Newsgroups: sci.environment,sci.energy
From: John De Armond
Subject: Neutron embrittlement (was Re: Nuclear Power and Climate Change)
Message-ID: <5lzr5f+@dixie.com>
Date: Thu, 07 Jan 93 20:30:05 GMT

kevin@nuchat.sccsi.com (Kevin Brown) writes:

>What's the current state of university research into nuclear power?  It
>seems to me that a university with a nuclear reactor would be a perfect
>candidate to perform a number of the required experiments needed to get
>the answers to these questions.

There are two problems.  One, many US universities have shut down or
are thinking of shutting down their NucE programs.  Sad.  Second and
more directly applicable to the problem, none of the typical University
research reactors can generate the sustained neutron flux needed for this
kind of testing.  A gross oversimplification is that neutron flux and
power output track.  Some specialized reactors such as DOE's Fast Flux
Test Reactor (FFTF) can and does do the job.

>If we don't know the answers to the questions you raise above, would it
>not be a good idea to get those answers?

The short answer is that neutron enbrittlement is well characterized
and has been for a long time.  I quote from a Westinghouse systems
overview book copyrighted in 1968:

	The effects of neutron irradiation on the physical properties of the
	vessel materials is of primary concern in reactor vessel operation.
	The reactor vessel is provided with specimen capsules, located
	between the thermal shield (heavy metal designed to absorb
	much of the gamma radiation and prevent gamma heating of ex-vessel
	components) and vessel wall opposite the center of the core.
	The capsules contain tensile Charpy V-notch and wedge-opening-loading
	specimens taken from hte reactor vessel shell plates and associated
	weld materials and heat-affected zone.  Dosimeters and thermal
	monitors are included to permit evaluation of the neutron flux
	and temperatures experienced by the specimens.  By comparing
	test data from the specimens removed periodically during refueling
	shutdowns with the unirradiated specimen data provided, the
	effects of neutron irradiation on the material properties can
	be determined.

This describes the system installed in all Westinghouse PWRs.  Other
reactor brands use similar systems.  Note that these capsules are
located next to the core where they receive significantly more irradiation
than the reactor pot.  It is from these very specimens that it was
extrapolated that neutron embrittlement MIGHT be a problem toward the
design life of the pot.  Several steps have been taken to mitigate the
problem.  But first a little review of what the problem really is.

Steel exhibits a property called the Nil Ductility Transition (NDI).  Below
the transition temperature, steel loses its ductility and becomes brittle.
The temperature varies with allow but is usually fairly low, below
the freezing point of water.  Long term neutron irradiation causes
crystal lattice defects in the steel which raises the NDT temperature.
The concern is that if the NDT temperature rises above that of cooling
water, in the event of an accident, cold cooling water could shock
the reactor while it was below NDT and thus cause it to fracture.
Under no scenario of irradiation is the NDT predicted rise above
approximately ambient temperature.

Several simple steps have been taken to address the problem.  Because
the rise in NDT is directly related to the total integrated fast neutron
flux dose, the first simple step was reduce the enrichment of the fuel
in peripheral fuel channels.  This reduces the generation of fast
neutrons AND provides shielding from fast neutrons in the interior
of the core.  Next, for reactors whose capsules show the possibility
of problems, they have been derated, typically to 95% full power.
Again this markedly reduces the fast flux impinging on the pot.
Lastly and most expensively, the sources of emergency cooling water
now have  heaters installed so that the cooling water is hotter than
any possible NDT temperature.  At Sequoyah in Chattanooga, TN, the
first plant I worked at,  for example, we installed heaters during
construction in the mid 70s.

The other important fact is that neutron embrittlement is completely
reversed by annealing.  The high temperature allows displaced
atoms in the crystal lattice to snap back into place.  If embrittlement
becomes a problem, the reactor can be annealed in place.  The process
will involve adapting the same equipment used to anneal the reactor
after fabrication to a radiation environment.  Not particularly
pleasant for the workers but certainly doable.  Massive literature
on the subject is available.  Reports of work at the FFTF on the
subject are of great interest.  Much work has been done on neutron
embrittlement in relation to hot fusion research because most of the
energy from a hot fusion reaction is postulated to come from fast
neutrons that will heavily irradiate the reactor components.

John