[ RadSafe ] What is a Travelling Wave Reactor?

[Brennan, Mike (DOH)]

Hi, George. 

I am not disagreeing with any of your points, and I don't claim to be an
expert in this area; just knocking some ideas around.

I don't think we are at the point where some lines of research should be
opposed because they might lead to using fissile material that might be
used in some other type of reactor.  It is entirely possible that what
is learned in researching TWR will improve IFR tech.  Bright people
working on challenging problems is often a good thing.

I think the not having to reprocess fuel might have a bigger energy
savings than we normally consider.  Also, leaving the fission products
in the core for up to 60 years has the non-trivial advantage of having
the decay heat in your reactor, where it is a plus, rather than in the
cooling pool, where it can be a negative.  Also, I can see that you
might get higher burn-up than expected, as there would still be some
fissioning going on in the "depleted" region, as neutrons leaked into
it.  

I would give TWR credit as being breeding technology, as the initial
load-out of fissile material, enough to sustain criticality for (let's
say) a year, can be used to create enough fuel from non-fissile stock to
keep the reactor running for 60 years (or indefinitely, if you design
the system so you can load new fuel in while the system is running, and
the "wave" can cross from the old fuel to the new without dying.
Generation 2.0 TWR)

I also don't see why you couldn't incorporate breeder blankets into the
shielding of a TWR.  There are going to be neutrons leaking, so you
might as well catch them in something useful.

I don't know if this horse will win any races; I am not inclined to bet
on it myself, at this point.  But it might be interesting to watch it
run around in the field for a while.

And if Bill Gates bets one way, I certainly would think hard before
betting the other. 



-----Original Message-----
From: radsafe-bounces at health.phys.iit.edu
[mailto:radsafe-bounces at health.phys.iit.edu] On Behalf Of George
Stanford
Sent: Sunday, March 28, 2010 8:58 PM
To: radsafe at health.phys.iit.edu
Subject: Re: [ RadSafe ] What is a Travelling Wave Reactor?

All:

      From the available info about the TWR, one can make some ball-park
calculations.  Some assumptions are necessary, because better numbers
have not, to my knowledge, been revealed.  If anyone has better info,
please come forward.

Fact 1:  In generating 1 GWe-yr of energy, any nuclear reactor
necessarily fissions about 1 tonne of heavy metal, creating 1 tonne of
fission products.

Fact 2:  The TWR uses metallic fuel and is cooled by liquid sodium.  It
is based on the technology of the IFR  (Integral Fast Reactor),
developed at Argonne National Laboratory in the '80s and '90s.  In
effect, the TWR is a very large IFR (in 
size, not in GWe) that forgoes reprocessing, storing its fission
products in the used part of the core (behind the traveling wave).  This
pushes the disposal problem perhaps 60 or more years into the future,
Unlike the IFR, the TWR 
does not completely burn its fuel, and leaves behind a mixture of
transuranic actinides -- which perhaps eventually could be recycled (not
clear).

Fact 3.  In commercial readiness, the TWR is at least a decade behind
the IFR.

Assumption 1:  A TWR will operate  for the predicted 60 years without
refueling.

      At the end of its life, therefore, it will contain 60 tonnes of
fission products mixed in with 240 tonnes of heavy metal (uranium and
transuranics) (see below).

Assumption 2:  No net breeding.
      Once started, a TWR will presumably create enough fissile material
(Pu-239) to sustain itself throughout its useful life, but no net
breeding potential is claimed.

Assumption 3:  The TWR will achieve a burnup of 25%.
      This is a guess, approximately what might be achieved in an IFR in
a single pass.  (LWRs achieve 4-5%.)

Assumption 4:  The enrichment of the initial critical zone is 20% (i.e.,
it's 20% fissile).
      This too is a guess, based on the 20% enrichment that a normal IFR
needs.

Assumption 5:  The initial fissile loading is 4 tonnes per GWe.
      This is still another guess, based on the approximate fissile
loading of an IFR core.  (An IFR plant also has another 4 tonnes of
fissile in the ex-core inventory, which a TWR does not have.)

      The above facts and assumptions lead to the following conclusions:

1.  The initial core loading will consist of 300 tonnes of heavy metal
(mainly U-238 -- or could be Th-232): 60 tonnes destined to be burned,
plus 240 tonnes that will be left over, unused, after 60 years
(Assumption 3),
     Note:  An IFR core has about 20 tonnes of heavy metal per GWe, and
another 20 tonnes or so in ex-core inventory.

2.  The initial 4 tonnes of fissile could come from three sources.  
	(a) It can consist of excess weapons Pu. (b) It can be Pu
recovered from LWR spent fuel.  Or (c), it can be 20 tonnes of uranium
enriched to 20% U-235.

(a) Weapons Pu.

      The United States has about 85 tonnes of weapons Pu, only part of
which is declared to be "excess" 
(<http://fas.org/sgp/othergov/doe/pu50yb.html>). 
That would be enough to prime about 10 IFRs or 20 TWRs -- a worthwhile
contribution to the longer-term energy supply, but not a major one.

(b)  LWR Spent Fuel.

      The United States is projected to have about 85,000 tonnes of
heavy metal (HM) in commercial spent fuel 
(<http://snipurl.com/v40kv>) by 2020, containing perhaps 680 tonnes of
fissile Pu.  That would be enough fissile to start up 170 TWRs or 85
IFRs.  For talking purposes, suppose either 170 TWRs or 85 IFRs
magically spring into existence 
in 2020, and no more fissile Pu comes from LWRs, and also assume for a
moment that enriched uranium is not available.

      Now IFRs can breed, with a doubling time of less than 15 years,
whereas TWRs do not breed.  In the TWR case, therefore, the nuclear
capacity would remain at 170 GWe from 2020 on,  The IFRs, however, would
catch up in 15 years, reaching 170 GWe by 1035, 340 GWe by 2050, and so
on.

      Fact: Every tonne of fissile invested in a non-breeding reactor is
a tonne of fissile unavailable for use in a reactor type that has growth
potential.

(c)  Enriched uranium.

      When the supply of fissile from LWRs is exhausted, the growth of a
non-breeding TWR fleet is over unless there is some other source of
fissile material -- and then there's no fissile to get a fleet of
breeders going either.  As of 
now, the only other carrier of fissile material is enriched uranium.

      To get the twenty tonnes of  20%-enriched uranium needed to prime
a TWR, one must mine 800 tonnes of natural uranium.  The global uranium
reserves could support a growing TWR fleet for perhaps a century or
more, but that would mean an expanding worldwide enrichment capacity, to
the distress of arms-control advocates -- a capacity that could be
reduced and eliminated much sooner with IFRs.

                                       *     *     *     *

      Postponement of reprocessing or waste disposal is not an obvious
advantage, and brings with it eventually a significant extra
waste-management effort.  The TWR seems to have no significant
capability that is not shared by 
the IFR, and it has a number of inherent disadvantages.  Moreover the
IFR is almost ready for prime time now, whereas the TWR development is
about where the IFR was in 1980.  Yes, there are non-trivial technical
issues.

      Will TerraPower sell enough TWRs to recoup Mr. Gates' investment?
I don't know, of course.  But the TWR's lack of breeding alone makes it
look like a second-best product, even if it can be made to work as hoped
-- one that would 
have no market at all but for official failure to permit the IFR to come
to fruition.

      That's how I see it now.  Comments and better information welcome.

      --  George Stanford
      Reactor physicist, retired

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