Thursday, March 24, 2011

Radiation and Humans

Even after a week, the “Chicken Little / Sky Is Falling” reporting continues on the nuclear power plant accident in Japan. This has (1) completely overshadowed the true disaster (the tsunami which devastated a far larger area at least as thoroughly as the two atomic bombings in World War 2), and (2) has induced people to panic and do stupid things (like buying potassium iodide over here in the US).

There is too much ignorance about nuclear technology and radiation characteristics, out among the population in general, and the media reporters in particular. There is also too much ratings/money chasing sensationalism among the reporters. It dis-incentivizes telling the truth, and that’s a bad thing.

As the two previous articles indicate, not all nuclear radiation is the same. There are different intensity levels and different decay times. There is also a huge dilution effect that renders a potent local release very dilute and relatively inoffensive when spread across an ocean and around the world.

Long-Term Exposure Limits

Furthermore, not all human medical responses to radiation are the same. The steady slow accumulation of general “background” radiation exposure has one set of limits, for long-term late-in-life effects. Further, these limits have evolved substantially since nuclear age began. These are depicted in figure 1. The current numbers are 5000 milli-REMS (mR) for adults in any one year, with a lifetime accumulation limit of 1000 mREM x age in years. For minors (under age 18), the limit is 500 mR per year. For fetuses (and thus pregnant women), it is also 500mR per year, with the added restriction of no more than 50 mR in any one month. (See also units of measure note below.)

This 5000 mR annual limit for adults has evolved from earlier, higher limits set when we knew a lot less about the medical effects of radiation. During World War 2 and up to 1950, it was 25,000 mR per year. Between 1950 and 1957, this was reduced to 15,000 mR accumulated in any one year. The current level was set in 1957. The minor and fetus limits are newer. Today, astronauts get to absorb the old World War 2 limit in any one mission, which really means they shouldn’t do that more than once a year.

Steady Background Radiation

These exposure limits apply to general background (steady) exposure. That radiation has a natural component and a man-made component. The natural component includes natural radioactive materials in the Earth’s crust, plus cosmic rays from space. It is there all the time, and varies widely from location to location. The average for the entire Earth is in the neighborhood of 240 mR per year, and 300 in the US. Very locally at very few locations (such as uranium mines), it can be 10 or even 100 times more.

The man-made component to background radiation includes the well-known atomic weapons testing fallout, the leakage of radiation from nuclear plant accidents, and leaks from nuclear industry processes in general. When nuclear weapons were being tested above ground 1945-1963, there was a significant radioactive fallout contribution, the nuclear total peaking in 1963 at about 15 mR per year. Since atmospheric testing ceased in 1963, the nuclear total has dropped back to about 0.5 mR per year, practically insignificant compared to the natural background, and that includes accidents like Chernobyl.

It also includes the effects of accidents like Three Mile Island (which leaked almost nothing) and the current crisis in Japan (which has leaked something, but nothing like Chernobyl). This figure alone explains why it is silly to fear radiation exposure in the US, no matter what happens in Japan.

The man-made component of background radiation also includes what amounts to fallout from the burning of coal for electric power. Radioactive elements are included in coal deposits, and these are in both the gas plume and especially the fly ash. About 1/3 (around 120 mR per year) of the total background comes from coal plants. Most folks do not know or appreciate the fact that coal plants are a greater radiation hazard than all the nuclear bomb explosions or reactor accidents that ever were, combined. But you can clearly see it in figure 2.

The other exposure isn’t really background, because not all of us are exposed to it: medical imaging that uses radiation (X-ray or nuclear radiation). This can be up to around another 100 mR per year. This wasn’t as widespread decades ago, and a lot of these technologies didn’t even exist then, so in figure 2, I only show this on the modern exposure.

Figure 1 – Allowable Exposure Limits for Background Radiation

Figure 2 – Typical Background “Then and Now”

Short-Term High-Level Exposures

Nuclear bombs and reactor accidents release localized high doses of radiation that decay over time, as depicted in figure 3. These present risks of relatively prompt casualties, as well as enhanced risks of long-term injury. The human medical response to this type of incident is simply different. The arbitrarily-chosen standard is the dose accumulated in one week (168 hours). Table 1 is an abridged description of these effects. These take the form of a list of mR per hour rates, with the total dose absorbed (not shown) being the average hourly rate multiplied by 168 hours. (These weeklong accumulations are far higher than any yearly background exposure limits, even from World War 2.)

The peak hourly rate is crudely about twice the average hourly rate. Peak hourly rates are directly comparable to hourly dose rates reported or predicted for the fallout from nuclear weapon explosions. For bombs ranging from 200 kilotons to 1 megaton yield, “very close in” (a few miles) the typical peak dose rate is around 500,000 mR per hour within 1 or 2 hours of the blast. “Far away” (dozens to hundreds of miles) the peak dose rate is closer to 500 mR/hour, something like 9 to 14 hours after the blast. Decay to far more survivable levels happens in a week or three. Local leakage exposures from the worst nuclear reactor accidents are hundreds of times less intense than that from bombs.

One Numerical Example

Say the average hourly dose rate was 179 mR/ per hour, and this was received over one week. The total absorbed dose would be some 30,000 mR. Compare that to the World War 2 dose of maximum 25,000 mR in one year. It is 5 times the modern limit of 5000 mR per year for an adult, and 50 times the modern limit for a child. Close to a major event, you can accumulate more than a year’s dose very quickly. This stuff is very dangerous, make no mistake about that.

Figure 3 – The Nature of Decaying High-Level Short-Term Exposures

Table 1 – Medical Consequences of High-Level Short-Term Exposures (Abridged) (click on table to see a larger one, browser "back" returns you to article)

Conclusions Regarding the Dangers Posed by the Japanese Reactor Accident

The workers in the plant in Japan trying to rectify the situation are at very serious risk. Adults a few miles away are at only a little risk, but infants and fetuses are at some risk. Dozens to a hundred miles away from the leaking plant, there is almost no risk at all from airborne radiation. The only credible risks would be contaminated food or water, and they’re fairly small, excepting maybe for infants.

Quite frankly, the human-made background radiation from coal plants is the more serious risk.

A Note On Units of Measure
There are specific definitions for units of radiation known as the “Roentgen”, the “rad”, and the “REM”, but quite frankly, they are all equivalent. In the literature, they are all reported as the abbreviation “R”, reflecting that interchangeability. The milli-REM ( mR) used here is 1/1000 of a REM (or rad or Roentgen). Two other common units are the Sievert (Sv) and the Gray (Gy). Effectively, 1 Sievert is 100 R, and so also is 1 Gray. The Sievert is actually 100 REM while the Gray is 100 Rad, but effectively the REM and the rad are the same (just “R”).

A Roentgen is the energy of gamma radiation contained within 1 cc of air. A rad (radiation absorbed dose) is the radiation energy transferred to some mass of material, typically for us, a human. REM (Roentgen equivalent man) is the rad dose multiplied by a quality factor that models the biological impact. For gamma radiation and beta particle radiation, the quality factor is 1 (1 rad = 1 REM).

Thursday, March 17, 2011

Follow-Up on the Japan Nuclear Crisis

The egregious “Chicken Little / The Sky Is Falling” reporting continues unabated over Japan’s nuclear plant crisis. I am getting very disgusted with all the fear-mongering-for-profit.

News media: please learn something about what you are talking about, before opening mouth and inserting foot. The run you have precipitated on anti-radiation iodine pills in the US is completely ridiculous, even insane.

It’s not that there aren’t serious risks at this plant, because there are. But, they have little to do with what gets splattered all over the evening news. Please refer to my previous posting on this subject (15 March 2011), and to figure 1 below.

Even if the reactor cores completely melt down, there is no risk of a Chernobyl-style intense-radiation event. Most, although maybe not all, the intensely-radioactive debris will remain within the outer (#3) containment, even if it is breached.

Breaching the #3 containment into the atmosphere to leak copious amounts of dangerous stuff is extremely unlikely in the extreme. This is for precisely the same reason that most of the fibrous “tire snot” sealing materials stay inside the tire, even after a blowout.

In any event, breaching that #3 outer containment is unlikely, because it is so very tough. This type of concrete and steel construction is pretty much the same as those concrete and steel structures still standing at ground zero of the Hiroshima and Nagasaki atom bomb explosions in 1945.

There, that should “calibrate” your gut feel for just how tough these structures really are.

Breaching the floor of the #3 containment, and releasing dangerous stuff into the earth and groundwater is more likely, but only in the event of a full core meltdown, which is still pretty unlikely, in spite of the severity of the disaster in Japan.

However, this is not atmospheric dispersion, and it stays more or less confined in a definite and geographically-small area. Radioactive fallout from the air is just not a risk more than a few miles from the plant, no matter what happens.

The real risk in this particular disaster is not the reactor and its containment structures, it is the spent fuel rod disposal ponds. These have little or no containment. If they go dry, and the spent fuel rods overheat and catch fire, that really is atmospheric dispersal of some very dangerous stuff.

That is the only thing anybody should be seriously worried about.

There are some lessons to be learned here. They are applicable to retrofits of existing nuclear plants, and to new plant designs. They are also nothing but plain old “horse sense”.

The Japanese plant was designed to withstand a Richter magnitude 8.2 earthquake, and quite successfully withstood magnitude 8.9. However, the geologic record (not the historical record) suggests that the Pacific Ring of Fire experiences quakes substantially above magnitude 9.

Not much can be done to “harden” existing plants. But, pick any number you want for design criteria for future plants. How about 9.5?

The problem in Japan wasn’t so much earthquake damage, it was the tsunami. Nobody planned for a 30-foot wave to sweep away the electrical grid and to disable all the back-up generators and water supplies. They haven’t seen a tsunami like that in Japan since 1700 AD.

Yet, the geologic record suggests that there actually have been far higher tsunami waves all around the Pacific, and in other oceans as well, including the Gulf of Mexico. Pick any number you want, but numbers on the order of 100 to 200 feet tall seem to have happened multiple times, all around the world in the geologic record. Just not within recorded history.

The other problem is the lack of multiple containment, around the spent fuel rod storage ponds. This is retro-fittable at every plant in the world today. It should be done as soon as is practical. It’s a safety thing, the “bottom line” is (and should always be) secondary to that consideration.

Figure 1 – Reactor Containments vs Spent Fuel Ponds

Tuesday, March 15, 2011

On the Nuclear Crisis in Japan

I am very displeased at the sensationalized, inaccurate reporting of this crisis in Japan, especially from the for-profit radio and television media. This is a disservice for the citizenry, and needs to stop. Now.

We all know that only bad news sells, and the more it is exaggerated, the more likely it is folks will sit through the commercials to hear the rest of the story. Besides that, the technical inaccuracies are quite egregious at times.

When there is a problem with a nuclear reactor, the appropriate expert to consult is a real nuclear engineer, not a nuclear physicist or a nuclear plant security expert. They did not study the actual engineering design of reactor systems and equipment, only the nuclear engineer did.

These exaggerations and inaccuracies feed into public fears, which in turn lets politicians take advantage of these fears, just to further their own careers. They do this instead of doing actual good for the people (which they swore to do, but so often fail).

This sensationalized, inaccurate reporting could put a damper on efforts toward increased US use of nuclear energy, at a time when we so desperately need more of it. Some truth about what is really happening in Japan could allay public fears and let our country get on with what it must do.

I personally am no nuclear engineer, but I am an experienced mechanical engineer. Here is some of what I do know about nuclear plants.

About Radiation

First, not all “nuclear radiation” is the same. There is more than one type of radiation, and more than one intensity level.

Some reactor fuel materials and their daughter products are quite intensely radioactive, and stay that way for long periods of time. These are generally solid materials, and in modern designs, not very flammable.

Other materials, such as core structures, containment vessels, and cooling water, can be “activated” into being radioactive materials by exposure to the nuclear radiation coming from the reactor core. These are also generally solid materials, and their radioactivity is initially far less intense, and decays far more quickly into insignificance.

The most innocuous of these are steam and air, harmless within minutes, and gaseous in form, so they cannot drop as “fallout” from the sky. See figure 1.

Figure 1 - Not All Radiation Is The Same

About Heat Production

Both the fission reaction and radioactive decay create heat within the active materials. Fission creates by far the most heat, but this ends immediately when the control rods are inserted to “kill” the fission reaction.

Radioactive decay produces less heat, but it is persistent for a time , until the radioactivity decays. Depending upon the material, this can be a very long time.

Reactor fuel and daughter products typically require many, many years to decay. This is in part why spent fuel rods are placed in pools of water: to keep them cool in spite of the heat produced by radioactive decay. The other part is that the water is a shield to absorb the radiation and protect the environment.

When a reactor is shut down by inserting the control rods, its core requires considerable cooling for a time measured in days to weeks, to offset the heat of radioactive decay. There is no fission heat being produced at all after shutdown.

Modern Reactor Designs

These typically have three very stout layers of containment: the fuel rod assembly tube, the reactor vessel itself, and a containment vessel; surrounding the reactor vessel and its closely-associated equipment. Some plants place this inside an ordinary building, others do not. See figure 2.

Figure 2 - Modern Reactor Designs Have 3 Layers of Containment

In these designs, none of the reactor fuel or core materials are flammable. The fuel is metal oxide, and is contained within a tube of exotic metal alloy. The fuel rod tube material actually melts before the fuel pellets themselves melt.

Fuel assemblies plus control rods, immersed in water, constitute the “reactor core” that is contained inside the reactor vessel. Water goes in and comes out as steam, because of heat produced in the core. This steam can be used (indirectly for radiation safety purposes) to generate electricity.

The first layer of containment is the fuel rod tube itself. Only if this gets hot enough to melt, are any of the still solid (or at worst molten) fuel and daughter products able to escape the tube.

The second layer of containment is the reactor vessel, which is a very stout steel item. It would take extreme temperatures and pressures to broach this vessel.

Pressure control in the reactor vessel is by venting, which releases the slightly radioactive steam and air, and little- to-none of the fuel or daughter product and fuel assembly tube materials. For post-shutdown cooling, water is pumped through the core in this vessel, whether that core is intact or not.

The third layer of containment is an extremely strong concrete and steel shell, built around the reactor and its associated equipment. These things are built to withstand impacting aircraft, explosive attack, tornadoes, and just about anything else but a direct hit with a large nuclear bomb.

If the reactor vessel does fail, the extremely dangerous mess is still contained within this shell. Again, pressure control is by venting the relatively innocuous radioactive steam and air. The truly dangerous stuff is nongaseous, and gets almost entirely contained within.

Many of the fuel assembly tubes, and maybe one of the reactor vessels, have broken in Japan because the plant’s core cooling ability was destroyed by the tsunami. None of the containment structures have failed, nor is it reasonable to think they ever will.

There cannot, and will not, be release of a disastrous quantity of the intensely-radioactive fuel and daughter products from the crisis in Japan. For pressure control, a very tiny amount of this material will be released, aerosolized along with the steam.

There will be a much larger release of the far-less-dangerous “activated” structural materials. Taken together, the danger of the released radiation is actually fairly low, and easily decontaminated by ordinary-but-prompt showers, and simple wash-downs of hardware.

Comparison to Chernobyl ?

This comparison is unreasonable fear-mongering. The reactor designs are completely different. The simple uncontained pile reactor at Chernobyl was a 1950’s Cold War legacy that should have been dismantled decades before it exploded . See Figure 3.

Today, no responsible or ethical engineer would design a reactor like that. It has no safety features or containment, and the operating characteristics are far less stable.

At Chernobyl, they lost control of the reactor, let it get too hot, and literally caught the core materials on fire. Both the graphite block structure and metallic reactor fuel were chemically flammable in air, and that is exactly what happened.

Without any containment at all, this fire produced enormous quantities of intensely radioactive smoke directly in the air. This smoke was composed of particles of graphite, reactor fuel, and daughter products, and exposure to it caused death within hours to weeks.

The proper comparison is to Three Mile Island, which did suffer a core meltdown inside the reactor vessel. However, at Three Mile Island, neither the reactor vessel nor the containment structure were breached. The only radiation released was the relatively innocuous and short-lived radioactive steam, and that at about the level of an ordinary chest X-ray to those exposed.

Figure 3 - The Antique and Unsafe Chernobyl Design

What Really Went Wrong In Japan
What went wrong in Japan had nothing to do with reactor core or containment design. It was an unanticipated wave size for the tsunami, which destroyed the post-shutdown cooling capabilities, specifically their electric power supplies. These were necessarily located outside the containment.

If we just design for taller tsunami waves, this problem with post-shutdown cooling capabilities never happens again. Simple as that.