Radioactive gases produced by fission, such as vaporized iodine-131 and xenon-135, were controlled in the stainless steel reactor tank by a bellows structure at the top, giving the tightly sealed system room to expand. Helium was kept over the reactor core to prevent air from leaking in and reacting with the sodium. The fission gases were piped off and compressed into holding tanks for controlled release into the environment after having been held long enough for the radioactivity to have decayed away. There were outer space reactors, rocket engines, and military systems being developed at Santa Susana, and all had their considerations of performance, weight, and reliability, but this SRE was to be a prototype civilian power plant. As such, the prevention of harm to the public was a primary and noble design consideration.
All newly designed sub-systems were actively tested at Santa Susana before they were integrated into this new type of power reactor. It was an experimental setup stacked with many unknowns, and there was a lot to learn about graphite-moderated, sodium-cooled reactors. The operation log shows that almost immediately there was trouble with the sodium pumps. Hours after the first power startup on April 25, 1957, the shaft seal on the main primary pump failed. A few weeks later, tetralin was leaking from an auxiliary secondary pump. A week later, the main secondary pump was replaced. In August, the sodium was found to be contaminated with tetralin. In November, the cold trap was clogged with sodium oxide. Air was getting in somewhere. In January 1958, sodium smoke filled the high bay, and men had to go in with oxygen masks to find the leaking bellows valves. In May the shaft seal on an auxiliary primary pump failed, and out of a main primary pump they could smell the strong odor of tetralin. Two months later all the pumps were taken apart and the ball bearings were replaced. The entire main primary pump was replaced with a spare. A month later, the electromagnetic pump clogged with sodium oxide again, and in September the main secondary pump was leaking tetralin. By April 1959 the main primary pump was leaking tetralin from the sodium seal, and the entire unit had to be replaced. A month later, a tetralin fire was extinguished without causing reportable damage.[140]
So, pumping sodium around in a nuclear reactor was not as easy as it seemed on paper, and the SRE was being taken apart and worked on all the time. That is the life of an experimental reactor, and Rickover’s insistence on no sodium in his precious Nautilus seemed to make sense if you were outside looking in. Every time a component from the reactor tank or the primary coolant loop was removed for repair or examination, the sodium frozen to it had to be cleaned off. For this purpose, the pump or the fuel assembly was moved to the wash cell, a special setup behind an atmospherically sealed hot-cell window. Using remote arms, a technician would hose down the sodium coating with warm water. It would instantly turn into hot sodium hydroxide and wash down the drain at the bottom of the stainless steel hot-cell chamber, leaving the once-contaminated piece bright and sparkling clean.
Real trouble did not start until RUN 13, from May 27, 1959 to June 3, 1959. The crew was supposed to run the coolant outlet temperature up to 1,000°F to see if the system could stand to work at higher power. They hoped to log 150 megawatt days. All was well until two days after startup at 11:24 a.m., when the reactor scrammed due to an abnormal sodium flow rate. Not hesitating to contemplate why the flow rate was wrong, the operating crew restarted the reactor immediately and ran it back up to power. There it stayed until 9:00 the next morning, May 30. At that point, the reactor system went squirrelly.
First, the reactor inlet temperature began to rise slowly over three days. On June 1, the temperature difference across the heat exchanger rose sharply, indicating that something wasn’t working. The thermocouple in one fuel assembly, number 67, showed a temperature increase from a normal 860°F to 945°F. The temperature in the graphite abruptly jumped by 30°F, also on May 30, and the thermocouple in fuel assembly number 16 showed a similar increase in temperature. They did not notice it at the time, but the automatic control-rod positioner was compensating for a slow increase in reactivity in the reactor. Obviously, something had occurred that was impairing the coolant flow, and by June 2 the main primary pump casing was reeking of leaking tetralin. The reactor was shut down on June 3 to examine the fuel and repair the coolant pump.
As the pump was being torn down, fuel assembly number 56 was removed using the impressive automatic fuel-removal machine and transferred to the wash cell for examination. To quote the accident report exactly, “During the washing operation a pressure excursion occurred of sufficient magnitude to sever the fuel hanger rod and lift the shield plug out of the wash cell.” Translation: The damned thing exploded and put the wash cell out of commission for a year. Nobody was killed, thanks to the three-foot-thick window and aggressive ventilation.[141]
Retrospective analysis would find that the vent at the bottom of the fuel assembly, where coolant was supposed to flow in and past the hot fuel rods, had been blocked by a substance technically referred to as “black stuff,” and this left a large remainder of sodium in the bottom of the assembly. When the technician aimed the hose into the top of the assembly, the big sodium wad went off like a hand grenade. This was not noted at the time, and number 56 was put back in the reactor. Damage to the wash cell diverted attention from further identification of the black stuff, but a working explanation was that it was residue from tetralin decomposed in the hot coolant. There was not supposed to be any tetralin in the coolant, but three pints of it were found in the cold trap plus a couple of quarts of naphthalene crystals, or tetralin with the extra hydrogens stripped off. No connection in particular was seen between these contaminations and the strange behavior at the end of RUN 13. The troublesome tetralin-cooled seal on one pump, the main primary, was replaced with a nack-cooled sodium seal, and the reactor was ready for RUN 14.[142]
RUN 14 was started on July 12, 1959. The experimenters expected some trouble with the fuel-channel outlet temperature, but they were not sure why. Perhaps if they could intensify the effect, the cause would snap into focus. The reactor was brought smoothly to criticality at 6:50 a.m. At 8:35 they increased the power level to a modest 500 kilowatts, and the graphite temperature started to flop around wildly, running up and down by about 10°F, and various fuel-channel temperatures started to diverge by 200°F. Not seeing this as a problem, they kept going until 11:42 when the reactor, hinting that something was amiss, scrammed due to a loss of sodium flow in the primary loop.
At this point I must find fault with the way they were operating the SRE. Something about the reactor was not working, and yet they kept restarting it without knowing why. Today, this disregard of trouble signs would be unheard of, I hope. You would never restart a reactor without knowing exactly why it scrammed. There would be inquiries, hearings, lost licenses, and firings, but apparently not in 1959, when the screws of bureaucracy weren’t tightened as they seem now. Start her up again and let’s see if that was just a fluke. By 12:15 they had SRE back up to power and were increasing the power and the outlet temperature. At 1.5 megawatts the temperature was fluctuating inexplicably by 30°F.
140
I can’t tell exactly where the fire occurred. It was in the “XX vault,” where X indicates an unreadable character in the SRE operations log summary. There was an NAS vault, a main pipe vault, an aux pipe vault, and a primary sodium fill tank vault. There was no tetralin in any of these locations.
141
The window was three feet thick, but not the glass. Hot cell windows were made of two very thick panes of glass, mounted in parallel and spaced about three feet apart in a sealed tank, very much like an aquarium. The space between the glass windows is filled with zinc bromide dissolved in water. Objects in the hot cell are given a yellow cast by looking through the zinc solution, which is an excellent high-energy gamma-ray shield.
142
The “nack” cooled seal used a mixture of sodium and potassium, or NaK, as the cooling fluid into the sodium seal to keep it frozen solid. Nack is liquid at room temperature, and it flows as freely as water. It was used in the fuel pins to fill the space between the solid uranium fuel pellets and the stainless steel tubes in which they were encased, giving a thermal coupling that would ensure heat flow to the coolant, which flowed around the tubes. The tetralin was a low-viscosity solvent, and it was hard to keep it from finding a way past the sodium seal and into the coolant. The nack would leak less, and if it did, so what? In all the other motorized sodium pumps in the system, tetralin was retained as a coolant.