Exploring the regions of deep space beyond Mars means sending probes where solar power isn’t practical. Since the 1960s, NASA has equipped its Apollo missions and unmanned explorers with Radioisotope Thermal Generators (RTGs). These have worked very well, but they run on plutonium 238, which is currently in short supply. Therefore, the Los Alamos National Laboratory is developing a new small nuclear reactor for spacecraft that uses uranium instead of plutonium to power Stirling engines and generate electricity.

Los Alamos sees its reactor as a supplement to RTGs, allowing scarce plutonium to be held in reserve for missions where there’s no substitute. The finished product that will be installed in a spacecraft is a very simple design consisting of a 12-inch (30 cm) high and 4-inch (10 cm) wide hollow reactor core containing a cylinder of enriched uranium with a weight of 50 lbs (23 kg) and diameter of around 4 inches (10 cm).

This is surrounded by a Beryllium reflector with a mechanism at one end for removing and inserting a single rod of boron carbide, which starts and stops the reactor, respectively. The reflector catches escaping neutrons and bounces them back into the core, improving the efficiency of the reaction.

Side view of the Los Alamos reactor

Behind the reactor is a radiation shield, which protects the rest of the spacecraft from the core’s radiation. Heat pipes are used to transfer heat energy from the core to a number of Stirling engines – a closed-loop engine that runs on heat differences. This cools the reactor and powered the Stirling engines, which are used to run a dynamo to generate electricity.

At the Nevada National Security Site’s Device Assembly Facility near Las Vegas, engineers from Los Alamos, the NASA Glenn Research Center and National Security Technologies LLC conducted a Demonstration Using Flattop Fissions (DUFF) experiment that produced 24 watts of electricity using a pair of free-piston Stirling engines. The DUFF is a sort of "lab bench nuclear reactor," similar to experimental setups developed during the Second World War. This DUFF is the first demonstration of a space nuclear reactor to produce electricity in the United States since 1965.

Cutaway view of the Los Alamos reactor

“The nuclear characteristics and thermal power level of the experiment are remarkably similar to our space reactor flight concept,” said Los Alamos engineer David Poston. “The biggest difference between DUFF and a possible flight system is that the Stirling input temperature would need to be hotter to attain the required efficiency and power output needed for space missions.”

Though successful, the Stirling engine system used in the experiment isn't considered enough for practical purposes. For missions beyond Jupiter, much more power is needed. “The heat pipe and Stirling engine used in this test are meant to represent one module that could be used in a space system,” said Marc Gibson of NASA Glenn. “A flight system might use several modules to produce approximately one kilowatt of electricity.”

The Los Alamos video below explains the reactor's operation.

Source: Los Alamos National Laboratory

Update: This article was amended on Nov. 30, 2012, to clarify the functioning of the reactor.