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.

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