From: millerj@gn.ecn.purdue.edu (Snide Pucker)
Newsgroups: sci.energy
Subject: Inherently/Passively Safe Nukes (was Re: ZERO Nuclear impact)
Date: 30 Jul 92 17:37:20 GMT
Organization: Purdue University School of Nuclear Engineering

In article <20408@sbsvax.cs.uni-sb.de> dietz@cs.rochester.edu (Paul Dietz) writes:
#>In article <1992Jul29.160427.17369@cerberus.ulaval.ca>, yergeau@cornu.phy.ulaval.ca (Francois Yergeau) writes:
#>
#>|> One such design is not only ready for construction, but in actual
#>|> operation in many places in the world.  It's the CANDU, which uses
#>|> heavy water as both the coolant and the moderator.  This gives it two
#>|> nice advantages:
#>...
#>|> 2) It is "inherently safe" in the sense that accidental loss of coolant
#>|>    automatically results in the reactor stopping through the loss of
#>|>    the moderator.

   As Paul mentions, "big deal."  Any water-cooled reactor has this safety
   mechanism (not labeled, inappropriately by the way, "inherently safe").
   It is the decay heat that is the bane of Light and Heavy Water Reactors;
   it becomes difficult to cool something when you lose your coolant.


#>|> This is my understanding of the expression "inherently safe": you are
#>|> _passively_ protected against a runaway reactor by the fact that
#>|> coolant and moderator are one and the same fluid; you cannot lose one
#>|> without losing the other, and if that happens, the chain reaction stops
#>|> automatically.

   Sorta.  The products of the fission reactions before "shutting down"
   the reactor do a rather good job at maintaining roughly 8% of the 
   original power production.  8% of 3000MWt is quite a bit of power.


#>In a "passively safe" reactor you are protected against many
#>other failure modes as well.  The property you describe above
#>is not unique to CANDU;  all existing light water reactors have
#>the same property.  Nor does that design necessarily protect
#>against overpower accidents; if the control rods are blown out
#>of a reactor (never mind how) on a time scale short compared
#>to the time needed for heat to conduct out of the fuel rods,
#>the fuel rods can damage themselves in the transient.

   Actually, reactors can be (are?) designed to compensate for 
   significant reactivity (overpower) transients.  Water has a rather
   short average neutron lifetime so the reactivity has very little
   real time to build up before feedback mechanisms can kick in.
   Graphite-moderated reactors, with their very long neutron lifetimes,
   must worry about this to a great extent.

   Standard nuke definitions:
     inherently, or passively, safe--requiring no intervention on the
       operators' part to maintain the integrity of the reactor and its
       components during both typical and atypical transient situations
       utilizing passive safety mechanisms 

     passive safety feature--feature relying on natural (i.e. gravity,
       entropy) mechanisms that are automatically engaged when needed 
       Examples: ice storage in contact with the containment walls to
       provide a passive heat sink (provided there is ice at the time),
       negative feedback effect of coolant voiding in water-moderated
       reactors, more on request.


Included because everyone should know this:
#>In any case, the fashionable accident scenario is loss of coolant
#>or loss of circulation, not loss of control of the chain reaction.
#>Afterheat from fission products can melt the fuel just fine, thank
#>you, even if the chain reaction has been stopped.  That's what
#>happened at TMI, you know.
i.e. TMI wasn't a "nuclear" accident but a heat transfer accident


#>For a better example of a passively safe reactor, see either
#>the Integral Fast Reactor (safe against transient overpower and
#>loss of circulation with failure to scram, although not against
#>loss of coolant) or various high temperature helium cooled
#>reactor designs.

   One need not get so exotic for passively safe reactors, although
   those are good examples.  For so-called evolutionary passive
   reactors, try General Electric's AdvancedBWR (or is the passive
   one the SimplifiedBWR?).  Basically, it is _designed_ (key word in the
   nuclear industry nowadays) to be a closed heat source.  Just supply
   the feedwater and get steam.  This is _highly_ simplified (maybe it
   is the SBWR =), but you get the general idea (reference and more
   details on request).

Ever trying to keep the post size down,
Jerry


-- 
---
   Jerry W. Miller                   #   "Opinions?  Of course they
   millerj@gn.ecn.purdue.edu         #    are mine, do YOU want them?"
   Purdue University School of Nuclear Engineering