Los Alamos self regulating reactors from tens of kilowatts for NASA to several megawatts

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

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