Friday, May 13, 2011

TEPCO concedes severe fuel damage at Fukushima

For the first time utility says fuel has "melted" and may be "crumbled" at bottom of the reactor pressure vessel for reactor 1

water cooling systemThe fuel assemblies in Fukushima reactor unit 1 are only partially covered by water. A TEPCO spokesman, Junichi Matsumoto, told a press conference in Tokyo Thursday May 12 that plant workers have poured enough water into the reactor to raise it to a level half covering the fuel assemblies, but that a faulty water level meter prevented the utility from making an accurate assessment of just how much water has been inside the damaged facility since the March 11 earthquake.

This means the millions of gallons of water that have been injected into the reactor pressure vessel (RPV) have leaked out of it. However, TEPCO is unsure about its migration path. One theory is that the nozzles, or guides, that allow control rods to enter the RPV from the bottom of the RPV were damaged or that other unknown breaches of the RPV may have taken place possibly as a result of contact with the melted fuel. This could include evaporation that accompanied venting the hydrogen from the reactor.

Damage worse than expected

The Wall Street Journal reported that overall damage to the reactor is much worse than previously reported raising questions about the damage to the reactor that might have been caused by the earthquake. TEPCO also said that similar levels of damage might be possible at reactor units 2 & 3.

Until now TEPCO had reported that the damage to the reactor was caused indirectly by the 15 meter high tsunami which swept over the five meter sea wall and destroyed diesel fuel for emergency generators and thus the loss of electricity for the cooling system.

This caused heat inside the RPV to rise beyond the melt point of the zirconium fuel cladding (1,855 C) and also the uranium oxide fuel (2,800 C). By comparison, the average operating temperature of the BWR design reactor is about 315 C.

BWR schematic (Image source: Wikicommons)

The new information means TEPCO's decommissioning plan for reactor units 1-3 may have to be revised causing further delay in stabilization of the damaged plants. At the press conference, spokesman Matsumoto said, "We can't deny the possibility that a hole in the pressure vessel caused water to leak."

The good news he said is that there is no danger of another hydrogen explosion. The temperatures recorded inside the RPVs for reactors 1, 2,& 3 on May 11 were - #1: 237F; #2: 239F, and #3:429F.

Another implication of the new damage assessment is that Japanese claims that the BWR reactor design withstood the impact of the 9.0 Richter scale earthquake may have to be reviewed once the units are in cold shutdown. It may turn out that there was more extensive damage to the cooling system pumps and pipes from the earthquake itself. The outcome of the review could raise earthquake safety and structural issues for other BWR reactors worldwide.GE BWR nuclear fuel assembly 2

How to cool down three hot reactors?

As far as getting the reactors into a state of cold shutdown, TEPCO had been planning to fill the primary containment structures that surround the RPV with water. This strategy was called the "water tomb" method.

Tadashi Narabayashi, a professor of nuclear engineering at Hokkaido University, told the Bloomberg wire service the "water tomb" idea won't work. He said a water circulation system is needed for the reactors.

Getting one in place won't be easy. The areas under the RPV are higher radioactive especially if they've been drowned by massive leaks from inside the RPV. According to TEPCO, 10.4 million liters (2.75 million gallons) of water are unaccounted for. The leaks would have to be fixed before TEPCO could install a cooling system and bring the reactor to cold shutdown.

Where has the water gone?

Evaporating steam fukushima Did this water boil off as steam? If temperatures did reach 2,800 C, the water could have evaporated very quickly during the first week of the crisis.

The water inside the RPV must be eight meters high to cover the fuel assemblies. In fact, the height of the water may be less important than the amount of crumbled fuel that has fallen into the base of the RPV and remains submerged there.

In any case. the amount of water leaking from the RPV is much greater than TEPCO has previously indicated was the case. It raises the question of whether a "meltdown" has occurred and how bad the situation real is inside the RPV.

What’s a meltdown and what’s not?

meltdownDefinitions of a "meltdown" vary, but it is generally understood in the nuclear industry to mean the fuel has become deformed by heat, partially melted, and some of it will have found its way outside the RPV.

It isn't clear in the case of the new information released by TEPCO this week that any of the fuel has migrated outside the RPV. However, TEPCO said in its press conference that 55% of the fuel in reactor #1 is destroyed by excess heat. Also, TEPCO said that it believes 90% of the fuel is still inside the reactor. Did the other 10% vaporize or if not, where did it go? If it migrated from the RPV to the primary containment what was the pathway?

TEPCO also said there are no cracks or ruptures of the primary containment structure surrounding the RPV. This means the risk of a massive release of radioactivity is not very high.

No China Syndrome

TEPCO spokesman Matsumoto said "we are not seeing a case of a 'China syndrome," which refers to the scenario of molten fuel melting its way through the RPV and through the primary containment structure.

While the hot fuel has a temperature of well over 2,000 C, the melt point of the steel for the RPV is much lower at about 1,400 C. However, if there is water in the bottom because of TEPCO's injection of 90 tons of water a day, or 217,000 gallons, it would rapidly cool any melting fuel and prevent it from impacting the steel RPV.

seesawThe New York Times reported that in a briefing to the U.S. Nuclear Regulatory Commission this week, William Borchardt said the new information from TEPCO was not a major development. He told the newspaper the condition of the reactors was "static," if not exactly stable.

With conditions in a see saw of progress and the revelation of news problems, it is clear TEPCO is a long way from being out of the woods at Fukushima.

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4 comments:

uvdiv said...

I am not an engineer.

Also, TEPCO said that it believes 90% of the fuel is still inside the reactor. Did the other 10% vaporize or if not, where did it go?

I think they mean that ~10% of the fuel melted through the RPV into the containment vessel. Look at "Interactive Graphics" label (2) in the WSJ article you linked. Vaporization doesn't seem realistic (except for the volatile FPs which we already know escaped).

According to TEPCO, 10.4 million liters (2.75 million gallons) of water are unaccounted for. [...] Did this water boil off as steam? If temperatures did reach 2,800 C, the water could have evaporated very quickly during the first week of the crisis.

It doesn't work that way. The amount of steam generated is limited by heating power (from decay heat), not temperature. That the fuel temperature reached 2,800 °C doesn't mean that the rate of heat production became higher; it means that the conduction of heat away from fuel was low. Long sections of fuel rods were above the water level, in contact only with steam.

The explanation in the WSJ article is that the water is leaking from the RPV into the containment vessel, and then (through piping maybe?) into the outer building, down to the basement. (I think some of this ended up outside in those concrete troughs they drained into the ocean, but maybe that was another reactor).

However, if there is water in the bottom because of TEPCO's injection of 90 tons of water a day, or 217,000 gallons, it would rapidly cool any melting fuel and prevent it from impacting the steel RPV.

The WSJ article says that the RPV is already breached. My understanding is that if there's a thick slab of failed fuel at the bottom of the RPV, water will only cool the slab's surface, not its interior. Uranium oxide is a not a good heat conductor.

netudiant said...

Once the fuel cladding is lost, there is a big temperature gap before the fuel pellets melt. Seen that the reactors are also stuffed with salt, it seems quite possible that the fuel, rahter than being gathered in a molten lump, is dispersed in salt at the bottom of the reactor, with periodic bits of water/steam flushing by and perhaps carrying particles of fuel material out of the RPV and the containments.

djysrv said...

Response to comments

UDIV - Thanks for your clarifications. These are useful updates.

Also, to others who have submitted comments, if you want public credit for them, you need to provide a verifiable identify such as a Blogger ID, Open ID, or similar mechanism.

I don't acknowledge anonymous comments, made up names, or a name with a link that goes to a nonworking URL. It's not my responsibility to look for your identity online.

You can also put a verifiable email address in the body of your comment in the form of [name] [at] [domain] [dot] [extension]

My feeling is you know who I am. If you want dialog, tell me who you are.

I always appreciate technical comments that correct errors.

Anonymous said...

This is the only blog I've seen that suggests that the fuel melted (>2800C) - is this a deduction or does it come from an official release?