Japan Earthquake: nuclear plants

marwood

21 microsievrts per hour according to the linked reference ''Tohoku Electric Power said, Onagawa (I long to) from a nuclear power plant (town Onagawa, Miyagi Prefecture Ishinomaki) on-site radiation monitoring system, the reference value of the reporting obligations prescribed by a Nuclear Disaster Special Measures Law about four times the radiation dose (21 microsievert per hour) is detected.''

The alleged source being the Fukushima Daiichi plant located approx 120 kilometers to the south. Given 120 kilometers of diffusion, assumedly by some sort of Brownian motion type process, the Fukushima release must have been very substantial.

turbo

We can't assume that, though. Particulate and aerosol releases can be very directional with well-concentrated plumes.

marwood

NHK TV showing video of smoke rising from Fukushima Daichi No 3
http://www.kcet.org/disasterinjapan/

Reno Deano

Just how do you compare CPM and MicroSV? Answer: you don't with out calibration standards. One is relative response to unknown source and the other is Dose. Sounds like technician talk!

rogerl

Guys. Supposed the fuel rods melted and become molten, do fission still occur in molten state or not anymore?

PietKuip

Immediately after the quake, the control rods were automatically inserted, and the fission chain reaction stopped. The heat production now is due to decay of the radioactive fission products, and is only about 10 % of the power when the reactor is on.

rogerl

I know. But supposed the controls rods were not inserted and the fuel rods melt and become molten and there is a tough cement underneath that prevent further falling underneath the plant. Would the uranium still fission when it's already in the molten state or would fission only occur when the uranium is solid?

Borek

For fission it doesn't matter whether the fuel is solid or melted.

Astronuc

The fission reaction would have eventually stopped but more slowly since the core configuration would have been disrupted. During operation of a BWR, some control rods are in the core for control of reactivity, i.e., to keep the core critical at steady state. The control rods are gradually withdraw during the cycle as the enriched uranium is depeleted (consumed) in a carefully devised sequence such that the core is always just near critical, otherwise the power is slowing increasing or decreasing depending on the need for power increase or decrease. Toward end of cycle, when much of the fissile material has been used, the control rods are all out (all rods out, ARO). The reactor can continue for some days afterward, and then it is shutdown for refueling and maintenance.

If the fuel melted it would displace the water and reduce the moderation, which would make the core slightly subcritical at some point in time, and the fission reaction would decrease.

However, we know that the control rods did insert in the Fukushima reactors, and the fission reactions ceased. The problem is that fission products continue to decay after the fission reaction is stopped, and that heat must be removed from the core/fuel following shutdown. The heat removal did occur for sometime after shutdown, first using power from emergency diesel generators, and then on batteries. However, at some point the cooling capacity was reduced or lost and some part of the core overheated.

Subsequently, Units 1 and 3 were flooded with seawater to maintain the cooling of the reactors.

From the continuity of matter property, what goes into a system, must come out elsewhere if mass in the system is constant. So, if some seawater goes in, it must come out somewhere as water or steam. It is the steam that is occasionally vented to keep the pressure down. However, this means that the system is no longer closed to the environment, and some fission products, mostly gases Xe and Kr, and perhaps some volatiles, e.g., I, will escape to the atmosphere.

The objective now is to cool the reactor core and minimize the release of radioactive substances to the environment.

LanceV

I still haven't found out what exactly they flooded with sea water.
Do they just replace lost water in the RPV by sea water? Or are they trying to cool the RPV from the outside by flooding the containment? Or both?
The use of boron suggests the former but I keep hearing about the latter.

Does anybody know more?

nlsherrill

So, when the news reports that there is a danger of radiation exposure...what exactly is being exposed? I think I know the very basics of the core, which is essentially uranium fuel rods that are bombarded by free neutrons right? Where does Xe, Kr, and I come from? Are these what uranium decays too? When people get radiation sickness/exposure, what is harming them? electromagnetic radiation or something else?

Clearly I have no clue what I'm talking about, but I would like too.

Reno Deano

Hypothetical People external to the plant and workers (real) near or within the plant would be the things being exposed to radiation. Eventhough the melted fuel losses its favorable geometry for substained criticality, some neutrons and high energy gammas will continue to prolong the fission process, but at a significantly lower pace and would be considered subcritical. The attached link discusses the fission fragment spectrum of radioisotopes.

http://hyperphysics.phy-astr.gsu.edu...e/fisfrag.html

Astronuc

Steam from the primary system would carry Xe and Kr (noble) gases, and perhaps some I and Br which are volatiles. It is expected that I and Br would form oxides, and react chemically to form iodates and bromates, if not iodides and bromides.

Xe, Kr, I, Br are fission products produced when U-236 (U235+n) fissions. There are other fission products such as Cs, Ba, La, . . . . and Rb, Sr, Y, . . . which are essentially in solid form, and the unused U, Pu, . . . which is in the fuel. Normally these are surrounded by a metal alloy of Zr, but that alloy has probably corroded/oxidized, and no longer performs its function, which is the keep the fuel (UO2 and fission products) separated from the coolant.

The fuel and fission products can oxidize into particles on the order of several microns, and this can be then dispersed in the coolant.

The noble gases can readily escape into the steam, and it is hoped that much of the fuel will remain intact.

There are also core components, e.g. control rods, and other structures that are made of stainless steel, typically SS304. The control rods contain boron carbide (B4C) and perhaps Hf, which are neutron absorbers used to control/limit the fission reaction or shutdown the reactor. If the temperatures in the core got to ~1300-1400 C, then the control rods could have melted. Above 1000 C, they could have gotten soft and deformed.

When people are exposed to radiation, it is usually beta and gamma radiation, or possibly alpha particles if isotopes of heavy elements, e.g., Rn, Ra, U or tranuranics were ingested or inhaled. Alpha particles are stopped by clothing or skin. Beta particles are more penetrating, and gamma photons are the most penetrating.

Ionizing radiation harms cells by radiolysis of the water (which forms peroxide and hydrogen), which can then chemically react with the complex molecules like DNA, RNA, proteins, vitamins, enzymes, coenzymes, . . . . which are necessary for cells to function.

A little radiation is not necessarily bad. Cells can be repared, or dead cells are simply discarded and replaced. The more radiation, the more cellular necrosis, the more one can become seriously ill. Some damaged cells may mutate into cancers. Nerve cells are particularly sensitive to radiation, and they are not so easily replaced.

TCups

Borek: I think the question is (at least mine is. . . ) this: if the core melts and becomes a molten mixture of fissile materials + melted control rods, even if the reactor's fuel has been "killed" with sea water and boron, is it still possible that the molten mixture might become "critical"?

And even if not, how long will the ongoing heat of decay (at 10% of the level of an operational reactor mentioned) persist in the absence of cooling water?

Finally, are either the primary steel containment vessel or the reinforced concrete secondary containment, or both up to the task of containment of a core meltdown and if so, for how long?

Astronuc

As far as I know, they were trying to inject seawater into the pressure vessel, ostensibly through the piping system used by the ECCS. Flooding containment is also a possibility, but that's mostly outside of the RPV which contains the core, and which is where the cooling water must go.

The steam is venting somehow, through pressure relief valves, and then the personnel have to vent the containment. So the seawater goes in, and some steam comes out, and there has to be a mass balance.

How they are cooling the seawater once it's heated is not clear.

If they pump seawater into the RPV/primary system, and it is flowing out into containment, that could mean an open valve or pipe break. HOWEVER, there is no information about the integrity of the RPV or piping within containment. We can only wait for further information from the site.

misnderstudge

well as a civil defence member (non physicist member) i don't think the risk is high at all, but in its history now is the closest to a risk there has been as far as i am aware. they will properly have to cold start it (i think no facts to back that up) which is very expensive and take months. i am surprised World Agency of Planetary Monitoring and Earthquake Risk Reduction,(WAPMERR) has said nothing at all. oh and the chance even if it was to go of a thermal explosion is so small it is practically 0

LanceV

Yes, but the question is can they inject water into the core? Maybe the pressure is to high. Or there is a danger of explosion when water hits the hot fuel rods.

At first I was under the impression that the sea water is circulated through the reactor. Maybe by extracting it via the suppression pool and then back into the sea. If the core is not molten this would probably result in only a minor pollution.

But it is hard to find any definite information. It seems that there is nobody twittering from the control room. Maybe they are busy atm.