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Re: Reactor Accidents
On May 16 Bill Field asked for information concerning reports by
current local residents of a reactor accident, possibly a meltdown,
that occurred in the early 60's at a reactor located near "Hallum",
Nebraska. His message is attached for reference.
On May 17 Luke McCormick identified the reactor as the Hallam Nuclear
Power Facility located at Hallam, Nebraska and provided information on
the reactor and its current status. His message is also attached for
reference.
There was never any accident at the Hallam Nuclear Power Facility
(HNPF).
Saying that an accident never occurred is very easy. However, trying
to prove that an accident never occurred is very difficult. Trying to
prove a null hypothesis is always difficult, and this is especially
true when you have local residents who are sure that an accident did
indeed occur and that someone probably covered it up. In order to try
to make a logical case out of what occurred it is necessary to go into
quite a bit of boring detail. Without the boring detail, one really
can't make a logical case for why what happened did happen. If I
hadn't been there and observed all the boring detail I'd probably
agree with the natives that there was probably an accident. After
all, the "corpse" is there at Hallam lying under a concrete slab which
is covered over with six feet of dirt. There aren't many ex-nuclear
power plants that can make a claim like that.
The HNPF was part of the old AEC Power Reactor Demonstration Program
which promoted the development of "advanced" design power reactors.
In this context "advanced" is probably best interpreted as a reactor
that did not employ a conventional PWR or BWR design. The
Demonstration Program included the construction of demonstration
nuclear power plants for a number of concepts including integral
superheat light water reactors, organic cooled and moderated reactors,
sodium cooled/graphite moderated reactors, gas cooled reactors, and
other concepts having greater intrinsic thermodynamic efficiency than
conventional BWRs and PWRs, or other unique design or operational
advantages. The Program was initiated during the late 1950s and
lasted into the mid-1960s. Reactors in this program were to be
constructed and started up by the AEC (actually, by the AEC's
contractor, the reactor vendor) but were subsequently to be operated
by the utilities which agreed to participate on a given unit. The
reactor vendor for the HNPF was Atomics International (AI) and, as
Luke McCormick pointed out, the utility was Consumers Public Power
District (now Nebraska Public Power District).
The HNPF was a 75 MWe sodium-cooled-graphite-moderated reactor and was
the demonstration plant for the sodium graphite commercial power plant
concept. The output temperature of the sodium exiting the core was, if
I remember correctly, around 945 degrees F and the delta T across the
core was 345 degrees. The plant had a good thermal efficiency. The
sodium from the reactor passed through a primary loop which exchanged
heat to non radioactive sodium in a secondary loop which was used to
generate steam. The steam generator had a superheater and the steam
produced was of high enough quality that it could be directly fed to a
more or less conventional turbine generator that was also fed by a
conventional coal fired boiler which was a separate power unit at the
site. The HNPF design was based on the experience gained from the
design and operation of an experimental reactor called the Sodium
Reactor Experiment (SRE) which was located at Santa Susana California,
operated from 1957-1964, and produced about 5 MWe for the Southern
California Edison grid.
Construction of the HNPF was completed in 1971 and the reactor was
taken critical in early 1962 and began a leisurely rise to power, with
thorough testing at each step of increase in power. This thorough
testing was due to the plant being the demo plant for the
sodium/graphite concept. The HNPF eventually was designated as being
in "commercial" operation in mid-1963. It had been producing power
for around a year by that point in time. As Luke mentions, the HNPF
was shutdown for inspection in late 1963. During this shutdown it was
verified that some of the moderator cans had developed small leaks.
The presence of the small leaks had been suspected from small
reactivity changes and other test data that had been observed. The
moderator can leaks were the "suspected" failure which Luke McCormick
mentioned.
An HNPF moderator can was a big graphite log encased in a stainless
steel can. When the graphite log was placed in the can, the remaining
air space between the log and the can was evacuated and then backfilled
with helium at significantly less than atmospheric pressure. A
stainless steel lid was then welded on the can. Unfortunately, there
were some stainless steel fingers attached to the can lid which caused
the can to stretch locally a little over the fingers. After the cover
was welded on, this local stretching, in conjunction with the less than
atmospheric pressure in the can, resulted in a local stress increase.
Subsequent thermal cycling caused a classic small stress rupture at the
stress point over the finger.
When a moderator can experienced a small crack as a result of a local
stress rupture nothing much happened. A moderator can was supported
from
the bottom and did not carry the weight of the graphite log and its
surrounding can. The stainless steel in the can was generally subject
to
a low stress except at the localized small areas where it was
stretched
by the lid fingers. Hence, small local stress rupture cracks did not
propagate. The function of the can was to keep sodium from soaking
into
the graphite. Sodium soaking into the graphite reduced the
effectiveness
of the graphite moderator and eventually could cause the graphite to
swell. Significant swelling over a period of time could lead to
mechanical clearance problems. About the only phenomena that were
actually observed at the HNPF were small decreases (not increases) in
reactivity and small temperature changes (in locations where
temperature
instrumentation was present which could sense such changes). There were
no fuel cladding failures and none would have been expected. Also,
there were no accident, or non-accident, transients experienced. In
short, nothing exciting happened. The fact that small cracks had
actually developed in moderator cans could not be verified until after
the reactor was shutdown and the moderator cans were visually inspected
for cracks.
As previously mentioned, the HNPF was the demo plant whose design was
to serve as the basis for commercial sodium graphite nuclear power
plants. Leaking moderator cans obviously were not an allowable design
feature for a successful demo project. Hence, the problem had to be
fixed. After the problem had been identified, the AEC initiated a
program to replace all moderator cans that had developed leaks and to
fix the problem so that no further stress ruptures would occur.
The reactor vendor accomplished the fix for the problem by venting the
area between the graphite moderator log and the stainless steel can to
the helium atmosphere above the sodium pool in the reactor. The
venting was accomplished by installing a snorkel tube on each moderator
can which extended from the top of the can to the helium atmosphere
above the sodium pool. This type of snorkel tube design had been
previously used at the SRE (the sodium graphite experimental plant that
preceded the HNPF). Hence, the SRE had not experienced the stress
rupture cracking problem encountered at the HNPF. The snorkel tubes
installed at the HNPF restored the internal pressure of the moderator
cans to essentially atmospheric pressure thus removing the excess
stress which had been due to the below atmospheric pressure helium
backfill pressure that had previously existed.
For those not familiar with sodium graphite reactor design (which
probably includes almost everybody these days), such a reactor had a
pool of sodium over the core and above the sodium pool was an
atmosphere
of helium at essentially atmospheric pressure. The reactor vessel for
such a reactor operated at a maximum pressure of around only 30 psi or
so (at the botton of the vessel) due to the static hydraulic head of
sodium present. This is, of course, quite different from the case of a
modern PWR with an internal operating pressure of over 2000 psi.
The moderator can restoration program was highly successful and all
leaking moderator cans were replaced and all moderator cans had snorkel
tubes installed to prevent recurrence of the problem in the future.
The reactor was made ready to start back up and resume power
operations. However, as mentioned by Luke McCormick, at that point the
AEC decided to cancel the sodium graphite reactor development program,
and the reactor was subsequently declared as being permanently
shutdown. Active decommissioning was commenced in 1966 and
decommissioning was completed in 1968. Entombment was chosen as the
HNPF decommissioning concept due the unusual design features of the
plant: namely, the below grade location of the concrete reactor vessel
vault and the primary sodium vaults. The HNPF is relatively unique
since it is one of the few reactor plants that used the entombment
decommissioning option.
The sodium graphite power reactor concept was not the only concept
whose
development was abandoned by the AEC in the 1964-1968 time frame. The
old
Power Reactor Demonstration Program was essentially discontinued, and
the
AEC chose to concentrate on fast breeder reactor development. There
were
a number of reasons for this action. One reason was that General
Electric and Westinghouse announced, in the 1963-1964 time frame, that
their LWR power reactor concepts were now fully commercial. About the
only U.S. non-LWR thermal neutron power reactor concept to survive to
the
1970s was the gas-cooled-graphite-moderated reactor concept; and, of
course, it ran into its problems in the 1980s.
In summary, the simple answer to Bill Field's question is that there
was no accident at the Hallam Nuclear Power Facility. Why do local
residents think there was an accident, and presumably a bad accident?
I don't know the answer to that question. I can think of several
reasons why they might have such an idea, but those reasons would be
pure conjecture on my part.
How is it that I know all this ancient history? The explanation is
very simple. I was an employee of the reactor vendor, Atomics
International (AI), and was intimately involved with the HNPF project
from 1961 through 1967. Some of my assignments included training
director for the startup crew, AEC licensed SRO for the HNPF, and
principal investigator for the initial decommissioning studies.
All opinions expressed here and all interpretations of information are
mine alone and should not be attributed to my current employer.
Standard disclaimers apply.
If you have any questions about the HNPF please give me a call. My
number is (708) 252-1562. I still keep in touch with some members
of the HNPF startup crew and the HNPF decommissioning crew. There
is nothing mysterious about the HNPF and its demise. There are
still a few people around who remember it quite well.
--------------------------
Bill Field's Message
Date: Thu, 16 May 96 15:19 CST
From: Field@amrf-po.pmeh.uiowa.edu
Subject: Reactor Accidents
Speaking of obscure reactor accidents. Is anyone familiar with a
reactor accident, "the locals call it a meltdown", that occurred in
the early 60's in the vicinity of Hallum, Nebraska. Residents of
Western Iowa and Eastern Nebraska frequently refer to it.
Thanks for your help....
Bill Field
R. William Field, Ph.D.
Department of Preventive Medicine University of Iowa
bill-field@uiowa.edu
---------------------------
Luke McCormick's Message
Date: Fri, 17 May 96 9:15:32 CDT
From: c0etxlim@mrd42.mrd.usace.army.mil (Luke I. McCormick) Subject:
Re: Reactor Accidents
The Hallam Nuclear Power Facility, built & operated by CPPD
(predecessor to Nebraska Public Power District) and the AEC went
on-line in May 1963, Reached
full operation in July 63. In sept. 63 the reactor was shut down for
inspection
there was a 'suspected failure' (no indication of what it was,
Cladding Failure maybe) In 1965 the fuel elements were removed the
reactor drained and a repair
program implemented. In jan 1966 the facility was ready for start-up.
In Aug 1966, AEC was no longer interested in the experimental program
and directed NPPD to prep for decommissionning. In 1968 the facility
was decommissioned, all fuel was removed from the site, equipment was
salvaged and
decontaminated. SOme components of the reactor system that couldn't
be
moved
were buried in vaults of concrete and the vaults 'sealed below the
earth' The surface above the isolation vaults was weatherproofed and
monitoring
wells sunk.
No problems yet at the facility, except for the giant pheasant. I
hadn't heard
of the meltdown before. I would have worried a little more about the
Atlas missile sites around the area than the reactor.
Luke McCormick c0cetxlim@mrd42.mrd.usace.army.mil
(a 2 hour scooter ride from the site)
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