No fiction, capsule shootout from a fantastic test reactor

Yes, way back during the 1950's a big company was  paid by the feds to engineer a mammoth test reactor.  Not stupid, they smelled a market and cheaply built a miniature test reactor of their own in an exotic area where brainy folks liked to live.  It was miniature only in the sense of core size, but power density and heat flux ultimately exceed that of the larger unit designed for the feds.

So, in the smaller machine they irradiated capsules for dollars.  It was a fantastic machine with a power density exceeding that of the mammoth machine but in a much smaller overall volume.

Its long and narrow pressure vessel was in a much larger deep pool. For shutdown, emergency or otherwise, large valves above the core vented to the pool yielding fast depressurization from several atmospheres. The primary pump kept running until operators turned it off if they felt like it.  Also, it is pertinent that normal flow was downward.

During plant shutdown there was an ingenious arrangement for decay heat removal to the surrounding pool. There were large check valves below the core and large solenoid valves above the core that so that with the system shut down and depressurized and the primary pump turned off, there was upward flow thorough the core by thermal convection. It was a neat, cheap and effective arrangement.

So, think about the following: The miniature test reactor ran at full power for several weeks when an automatic shutdown was initiated for some reason.  Simultaneously a lot of stuff happened; poison rods dropped into the core to insure shutdown, the upper solenoid valves opened and depressurized the system, and more stuff took place.  The primary pump was turned off.  The lower

Now, it is not easy to imagine everything, but it was late at night or early morning, and human beings can do strange things. The game was to get the machine started up very soon because there were factors that could prevent startup for many hours if it was not promptly executed. So, with the primary pump still turned off, the game was to pressurize the system, turn the pump on, and then return to full power.  Well, there were problems of sorts.  Thus the system was pressurized for several minutes and then depressurized.  This happened many times before the primary pump was turned on and the reactor was stated up.

A week or so later the reactor was shut down for normal core changes.  Several capsules was found above the core.  Imagine why that happened.

There is more fiction.  There were battles among gangs about what to do about capsule shootout.  One gang felt that the feds should immediately be informed.  One non-involved sect leaked hints to the feds but their story was garbled, and besides, the feds did not want to hear stuff like that because their inspectors always knew what was going on.

Some insisted that the capsules should be locked in place with an upper fitting.  That proposal led to fears that changes would have to be cleared with the feds who would ask why. And maybe the feds would ask about reactivity insertion via shot out capsules; like how many and how fast and maybe even how destructive and more.

One manager with substantially more authority than brains arranged for a viewing port on the top head of the reactor vessel and movies were taken while the primary pump was turned on and off.  Of course, that revealed nothing.  Little was written on the operating log.  If the fed inspectors ever knew about this, they did not dig into it.

Time seems to heal a lot.  So, after many months design changes led to the capsules being locked in place.  The feds were conveniently disinterested.  

Nobody ever suggested that the system should never be pressurized for extended times without the primary pump in operation.  Somebody asked, "Even with the capsules locked in place, what would happen if the system was suddenly depressurized after an hour or more of pressurization without primary flow?"  That was an inconvenient question that was not discussed. The feds never came close to considering such possibilities. I may extend this fiction to considering that, but I need a rest.

So, I'm not rested, but I am impatient. 

Consider the heat transfer and temperature buildups in the arrangement that I have partially described.  Following operation at power the unit shuts down.  It is possible to maintain the unit at a pressure of several atmospheres with the primary pump turned off; and this is what happened.  The decay heat at the time of shutdown is substantial and rapidity becomes less but still averages more than one megawatt over the next hour.  

The flow coasts down and is zero in less than 10 seconds.  After all, this is fiction so I need not perform even approximate calculations. The water within the narrow channels reaches the saturation temperature in less than one minute and then becomes steam.  I don't feel like doing a lot of work, so I will suppose that the fuel plates never get to 1100 degrees Fahrenheit where they would melt. I won't even suppose that the plates will become weak and deform substantially.

Now, even with my most forceful imagination, I cannot conceive of significant natural circulation in the piping arrangement.  Normally, the flow enters the top of the reactor vessel and flows downward.  Flow exits the reactor vessel at its lower end and the piping is arranged so that flow is upward to the top end of the pool.  At that point the piping is directed to the primary heat exchanger and then to the primary cooling pump, etc.  There will be heat transfer from the reactor vessel and its discharge pipes to the surrounding pool, but this will not be sufficient to enable significant natural circulation.

So, what do I imagine is going on?  I imagine that there is burping of steam and water into and out of both ends of the reactor core and that taxes my imagination, but otherwise I get into deformation and even melting and I do not want to imagine that hard.  Top end burping is more easily visualized than lower end (anti-gravity) burping.

Having rested, it really takes a bit less imagination to visualize a shutdown heat flux of 12,000 British Thermal Units per square foot per hour.  Also imagine an assembly of flat plates with spacing of about 0.01 feet that is the cooling channel for water flow.  With no flow, how fast does that water heat up?  That is easy.  A square foot of the channel contains 0.01 times of about 62.5 or so pounds of water,or 0.6 pounds.  Then, it heats up at a rate of 2X12,000/0.6=40,000 degrees Fahrenheit per hour or about  11 per second.  That is pretty fast. (I doubled the heat flux because the narrow channel is heated from two sides.)  

Suppose I pressurize this arrangement to around 10 atmospheres.  Then the water boils at about 350 Fahrenheit where it takes about 870 Btu to convert one pound of water to steam.  Things happen pretty fast.  If I start at 100 F, it takes less than one-half minute to get to boiling. And when I start boiling, it takes only 24,000/0.6X870=46 seconds to boil all of the water in the channel. In about a minute, I have gone from a water-filled channel to a steam filled channel.

Now, I begin to increase the pressure of the system as I add energy.   I'll imagine that I have a surge tank that is pressurized to 10 atmospheres with a  gas space above.  Even so, adding heat from the core will further pressurize this system. 

Well, I will not even think about these possibilities when I license this rig with the feds.  It is their job to know it all and if you don't believe that, ask them.

Anyway, I imagine that I have installed rupture discs set at a bit of overpressure, maybe 10 psi or more. Once in a while those burst during startup evolutions.  At that time the decay heat fluxes will be substantially less than 12,000, but even at 1200, the time to boiling and steaming is less than 15 minutes, so there is not a lot of time for messing around with a bottled up system.  Even at 600 I can have bad times. Even fiction can be troubling.

They say that readers will drop the book if it strays a lot.  Too bad.  Here we go further into this ridiculous stuff.  It turns out that the miniature test reactor became a money-maker.  To make more money it had to operate at a higher power. That meant that it was necessary to talk to the feds.  And there were other problems. There were those rupture disks that were troublesome.  There was the matter of the heat transfer correlation that was a barrier to power level increase.  (An outsider had squeezed onto the payroll and he pointed out an error in that correlation, but it was not acceptable to challenge the corporate expert who was an economic asset in far bigger games.)

However, in time these matters and others were taken care of, and the increase in power level went ahead.  Rupture discs were tolerated. Operation at heat fluxes in the range of
one million British Thermal Units per square foot per hour was now allowed.

Following a very meticulous power ascension game that lasted several months, the reactor resumed commercial operation at the increased power. Operation was not flawless, but profits and bonuses prevailed.

However, the rupture discs continued to delay startups.  Unknown to those who operated the reactor and also to those who pursued analysis of its operating experience, a very secretive high level corporate panel determined that operation of the facility was beyond the level of risks that the corporation ought to tolerate.  The panel's review of the rupture disc events led to a much larger scope of its investigations. Indeed, the reactor was located in an exotic area where brainy folks liked to live and that area was closing in on the site. The panel reported that operation of the reactor should cease.  However, an abrupt action could lead to a whole set of business and public relations problems.

Nature came to the rescue.  Earthquake faults, such as the New Madrid are widespread. Thus it was easy to get the feds to declare that the reactor was very close to the splay of the splay etc. The analysts immediately prepared responses to the orders from the feds.  Much to their surprise, headquarters had no interest in fighting the feds, and the reactor has never again operated.