Editing Nuclear electric rocket (section)

==Concepts==
===Pebble bed reactor combined with gas turbine===
A [[pebble bed reactor]] using high mass-flow gaseous [[nitrogen]] coolant near normal atmospheric pressures is a possible heat source. Power generation could be accomplished with [[gas turbine]] technology, which is well developed. [[Nuclear fuel]] would be highly enriched [[uranium]] encapsulated in low-[[boron]] [[graphite]] balls probably 5–10&nbsp;cm in diameter. The graphite would also moderate the [[neutron]]s of the nuclear reaction.

This style of reactor can be designed to be inherently safe.  As it heats, the graphite expands, separating the fuel and reducing the reactor's criticality.  This property can simplify the operating controls to a single valve throttling the turbine.  When closed, the reactor heats, but produces less power.  When open, the reactor cools, but becomes more critical and produces more power.

The graphite encapsulation simplifies refueling and waste handling. Graphite is mechanically strong, and resists high temperatures.  This reduces the risk of an unplanned release of radioactive elements, including [[fission product]]s. Since this style of reactor produces high power without heavy castings to contain high pressures, it is well suited to power spacecraft.<ref>{{Cite web |last=Wang |first=Chunyun |date=August 31, 2003 |title=Design, Analysis and Optimization of the Power Conversion System for the Modular Pebble Bed Reactor System |url=https://dspace.mit.edu/bitstream/handle/1721.1/17026/54495585-MIT.pdf?sequence=2&isAllowed=y}}</ref>

===Novel electric propulsion concepts===
A variety of electric propulsion technologies have been proposed for use with high power nuclear electrical generation systems, including [[VASIMR]], [[Dual-Stage_4-Grid|DS4G]], and [[pulsed inductive thruster]] (PIT). PIT and VASIMR are unique in their ability to trade between power usage, specific impulse (a measure of efficiency, see [[specific impulse]]) and thrust in-flight. PIT has the additional advantage of not needing conditioned power.{{citation_needed|date=July 2019}}

===Electrical generation===
A number of heat-to-electricity schemes have been proposed. In the near term, [[Rankine cycle]], [[Brayton cycle]], and [[Stirling cycle]] generators go through an intermediate mechanical phase, with attendant energy losses. More exotic technologies have also been proposed: [[thermoelectrics|thermoelectric]] (including [[graphene]]-based thermal power conversion<ref>Technology Review, March 5, 2012: ''[http://www.technologyreview.com/view/427140/graphene-battery-turns-ambient-heat-into-electric-current/ Graphene Battery Turns Ambient Heat Into Electric Current] {{Webarchive|url=https://web.archive.org/web/20151208171726/http://www.technologyreview.com/view/427140/graphene-battery-turns-ambient-heat-into-electric-current/ |date=2015-12-08 }}''</ref><ref>Scientific Reports, Aug. 22, 2012: ''[http://www.nature.com/srep/2013/130311/srep01383/full/srep01383.html Graphene-based photovoltaic cells for near-field thermal energy conversion]''</ref><ref>MIT News, Oct. 7, 2011: ''[http://newsoffice.mit.edu/2011/graphene-thermoelectric-1007 Graphene shows unusual thermoelectric response to light]''</ref>), [[pyroelectricity|pyroelectric]], [[thermophotovoltaic]], [[thermionic converter|thermionic]] and [[magnetohydrodynamic]] type [[thermoelectric materials]].