APR: your source for nuclear news and analysis since April 16, 2010

Friday, April 1, 2011

Friday evening update / Fukushima Daiichi

Latest JAIF status shows fairly stable temperature has been achieved at No. 1 plant. Other data are essentially unchanged. For the first time, it appears as if we can say that there is nearly a stable situation at the site.. there are no massive emergencies that are deflecting efforts and forcing evacuations as had been occurring daily during the first week plus of the accident sequence, and work is focused on a few very important efforts regarding cooling water and removal of contaminated water from places it shouldn't be.


News has just come out just a short time ago that TEPCO will in fact be getting the "mega barge" that had most recently been used as a massive fishing platform. This barge, measuring 136 meters by 46 meters, can hold 18,000 tons of water. It will first go to a shipyard for some modifications and then be sent to Fukushima Daiichi.

Seawater circulating systems are under repair at the site concurrent with dewatering efforts and freshwater injection efforts. The intention is, and has been, to get the plants off the "feed and bleed" they are on now and use recirculating water systems that can keep the contaminated water contained in the primary system and use the normal heat sink -- the ocean -- to transfer the heat out via an intermediate seawater system.

Many people are seemingly still surprised that the reactor cores could be giving off THAT much heat this long after shutdown. (Of course this "shutdown" term I use assumes shutdown at initial scram and no recriticality due to dislocation / melt of control rods.. but we won't even speculate at that possibility now.) The short story is that there's a lot of heat being generated still, even with the reactors shut down.

Immediately after a full scram, the thermal power output of the core is roughly seven percent of the power at the time of the scram. To use a very standard size of very modern plant that has a reactor thermal power of about 3300 MWt and an electrical output to the grid of about 1000 MWe, that means that the output right after the full scram is still 225 megawatts. Let's take some information from old reliable WASH-1250 that we've quoted so many times before during this evolving story and show some times after shutdown and thermal power outputs for this same theoretical reactor plant size.

t=0 power=225000 kw
t=1 day power = 17400 kw
t=5 days power = 9720 kw
t=15 days power = 5600 kw
t=30 days power = 4060 kw
t=60 days power = 2350 kw
t=120 days power = 1740 kw
t=210 days power = 1100 kw
t=365 days power = 659 kw
t=1097 days power = 204 kw
t=3653 days power = 67 kw

There are a lot of assumptions about the core power history used to create these data. and of course these are a lot higher than you'd find for the Fukushima Daiichi plants which are rated much lower (No. 1 is rated 1380 MWt / 480 MWe, Nos. 2 through 4 are all rated 2381 MWt / 781 MWe) but you will certainly get an idea of the relative heat still being generated even this long after shutdown, and why there is so much focus on core cooling still being made.

Here is an interesting related fact: The irradiated and now radioactive iron in a nuclear plant generates heat from the decay process. According to a large set of books* we have here covering the disposal of decommissioned US submarine reactors, the irradiated structural materials in a typical navy sub reactor plant are giving off roughly 800 watts of heat six months after the last reactor operation. The manual points out that this is about the output of a small hair dryer. So as you can see, even though the effect here is tiny, heat is a common theme in reactor plants .. even ones that are long since shut down.

10:25 PM Eastern 4/1

* Title- Final Environmental Impact Statement - Disposal of Decommissioned, Defueled Naval Submarine Reactor Plants. Volumes 1, 2 (two books) and 3. Part of APRA technical library.

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