Archive for June, 2016

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Manhattan Project National Historical Park, Part I: B Reactor

June 29, 2016

In November 2015, the US National Park Service and Department of Energy came to an agreement outlining a new national park, one that would focus on the history of the American effort in World War II to develop nuclear energy for warfare (the “Manhattan Project”).  B Reactor at Hanford is already open with scheduled tours managed under this arrangement.  It was the world’s first plutonium production reactor, designed and built under truly remarkable wartime circumstances, and it operated from 1943 to 1968.

By special arrangement with Colleen French, the DOE’s park coordinator at B Reactor, I was able to visit the reactor at my own pace with a small group of nuclear enthusiasts in March.  Geiger counters, scintillators, and gamma spectrometers also came along, although there was some official resistance to their presence.  And this brings me to the two questions I hoped to answer in visiting this place: firstly, how are the NPS and DOE handling the interpretive challenges inherent in opening a radiation facility to the general public of all ages; and secondly, will hardcore “nukeheads” like me find a sufficiently authentic and engaging experience given the constraints imposed by preparing the site for the public.  My experience at B Reactor was heartening.  The reactor remains an interesting radiation environment (see photo galleries below), and its staff have made rational choices in seeking balance between public safety and respect for the authentic realities of the place.  For nerds with the right instrumentation, the radiation signatures in various parts of the building tell little stories about what happened there.  Reactor equipment has been lovingly left intact throughout–down to the decommissioning tags from 1968.

The radiation signatures at B Reactor were thrilling to me, like little ghosts of the past jumping out to whisper their secrets, but of course, radiation is sometimes feared and loathed.  I empathize with administrators who worry that the crackle of a Geiger counter might repulse or anger some visitors.  My own view is that all kinds of genuine reactions, ranging from enthusiasm to fear, are valid, and all should be tolerated.  Scientifically-informed judgement should guide how safety is established at such sites, but it is still possible to be welcoming and accommodating toward visitors expressing a broad spectrum of reactions, including both the occasional phobia and the occasional super-demanding “nukehead” (e.g., me).  The sites in the nascent Manhattan Project National Historical Park belong to all of us–the enthusiastic and the timid, the plant operators and the “downwinders,” the bombers and the bombed.  Uniting us all is interest in the history, and I am encouraged by the respect for history I witnessed at B Reactor this year.  Best wishes to the other Manhattan Project park sites as they open doors to the public.

Now what you probably came here for: captioned photo galleries!

Reactor operating position and safety systems

B Reactor offers a window into the minds of reactor designers who had never before worked at the power scale envisioned for plutonium production, but who still thought of a surprisingly comprehensive suite of instrumentation, controls, and safety systems, many of which have analogous descendants in modern reactors.  Notable are multiple ranges of power measurement instruments, flux profiling and distribution control in the core, gravity-dropped safety rods, a backup gravity-operated shutdown system in case the core sustained mechanical damage, emergency core cooling tanks in case of a water delivery failure, electrical and hydraulic redundancy in the horizontal control rod system, seismometer SCRAM in case of earthquake or war; and individual fuel channel pressure measurements.

Horizontal control rods

Horizontal control rods were used to regulate the reactor power and adjust the flux distribution in the core.  Some of the rods were hydraulically driven, others electrically driven.  The “inner rod room” lies directly above the control room and is still quite radioactive and off limits (even to me).  This is where withdrawn rods would actually reside after exiting the core.  Their drive mechanisms are on the other side of a heavily-shielded wall, the “outer rod room” (shown in most of these photos).  Radiation is detectable in the outer rod room, and particularly in a floor drain under it.  The radiation here mostly comes from cobalt-60, a product of neutron activation of steel.

Reactor discharge face

Irradiated nuclear fuel slugs would be pushed out the back of B Reactor into a water-filled trough.  This is a truly exciting part of B Reactor, since the radiation levels are bordering on high even today.  The gamma spectra reveal the activation nuclide europium-152, which we know accumulated in the cooling water system (see below) but could also be formed in the pile graphite and shielding concrete; and long-lived fission product cesium-137.  The Cs-137 was formed in fuel and subsequently escaped through ruptures and leaks in the fuel cladding.

Irradiated fuel storage pool

After being irradiated, short-lived radioactivity in the fuel was allowed to decay for several months before chemical processing to recover the plutonium was undertaken (typically, unless one was doing a “green run,” in which case you would process it right away).  In common with the reactor discharge face area, radiation levels in the fuel storage pool at B Reactor remain a little too high for public access.  However, the wooden decking over the pool can be viewed through a window.  The cause for the high residual radioactivity is none other than our old friend, cesium-137, which escaped from damaged fuel.

Above and below the reactor

At the “pile top” we find the gravity-aided vertical safety rod (VSR) mechanisms, as well as hoppers full of boron carbide balls–a last-ditch shutdown feature in case the VSR guide tubes warped from thermal-mechanical damage in the core.  Below the reactor is a small basement (the “Beckman room”) where reactor flux measuring instruments were located.  Today, the basement contains an impressive stash of radioactive tools and fuel handling equipment, probably left in position from shutdown in 1968.

Cooling water systems

B Reactor employed a once-through cooling circuit: water was drawn from the Columbia River, treated, pumped through the reactor’s process tubes, allowed to “cool down,” both thermally and radiologically, in an outdoor basin, and then discharged back into the river.  The discharge water sampling station in B Reactor allowed chemists to monitor their effluent, alerting them to damaged fuel or water treatment problems.  Today, the sampling station remains a bit radioactive, with the rare-earth activation nuclide europium-152 being wholly responsible for the measured gamma radiation there.

Reactor grounds

The back yard of B Reactor has some interesting stuff, like pallets full of channelized reactor graphite and drums full of unused boron carbide shutdown balls.  Several railroad cars used to transport irradiated fuel are now permanently displayed on the grounds, and these are sizzlin’, producing peak gamma exposure readings in excess of 5 mR/h, all due to residual Cs-137.

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2015 Photos from Chernobyl

June 11, 2016

I have been extremely slothful in attending to my blog, and if anyone still reads it, I apologize and thank you for your patience!  I’m attempting to catch up for the last few years in my spare time, posting the content and photos I’ve intended to publish more punctually but somehow haven’t found the time to do yet.  The following images were taken at Chernobyl Nuclear Power Plant in September of 2015 (with a couple from 2013, another trip I somehow managed not to document on my blog).  Amazing progress has been made on the New Safe Confinement.

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Herb Anderson’s “Live Block” of the Chicago Pile

June 4, 2016

 

They don’t give out spent nuclear fuel as a memento anymore.  But on the tenth anniversary of the first nuclear reactor (the Chicago Pile) going critical, pile physicist Herbert L. Anderson was presented with this handsome “live block” of graphite and uranium metal fuel, piping hot and right out of the reactor core.  With an estimated two millicuries of Cs-137 then distinguishing it from the natural uranium whence it was made, the unique artifact spent the next sixty years as part of Anderson’s home decor, a reminder of his pivotal role in one of the 20th century’s greatest triumphs in physics.  Herb’s wife Betsy kindly gave it to me in 2014 with the hope that new understanding and appreciation would follow.

Now, having had nearly two years to get to know this artifact, I can share some preliminary findings about it–and a few lingering questions as well.  I am grateful for ongoing partnerships with the University of Missouri and the Vinca Institute of Nuclear Sciences that are bringing new details to light about its metallurgy and history, and I am grateful for past assistance from the University of New Mexico here in Albuquerque.  I am actively searching for ways to bring this piece of the first reactor to an appreciative public audience.  So, dear reader, if you have suggestions or information that will help with either the technical understanding of the artifact, or its accommodation in a museum for the upcoming 75th anniversary of the Manhattan Project, please get in touch.

Part I: Basic physical description

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This is a “live block” (meaning a piece of graphite with nuclear fuel installed in it), distinguished from the “dead blocks” of pure graphite that were interspersed or used as reflectors in the Chicago Pile.  Several museums possess “dead blocks”; to my knowledge, these include the American Museum of Science and Energy (Oak Ridge), the Bradbury Science Museum (Los Alamos), the National Atomic Testing Museum (Las Vegas), and the National Museum of Nuclear Science and History (Albuquerque).  My friend Kelly Michaels has an excellent photo set of these artifacts.  Pieces of Chicago Pile fuel also survive independently;  most notably, this piece once belonging to Alvin Weinberg.  However, the Herb Anderson “live block” is unique, to my knowledge, in that it contains fuel and moderator together.  The block’s measured dimensions, including fuel dimensions and those of the decorative housing, are available in a SolidWorks model to interested parties (please contact me).

The “T01” lot stamp appearing on the right face of the graphite block indicates that the graphite is AGOT made by the National Carbon Company, one of at least six types of graphite used to build the Pile.  AGOT had the lowest neutron absorption of all of these types, so was preferred for the pile’s core region.  About 2/3 of the CP-1 pile consisted of AGOT.  This grade of nuclear graphite went on to be used in the Graphite Reactor at Oak Ridge and the plutonium production reactors at Hanford.

The fuel is unclad uranium metal in cylindrical elements that bear identifying stamp marks on the front faces.  When I replaced the original cracked acrylic housing around the artifact, I was able to weigh the fuel elements directly.  The left element weighed 2.564 kg, and the right one, 2.553 kg.  The left element stamp reads “M230/L101/P2” while the right one reads “M170/L79/P1”.  The significance of these marks remains unknown to me.  I believe that if someone is able to assist in their interpretation, we might learn which of the three recorded contributing manufacturers of U metal produced this fuel.  It should be noted that metal fuel was a small minority of the Chicago Pile fuel, amounting to just 5.4 metric tons; the vast majority of the fuel was pressed-oxide “pseudosphere” elements.  Metal was made variously by Westinghouse, Metal Hydrides Corp., or the Ames Process.

Another question raised by this artifact is that it contains cylindrical metal fuel placed into chamfered recesses in the graphite designed for receiving “pseudosphere” oxide fuel.  As such, the cylinders cannot remain centered or upright in the recesses without the assistance of some acrylic supports that may be seen in the x-ray image.  I am quite sure that acrylic was not part of the original pile construction!  One is tempted to question, then, whether this fuel-and-stringer combination is original.  It could be that most graphite live blocks were machined for pseudosphere fuel, but when metal became available, the pseudosphere live blocks were used anyway (perhaps with graphite inserts serving the mechanical function of the acrylic supports, which begs the question of why the artifact contains acrylic instead; or perhaps without any supports, the fuel cylinders simply being dropped awkwardly into the recesses).  A lack of detailed photos from the construction of CP-1 makes the question hard to answer.

Part II: Gamma spectrometric estimate of fuel burnup

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Mentioned earlier is the fact that this fuel contains cesium-137.  In fact, the external radiation signatures are dominated by this long-lived fission product.  Without a doubt, then, the fuel has been significantly exposed to reactor operation.  By comparing count rates in the Pa-234m gamma peaks to that in the Cs-137 peak at 662 keV, we can determine the quantity of Cs-137 remaining in the fuel under the assumption that the Pa-234m is in equilibrium with its U-238 parent.  This will motivate the estimation of fuel burnup range under various assumptions about the artifact’s history.  I performed the requisite experiments with my PGT germanium detector and obtained the spectrum shown above, leading to an estimated activity of 540 microcuries of Cs-137 distributed throughout the total fuel at the time of measurement.  Here are a few historical scenarios and the fuel burnup roughly corresponding to them:

  • The fuel operates in CP-1 only (December 1942-February 1943):  163 kWd/MTU
  • The fuel operates in CP-1 and its reconstruction in the Red Gate Woods (CP-2), and is removed from the operating reactor before being presented to Herb Anderson in November 1952 at the Tenth Anniversary celebration in Chicago: 132 kWd/MTU
  • The fuel was removed from the pile (CP-2) when it was decommissioned in 1954, and somehow was then integrated into the artifact: 127 kWd/MTU

There are challenges with all three potential histories.  The first is very unrealistic, given the known operating conditions of CP-1 in the brief months it was in use.  Intermittently critical, with a peak power of ~200 W achieved on one day only, the burnup in the fuel attested by these calculations is many thousands of times greater than what is possible according to the conventional history of that Pile.  The second scenario is supported by both the burnup calculation (even though I am aware of no formal operating records from CP-2) and the description given by Mrs. Anderson of how Herb got the item, but it leads to two big puzzles, firstly concerning how the fuel was removed from the reactor while the reactor was still in service, as the pile was not designed to be easily disassembled in the CP-2 instantiation; and secondly concerning the high activity levels of the discharged fuel when it must have been released from government custody to Anderson.  The third explanation avoids the issue of taking apart the reactor just to obtain a souvenir as the reactor was disassembled during decommissioning; however, it is historically inconsistent with the story told by Mrs. Anderson.  So what this gamma spectrometry measurement allows us to say with certainty is that the fuel was used in CP-2 (as well as the original pile, presumably).  Beyond that, plenty of thought-provoking questions remain.

Part III: Neutron multiplication properties

It would seem there is no greater aspiration for a piece of the world’s first nuclear reactor than to return, momentarily, to the task originally undertaken with so much fanfare: multiplying neutrons in fission chain reactions.  These three photos above show some multichannel-scaling apparatus to look at fission in the CP-1 block (set up in my kitchen, because this is a “cooking” project of sorts).  We are going to examine the time correlation between neutron counts in a bank of two He-3 proportional counters next to our specimen.  Both counter tubes and the specimen are reflected by polyethylene blocks to trap neutrons in the system as best we can.  Highly-correlated counts point to fission “chains”, in which a fission event causatively leads to successive ones on a time scale controlled by the neutron transport properties of the specimen and surroundings.  I’ll measure correlation by way of excess variance, or the Feynman Y-statistic: the difference between the measured variance-to-mean ratio of counts accumulated in a certain time window interval and unity (which corresponds to idealized, uncorrelated, Poisson-distributed counts).  We’ll look at the CP-1 live block by itself and with a small additional neutron source present.  We will also look at the neutron source alone, a lead brick, and the empty polyethylene cavity.  Results and commentary below.

So what the fuck does this mean?  Firstly, the CP-1 block by itself produces strongly time-correlated neutrons (purple data) on a measurement scale of about a millisecond or greater, while the little homemade AmBe neutron source is pretty much stochastic (red data).  (Note, though, that the AmBe source is about five times stronger a neutron source than the block.)  Putting the block in with the AmBe source slightly reduces the neutron count (~12%) versus the source alone, but produces excess correlation of nearly 30% of the block by itself, indicating the presence of induced fission.  The high correlation in the block itself may be attributed to spontaneous fission (SF) as a minor decay mode of U-238, as well as a smaller contribution of spallation and fission induced by secondary cosmic rays.  These neutron sources each produce a burst of neutrons, and are also closely coupled to successive induced fissions.  The AmBe source, by contrast, is driven by radioactive decay: alpha particles slam into beryllium.  Notice the curvature of the data in all cases: it rises as we lengthen the counting window.  That is to say, there is more neutron correlation as the window gets longer.  Neutrons take their time moving through materials, scattering, slowing down, and finally reaching the detector, and neutrons produced in coincidence will not register as such unless the window is long enough to account for their random meanderings through material.  Finally, just to illustrate fission and other fission-like reactions in something other than uranium, I put a 20-pound lead brick in the counter.  Now you may believe that lead is not a fissionable material, but under the right conditions–such as when a 500-MeV electron in the secondary cosmic ray spectrum hits it–the lead nucleus can split up by fission or by a somewhat similar process called spallation, cooking off a distribution of neutrons.  And that is why we see highly-correlated neutrons (green data) being emitted by lead.  Again note the upper right graph, though: lead is a very weak source of neutrons even though the ones that are emitted are highly time-correlated.

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