Passive safety

There's no doubt that the Fukushima catastrophe has decimated public support for nuclear power.

[Asahi Shimbun] Nuclear power opponents increase in 7 countries

NuScale RPV and containment vessel

Suppose you are a proponent of large-scale nuclear expansion, and you are arguing your case with someone who has become opposed to it after the Fukushima disaster. They cite Fukushima as an example of -- if not danger, the unreliability, and potential for economic devestation, of nuclear power plants. Setting aside the issue of operating Gen-2 reactors, what's your defense of new construction -- for building Gen-3.5, Gen-4 NPPs? Granted, human engineering is never perfect; but how would you make a convincing case that nuclear power can become more reliable than the engineering of 50 years ago?

I think the first place to start is with decay heat removal. Fukushima's failure to do this under extreme conditions directly caused much of the disaster: the core drying up and uncovering fuel, causing it to overheat; the high temperatures allowing zirconium oxidation, leading to hydrogen explosions breaching containment; the overheated fuel melting, releasing fission products; the melted fuel (probably) boring through reactor pressure vessels, resulting (again) in radiological release.

So then: how fundamentally can you "eliminate" the design limitations involving decay heat removal? How far can you go towards making heat removal passive and intrinsically safe?

I'm not an engineer, but I'd like to provoke debate by suggesting one solution. It's based on very basic physics, and there's already a Gen-3.5 design that exemplifies it.

The concept is to put small reactors in very large, atmospheric-pressure water pools. Large enough that they can reject decay heat indefinitely by boiling away into the atmosphere. At least one small/modular reactor (SMR), NuScale, seems designed to do this:

[NuScale] NuScale's Passive Safety Approach

[NuScale] Introduction to NuScale Design

The NuScale design has twelve 150 MWt/45 MWe reactors, each in their own steel containments, submerged in a common 4 million gallon (15,000 m³) pool. With about 20 megawatt-days of decay heat per reactor over the first 30 days (see above), or 240 MWt-days combined, the pool is more than enough to absorb all of it through evaporative cooling. (It would take at least 396 MWt-days to evaporate all of it, just by multiplying by the heat of vaporization).

This is described by NuScale as a "30-day supply" of cooling water. By their reasoning, this is all that is needed: air cooling is sufficient afterwards, when each core is down to just <400 kW of decay heat.

So, this design's emergency heat rejection seemingly does not depend on

• External water supply (Fukushima: lost seawater pumps)
• External AC power (Fukushima: lost power lines, diesel generators)
• Battery power (Fukushima: batteries did not last long enough)
• Safety-critical piping (Fukushima: possibly damaged by earthquakes)

On that last point: NuScale argues their design eliminates "large break" loss-of-coolant accidents, since the reactors are completely submerged in a pool. The core can always be flooded simply by opening a valve in the containment.

Should this type of SMR be the nuclear industry's response to Fukushima? What do you think?

NuScale plant layout:

Japanese parents in arms over school radiation standards: govt. raised from 1 mSv/yr to 20 mSv/yr

Being reported by many sources, including:

[NYT] Japanese Parents Assail Government Over Radiation

[ABC (Australia)] Outrage as Japan lifts radiation limit for kids

At issue are updated government guidelines that allow schoolchildren to be exposed to radiation doses that are more than 20 times the previously permissible levels. That dose is equal to the international standard for adult nuclear power plant workers.

(I'd earlier blogged about the top radiation advisor who resigned in protest over this decision).

As ABC writes, the government position is one of perceived necessity: contamination is widespread enough that the old 1 mSv/year would demand an exodus.

The government says it had no choice but to raise the legal exposure limit, saying about three-quarters of the schools in Fukushima have radiation levels above the old safety level of one millisievert.

I'm not even aware of a map at this resolution: the ones available (like in my previous post) stop at 5 mSv/year external dose, which already covers a very wide area.

Like a sacrificial offering to an angry mob, an education ministry official was bundled outside to speak to the demonstrators, although he had very little to offer them at all.

The hapless official's words only seemed to anger the protesters further.

"The current radiation levels for schools in Fukushima pose no health risks to kids at all," the official said.

I find it bizarre that the government is claiming external doses of tens of mSv -- at the level of CT scans, or possibly several CT scans -- are absolutely risk-free. While there's little empirical data at this level, it's widlely believed that doses these high cause nonzero excess risk of cancer and cancer fatalities. See for instance an authoritative reference from a radiological professional society -- these for adult doses:

Radiation Exposure in X-ray and CT Examinations

This is a very messy situation. The high radiation levels in parts of Fukushima are, apparently, likely to cause cancer deaths. And this excess risk is relatively small compared to other risk factors. So how does one react to this? Should government apply ordinary risk-tolerance standards, doing nothing, and allow a few tens or perhaps hundreds of radiation deaths (epidemiologically undetectable)? Or should they mount a massively expensive and destructive evacuation effort over a large area (the 5 mSv/year external dose range currently encompasses about 1 million people), itself probably causing serious health hazards to the displaced? Or should it leave people to make their own risk judgements (whether rational or not), informing them of the situation accurately?

(One of the commenters on this blog suggested, correctly, that to evacuate at this risk level, one should equally evacuate most cities near fossil fuel plants, factories, major highways. Which is strictly accurate, if you weigh all risks equally without regards to source. But then it does not seem people choose to view radiation dangers in the same way as other dangers.)

What I think they should not be doing is denying the problem like the Japanese officials are at this point. This is either incompetent or unconscionable.

From some extrapolations of future dose (over more than one year), see the new ISRN report (pdf, in French). In my previous post I also made a graph charting the decay of dose rates from cesium. (Other isotopes are mostly decayed away at this point).

ISRN on Fukushima radiation effects; I-131 thyroid exposure by inhalation

French public radiation-protection agency Institut de radioprotection et de sûreté nucléaire released a report Monday (in French only). Based on the French standard of 10 mSv/year to the public after a nuclear accident, they are calling for further evacuations of an additional 70,000 Fukushima residents.

[AFP] 70,000 more should evacuate after Fukushima: Watchdog

Staying in this area means the inhabitants would be exposed to radiation of more than 10 millisieverts (mSv)in the year following the disaster, according to the IRSN.

This level is used in French safety guidelines for protecting civilian populations after a nuclear accident. In France, 10 mSv is three times the normal background radiation from natural sources.

"Ten mSV is not a dangerous dose in and of itself, it's more a precautionary dose," said Champion, noting however that this figure that does not include any additional doses from contaminated food or water.

(If I remember right, Japan is using a 20 mSv/year standard).

(summary) [IRSN] Gestion radiologique des territoires contaminés à la suite de l’accident de Fukushima : l’IRSN rend public son rapport d’analyse de la situation

(report) [ISRN] Evaluation Au 66^eme Jour Des Doses Externes Projetees Pour Les Populations Vivant Dans La Zone De Retombee Nord-Ouest De L’Accident Nucleaire De Fukushima - Impact Des Mesures D’Evacuation Des Populations

Since the report is in French only for now, my comprehension is limited. Here is the map they are using, similar to a dozen other maps I've linked on this blog (except that this one depicts city population sizes). Their recommended evacuation is on the green (10 mSv in 1st year) line, following French standards. (Note that if the standard were more conservative by just a factor of two, several major cities would be encompassed, including over one million people. Or: about one million people will be exposed to ~5 mSv external dose this year, the equivalent of a CT scan.)

By their accounting, this corresponds to a combined ¹³⁴Cs/¹³⁷Cs deposition level of 600,000 Bq/km², or an outdoors dose rate of 2 μSv/hr. (There's no inconsistency there; people do not spend all their time outdoors, and buildings shield gamma radiation somewhat.) I made a graph of how this dose rate will decay over time:

(¹³⁴Cs has a t_1/2 of 2.07 years, and ¹³⁷Cs has a t_1/2 = 30.08 years. Based on surveys (two are below), the ¹³⁷Cs/¹³⁴Cs activity ratio is currently about 1.2. Comparing the decay radiation (¹³⁴Cs, ¹³⁷Cs), ¹³⁴Cs gammas are about 2.7 times more energetic (there's also a small contribution from beta rays, if they're not shielded). So the initial dose ratio is around 2.5 in favor of ¹³⁴Cs; this goes away fairly quickly.)

[MEXT] Readings of dust sampling

Interim Report on Radiation Survey in Iitate Village area conducted on March 28th and 29th

The report also mentions potential ¹³¹I thyroid doses to children in excess of 1 Sievert. (Note that Sieverts are intensive quantities, they have units of J/kg. A dose to an individual organ is not the same as a whole-body dose.) This is the main risk factor for thyroid cancer, one of the most destructive effects of the Chernobyl disaster. (I don't know off hand what doses were involved there, or what thyroid doses are considered dangerous).

¹³¹I ingestion (e.g. through contaminated milk) was probably limited by food bans. But, in Nuclear Safety Commission data I found (in Japanese), they're talking about internal doses through inhalation of iodine from air:

[NSC] 文部科学省　緊急時迅速放射能影響予測ネットワークシステム（SPEEDI）による計算結果

This map is made on the (conservative) assumption of an infant staying outdoors 24 hours a day (it's cumulative over the first month or so):

Not sure what to make of this.

Kirk Sorensen founds Flibe Energy to commercialize the molten-salt nuclear reactor

[Energy from Thorium] Third TEA Conference: Theory Into Practice

...The most significant was the announcement by Kirk Sorensen, founder of this blog, that he has left his job as chief nuclear technologist at Teledyne Brown to found a company, Flibe Energy, dedicated to building commercial liquid-fluoride thorium reactors (LFTRs).

Flibe Energy, Inc.

(FLiBe = Fluorine, Lithium, Beryllium, as mixed LiF/BeF₂ salt)

Apparently these are his slides from the TEAC3 conference:

[Flibe Energy, Inc.] Introduction to Flibe Energy

Congratulations to Kirk, and best of luck in your endeavor!

This and that: GAO on Yucca decision; BRC nuclear waste report; TEPCO data released; CBO on Navy nuclear destroyers

As has been pretty obvious all along, the Administration's sabotage of the Yucca Mountain repository was a purely political act with no real justification. The Government Accountability Office investigated and discovered the obvious:

[NYT] GAO: Death of Yucca Mountain Caused by Political Maneuvering

The Obama administration's rushed efforts to shut down Yucca Mountain were strictly political and could set back the opening of a nuclear waste repository by more than 20 years, according to a new report by a federal watchdog.

The administration killed the repository program last year without citing technical or safety issues, and restarting the costly and time-consuming process of finding a permanent repository or an alternative solution could take decades and cost billions of additional dollars, the Government Accountability Office reported yesterday.

Separately, the "Blue Ribbon Commission" (which I have claimed was stacked for whitewashing the Administration's political position on Yucca Mountain) released its draft report, which looks predictably bureaucratic:

[BRC] May 13, 2011 Full Commission Meeting | Information, Documents & Presentations

I'm not editorializing on it, but Jim Hopf and Charles Barton are.

TEPCO published thousands of pages of previously unreleased data (WSJ says some of it is redacted):

[WSJ] Documents Illustrate Desperate Hours at Plant

[TEPCO] 東北地方太平洋沖地震発生当時の福島第一原子力発電所プラントデータ集 (plant data -- the biggest collection, I think) (accompanying press release)

[TEPCO] Submission of Report of Analysis of Observed Seismic Data [...]

[TEPCO] Submission of report [...] on the damage of electric facilities [...]

For example: temperatures measurements from unit #3

TEPCO press releases (and attached reports/data) are collected here here.

Reported in Forbes, the Congressional Budget Office is looking at the economics of a much larger US nuclear navy -- encompasing nuclear destroyers, amphibious assault ships, and others.

[CBO] The Cost-Effectiveness of Nuclear Power for Navy Surface Ships

To assess the relative costs of using nuclear versus conventional propulsion for ships other than carriers and submarines, the Congressional Budget Office (CBO) developed a hypothetical future fleet, based on the Navy's shipbuilding plan, of new destroyers and amphibious warfare ships that are candidates for nuclear propulsion systems. Specifically, CBO chose for its analysis the Navy's planned new version of the DDG-51 destroyer and its replacement, the DDG(X); the LH(X) amphibious assault ship; and the LSD(X) amphibious dock landing ship. CBO then estimated the life-cycle costs for each ship in that fleet—that is, the costs over the ship's entire 40-year service life, beginning with its acquisition and progressing through the annual expenditures over 40 years for its fuel, personnel, and other operations and support and, finally, its disposal. CBO compared lifecycle costs under two alternative versions of the fleet: Each version comprised the same number of ships of each class but differed in whether the ships were powered by conventional systems that used petroleum-based fuels or by nuclear reactors.

Their result is that conventional (fossil fuelled) ships of these size likely remain cheaper than nuclear versions. Here's their sensitivity to oil prices:

The higher oil prices are probably not realistic over long periods, because synthetic oil (from coal or gas, or possibly biomass) can be produced for less.

One of the CBO's caveats is that it assumes the same power consumption; but without fuel limitations, nuclear ships can travel much faster, using more power. They say the costs would break even for destroyers, if their energy consumption more then doubled.

Unit #4 spent fuel pool in danger of structural collapse?

I missed this news, from over two weeks ago. A TEPCO press release from April 30th reports efforts to reinforce "severely damaged" walls supporting the unit #4 spent fuel storage pool:

[TEPCO] Installation of support structure under the spent fuel pool of Unit 4

Since the walls of the reactor building, Unit 4 are severely damaged, we have been evaluating the seismic capacity to confirm the soundness of the building. As the result of the evaluation, we have decided to install support structure under the spent fuel pool to enhance safety of the building.

There is a brief mention of the hazard in a Reuters article:

Officials also remain worried about structural damage to the No. 4 reactor and whether its storage pool for spent fuel rods has sufficient support. A strong aftershock could topple the structure and spill and scatter radioactive fuel on the ground, compounding the crisis, experts have said.

[NISA spokesman Hidehiko] Nishiyama said that Japanese officials "don't believe it is in danger of immediate collapse," but want to shore up the No. 4 reactor with new steel girders and cement.

[Reuters] Japan readies new tactics for Fukushima after setback

Fukushima updates

TEPCO has confirmed substantial fuel damage in unit #1. A large part of the fuel has melted and is believed to be accumulating at the base of the reactor pressure vessel [1,2]. It is further suspected that this fuel melted holes [2], several centimeters wide [3], in the base of the steel RPV, leaking both molten fuel and water. (At Three Mile Island-2, the RPV was not breached like this). TEPCO engineers say the fuel mass seems to be contained in the steel containment structure [1]. However contaminated water is leaking out in large quantities, possibly through the suppression pool (torus), accumulating in the basement [1], and possibly leaking outside. There is at least 12 feet of water in the basement of unit #1 reactor building [4]. A robot in this building measured dose rates of up to 2 Sv/hr near an entrance [5].

Here is a graphic of this situation from the WSJ [1]:

And another from the Asahi Shimbun [6], which postulates this water is leaking from the basement into outdoor drains. (I can not read Japanese, but it looks like these arrows have question marks ('?') on their labels).

An elderly worker ("in his 60s") was killed at the Fukushima Daiichi site, apparently of a heart attack after carrying heavy equipment. There have already been numerous injuries and health problems among workers here, disclosed in scattered TEPCO press releases (but not aggregated anywhere, to my knowledge). For example, here is one of the casualty reports, from March 24:

Casualty

-2 workers of cooperative firm were injured at the occurrence of the earthquake, and were transported to the hospital on March 11th.

-4 workers were injured and transported to the hospital after explosive sound and white smoke were confirmed around the Unit 1on March 11th.

-Presence of 2 TEPCO employees at the site is not confirmed on March 11th.

-1 TEPCO employee who was not able to stand by his own holding left chest with his hand, was transported to the hospital by an ambulance on March 12th.

-1 subcontract worker at the key earthquake-proof building was unconscious and transported to the hospital by an ambulance on March 12th.

-The radiation exposure of 1 TEPCO employee, who was working inside the reactor building, exceeded 100mSv and he was transported to the hospital on March 12th.

-2 TEPCO employees felt bad during their operation in the central control rooms of Unit 1 and 2 while wearing full masks, and were transferred to Fukushima Daini Nuclear Power Station for consultation with a medical advisor on March 13th.

-11 workers were injured and transported to Fukushima Daini Nuclear Power Station etc. after explosive sound and white smoke were confirmed around the Unit 3. One of the workers was transported to the FUKUSHIMA Medical University Hospital on March 14th.

-At approximately 10 pm on March 22nd, 1 worker who had been working on setting up a temporary power panel in the common pool was injured and transported to Fukushima Daini Nuclear Power Station where the industrial doctor is.

-At approximately 1 am on March 23rd, 1 worker who had been working on transporting a temporary power panel in the common pool was injured and transported to Fukushima Daini Nuclear Power Station where the industrial doctor is.

http://www.tepco.co.jp/en/press/corp-com/release/11032401-e.html

More updates on joint Japanese/American airborne monitoring [7]. On the left is measured external dose rates at 1 meter height (in micro-Sieverts per hour, μSv/hr); on the right is estimated ¹³⁷Cs deposition rates in Bq/m². (Click to enlarge)

You can compare with Chernobyl ¹³⁷Cs deposition, keeping in mind the scales are not the same, that 1 Ci/km² = 37 kBq/m², and that the Chernobyl map was made several years after the disaster.

MEXT monitoring seems to have detected several other isotopes away (>20 km) from the Fukushiam Daiichi site -- comparatively small amounts of ⁹⁵Nb, ^110mAg, and ¹⁴⁰La [8]. These are not volatile elements (or isotopes which are decay products of isotopes of volatile elements, as far as I can tell); I am not sure what mechanism distributed these isotopes, or how to interpret this. The academic radiological survey also detected ¹⁴⁰La, and inconclusively, ⁹⁹Mo and ¹⁴⁰Ba (parent of ¹⁴⁰La); however it did not quantify the amounts present.

[1] [WSJ] At Reactor, Damage Worse Than Feared

[2] [Telegraph] Nuclear meltdown at Fukushima plant

[3] [Reuters] Fukushima reactor has a hole, leading to leakage

[4] [NYT] Japanese Worker’s Death Not Linked to Radiation

[5] [Yomiuri Shimbun] http://www.yomiuri.co.jp/science/news/20110514-OYT1T00768.htm

[6] [Asahi Shimbun] http://www.asahi.com/national/update/0514/TKY201105140387.html

[7] [MEXT/DOE] Results of Airborne Monitoring

[8] [MEXT] Readings of dust sampling