The Manhattan Project

Crawford Greenewalt's Interview

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Crawford Greenewalt was an American chemical engineer for the Dupont Company who acted as the liaison between the physicists at the Chicago Met Lab and the company's engineers in Wilmington, Delaware during the Manhattan Project. The challenge was to translate the scientists’ theoretical ideas into workable blueprints for the production of plutonium on a massive scale at the B Reactor being built in Hanford, WA. In this interview, Greenewalt discusses his role as a member of DuPont's review committee, which evaluated the different methods of fissile material production. Greenewalt, who was present at the University of Chicago when the first artificial self-sustaining nuclear reaction was set off, recalls the relatively calm atmosphere in the laboratory that day.
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Crawford Greenewalt: My first contact was to go out to Chicago with a very large group of people and I have forgotten how many there were, perhaps fifteen or twenty as I recall, it may have been less than that. That is surely in the record too, where we were all exposed to this Chicago development.

Stephane Groueff: By whom?

Greenewalt: By the University of Chicago people, Dr. [Arthur Holly] Compton and his associates out there.

Groueff: [Enrico] Fermi?

Greenewalt: Fermi was there yes and many others, [Eugene] Wigner.

Groueff: Wigner.

Greenewalt: This was our first introduction to it; I have forgotten how long it was, two days. I do not remember the time very well, I do not remember the day it was but it seems to me November eighteenth is a little late. I think this must have happened before then.

Groueff: It is November I think it is when Groves came here to Wilmington [Delaware].

Greenewalt: I have a feeling that this visit that the group made to Chicago was before November eighteenth.

Groueff: Right.

Greenewalt: But I am not sure, all I remember is that when I was on this reviewing committee we were in Chicago on Thanksgiving Day.

Groueff: That must have been before.

Greenewalt: Yes. Well there was a group of people that went out to look at this thing to see what it was all about because of course Groves could not give us the technical detail of what was going on. Then I think when Groves came on the eighteenth it was to ask DuPont Company to undertake some part of it. My recollection is that at that time, well I do not know this, because I was not involved, I think the first idea was that we were asked to take care of the chemical separation process alone.

Groueff: And not the construction of the pile?

Greenewalt: I do not think so, I think the first idea was the chemical separation process, and then as time went on they asked us to take on the whole thing. My own involvement started with this group of people that went out to Chicago to hear it. Then my second involvement was that I was appointed on this reviewing committee by General Groves and Dr. Conant. We saw the separation work at Columbia and we revisited Chicago and went to California to see the magnetic separation work.

We came back through Chicago on that trip and that was the famous day of December second, when the first chain reaction was conducted under the West Stands [of Stagg Field]. That was the beginning of it. Then I think sometime after that we were asked to take on the whole thing—that is, the pile and the separation plant, the whole Hanford installation. Here I am really quite vague, but this must be all thoroughly documented, this part of it.

Groueff: From what I found is that you were part of this this reviewing committee of [W.K.] Lewis and a group of DuPont people and your task was to—

Greenewalt: This was not a DuPont committee.

Groueff: No.

Greenewalt: It was a committee appointed by Groves and Conant, with Dr. Lewis of MIT as chairman. There was Roger Williams, Tom Gary and I. There was a fifth member, [Eger] Murphree, who at that time was Vice President of Standard Oil who became ill and could not go. So he did not participate although he had been appointed a member of this committee. This was not a DuPont committee; this was a group of presumably skillful people who were to review these three methods and to report on their general feasibly.

Groueff: How did they look to you at that time?

Greenewalt: Well now, that report is in the record somewhere. I am trusting my recollection as I remember the method that seemed the surest was the—

Groueff: Electromagnetic?

Greenewalt: No, the separation process, the diffusion process. On the other hand it was an exceedingly complex thing but the advantage of it was if you could construct a single diffusion cell the diffusion process is simply a multiplicity of those. While the control would be difficult we felt that that was the most feasible.

The difficulty with the magnetic separation process at that time is that the yields were exceedingly low and when you calculated all the basis of what they had attained at that time, the power requirement and equipment requirement was simply an enormous job, unbelievable.

The plutonium process, our feeling was that that was the least certain in terms of technical success of the three but it had the virtue of not involving an enormous complexity of units as the diffusion process did. If it worked at all it would be more straightforward to build and I think we thought at the time that it would probably be less expensive to build and quicker to build than the diffusion process. Now the way it turned out—

Groueff: Dr. Dunning in Columbia told me that at that time his diffusion was met with great skepticism by all the scientists and he thinks that one of the turning points was your visit there, this committee. He said when you arrived there the attitude was practically one of visit just to be polite and to spend a few hours just as a formality. He had the impression that—

Greenewalt: He said this was our idea?

Groueff: Once you arrived when you saw the results that you all got very excited about the whole thing and he thinks the turning point of the acceptance of diffusion idea was the report of your committee.

Greenewalt: Well that last may be so but we had no preconceived notion at all.

Groueff: No?

Greenewalt: No. We were asked to review these three methods. So far as we were concerned, they were experimental projects that were being carried along simultaneously. We had no preconceived notion at all. Dunning is romancing a little bit.

Groueff: Probably he thinks about the other people and he felt sort of frustrated that his older colleagues like [Harold] Urey and—

Greenewalt: Heavens, Urey was in charge.

Groueff: Urey was in charge, but Dean G.B. Pegram at that time was supporting it but somehow the responsible people were not doing too much.

Greenewalt: That has nothing to do with us. We dealt with Urey who was the leader of the project, Dunning was there of course. But we came to review it as an experimental operation and our job was to do our best to determine whether this was feasible for the construction of a plant.

We concluded it was, although there were serious gaps at that time in their information. For example, this very important question of the barriers for the diffusion process—that had not been solved at that time. But on the other hand they had some experimental work and it was indicative that it could be solved so our view was that this is straightforward. Of course the separation in each diffusion cell would be minute but by simply stringing a great many of them together it could be done. This was no more than distillation or things that you were familiar with. There were of course very great problems; that is a thing as big as this has to have no leaks.

Groueff: But you did not think it was impossible?

Greenewalt: No, not at all.

Groueff: What was the recommendation of your committee?

Greenewalt: We recommended that both plutonium and the diffusion process be pursued with all possible dispatch.

Groueff: And the electromagnetic?

Greenwalt: Well I think our recommendation there, which was not accepted by the way, was that it be left as it is until [Ernest] Lawrence could demonstrate greater yields that he at that time could.

Groueff: I see. Which pile did you review at that time? Was it heavy water or—?

Greenwalt: At that time the one on which the most work had been done was the graphite reactor. But, we were told about the possibilities of the heavy water moderation. I think one of our recommendations was that the [00:45:00] development of methods for recovering heavy water be pressed also.

At the moment there just was not enough heavy water in existence to moderate a unit even if you wanted to do it that way. We thought that was potentially good and that work should go forward on developing ways of recovering heavy water in larger quantities. The only reactor that had actually been developed and put before us was the graphite reactor.

At that time there were two methods of cooling being proposed. One was the light water cooling and the other was helium cooling. Well we did not cross that bridge; this was a question of more engineering work that would have to be done to decide which of those two had the greater feasibility. The decision to go on with the water-cooled graphite reactor was not made until later and we did not comment, I do not think we did, on the two rectors. Now I do not know whether this report of ours has been declassified or not, it is there, three pages.

Groueff: Mr. Greenwalt, do you remember clearly the morning of the day of December the second when you came back from California?

Greenewalt: Yes.

Groueff: And the whole committee, you went to see Compton, and then what happened, how did you learn first that Fermi is doing the experiment that same day?

Greenewalt: Well Compton told us that morning as I recall that this afternoon they would make an attempt to start the chain reaction. Why he picked me out of the three I do not know but he did.

Groueff: He says because you were the youngest and you will remember the longest.

Greenewalt: Perhaps the most expendable.

Groueff: Were you personally very excited or what was your reaction to that?

Greenewalt: My participation in this has been greatly exaggerated. Compton has a long story. Of course it was an exciting thing because actually I never regard this particular experiment as really critical. That is there are many first experiments that do not work and this does not mean that it cannot be made to work. This is just the first try and it was interesting, but I did not consider it as critical in determining whether or not the project fell flat or whether it went on.

I am quite sure that if it had not worked, if the experiment had failed for some reason or other, they would have found some way of trying again. It never struck me that this was crucial as to whether it worked or whether it did not work. Now the fact of the matter is that it worked and of course it was very exciting to see it go. But the whole atmosphere in that place was one of relative calm.

Groueff: It was not very tense.

Greenewalt: Not overtly so, no.

Groueff: No, weren’t you a little bit worried that some unusual explosion may occur or that the whole of Chicago may disintegrate? This has never been done before and nobody knew—

Greenewalt: Of course there was that chance and there was always that chance, you heard about the Suicide Squad, what they called the Suicide Squad, did they tell you that story?

Groueff: No.

Greenewalt: Well of course there was always the remote chance that the reaction would get out of control. The physicists and Fermi had thought the chance was exceedingly remote, but on the other hand no matter how remote they wanted to take all the precautions they could. The last resort precaution was that they had a platform at the corner of the pile, level with the top of it. As you may know cadmium is a very strong neutron absorber. As a matter of fact the absorbing material was cadmium, a very strong neutron absorber.

These fellows, there were two or three that were on this platform at the end and they had on the platform buckets of strong cadmium nitrate solution. The idea was that if the reaction could not be controlled by the control rods and seemed to be getting away they were to run out on top of the structure and pour the buckets of cadmium nitrate.

Groueff: Physically, by hand?

Greenewalt: Physically, by hand pour it in. This was supposed to stop the reaction, but this was simply the remote precaution. Well, it was not necessary at all and I do not think I had any feeling of fear.

Groueff: You did not?

Greenewalt: No.

Groueff: You were a spectator.

Greenewalt: If I had, I would have said I would not go.

Groueff: When was that? In the afternoon?

Greenewalt: Yes in the afternoon.

Groueff: And how long did it last until you realized it was a success?

Greenewalt: Not very long, I know because we took the train back that afternoon. I would guess a half an hour.

Groueff: And after that you went back with your colleagues.

Greenewalt: I walked back with Compton and collected our colleagues and we went back to Wilmington.

Groueff: Were you very particularly happy or elated?

Greenewalt: Yes I think it is always comforting to know that at least one crucial part of the experiment works, but of course this had to work otherwise the whole idea was no good.

Groueff: I see.

Greenewalt: But on the other hand I think the atmosphere at the time was that this is the first try, the first experiment. Heavens, if you do an experiment in the laboratory you do not simply say that if the first experiment works, we are through. You try again. Well I suppose if I felt anything I thought it will certainly be very fortunate if this works, but if it does not work—

Groueff: You will try again.

Greenewalt: They’ll doubtless try again because after all the question really was, the physicists were perfectly certain that a sustaining chain reaction could be developed, the problem was the purity of the materials. Now they did their best to make the graphite and uranium just as pure as possible but more could be done. So, if it did not work they would have analyzed and found out why and you would have learned something by how close it came to criticality and then presume later they would have done something about it. Our reviewing committee had actually written our report before we got to Chicago.

Groueff: That did not influence you?

Greenewalt: No, as a matter of fact this has been misstated in some of the published accounts. Our report was written and we were ready to turn it in but before we got to Chicago, actually wrote it on the long train trip east.

Groueff: I see, of course this experiment probably psychologically helped a lot.

Greenewalt: Enormously, the point I am really making is a negative one. Had it not worked it would not have spelt the doom of the project. The fact that it did work of course said well at least that hurdle is behind you and now we can go on to something else.

Groueff: After this experiment what was your particular job?

Greenewalt: At that time we had still not undertaken to the DuPont Company. These dates I am not very certain of, but at that time the DuPont Company had made no commitments as to whether they would do this thing or not. That came later and I do not know when it came, not very much later.

Groueff: Do you remember a meeting described in a rather colorful way in one of the books where General Groves said that—

Greenewalt: It was Groves’ book that had most of these meetings.

Groueff: Meeting of the Board of Directors and in front of every member of the Board there was a paper put face down and he made his expose of the situation. He said it was a very important thing for the national defense and he was asked by General Groves who was talking from Secretary [Henry] Stimson and even from the White House and had to vote whether DuPont would be involved or not. Every director voted unanimously without turning the paper to see the detailed information, which Groves probably preferred not to give. Were you a director then?

Greenewalt: Yes I was a director then. There is always a tendency to overdramatize things.

Groueff: Yes.

Greenewalt: On the other hand I think you have to put this in perspective. At our board meetings, the Executive Committee, which is the operating head of the DuPont Company, makes its report of actions taken during the month. There is very rarely any question raised with respect to the Board. At the Board meeting in question here, the question really was should we undertake this matter or should we not. The detail that was face down as you say – I had forgotten this detail, it would not have been very important – simply gave the background of what we were expected to do. Of course this is highly secret, greatly classified and I think most of the directors felt that they would rather not know it; they would rather not be exposed.

Groueff: How much were they told?

Greenewalt: They were told the full story—that is, not the technical details. There is no use; they would not have understood them.

Groueff: Building a weapon—

Greenewalt: We were not asked to build an atomic bomb.

Groueff: To build the material.

Greenewalt: We were asked to produce the material and there were representations from the highest source, which we demanded from President Roosevelt that this was really an essential item in the defense to the United States. With that assurance from the President it became a question of patriotic duty to do it so we did, that is all there was to it.

Groueff: I want to establish to what proportion the odds were against logical success.

Greenewalt: You cannot put it in that way.

Groueff: In the light of the existing information or knowledge, was it something that you would consider then as very difficult, impossible, difficult or just one of those jobs?

Greenewalt: Well, there is no doubt about it; it was an exceedingly difficult undertaking. At that time with the experimental work in the condition that it was there was no assurance that any of these elements would operate. There was no assurance whatever that you could make a reactor run at the high power levels that were required to produce plutonium. There was no assurance that the plutonium, if produced, could be separated. There certainly was no assurance at that time that an atomic bomb could be made, even given the plutonium so that this was simply a large series of gambles. But on the other hand you weigh these things against the security of the United States and if the gamble is a great one you do it nonetheless if there is even a small chance of producing the result.

Groueff: Also wasn’t it against the methods and practices of DuPont Company to start such a major effort without pilot plants, without established or proven methods?

Greenewalt: But we did have pilot plants. We had the installation at Oak Ridge.

Groueff: Was it already built?

Greenewalt: No, but it could be built and it was built and the reactor at Oak Ridge was operated and the separation process was operated at Oak Ridge before the Hanford plant came into being. To the extent that you could, and of course we were very firm on that recognizing the fact that experimentation on a full sized plant is exceedingly difficult, expensive, and time consuming. In taking the job on we were certainly going to build pilot plants wherever we could and whatever time we had and we were successfully. That is we had a large installation at Oak Ridge, which was a pilot plant for the reactor, not very much power as I recall—It was about a thousand kilowatts.

The reactor at Oak Ridge and of course the chemical separation process operated on very small acts of concentrations. To that extent you could check it out. Of course many of the special machines that you had to develop to run this thing remotely could be checked out under those circumstances. It is not as if we did not have a pilot plant, we did. But at the time we took the job on, there was vitally nothing.

Groueff: When General Groves offered you the job, he did not offer you very much in the way of proven theories or experiments.

Greenewalt: No, and as a matter of fact he left it to us to satisfy ourselves. Actually he was not asking us to take this word or the scientific position at the time, but he facilitated our finding out for ourselves.

Groueff: Then say yes or no?

Greenewalt: Yes as a matter of fact I do not think we would have taken the thing on General Groves’ say-so. Of course we had made the initial review when a dozen or more of us went out to Chicago the first time. Then our reviewing committee went again and there were three DuPont people on it so that we would get a second impression. We were relying really on our own review of the technical position and not on General Groves or Dr. Compton or Dr. Conant.

Groueff: Who assigned you to be the DuPont representative?

Greenewalt: I do not know.

Groueff: And you do not know why you were?

Greenewalt: Stein must have done it because I think he knew me better than anybody else.

Groueff: He was the first one who briefed you on the negotiations with Groves and the government?

Greenewalt: I do not think the first time around I was briefed at all. I was simply called in and said, “A group was going to Chicago to see this will, you go?” We went.

When you wrote something it was stamped “Secret” or “Confidential.”  You went through all of this rigmarole about keeping it the safe. Nobody kept any private diaries so far as I know, because it would have been against all the rules.

Groueff: In order to describe at that time, I’d like to ask you a couple of personal questions. I understand that you are very interested in humming birds?

Greenewalt: I wasn’t at that time.

Groueff: Oh, that’s a new hobby?

Greenewalt: Yes, but let’s stick to this, because this was twenty years ago. You see I had no official position until after the DuPont Company agreed to take it over.

Groueff: What was your position at that time?

Greenewalt: I was Technical Director of one of our industrial departments. It’s the department that is now INB. It was then Grasselli and I was Technical Director of that department.

Groueff: As a chemist?

Greenewalt: No. I was Technical Director.

Groueff: But your background is chemical engineering?

Greenewalt: Chemical Engineer. Yes.

Groueff: So it could have been some other director appointed, but it just happened—

Greenewalt: Yes.

Groueff: And after you finished your job with the Reviewing Committee, what did you do after that?

Greenewalt: I went back to Grasselli. The first time I was officially involved was after the DuPont Company agreed to undertake this task. Then of course there was the matter of recruiting throughout the company to get people to staff this work. And it was at that time that I came into it officially, Roger Williams was the head of it. They asked me to do a technical liaison job. My responsibility was to take the information from the scientific effort in Chicago and translate it into terms that our engineering and technical people could use to design and build a plant.

Groueff: So you traveled between Chicago and here? Where did you live at that time, in Wilmington?

Greenewalt: I lived in Wilmington. That was my home, but I used to spend half of each week in Chicago and half here.

Groueff: How did you commute?

Greenewalt: By train.

Groueff: And did you go later to Hanford?

Greenewalt: Yes, only for the startup period.

Groueff: For the operations?

Greenewalt: Yes.

Groueff: But not during the construction?

Greenewalt: No, but I did visit there simply to look. The construction was the engineering department’s job. Mine was a technical job.

Groueff: And your headquarters were always here in Wilmington?

Greenewalt: Yes. I used to go to Hanford and to Oak Ridge when the semi-works pilot plant was underway.

Groueff: DuPont was not operating the semi-plant?

Greenewalt: Literally speaking the pilot plant at Oak Ridge was under the jurisdiction of the University of Chicago, because we felt that inasmuch as they had the responsibility for the scientific work, they should have the responsibility for the pilot plant work. But actually although a representative of the University of Chicago was a Director at Oak Ridge, the thing was very largely staffed with DuPont people.

Unidentified Male: Who was [Martin D.] Whitaker? He was later President of Lehigh. Wasn’t he down at Clinton working?

Greenewalt: Yes, but he was initially in Chicago as a physicist. He was a physicist by training and he was there, and he was selected by the University of Chicago to be director of this plant.

Groueff: But the model for the big plant in Hanford wasn’t built after the semi-works?

Greenewalt: No. The commitment was made as to the kind of reactor we would do before we had any pilot experience. And there were a whole raft of decisions that had to be made.

Groueff: Like with the cooling and those kinds of things?

Greenewalt: The first thing that we had to do was to decide between the methods of cooling, whether we would have a helium-cooled reactor or a water-cooled reactor. This was a decision that was based in part on the scientific information and in part on the engineering problem of building it. These decisions had to be made simply on the basis of the best information you had, but without any pilot plant. We decided on the water-cooled reactor. A similar decision was the kind of separation process one was going to use. There were two or three possibilities.

This was of course where our industrial experience became quite important, because all of us, I particularly, realized that there simply wasn’t the time to work out three or four alternate methods and then decide which was the best. What you had to do was to decide on something that you were sure would work, and then put all of your effort on it. This is something that the people of Chicago found difficult sometimes to understand. But with time pressing and the necessity of getting a plant built just as promptly as possible, you simply had to make the best decision you could as to which process you were going to use and then simply concentrate on that in exclusion of everything else and to hope that you were right.

Groueff: You were the man of DuPont who had to see the different designs in Chicago, and then to bring it back here for a decision? Is that oversimplified?

Greenewalt: That’s oversimplified. I think my problem was to be fully aware of and understand the various possibilities that were being worked on in Chicago. And then at the appropriate time to see to it the decision was taken.

Groueff: The final design that was chosen for the pilot, was that based on Wigner’s group design?

Greenewalt: Wigner’s group had done a simply marvelous presentation of the water-cooled reactor. They had done it extremely well and they had thought of and considered many of the engineering problems. It was an excellent job. So we took that and applied then our own engineering criteria to that along with a similar study for the helium-cooled pile that others had done in Chicago.

We concluded taking everything into account with the scientific information as far as it went, plus the engineering problems involved, that the water-cooled reactor was the one that could be built. So all work on the helium-cooled reactor was stopped, and everything was concentrated on the water-cooled reactor. And the same thing happened a little later on with the separation process.

Groueff: Was Fermi also working on the water-cooled reactor?

Greenewalt: Fermi was concerned solely with the general physics of reactor operations. He was not involved in the engineering studies at all. Wigner’s group, which was really separate from Fermi’s, had taken it upon themselves to make this study of the water-cooled reaction. There was another group that I’ve forgotten.

Groueff: It was separate. They didn’t do that together?

Greenewalt: A separate group that was developing information on the helium-cooled reactor, and Fermi was really supplying the basic experimental information to both groups. But he had no part as I recall in the development of either design.

Groueff: I see. And Dr. Szilard, did he have any part?

Greenewalt: He had a reactor design that he promoted for a while, but it never really was seriously considered. It was to be cooled with a molten metal. My vague feeling is it was bismuth, but I can’t remember now. That’s in the record. 

Groueff: Yeah, but he didn’t work with Wigner’s group?

Greenewalt: No, because Szilard was more or less of a freelance and he played really a very little part in the scientific development.

Groueff: Probably in the beginning with deciding the people?

Greenewalt: Yes that’s right when he was at Chicago, but he played no important part. The key people in the reactor end of it were Fermi and Wigner. And the key person on the separation end of it was Seaborg.

Groueff: Yes. What was the role of Dr. Compton?

Greenewalt: He was the boss.

Groueff: He was involved more in the executive end of it?

Greenewalt: Yes.

Groueff: But he didn’t work in the laboratory with Wigner and Fermi?

Greenewalt: No, but of course he was a Physicist and a Nobel Prize winner and certainly had the understanding to appraise these things. He was the Director.

Groueff: And when you were in Chicago you worked closely with Wigner and his group?

Greenwalt: With all of them.

Groueff: And with Compton also?

Greenewalt:  My job really was to have a feel for the experimental work that was being done and to be sure that it would give us the answers that we needed when we needed them. This is the important thing, because the people in Chicago had not the remotest concept of what was involved in building a plant of this sort. This was a huge thing. The important thing was that the information to enable our engineering department and to design the various elements of it, that that would be produced when it was needed and that it be adequate. There were many, many things.

For example, one of the most troublesome problems actually was a question of shielding. This was a major thing and there were all sorts of ideas on how the shielding was to be done. I remember pushing that one and actually making a number of proposals as to how the shielding should be built, because this came very early in the design. The shields are huge things if you’ve never seen a reactor.

Groueff: Concrete shields?

Greenewalt: No, this wasn’t concrete. Concrete wouldn’t work. It would work but my Lord you would have to have had—

Groueff: Very thick walls?

Greenewalt: Yes, you see there was a dual problem here. One had to shield not only against beta and gamma radiation, but also against neutrons. The shielding against neutrons is an exceedingly difficult thing.

Groueff: And nobody in the world had experience with that of course then? It was brand new.

Greenewalt: They had experience on an exceedingly small scale. To shield against the neutron flux that would come out of the pile was something that nobody knew about. So the problem was so far as the gamma radiation and the beta radiation and the alpha radiation as far as the fission products are concerned, that was just a question of mass. And you could produce that mass any way with lead or with concrete.

Let me show you the difference. In the chemical separation plant all you had to deal with were these various kinds of radiation for the fission products. The shielding there was concrete, which was the cheapest way of getting sufficient mass to protect the people. In the reactor, concrete wouldn’t have done you any good because the neutrons had very high velocities and simply would penetrate any normal shielding material. So they had to be slowed down and then absorbed. And so the problem was, “How do you slow down the neutrons?” You use light atoms like hydrogen.

One proposal was to use the light atom in the form of water. That is a hydrogen in water. We objected strenuously to that because this would have involved millions of feet of leak-proof welding, because the water could not be allowed to escape. So the idea was put forward to put the hydrogen in the form of masonite. Do you know what masonite is?

Groueff: No.

Greenewalt: Masonite is wood in effect. But it’s a form of wood that is compressed to a very high density. It’s a building material that was quite commonly used. The point is that this is primarily cellulose. Cellulose is carbon, hydrogen, and oxygen so that while the hydrogen content was not as high as it was in water. Nonetheless it was there as a solid material with no leak problems.

The shield that we finally came up with was alternate steel plates and masonite. So the role was that the steel would absorb all of the other radiation. The masonite would slow down the neutrons to what they call thermal velocity. And at thermal velocity any number of things will absorb. So you slow them down in the masonite and then they absorb in the iron. In this way you slow down the neutron flux to thermal velocity so that it could be captured in the iron. This problem of how you get your light element in the shield was a very difficult one, and we argued loud and long.

Groueff: Where was that, in Chicago?

Greenewalt: Yeah in Chicago. I remember Wigner was very strong for using water. I was equally strong for not using water, simply because the water would have to be circulated and it would become highly radioactive and so it could leak. If it leaked you’d have radioactive water all over the place. I said that hydrogen has to be in some solid form. Finally after arguing this thing out we settled on this steel masonite combination.

Groueff: Did any particular individual play a role in suggesting the masonite to solve the problem?

Greenewalt: I don’t know that.

Groueff: It was just one of those collective works. What other difficult problems did you have? This morning we talked about the canning of the slugs.

Greenewalt: That was the thing that we never really solved until we were actually at Hanford, and this was very troublesome. We had a technique for doing it. The problem was to make these coatings so perfect and you had thousands of these things to put in cans. The process had to be perfect enough so that you could put a thousand of them into cans and not have to reject more than a small percentage. In the first place uranium is scarce and very expensive and you couldn’t have any imperfections. It was a very big problem and it actually was not solved until we were actually out there. It was solved just in the nick of time.

Groueff: [Walter O.] Mr. Simon told us that this particular problem was solved in a very empirical way by producing a thousand slugs a day and to see which ones would work.

Greenewalt: That’s right.

Groueff: Rather than let the scientists propose to prepare the perfect slug in the laboratory and then copy it.

Greenwalt: This had all been done. All the laboratory work had been done. The problem was a mass production operation with an extraordinary high degree of quality control. The thing that you could do once or twice in the laboratory doesn’t follow at all. When you have workmen doing it routinely in the plant the quality will stay good. This was the problem.

You can get this from Glenn Seaborg, but the other decision that had to be made was what separation process you were going to use. Here again, there were two major ones and a couple of minor ones that were being developed. Each one of these was absorbing a great deal of technical effort. When the time came we simply couldn’t afford to carry these things alone. We must decide. I remember that was another famous day when we got together with Seaborg and the decision was made.

Groueff: In Chicago?

Greenewalt: In Chicago. We would concentrate on this one and just make it work.

Groueff:  Who was making the decisions on the DuPont side, you?

Greenewalt: No. I wouldn’t put it that way. My job was a liaison job. My job was simply to produce the information. Any decisions that we had to make I participated in. Of course we all had notions. There were the operating people. There were the engineers that had the design to do and the procurement to do.

For example take this shielding again. It turned out to be a very real problem to find somebody who could machine these composite blocks of steel of masonite with the necessary precision. You couldn’t have cracks out of which you could get neutron flux. The design had to be made perfect. The procurement had to be done, and all of this took time. So put this all into sequence took the cooperation of a lot of different people. So you won’t find any “God” in this enterprise.

Groueff: Did DuPont and the scientists disagree on some techniques. Who was the immediate superior? Was it Compton or General Groves?

Greenewalt: When it came to what we would put into the plant, so long as it was our responsibility and so long as the thing was technically sound, the decision was ours.

Groueff: I see. To what extent did General Groves participate?

Greenewalt: Scarcely at all.

Groueff: He gave you a carte blanche?

Greenewalt: A carte blanche and off we went.

Groueff: And he kept it?

Greenewalt: Oh yes. As a matter of fact he was extremely helpful. He backed us up and there were times that [Gil] Church can tell you about if he hasn’t already done so where we had to make an issue with the Manpower Commission to get enough workers to do the construction.

Unidentified Male: He was a driving force to help in the procurement.

Greenewalt: Yeah. He backed us up wonderfully and let us alone. We would have our fusses with the Chicago people. The basic difficulty was really based upon ignorance on their part as to what one had to do to build a plant. I remember very well Fermi saying to me, “You are doing this the wrong way. You should go out there and build a reactor and it won’t work. You’ll find out why it won’t work and then you’ll build a second reactor and it will work.” 

I just laughed at him. I said, “Enrico, we haven’t time for this. The first reactor must work if it’s humanly possible to make it work.” But they had no conception of what it took to build one of these things. Their idea was that you could do an experiment on this large scale. And if it failed you did another experiment. We were at the stage with three quarters of a million dollars involved. If we had built that first reactor and it had not worked, I think we might have taken perhaps another year to build a new one. At that time everyone was telling us that the Germans were ahead of us.

There’s one other thing that you ought to get the story of, because this is a very exciting one. [Dale] Babcock’s the fellow, if you haven’t got it from him, and that’s the xenon poisoning.

Groueff: Yes, he wrote a paper on that. I haven’t read it yet. Because I understand the whole work was stopped?

Greenewalt: We thought at one time that the whole thing was done.

Groueff: Why, because of the chain reaction stopped?

Greenewalt: Yeah. We were always fearful of this. Babcock can tell you the story, but we were always worrying down here. What can happen? Can we foresee untoward events and take them into account? We were imbued with the idea that this thing has to work at the beginning. We had no second chance. And so we spent our time worrying about eventualities that could ruin us.

One of the things that was completely unknown was the so called poisoning effect of the fission products. Any one of these many things that are produced in fission could have a very high neutron absorption, and therefore soak up all the neutrons to stop the chain reaction. There was no experimental way of demonstrating, because to do it properly you had to have the commercial flux. You had to operate at two hundred and fifty thousand kilowatts before you knew. 

So the best thing that we could do was to allow room in the reactor to put in more uranium. This was all that we could do about it. But the whole thing was considered very carefully and the possibility was taken into account and we had to decide what can we do if this happens? It did happen. I was there that night.

Groueff: In Hanford?

Greenewalt: Yeah.

Groueff: Could you describe how it happened and your reaction when you learned of it?

Greenewalt: You pulled out the control rods and the power came on. Everything seemed to be working fine. Then you’d pull them out a little more and the thing went up to ten thousand kilowatts and it died. You had the control rods all the way out. Then the whole thing began to decay and there was no more power.

Groueff: At first you didn’t know why?

Greenewalt: We didn’t know why. We had all sorts of ideas. We thought water might have leaked. The first thing we did was to dry it out by blowing hot air through it. That wasn’t it. Then we tried again and determined quite clearly that this must be actual neutron absorption by one of the fission products. John Wheeler actually was out there at the time and Fermi too figured out what it must be. Then we thanked our lucky stars that we had those extra holes.

Groueff: How long did the stoppage last?

Greenewalt: It depends on how fast you came up to power.

Groueff:  No, was it for several days?

Greenewalt: No. It happened right away, as you pull out the rods.

Groueff: No, I mean that you found the cause.

Greenewalt: It was diagnosed in an extraordinarily short period of time. Babcock has that story. That’s one of the most exciting stories in this whole thing.

Groueff: That was one of the lowest points?

Greenewalt: We thought that we were through. And here was a seven hundred and fifty million dollar monument. We thought that we might be through. We didn’t know at the time. Fortunately we had made this provision for extra material. If we hadn’t made that provision we really had to rebuild the whole thing.

Groueff:  So when you put in some extra uranium in it worked?

Greenewalt: Yeah, and we got up to power and there we were.

Groueff: Looking back you were right.

Greenewalt: Babcock has that whole story, because he actually within the last six months or so took the trouble to go back into all of it. He’s got it all there. That was really something.

[Audio had been turned off and then turned back on again in the middle of a sentence].

Greenewalt: By ego I mean that Fermi had complete confidence in his own mental powers. I don’t mean that he lacked ego in that sense, but he didn’t have ego in the disagreeable sense. He did his job and did it extraordinarily well with a great deal of modesty. 

Groueff: Was he an easy and pleasant man?

Greenewalt: When you got to know him. This took time. I don’t know whether you would call it shy or aloof.

Groueff: He wasn’t a typical exuberant Italian fellow?

Greenewalt: No. He was very hard to know. We finally became very good friends, and he paid me a very great compliment once. He said when this was all over he asked me to come to Chicago and join him in the Institute for Nuclear Studies. But this was a long time coming. Not because he was unfriendly, but simply because he was—

Groueff: He was reserved.

Greenewalt: Yes, he was reserved. Awonderful person.

Groueff: How was Wigner?

Greenewalt: You talk to Wigner sometime and I think you’ll see for yourself.

Groueff: I’ve never met him. 

Greenewalt: He’s a curious person.

Groueff: He was more difficult than Fermi to work with?

Greenewalt: Much more difficult to work with, but extraordinarily able. I was delighted to see him get the Nobel Prize last year.

Groueff: And Szilard as you say, he was much more difficult?

Greenewalt: I’ll tell you since you haven’t got that machine going—

Groueff: I have it going, but I’ll stop.