"cold shutdown" that doesn't require coolant circulation?

mheslep

Interesting. Which comes about from dissolved N2 gas in the water, or some nitrate hanging about?

Hiddencamper

It is an (n,p) reaction:

O16 + n -> N16 + p

The oxygen is from the water in the reactor vessel.

See http://en.wikipedia.org/wiki/Nitrogen

N-16 is the reason we have a 3 foot thick concrete bioshield around BWR heater bays and turbines.

mheslep

Ah of course, I should have seen that.

Continuing, the fuel itself is an oxide. I would think that would create problems, rapidly braking the oxide bonds of the fuel in the conversion of O to N.

Hiddencamper

The fuel pellet is pretty much lost the moment you do your first heatup on the fuel. It's known to expand, crack, and under some very nasty transients or against heat limits, shatter/vaporize. Over time, due to changes in the composition of the fuel pellet itself, and changes in the cladding, your thermal limits become more limiting and your heat transfer rates get reduced. These are all accounted for in both core design and core modelling, and are validated in real time against actual plant data.

nikkkom

There is little tritium in BWRs, since they have almost no deuterium, and produce tritium by other means than D+n->T. Tritium production is only significant in heavy water reactors.

Not only that. A large BWR contains on the order of 50 thousands of individual fuel rods. With such a large number of rods, it's impractical to ensure that absolutely all of them stay watertight. Thus, BWRs are not stopped when tests indicate that just one single rod ruptured and water is now in touch with its fuel ceramic pellets, washing out some fission products.

Hiddencamper

Reactor water chemistry is regularly sampled for the difference between diffusion, and actual leakage/seepage/cracking of the fuel. Once ratios of specific elements like iodine and xenon are seen to go outside of normal, in a BWR you can perform suppression testing. What we've found is if you push control rods in near the suspected leakers, you will see a decrease in radioactive inventory in the reactor coolant system. If you then push in 1 or 2 face adjacent controls rods and possibly a diagonal rod it will greatly suppress the amount of leakage from the leaky bundle, almost returning it to 'normal' levels for the reactor. You can then continue operating the unit, albeit with lost effective full power days.

In a PWR, a fuel leak almost always requires the fuel be removed and replaced. PWRs cannot run with a rod full in to suppress it the way a BWR can.

mheslep

BTW, what happens to the continuously generated hydrogen, the H2 left behind (and the p when it neutralizes)?

rmattila

In a BWR, non-condensible gases end up in the condenser vacuum system, recombiners to recombine most O2 and H2 back to water, then to the off-gas system to be delayed and filtered, and eventually to the atmosphere through the stack.

Hydrogen has a nasty habit of moving with steam in the primary piping and accumulating in places where steam condenses (e.g. inside certain valves), causing fragility issues with certain steel materials.

Hiddencamper

Another note about this is BWRs usually inject hydrogen into their water to help protect the core and vessel from oxidation. This has some unpleasant side effects like increased radiation rates, fouling of venturis and instrument lines, and plating out of materials (could be good or bad), but is all in all beneficial for the plant as it prevents certain types of stress corrosion cracking.