id(t)ej, where d(t) is the delta-function, I is the net output of neutrons from a micro-pellet and ej is the total amount of first fissions produced by single thermonuclear neutron in j-th blanket (the coefficient of utilization).

Integration (1) according to the conditions of periodicity gives the formula:

Equation 2

In the case of a system of enclosed blankets, e2=0 can be assumed, and then the algebraical expressions for Ej are:
Equation 3

The blanket parameters M1 and M2 are the multiplications of fissions produced by one initial thermonuclear neutron from the micro-pellet. Note that it is possible to interpret M1 and M2/M1 parameters as multiplications in the first and second cascades of amplification, respectively.

If M = (E1 + E2)/e1I - the net fission multiplication in the system, then the formula for M is:

Equation 4

For a fixed value M it is possible to obtain from (4) the following:
Equation 5

The multiplication factor of neutrons in the coupled system (taking into consideration Eq.5) is:
Equation 6

The dependencies k21, Keff and M1 on k12 can be analysed under conditions where Dk11, Dk22 and M (M = 100) are fixed. These dependencies for four different cases {k11, k22} - (0.9, 0.9), (0.93, 0.93), (0.94, 0.94) and (0.95, 0.95) are shown in Fig.4-6.

Multiplication factor of the system versus k12

Figure 4. Multiplication factor of the system versus k12.


k21 as function of k12

Figure 5. k21 as function of k12.


M1 as function of k12

Figure 6. M1 as function of k12.


Transient neutron processes in a blankets

Figure 7. Transient neutron processes in a blankets.


It is obvious that the best parameters of cascade multiplication are provided for value k12=0. However, in the first case it requires an unrealistic high value for k21~0.9. In cases 2 to 4, the requirements for k21 are essentially reduced. However, the values of multiplication in the first cascade must be more than 10 (M1 ~ 20 and M2/M1 ~ 4, k12 =0, case 4). It should be noted that the minimum value of Keff of the system is provided for k12=0. In this case, Keff is the maximum value of {k11, k22}. When k12 is more than 10-3, the Keff of the system rapidly increases and the advantages of a cascade system in a safety sense are decreased.

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 [10]: e1=0.5; k11=k22 ~ 0.9410.002; k21=0.2450.002; k12= (1.65 0.12) 10-3; l1 =410-7 sec; l2 =3.510-4 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 [11] 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 ~31015 neutrons per pulse. Such neutron output could be provided by using two laser drivers with Elaser ~300 kJ/pulse [1]. 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.


The computed pulses in a blankets

Figure 8. The computed pulses in a blankets.


The computed pulses compared with algebraically calculated ones

Figure 9. The computed pulses compared with algebraically calculated ones.


Thus, the formulas for net released energy during a specific period were obtained. It should be noted that the rate of energy release in the blankets is essentially different over time. The majority of the energy is released during a very short interval of time Dt determined by the lifetime of a prompt neutrons in the system and its subcriticality. Then the rate of energy release declines to the "background" level, which is determined by the concentration of sources of delayed neutrons. Formulas for estimating the time distribution of fission pulses, background and a peak power for the blankets and the NPLD are derived in detail in Refs. 7 and 12. For example, the background (constant in time) level for the thermal blanket is 8.9 MW and 1.2 MW for the fast blanket.

Transient neutron processes in the fast and thermal blankets computed with the POKER code [13] 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 31015 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 [14] 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.


Conclusions

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 [15]).


References

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