Thread by @ArmsControlWonk

An image by @planet shows the steam plant at North Korea's reprocessing facility, which separates plutonium for nuclear weapons, is operating. *Sometimes* that indicates the plant is separating plutonium, but @DaveSchmerler and I think probably not *this* time.🧵

Why say anything at all if its nothing? There are many analyses that note the times the steam correlated with a reprocessing campaign. It is equally important to document the times steam DOES NOT correlate with reprocessing. Photo-interpretation is about documenting patterns.

Quick refresher: Yongbyon has a reprocessing plant (aka "Radiochemical Laboratory") to separate plutonium from spent fuel unloaded from a reactor. North Korea uses plutonium to make nuclear weapons. The reprocessing plant relies on steam served from a nearby thermal plant.

Why steam? Reprocessing facilities need heat, which at Yongbyon is supplied by steam. If the reprocessing plant is separating plutonium, then the thermal plant is supplying steam. Japan's Tokai reprocessing plant, FYI, also also uses steam.
inis.iaea.org/search/search.…

This is why, when North Korea "disabled" the Yongbyon nuclear complex in 2007 during the Six Party Talks, one of the steps that North Korea took was to "cut two of the four steam lines into the reprocessing facility." No steam, no separating plutonium.
files.ethz.ch/isn/55146/2008…

And this is also why @rafaelmgrossi, last summer, pointed to the operation of the steam plant as evidence that North Korea had reprocessed more spent fuel and, presumably, recovered more plutonium for its stockpile of nuclear weapons.
en.yna.co.kr/view/AEN202103…

Sometimes, however, the steam plant operates for other reasons. In 2018, the steam plant operated for about a month, which the @iaeaorg concluded was related to some operation other than reprocessing, such as waste consolidation.
iaea.org/sites/default/…

What's waste consolidation at a reprocessing facility, you ask? Boy, do I ever have a paper for you.
inis.iaea.org/collection/NCL…

In this case, the crucial difference is duration. It takes several months to reprocess a load of spent fuel. We have a single image of plume on one day. To conclude the plant is producing plutonium again, as it did last summer, require operation of the steam plant over months.

Neither @DaveSchmerler nor I expect the plant to operate for months because North Korea just completed a reprocessing campaign last year. North Korea doesn't have any spent fuel left to separate. The gas-graphite reactor is busy making more, but it won't be ready until next year.

This is, ultimately, a methods thread. To interpret exhaust from the steam plant's stack, we must know both when that exhaust signaled reprocessing and equally all the times it did not. This allows us to observe differences between operations, in this case indicated by duration.

In the past, such an approach was infeasible because Yongbyon might only be imaged every few months. One clear day with exhaust coming out of the stack might be the only picture taken during a reprocessing campaign! Today, though, we have frequent enough imagery to do it right.