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tahanson43206

Moderator

Registered: 2018-04-27

Posts: 19,356

Re: Hybrid nuclear fission fusion technologies

This recent post by Calliban is a good fit for this topic:

http://newmars.com/forums/viewtopic.php … 91#p202591

Chinese progress report

(th)

Mars_B4_Moon

Member

Registered: 2006-03-23

Posts: 9,776

Re: Hybrid nuclear fission fusion technologies

What if Titan Dragonfly had a Fusion Engine?

https://www.universetoday.com/161270/wh … on-engine/

In a little over four years, NASA’s Dragonfly mission will launch into space and begin its long journey towards Titan, Saturn’s largest moon. As part of the New Frontiers program, this quadcopter will explore Titan’s atmosphere, surface, and methane lakes for possible indications of life (aka. biosignatures). This will commence in 2034, with a science phase lasting for three years and three and a half months. The robotic explorer will rely on a nuclear battery – a Multi-Mission Radioisotope Thermal Generator (MMRTG) – to ensure its longevity.

Calliban

Member

From: Northern England, UK

Registered: 2019-08-18

Posts: 3,789

Re: Hybrid nuclear fission fusion technologies

What if Titan Dragonfly had a Fusion Engine?

https://www.universetoday.com/161270/wh … on-engine/

In a little over four years, NASA’s Dragonfly mission will launch into space and begin its long journey towards Titan, Saturn’s largest moon. As part of the New Frontiers program, this quadcopter will explore Titan’s atmosphere, surface, and methane lakes for possible indications of life (aka. biosignatures). This will commence in 2034, with a science phase lasting for three years and three and a half months. The robotic explorer will rely on a nuclear battery – a Multi-Mission Radioisotope Thermal Generator (MMRTG) – to ensure its longevity.

It will be interesting to see how the lattice confinement fusion experiments progress.  This technology could be used to produce very compact fast-fission reactors that could replace RTGs in many applications.

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"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."

Calliban

Member

From: Northern England, UK

Registered: 2019-08-18

Posts: 3,789

Re: Hybrid nuclear fission fusion technologies

Nuclear fusion is no panacea for our energy problems.
https://oilprice.com/Alternative-Energy … tmare.html

D-T fusion is by far the easiest to achieve on a commercial scale.  But 80% of the energy is released as neutrons, which smash into the reactor walls making them brittle and radioactive and creating a radioactive waste disposal problem.  A rogue state could use the abundant fast neutrons that a fusion reactor produces to breed plutonium for bombs.  Fusion reactors are energy hungry, consuming a great deal of power to initiate and sustain plasma heating and containment.  In tokamak fusion, the limitations on magnetic field strength also limit power density far beneath what would be achievable with a fission reactor.

In my opinion, these issues make a strong case for pursuing a hybrid approach to nuclear fusion.  This involves either using fission to provide the heat and/or compression to assist initiating inertial confinement fusion or using a tokamak with a DU blanket, breeding plutonium for downstream fission reactors to consume as fuel.  An IC hybrid fusion machine is particularly interesting.  It is energetically much easier to provide the compression conditions needed to initiate fusion than it is to provide the heating.  A few micrograms of fissile material at the heart of a pellet can provide hotspot heating needed for ignition, whilst external drivers provide compression.  This approach wouod allow construction of compact and powerful space drives needed for fast interplanetary travel.  On Earth, an IC hybrid that is triggered by fission, could function as a very compact powerplant.

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"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."

Calliban

Member

From: Northern England, UK

Registered: 2019-08-18

Posts: 3,789

Re: Hybrid nuclear fission fusion technologies

This figure explains why the fusion-fission hybrid reactor is so attractive as a fissile fuel factory in a world where uranium supply is scarce.
https://www.researchgate.net/figure/Neu … _283847146

The figure shows the number of neutrons resulting from fission of three different heavy actinide atoms.  For a neutron energy of 1E7ev, the number of neutrons released by fission is off the top of the chart.  D-T fusion releases neutrons with energy 1.4E7ev.  If that neutron impacts a plutonium-239 nucleus, the resulting fission will generate several neutrons.  Each of those neutrons may then be absorbed by a 238U nucleus in the blanket region, with each absorbed neutron breeding another 239Pu atom.  This is fissile and can be used to fuel other nuclear reactors.

So a single hybrid reactor, generating 1000MWe and driven by 240MW of fusion power, could produce enough fissile fuel for several additional 1000MWe light water reactors.  If those reactors have a conversion ratio of 0.8, then a 1GWe hybrid can breed enough makeup fuel for 25GWe of lightwater reactor capacity.  This means that every joule of energy generated by fusion is leveraged 100 times.  This is possible because the very high energy neutrons released by fusion can be used to breed a lot of useful plutonium.  It means that the fusion reaction never needs to perform particularly well, because the neutrons it generates do a lot of work.  The fusion part of the reactor never really needs to get beyond breakeven.  That is to say, if we use 1GJ of driver energy to generate 1GJ of fast neutrons, then a hybrid will work well.  With ICF fusion, this could probably be done right now if fusion pellets have small amounts of fissiles in their centres to generate the hotspot needed to ignite the pellet during laser compression.

Generally, the heavier the isotopes impacted by high energy neutrons, the more additional neutrons will be released by fission.  Ideally, we would seperate heavy actinides like americium and curium from spent nuclear fuel and use them to line the reaction chamber of a fusion-fission hybrid.  A 14MeV neutron fissioning a curium atom, would produce up to 10 free neutrons that then enter the blanket assemblies breeding plutonium.  The higher actinides that cause so much worry as long-lived radioactive waste may turn out to be a gift to future generations that will ensure their continued energy abundance.

On Mars, a hybrid fusion-fission reactor would come into its own.  We are going to need a lot of cheap energy to grow the colony and ensure high living standards.  Energy supply also needs to grow quickly as more colonists move to Mars.  But uranium would appear rare on Mars.  It may need to be imported.  We therefore need to get every joule of energy possible out of the uranium.  That requires breeding.  And that breeding needs to be 'fast' in order to grow power supply quickly.  The hybrid is ideal, because of the huge neutron fluxes it can produce.  This can make enough plutonium to expand the energy supply at any rate needed.

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"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."