Kilopower, Megapower reactors would revolutionize energy safety and space and military applications

NASA, Department of Energy, Los Alamos National Lab hoste a news conference at noon EST (9 a.m. PST) Thursday, Jan. 18, at the National Atomic Testing Museum in Las Vegas, to present the Kilopower reactors.

The Kilopower reactors are tiny nuclear reactors that have been redesigned for safe power.

The full power runs for the test reactors will be done this month and the next two months.

Before the reactors are started the uranium in them would only cause a person to experience 0.5% of their average yearly exposure in the event of a launch accident.

The reactors including shielding are the size of a wastepaper basket. They currently are testing two sizes (1 kilowatt and 10 kilowatt). They would send five of the ten-kilowatt units on a Mars mission.

There is also work to similar safe modular nuclear reactors with megawatts of power. There is a Megapower design of the Los Alamos reactors. This would use stirling engines for the power conversion. The Megapower design is described at the bottom of this article.

* A compact, low cost, fission reactor for exploration and science, scalable from 1 kW to 10 kW electric
* Novel integration of available U-235 fuel form, passive sodium heat pipes, and flight-ready Stirling convertors
* Would provide about 10x more power than the Multi-Mission Radioisotope Thermoelectric Generator
* Could be scaled up to provide modular option for human exploration missions to the Mars/Lunar surface
* Potentially enables Decadal Survey Planetary Science missions without reliance on limited plutonium dioxide fuel

* Full-scale nuclear system-level test of prototype U-235 reactor core coupled to flight-like Stirling convertors at relevant operating conditions
* Design concepts that show scalability to 10 kWe for Mars surface power

Leverages existing DOE/NNSA nuclear materials, manufacturing capabilities, test facilities, nuclear safety expertise, and DOE/NNSA cofunding

The International Space Station has about 400 kilowatts of solar power but the arrays in total are about the size of a football field. Mars gets one-third of the solar flux of Earth orbit. The night on Mars lasts ove 12 hours.

NASA is pushing forward on testing a key energy source that could literally “empower” human crews on the Mars surface, energizing habitats and running on-the-spot processing equipment to transform Red Planet resources into oxygen, water and fuel.

* A small pill size rod of Boron Carbide will turn on the reactor
* Beryllium oxide reflectors will enhance the nuclear reaction so less uranium is needed
* a 6-inch uranium core is used

Well established physics are used for a design targeted at needing almost no control system to safely generate up to 10 kilowatts of power for many years.

The agency’s Space Technology Mission Directorate (STMD) has provided multi-year funding to the Kilopower project. Testing is due to start in November and go through early next year, with NASA partnering with the Department of Energy’s (DOE) Nevada National Security Site to appraise fission power technologies.

“The reactor technology we are testing could be applicable to multiple NASA missions, and we ultimately hope that this is the first step for fission reactors to create a new paradigm of truly ambitious and inspiring space exploration,” adds David Poston, Los Alamos’ chief reactor designer. “Simplicity is essential to any first-of-a-kind engineering project – not necessarily the simplest design, but finding the simplest path through design, development, fabrication, safety and testing.”

Sun-independent power
The pioneering Kilopower reactor represents a small and simple approach for long-duration, sun-independent electric power for space or extraterrestrial surfaces. Offering prolonged life and reliability, such technology could produce from one to 10 kilowatts of electrical power, continuously for 10 years or more, Mason points out. (The average U.S. household runs on about five kilowatts of power). The prototype power system uses a solid, cast uranium-235 reactor core, about the size of a paper towel roll. Reactor heat is transferred via passive sodium heat pipes, with that heat then converted to electricity by high-efficiency Stirling engines. A Stirling engine uses heat to create pressure forces that move a piston, which is coupled to an alternator to produce electricity, similar in some respects to an automobile engine.

Having a space-rated fission power unit for Mars explorers would be a game changer, Mason adds. No worries about meeting power demands during the night or long, sunlight-reducing dust storms. “It solves those issues and provides a constant supply of power regardless of where you are located on Mars. Fission power could expand the possible landing sites on Mars to include the high northern latitudes, where ice may be present,” he points out.

Power options

NASA has flown a number of missions powered by radioisotope thermoelectric generators (RTGs) over the past five decades, such as onboard the two Viking Mars landers, the Curiosity rover now at work on the Red Planet, the Apollo expeditions to the moon, the two Voyager spacecraft, and the New Horizons probe to Pluto and beyond, as well as the just-concluded Cassini mission at Saturn. RTGs produce electricity passively with no moving parts, using the heat from the natural decay of their radioisotope heat source.

“What we are striving to do is give space missions an option beyond RTGs, which generally provide a couple hundred watts or so,” Mason says. “The big difference between all the great things we’ve done on Mars, and what we would need to do for a human mission to that planet, is power. This new technology could provide kilowatts and can eventually be evolved to provide hundreds of kilowatts, or even megawatts of power. We call it the Kilopower project because it gives us a near-term option to provide kilowatts for missions that previously were constrained to use less. But first things first, and our test program is the way to get started.”

The novel energy-providing technology also makes possible a modular option for human exploration of Mars. Small enough in size, multiple units could be delivered on a single Mars lander and operated independently for human surface missions.

Breadboard test
In step-wise fashion, with safety as a guiding principle, Mason says the Kilopower hardware will undergo a full-power test lasting some 28 hours.

Moving the power system from ground-testing into a space system is an achievable objective, says Don Palac, Kilopower project manager.

Lead Researcher Marc Gibson adds, “The upcoming Nevada testing will answer a lot of technical questions to prove out the feasibility of this technology, with the goal of moving it to a Technology Readiness Level of 5. It’s a breadboard test in a vacuum environment, operating the equipment at the relevant conditions.”

Looking into the future, Mason suggests that the technology would be ideal for furthering lunar exploration objectives too. “The technology doesn’t care. Moon and or Mars, this power system is agnostic to those environments.”

Los Alamos is working with NASA on nuclear fission systems (Kilopower and MegaPower) as a heat source that transfers heat via a heat pipe to a small Stirling engine-based power convertor to produce electricity from uranium. NASA has focused on the use of KiloPower for potential Mars human exploration. NASA has examined the need for power on Mars and determined that approximately 40 kilowatts would be needed. Five 10-kilowatt KiloPower reactors (four main reactors plus one spare) could solve this power requirement.

During steady state, a reactor operates with a neutron multiplication factor of ‘1.000’; that is, the number of neutrons in the core remains unchanged from one generation to the next generation.

Almost every perturbation in a reactor’s operation ultimately translates into either a positive or a negative reactivity insertion incident, defined as the state in which the core neutron multiplication factor deviates from its steady state value. Sudden and significant positive reactivity insertion can lead to runaway reactor kinetics, wherein temperatures can exceed thermal limits very rapidly.

Past development approaches relied on sophisticated control systems to reduce or eliminate such a likelihood. Luckily, reactors also have an inherent ability to self-correct via negative temperature reactivity feedback; reactor power automatically decreases as core temperature increases, and vice versa.

It has been known that strongly reflected small compact fast reactors, such as kiloPower, can be designed to maximize these mechanisms to a point of being totally self-regulating. the Los Alamos objective is to design-in self-regulation as the front-line feature in order to minimize technical and programmatic risk and to demonstrate via testing that self-regulation is both reliable and repeatable.

A scaled up 2 megawatt system would be expected to weigh about 35 metric tons. It would transportable by air and highway.

To that end, multi-scale and multi-physics simulations are relied upon to perform high fidelity design studies that explicitly examined
(a) how choices related to fabrication, alloying and bonding techniques would affect the internal crystalline structure of each nuclear component and in turn
(b) how that morphology affects that components thermal, mechanical and nuclear performance at conditions of interest.

Rapid prototyping and engineering demonstration

A key objective of the affordable strategy is that the nuclear components can be fabricated to the exacting tolerances demanded by the designers. This includes not only the physical dimensions, but also density and crystalline phase of the alloys.

The materials’ characteristics determine thermal and mechanical performance of the core, which in turn affects its nuclear performance. After several joint efforts, an exact replica of the kiloPower core was fabricated at Y-12 with depleted uranium. This provided needed experience and data on casting, machining and material characteristics of the reactor core.
The second phase involved engineering demonstrations where the DU core is assembled together with the rest of the system (including the heat pipes and Stirling engines) in the configuration needed for a flight space reactor. Finely controlled resistance heaters were used to closely mimic the nuclear heat profile that is expected in the nuclear core during regular operation.

Los Alamos National Laboratory, in partnership with NASA Research Centers and other DOE National Labs, is developing and rapidly maturing a suite of very small fission power sources to meet power needs that range from hundreds of Watts-electric (We) to 100 kWe.

These designs, commonly referred to as kiloPower reactors, are based on well-established physics that simultaneously simplifies reactor controls necessary to operate the plant and incorporates inherent safety features that guard against consequences of launch accidents and operational transients.

Full-scale nuclear test

The nuclear demonstration test will occur in late summer or early fall of 2017. The test will be conducted at the Device Assembly Facility at the Nevada National Security Site (NNSS).

It will be comprised of a ~32 kilogram enriched uranium reactor core (about the size of a circular oatmeal box) made from uranium metal going critical, and generating heat that will be transported by sodium heat pipes to Stirling engines that will produce electricity.

The test will include connecting heat pipes and Stirling engines enclosed in a vacuum chamber sitting on the top of a critical experiment stand. The critical experiment stand has a lower plate than can be raised and lowered.

On this plate will be stacked rings of Beryllium Oxide (BeO) that form the neutron reflector in the reactor concept. A critical mass is achieved by raising the BeO reflector to generate fission in the reactor core. Once fission has begun, the BeO reflector will be slowly raised to increase the temperature in the system to 800 degrees Centigrade.

The heat pipes will deliver heat from the core to the Stirling engines and allow the system to make ~250 watts of electricity. For the purpose of testing only, two of the eight Stirling engines will make electricity, the others will only discard heat.

The data gained will inform the engineers regarding startup and shutdown of the reactor, how the reactor performs at steady state, how the reactor load follows when Stirling engines are turned on and off and how the system behaves when all cooling is removed. This data will be essential to moving forward with a final design concept.

Lessons learned from the kiloPower development program are being leveraged to develop a Mega Watt class of reactors termed MegaPower reactors. These concepts all contain intrinsic safety features similar to those in kiloPower, including reactor self-regulation, low reactor core power density and the use of heat pipes for reactor core heat removal.

The use of these higher power reactors is for terrestrial applications, such as power in remote locations, or to power larger human planetary colonies.

The MegaPower reactor concept produces approximately two megawatts of electric power. The reactor would be attached to an open air Brayton cycle power conversion system. A Brayton power cycle uses air as the working fluid and as the means of ultimate heat removal.

“MegaPower” reactor patent – Mobile heat pipe cooled fast reactor system US 20160027536 A1

The development costs for more advanced reactor concepts are even less firm. For example, presenters from the LANL cited a FOAK range of $140 million to $325 million for their reactor heat pipe system, MegaPower, with an expectation that the power conversion system could be provided on a loan basis for the initial vSMR development and testing. Considering a $25 million to $50 million range for the power conversion and other process system design development, then advanced reactor FOAK development costs could range from $150 million
to $375 million.

MegaPower cost estimates include:
* Reactor technology development: $85 million to $125 million
* LEU fuel (16 to 19% enriched) depending on DOE fuel supply: $5 million to $45 million
* Development and test facility modifications: $50 million to $100 million
* Transport Security Armor development: $0 to $25 million
* NRC Licensing: $0 to $30 million
* Total estimated costs: $140 to $340 million