Power systems

A 10 MW electrical generator and 25 MW thermal nuclear reactor assembly. Astronaut shown for scale. Conventional nuclear reactors will be important both for precursor mission and for Icarus Itself. Although the technology has been available for decades, there has been little progress in developing a large working reactor for space applications.

Un réacteur nucléaire de 10 MWe avec une production thermique de 25 MW. L'astronaute est là pour donner une échelle. Des réacteurs nuclaires conventionels seront requis tant pour les missions préalables que l'exploration Interstellaire elle-même. Bien que la technologie soit développée depuis des décénies, il y a eu très peu de progrès à ce jour pour le développement de réacteurs nucléaire développant une puissance significative pour des applications spatiales.

Introduction

Primary power systems will extract energy from the ship exhaust using direct energy conversion. Secondary power systems, most likely fission reactors, will power secondary thrusters and serve as the equivalent of starter motors for the fusion reaction. They will also keep the ship warm during the long coast period, and restart the drives at target.

About 10 to 20 MWe are required to power the Icarus systems at peak load, depending on design details. An electrical power of less than 1 MWe, and a thermal power of 4MWt, should be sufficient to keep essential components warm and power a communication link with Earth during the coast phase. With a mass ratio of 2 kG/kWe, including all the cooling system, the power system could mass from 20 to 40 tonnes, including about 2 tonnes of nuclear fuel.

Primary power

For all of the Icarus designs, primary power is extracted from the drive using various direct energy conversion schemes. While the drive is in operation, it feeds all secondary power systems. However, both before the drive is started and during the coast phase of the trip to the target star, a secondary power system is required. If there are shutdowns of the drive for maintenance during the boost and stop phases, the secondary power source is also required, to start up the drive again. The primary power of Icarus reaches Gigawatt levels, while the secondary power systems are in the low Megawatt range. Primary power is explored in the Direct energy conversion chapter.

Secondary power sources

Nuclear fission reactors are an obvious power source for secondary power. Fissile materials are nature's batteries, charged up at their time of creation in supernovae explosions. Although it may not be possible to design reactors that extract all of the potential energy from these materials, what we can get out of them is enough for the needs of the Icarus spaceship. For a 1 MW power source, lasting 100 years, a minimum of 100 kg of radioactive material is required. In practice, extracting 10% of the energy (known as the burn up fraction) is currently feasible with breeder type reactors, so about 1000 kg of uranium or thorium should suffice for the needs of the ship. Higher burn fraction are probably possible with traveling wave reactor designs₍₁₎.

Available energies in radioactive materials₍₂₎

Uranium: 80 000 GJt/kg
Thorium: 79 400 GJt/kg
Plutonium: 2 239 GJt/kg
Tritium decay: 583 GJt/kg

The efficiency of the reactor might be about 40% or 0,4, and burn up fraction 10%.

So for a 1 MW_e reactor powered by uranium:

1 MW x 100 yrs x 8760 hrs/ year x 3600 seconds/hour = 3,13e9 MJ ÷ 8e5 GJ/kg ÷ 0,4 ÷ 0,1 ≈ 1000 kg of nuclear fuel

For Firefly, with an average power use of 4 MWt, the required nuclear fuel would therefore bet about 2 tonnes.

Conventional reactors

There are dozens of conventional nuclear reactor designs that might be used to power the secondary systems of the Icarus spaceship. By conventional, we mean with a thermal core and a heat engine based electrical generator using circulating fluids similar to existing reactor on Earth. However, there is no existing space borne nuclear reactor of this type, and no clear path to one at this time. The Kilopower₍₃₎ design for a new space nuclear reactor may change this soon. The specific mass for the Kilopower design is 154 kg/kWe, and the design target is 10 kWe per unit. For the 10 to 20 MWe required for Icarus, this would mean 1500 to 3000 tonnes of mass, which is way over our target of 20-40 tonnes. So we need to develop the technology further than Kilopower has.

A recent design, currently on hold, the SSTAR reactor developed at the Lawrence Livermore National Lab was a sealed reactor core 15 meters tall and weighing 500 metric tonnes that could produce 100 MWe of electrical power over 30 years₍₄₎, this mass did not include the turbo machinery, however. More advanced designs are under development by a number of other companies ₍₅₎.However, the market for such machines remains uncertain. Comparable reactors like the 150 MWe D2G have been in use by the US Navy in its nuclear destroyers and submarines for decades₍₆₎. NASA has produced hundreds of studies of nuclear reactors for space applications, some of them extremely detailed. The SAFE-400₍₇₎ design saw some prototype work done for a 100 kWe reactor, with an overall specific mass of 5kg/kWe. A more advanced reactor study made for NASA, the vapor core MHD system₍₈₎, proposed a 200 MWe reactor with a specific mass of 0,37 kg/kWe.

In practice, 200 MWe is far more power than what Icarus needs. On an ongoing basis, the highest power draw will be the communications system, which would use 5 MWe to power the transmitters at target. A group of reactors providing 10 MWe would be sufficient for this load and to charge the startup capacitors. Multiple reactors would provide both redundancy and an easier way to reduce power output down to a few MWt during the long cruise phase. With a specific mass of 2 kg/kWe, the reactors and associated cooling systems would weigh about 20 tonnes, including all the radiators, turbines and auxiliaries. 20 Tonnes should be added for redundant systems and extra fuel.

For comparison, Daedalus specified four reactors at 650 kWe each(2.5 MWe total) for the second stage, with a total mass of 38 tonnes, and four 15 MWe with a total mass of 60 tonnes for the first stage. This corresponds to specific masses of 15,2 and 1 kg/kWe.

The following table shows some simulation results for reactor design. It is expected that Very High Temperature nuclear Reactor based on the helium Brayton cycle should reach outlet temperatures of 1300-1400K₍₉₎. 2200 K is very much a stretch goal. An equal alternative might be a liquid metal cooled thorium reactor, with supercritical CO2₍₁₀₎ or sulfur₍₁₁₎ as a working fluid.

Power systems table 1

Cutaway view of a Brayton cycle nuclear reactor. The working fluid is Helium. All dimensions are approximate.

Helium leaves the reactor at high pressure and temperature. It runs through a turbine, than drives both a turbo-alternator to create electrical power and a compressor on a common shaft. The gas leaves the turbine at lower pressure and temperature, and runs through the radiator. Excess heat is dissipated by the radiator. The gas is then compressed and returns to the reactor to be heated again.

Radiator array and reactor

The radiator array is much larger than the reactor itself, with an overall efficiency of 40%. In fact the radiator is probably heavier than the reactor.

A model of the Brayton cycle reactor, with piping and a added heat exchanger for more efficient power production (ML)

Radioisotope thermoelectric generators

Radioisotope thermoelectric generators (RTGs) are currently the only working nuclear reactors in space. They are extremely reliable; the Voyager RTGs, for example, have been operating for 40 years. Plutonium²³⁸ is the most likely radioisotope, but Americium²⁴¹ might be a good alternative. However, at an initial power density of .57 W_t/gram, and with a half life of 87.5 years, the core of a 1 MWe thermoelectric generator would weigh almost 70 000 kg, so a very significant mass. Producing that amount of plutonium would be a challenge, for no mass advantage. In the case of americium, the power density is even lower, and a 1MWe generator might weigh in at 2000 tonnes. So RTGs are not a probable solution for Icarus.

Thermoelectric systems

The efficiency of the reactor might be about 10% or 0,1. Thermoelectric generators are not very efficient()

So for a 1 MW_e reactor at the end of the trip (need to double the mass)

1 MWe ÷ 0,1 = 10 MWt ÷ 0.57 Wt/g x 2 ≈ 70 000 kg

Fission fragment reactor

One interesting power source is the fission fragment reactor₍₁₂₎. Better known in its incarnation as the fission fragment rocket, a similar process, arranged as a reactor rather than as a drive, would allow for the production of electrical energy without the need of a thermal cycle. Direct energy conversion could produce electricity with efficiencies approaching 90%, eliminating most of the radiators and the associated turbomachinery for a very significant reduction in overall mass. However, for large ships such as the Icarus probes, these levels of efficiencies are not requirements, but would simply be advantageous, if available at the time the starships are built.

Small fusion reactors

If small fusion reactors were possible, we wouldn’t need such a big ship! ;-)

The mass for the secondary power fission reactors comes mostly from the cooling system, radiation shields and turbo alternators required as auxiliaries. The energy producing core is quite small. This would be true for fusion reactors as well, and the mass reduction would probably not be very significant.

System configuration

The design of the coolant loop part of the secondary power system is covered in the Thermal Control page. Energy recovery is also covered in its own page. Service power distribution however is specific to secondary power systems.

Power distribution

Either DC(Direct Current), AC (Alternating Current) or a mixture of both types of power distribution are possible for Icarus. As the ship structure is not conductive, being mainly made from composites and ceramics, it cannot be used as a ground return and all return current must be wired in the same way as the distribution. Using higher voltages allows for lower currents and lighter wires, and semiconductor switches can be used for both distribution and motor control. Superconducting power mains, motors and transformers can also be used to reduce equipment mass. Higher temperature superconductors will allow the use of higher temperature cooling, possibly allowing for cooling without requiring refrigeration cycles.

The largest loads on the secondary power system will be secondary propulsion, motors for the main power cooling system pumps and compressors, charging of the main ship power storage system and fuel injection at the drive.

Icarus Firefly power diagram

Secondary propulsion

Attitude control, in certain flight modes, requires secondary propulsion. Power is needed to drive the propellant to the required velocity. The higher the exhaust velocity, the higher the power, but the less propellant is used.

Daedalus proposed Hall Ion thrusters. High power levels were achieved by using the auxiliary storage capacitors from the main drive for short bursts of power, reducing secondary propellant needs. The Daedalus Hall thrusters for the second stage reaction control system used a thrust rating of 50 Megawatts per thruster, and given that the Icarus design is similar to the second stage of Project Daedalus, a similar thrust rating could be planned for. The 1-2000 kWh of the Icarus main Energy Storage System would provide a few minutes of 50 MW power, sufficient for all proposed maneuvers for the Icarus vehicle.

A thrust rating of 50 Megawatts per thruster is exceptionally high, considering that the largest Hall thruster flown in space to date is the 4.5 Kilowatt Aerojet BPT-4000 Hall thruster flown in 2010, and the largest Hall thruster test fired is the 72 Kilowatt NASA BP457 Hall Thruster, with nearly 3 Newtons of thrust. Nevertheless, there seems to be no real obstacle to developing such thrusters. Efficiency of 80% and more are achievable.

Propellants for secondary propulsion

The most likely secondary propellant is hydrogen, as it provides the highest specific impulse for the lowest mass. Liquid hydrogen can be stored in tanks, and fed to the thrusters using simple pumping systems. Argon or any other easily ionised gas are possible alternatives.

Power systems example

Power system metrics

References

(1) TerraPower, L. L. C. (2010). Traveling-wave reactors: a truly sustainable and full-scale resource for

global energy needs. In Proceeding of ICAPP (Vol. 2010).

(2) Wikipedia, Energy density

(3) Gibson, Marc A., Steven R. Oleson, Dave I. Poston, and Patrick McClure. "NASA's Kilopower reactor development and the path to higher power missions." (2017).

(4) Sienicki, J. J., A. Moisseytsev, W. S. Yang, D. C. Wade, A. Nikiforova, P. Hanania, H. J. Ryu et al. Status report on the Small Secure Transportable Autonomous Reactor (SSTAR)/Lead-cooled Fast Reactor (LFR) and supporting research and development. No. ANL-GENIV-089. Argonne National Lab.(ANL), Argonne, IL (United States), 2008.

(5) Article: Glaser, Alexander, M. V. Ramana, Ali Ahmad, and Robert Socolow. "Small Modular Reactors." (2013).

(6) Presentation: 60 Years of Marine Nuclear Power: 1955 – 2015 Part 2: United States Peter Lobner December 2015

(7) Poston, David I. "Nuclear Design of the SAFE-400a Space Fission Reactor." (2002).

(8) Anghaie, Samim. Development of Liquid-Vapor Core Reactors with MHD Generator for Space Power and Propulsion Applications. No. DOE/ID/13635. University of Florida (US), 2002.

(9) D. Chapin, S. Kiffer and J. Nestell, "The Very High Temperature Reactor: A Technical 1. Summary," MPR Associates Inc., June 2004.

(10) Ahn, Yoonhan, Seong Jun Bae, Minseok Kim, Seong Kuk Cho, Seungjoon Baik, Jeong Ik Lee, and Jae Eun Cha. "Review of supercritical CO2 power cycle technology and current status of research and development." Nuclear Engineering and Technology 47, no. 6 (2015): 647-661.

(11) Rousseau, Ian M., and Michael Driscoll. "Analysis of a High Temperature Supercritical Brayton Cycle for Space Exploration." Massachusetts Institute of Technology. Nuclear Engineering Department (2007).

(12) Clark, Rodney, and Robert Sheldon. "Dusty plasma based fission fragment nuclear reactor." In 41st AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, p. 4460. 2005.

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