TEPCO Rose

[PamW]

36. Spontaneous fission.

Last edited Sat Feb 23, 2013, 08:09 PM - Edit history (1)

RobertEarl stated:
I think I have figured out what good ol' Arnie has been trying to tell us. This: Fukushima reactor plutonium experienced spontaneous fission.

Just so you know; it was NOT a case of spontaneous fission not happening and then all of a sudden it happened and we got an explosion. That is NOT the case.

In any amount of Plutonium that contains Pu-240 and Pu-242; even weapons grade plutonium; spontaneous fission is happening ALL the time.

In any macroscopic amount of Pu-240 / Pu-242; there are huge numbers of atoms of Pu-240 / Pu-242. So even though the probability per atom is small; since there are a huge number of atoms; there is a constant background of neutrons due to these spontaneous fissions.

In weapons grade plutonium, the amount of Pu-240 / Pu-242 is low enough so that this background of neutrons is low enough. Even though when in a bomb the background neutrons will start multiplying as the bomb assembles, the original level is low enough that the bomb doesn't predetonate or disassemble before assembly is complete.

If you have reactor grade plutonium, the presence of more Pu-240 / Pu-242 means the background level is higher. This means that the amplification of the neutron population needed to get enough energy production to predetonate or disassemble the bomb core is less; because you started with a higher level to begin with. That means it takes less time, and the predetonation can "beat" the assembly and cause the bomb to fizzle out. That's why you can't make a bomb with reactor grade plutonium.

Arnie Gundersen was TOTALLY WRONG in EVERYTHING he said.

What you saw in the Unit-3 explosion was a hydrogen explosion that occurred OUTSIDE the containment building. As a result of that explosion, a bunch of non-radioactive structural material was dispersed. However, that explosion wasn't the source of the radioactive contamination as Gundersen claimed.

However, CTBTO claimed that it saw contamination from a damaged reactor. That is correct. So where did the contamination come from? The Fukushima reactors are BWRs. That means that the Rankine cycle working fluid, the water that turns to steam to drive the turbines goes through the reactor:

http://www.nrc.gov/reading-rm/basic-ref/students/animated-bwr.html

Contrast that with the PWR which has separate water loops for reactor coolant and Rankine working fluid:

http://www.nrc.gov/reading-rm/basic-ref/students/animated-pwr.html

You can see the difference. The reactor coolant in the PWR is that yellow loop which gives up its heat in the heat exchanger / steam generator; but is otherwise sealed off from the Rankine working fluid water that goes through the turbine.

When Fukushima lost its diesel generators due to the tsunami, it lost the power to run the cooling pumps. Therefore, the plant couldn't circulate water and dump heat to the environment benignly. After the quake, the reactor shut down, but due to the radioactivity, there's still heat being produced, and the reactor needs to be cooled. The plant didn't have any offsite power to run the pumps, but it did have those diesel generators which kicked in automatically.

So for the first hour after the earthquake, the pumps circulated water through the reactor, and then through some valves that bypass the turbine ( "turbine bypass valves" ) and sent steam directly to the condenser where it was cooled by that dark blue loop at right. The condenser acts like the heat exchanger in the PWR and allows heat to be transferred from reactor coolant to that dark blue loop at right. Because the two loops are isolated from each other; there's no radioactivity that is discharged by that dark blue loop. Therefore, the plant can cool the reactor without discharging radioactivity, the same as it does when it's at power and turning the turbine.

However, an hour or so after the earthquake, the tsunami hit. No question, the Japanese had a very poor design of the "balance of plant". They had the fuel tanks for those backup diesel generators sitting at dockside above ground for convenience of filling. The diesel generators and their electric switchgear were in the basement which flooded. The fuel tank got swept away by the tsunami water.

So the plant was left without power to run those pumps you see in the diagram.

So in order to cool the plant, the operators purposefully opened valves to vent steam to the environment. Unlike PWRs, where the steam is separate from the reactor coolant; the steam in a BWR is the same water that goes through the reactor. Because the reactor overheated and the tubes that contain the fuel oxidized; radioactive fission products that were created when the fission reaction was going on before the quake, got into the reactor coolant water. The operators were venting the steam made from that water to the atmosphere as a way to cool the reactor. This operation therefore meant that the operators were venting radioactive fission products to the atmosphere, on purpose.

It was the radioactivity from those venting operations that CTBTO captured and analyzed. The explosion really had little, if any; effect on putting radioactivity into the environment. It looks bad; but that's not what contaminated the environment. It was the venting by the operators.

PamW