Things are heating up all over in this week's episode of Manh(a)ttan ("The New World"). Not only is the X10 nuclear reactor in Tennessee firing up, romantic sparks are flying on several fronts, setting up a metaphorical chain reaction among all the convoluted personal relationships that one suspects will ultimately go critical.

Let's get right down to what's going on at the future Oak Ridge National Laboratory, where last week, rival physicists Helen Prins and Charlie Isaacs -- traveling undercover as a married couple -- found common ground in objecting to insufficient safety protocols at the facility. They were over-ruled by the head military contractor, Daniel Ellis, who surged ahead with plans to initiate criticality as scheduled. Now the big day has arrived, and although Charlie is granted the privilege of inserting the final uranium fuel rod to induce a self-sustaining chain reaction, he takes a pass, concerned at bearing any responsibility for the potential disaster that could ensue.

Helen has no such qualms, and quickly volunteers to do so in his stead, figuring, "Someone has to go down in the history books for this, it might as well be a woman." Everyone cheers, and nothing catastrophic happens. Helen tells the meticulous Charlie that he has no sense of humor and really needs to embrace life's unpredictability. "I'll take predictable over reckless any day," he retorts. She reminds him of the first nuclear reactor pile -- the famous "Chicago Pile" initiated on December 2, 1942 -- which Enrico Fermi located under the bleachers of the football stadium, lest the ensuing reaction run out of control and take half of Illinois with it. There weren't any safety regulations then; legend has it that Fermi's backup plan was a couple of graduate students ready to dump a vat of salt cadmium solution on the pile in the event of a runaway chain reaction, since this would absorb enough neutrons to dampen it. The experiment went off without a hitch, however. Instead of disaster, "they changed the world and toasted with martinis."

Helen expects a similar outcome this time. As Ellis announces that criticality has been reached, she suggests to Charlie that they go back to her room for a drink to celebrate. Yes, it's 9 AM and technically this is a dry town, but the resourceful physicist smuggled a few bottles of whiskey in her luggage for the occasion. Why not toast the dawn of the new world?

Oh, Helen. You should know better than to tempt fate. Ominous buzzing alarms begin to sound almost immediately after the criticality announcement. For some mysterious reason, the reactor has shut down. Ellis has his men remove the control rods, and assures Charlie they'll figure out what triggered the shutdown once they sort through all the data. It's just a minor technical glitch, he insists, no call to over-react and cause panic with an evacuation. Except then another alarm goes off, indicating a sharp rise in the core temperature at a rate of 5 degrees each second. Ellis has a cool-down procedure in place, but Charlie points out that this could actually melt the core. Ellis responds -- and I paraphrase -- yeah, yeah, you're a big-shot smarty pants, but this a factory, not a college lecture hall, and we've done just fine managing our reactor without you so far, so shut up and let us handle this.

Fortunately, Helen figures out that something is building up inside the reactor, probably related to one of the chemical byproducts of the fission reaction. The onsite expert on those byproducts? Ellis's assistant, Theodore Sinclair, who is a nuclear physicist reduced to fetching coffee because racism. Charlie and Helen find a bitter Theodore packing his things, having been banned from even being inside the reactor room after trying to help Charlie and Helen before. "You can't leave, the core temperature is 1000 degrees!" Charlie exclaims, to which Theo responds, "Why do you think I'm leaving?" But in the end, that reactor is his baby and they convince him to help.

It all comes down to how a nuclear chain reaction works. On average you want to get more than one neutron out than you put in, running the reactor right at the point of criticality. Yet you don't want the neutron recapture rate to be too high. Helen is right: it's all about the byproducts, like radioactive iodine, which decays over six hours into radioactive xenon (xenon-135). Xenon-135 has a very high neutron capture rate, a unit of area measured in barns, "as in the broadside of a barn," says UCLA physicist David Saltzberg, who provided some technical consultation for this episode. So the reactor stalls out; there aren't enough neutrons being pumped out to sustain the chain reaction, even though, for the first six hours, it runs just fine. The good news is that xenon-135 will eventually decay -- but it takes nine hours. "Imagine you're driving a car, but when you hit the gas it takes six hours before the car starts to accelerate," says Saltzberg. "Then you hit the brakes and it takes nine hours before things start to slow down. This is how you drive a nuclear reactor."

That's how Theo figures out that xenon-135 is the likely culprit: he knows there has to be a chemical byproduct with a neutron recapture rate of 2.2 to 10<6> barns (around two million barns, compared to about 500 barns for uranium), and xenon-135 fits the bill. This rare isotope is poisoning the reaction, recapturing neutrons "faster than we can supply them." As Theo tells Charlie and Helen, "Sometimes the most crucial elements in a reaction are pretty much invisible. Sometimes they're barely allowed in the building." An incident very much like this occurred at the Hanford nuclear facility in Washington State in the 1940s. The reactor died, and Eugene Wigner John Wheeler [correction] ended up figuring out the problem -- in part because Fermi, in a different time zone, was asleep at the time.

The solution is counter-intuitive: taking Theodore's advice, Charlie tells Ellis to add more fuel to the reactor, since it was taking the fuel rods out that caused the temperature to rise in the first place. According to Saltzberg, this is what happened at the Chernobyl facility in 1986: the operators didn't realize that xenon poisoning was the reason they had lost power, so they pulled out the control rods. This actually increased the neutron flux, over-heating the core, and ultimately resulted in one of the worst nuclear accidents in history. (By 1986, frankly, any qualified nuclear operator should have known about xenon poisoning.) That doesn't happen here, because Theodore's solution saves the day.

While all this drama is playing out in Tennessee, Charlie's wife, Abby, is chagrined to learn that the woman traveling with her husband is not only single, but pretty, and a PhD. She unloads to her next-door neighbor and new BFF, Elodie, who assures her she has nothing to worry about: "If your husband prefers hamburger to filet, he deserves an empty stomach." Abby lets slip that she has heard Elodie's love-making with her husband, and is shocked when Elodie cheerfully confesses that she fakes her orgasms: "I only get to the top of that hill climbing alone." She suggests a girl's night out and loans Abby an outfit, since her own are pathetically proper and conservative.

Abby (Rachel Brosnahan) and Elodie (Carole Weyers) might just be More Than BFFs.

They end up at seedy local bar, masquerading as "Anne and Diane," a couple of WACs from the base. One of Abby's dance partners gives her a 90% proof drink, with predictable results. He lures the tipsy Abby into a back hallway, but the watchful Elodie intervenes and takes her home. She is tucking Abby in, with Abby still protesting that she was having fun and isn't the least bit tired, teasingly asking Elodie to dance with her. Elodie leans in and kisses her -- and let's just say that Abby responds with enthusiasm to this introduction to the Sapphic arts.

Her husband Charlie has been bonding with Helen in an Oak Ridge coffee shop, Helen sneaking shots of whiskey into the cups to help him relax after the day's events. He's still not sure they won't charbroil the state of Tennessee before this is over. Helen reveals that she is actually Dutch by birth: her mother is buried in Holland, and her father fled to London when the war broke out. Charlie confesses that his mother lives in St. Louis and is father is an inmate in a state penitentiary -- something he has never even told his wife.

Once the confessions start, they just keep coming. We learn that Helen had become pregnant the year before by a classics professor at Princeton, who gallantly proposed marriage. And she might have accepted, had Frank Winter not offered her the chance to be the only female physicist at Los Alamos. Her beau refused to accompany her, "so I laid down on a piece of metal in a duplex in Teaneck" and terminated her pregnancy. Charlie says it must have been a complicated choice. Her response lays out the dilemma facing any woman physicist of that era: "When the war is over, you'll get tenure anywhere you want. Even Theodore. He'll be fine. Academics choose a black man over a woman every time. I'll fight to get an adjunct job at Podunk Junior College. But I don't give a shit. Because for however long the world lasts, I get to do what I love." Her dream just comes with a very high price tag.

They end up drinking and bonding some more in her hotel room, with flirtatious overtones -- but the revels are interrupted by a soldier calling them back to the facility. Now that the reactor has settled back down, they're bringing it back to critical. The next day, Helen bids farewell to Theodore, who has been allowed back into the reactor room, provided he "remembers his place." Why does he stay? "This is as close as I can get to wherever you're building what we both know you people are building," he admits, and asks her to pass his work on neutron reactions to Frank Winter. "If he found a way to get a woman there, he can find a way to get me there -- no offense." Helen decides to be cool about that, figuring "a rising tide raises all boats."

Romance is blossoming for the Winters' rebellious teenaged daughter, Callie, who is horrified at Liza's insistence that she be involved with some sort of Girl Scout troop on base, requiring her to wear a less-than-flattering uniform. "You look like a leprechaun," Liza teasingly concedes, "But you're our leprechaun." When she appeals to Frank, he admits it sucks, but assures her that once the war is over they'll be back in Princeton as if none of it ever happened. They all just need to get through this stage the best they can. Callie's coping mechanism of the moment: Private Cole Dunlavey, who became smitten with her a couple of episodes ago. They end up sneaking off and sharing a kiss or two, after which they lay back and gaze at the stars and she tells him all about the Greek mythology behind the Taurus constellation.

Meanwhile, Liza is concerned because the maid, Paloma -- i.e., Frank's mistress -- hasn't shown up for work for two days, despite always being punctual. She and Frank drive out to the woman's home to check on her. They learn that Paloma's brother was a soldier, just killed in action in the Pacific.

Frank Winter (John Benjamin Hickey) comforts Paloma (Tailinh Agoyo) on the loss of her brother.

Worse, the military won't return his body -- standard procedure in warfare -- so the tribe can't give him a proper burial according to their Native American tradition. Paloma (through Liza, who has learned a bit of Spanish) asks Frank to escort them onto the base. There is a shrine on those grounds, where the tribe can perform a "releasing rite," if only they had access. Naturally, the military took over the land for the Manhattan Project with no regard for the tribe's religious traditions -- a point driven home by a tribe member named Aniceto (David Midthunder), who quotes Robert Frost:

Before I built a wall I'd ask to know

What I was walling in or walling out,

And to whom I was like to give offence.

After the rite, there is an awkward moment when a grief-stricken Paloma throws her arms around Frank and he comforts her, as Liza looks on in shock. Paloma hugs her too, but Liza is beginning to suspect there is more to Frank's relationship with their maid than he lets on. But not for long -- Paloma quits the very next day, telling Liza she's no longer comfortable working for the Winters. It's unclear just how much Liza knows, as opposed to merely suspects; she's certainly not eager to find a replacement.

It's an awkward morning after for Abby, too, who sleeps until noon before waking up -- alone, no sign of Elodie apart from the rumpled bedclothes -- with a heck of a hangover. Is it me, or is the nanny just a wee bit judgy when she finds the lady of the house still naked in bed in the middle of the day?

Paul Crosley (Harry Lloyd) gets an unexpcted bonus for working late.

Charlie and Helen return to base camp that evening, and when Helen asks whether they're supposed to go back to being sworn enemies now, he dodges the question. He also refuses her offer of more drinks later. Helen takes Charlie's rejection in stride, hefting her suitcases and marching into the Winter lab with her head held high. She pulls out the envelope Theodore gave her, considers a moment, and then tosses it into the trash. Nooo! What happened to a rising tide lifting all ships? I guess Helen has decided to look out for Helen.

Conveniently, Paul Crosley is also there, working late in an ambitious attempt to impress Winter. Noticing the whiskey she pulls out of the case, he admits he could use a drink. She suggests they could just skip the drink. She wants Charlie, but if she can't have him -- well, Crosley is a decent consolation prize. The girl has needs.

Charlie finds Abby in bed, and professes his ardent love for her, unaware of her tryst with Elodie. But he needs to get back to the lab to ponder the implications of the xenon-135 poisoning for Ackley lab. On the drive home, he and Helen had car trouble because the gas station attendant had watered down the gasoline. "Even a Cadillac won't run if the fuel's no good," Helen observes -- and Charlie realizes that Ackley's Thin Man design for the bomb is based on the plutonium produced by the cyclotron in Fermi's Chicago lab. The plutonium being produced in the reactor comes with lots of byproducts, like iodine and xenon -- the equivalent of water in the gas. "What if reactor plutonium isn't pure?" he wonders. Helen shrugs and admits Frank Winter had already accounted for this because he always assumes the worst: "The implosion model could run on crude oil."

And there is the key to how the B-plan will become the A-plan for the Manhattan Project. The reason Thin Man ultimately didn't work came down to the difficulties of scaling up from making small amounts (micrograms) of plutonium with the cyclotron to making larger amounts (kilograms) in a nuclear reactor. Remember that the most common form of uranium is U-238. If you inject an extra neutron, it becomes U-239, a highly unstable isotope that rapidly decays into neptunium, and then decays again into Pu-239, an excellent fissile material for bomb purposes.

Once you get Pu-239, ideally you should remove it right away, but it's a time-consuming process, spanning a couple of months, to get sufficient material for a bomb. While you're waiting, lots of extra neutrons continue to fly around the reactor, and some of them get absorbed by the plutonium. It becomes Pu-240. Pu-240 is fissile, which is good for a bomb, but it undergoes spontaneous fission, which is bad. It happens so quickly there is not enough time to assemble the pieces of the gun model used in the Thin Man design. The chain reaction kicks off too soon.

"It's a linear versus square problem," Saltzberg explains. Pu-239's neutron capture rate is linear with time; that of Pu-240 is exponential with time. The longer Pu-239 sits in the reactor, the more likely it is to absorb extra neutrons and become Pu-240, which absorbs neutrons exponentially. "That's why the small amounts made in the cyclotron were fine, because there were nowhere near these quadratic effects, where twice the exposure makes four times the Pu-240," explains Saltzberg. The larger amounts made in the nuclear reactor necessarily required longer exposure to the hot neutron fluxes, and hence there was more contamination. And it was far too costly to then try to separate Pu-239 and Pu-240, because they are so chemically similar.

Physicist Emilio Segre discovered this when he measured the neutron capture rates for Pu-239 and Pu-240; he realized immediately that Thin Man wouldn't work because the contamination would be too high and the design required pure Pu-239. It's not really a spoiler to say so: while the show's writers might tweak the history and the science for their narrative ends -- Manh(a)ttan is not, nor was ever intended to be, a documentary -- they're probably not going to rewrite history entirely. As Saltzberg put it, this is a world "where the laws of physics still apply. And they [the writers] use real stories for inspiration."

For those who are interested in the actual history of the Manhattan Project, there's an abundance of archival information publicly available, and countless books, from popular to technical levels. Saltzberg recommends three books in particular: The Making of the Atomic Bomb, by Richard Rhodes (which deservedly won a Pulitzer); Critical Assembly, by Lillian Hoddeson (a bit more technical than Rhodes); and Now It Can Be Told, a memoir by General Leslie Groves, the Grand Poobah of the entire Manhattan Project.