Nuclear Reactors

Unlike in a bomb, in a nuclear reactor you want the fission reaction to be controlled, not to run away. Control Rods Cd-113 or boron. Cooling water. Containment building. Moderator C-12 or H-2
coolant
fusion reactors progress also
and the cold fusion debacle. relate to scientific method (it worked in this case, despite the press), peer review, science and the press/public
Three-Mile Island accident

On 26 April 1986, at 1:24 a.m., Reactor #4 at the Chernobyl Nuclear Power Station exploded. Chernobyl is located in Ukraine, 100 km to the north of Kiev, a city of two and a half million people. The explosion was not a "nuclear" explosion in the sense that the reator fuel never achieved a complete chain reaction as in an atomic weapon. The fuel was only enriched to 2% U-235, not enough for a nuclear explosion to occur. Even if the fuel were highly enriched, the reactor fuel assembly would have blown itself apart before a complete reaction took place. Recall from our discussion of nuclear weapons how carefully a nuclear bomb must be designed and constructed to prevent the initial stages of its explosion from snuffing out the reaction, "fizzling". The actual explosion was energetic enough to blow up half of the huge building containing Reactors 2 and 4, but not the surrounding countryside.

Even though the explosion took place early on the 26th, no one outside the USSR knew of it until the 28th, when the edge of the radiation fallout cloud reached Scandinavia and was detected in Sweden. Although Mikhail Gorbachev had come to power the year before and had declared a new age of perestroika (restructuring) and glasnost (openness), the usual Soviet habit of concealing embarassing incidents prevailed until the western European states pressed for answers.
The Soviet media (still tightly under government control) shifted from silence to celebrating the real heroism and sacrifice of the firefighters and soldiers dealing with the situation at the reactor: the explosion was only the beginning.

In the following months the investigation revealed that the reactor operators had caused the explosion, not mechanical or electronic failure. The operators were conducting a test to determine how long, in the event of an emergency reactor shutdown, the generator turbines would continue spinning by their own inertia. Why? The reactor has emergency systems to provide cooling water to the core in case the normal cooling system malfunctions. These emergency systems normally run off electricity from the electrical grid. In the event that the external electrical supply fails, there are electric generators run by diesel engines to provide electricity. These generators require some time to start up, however, so the plant operators were testing how long the emergency systems could be kept running using the plant's main generators after the reactor has been shut down. Running the emergency systems off the main generators as they coast down would give them time to start the diesel generators. Ironically, the world's worst nuclear accident was caused by the test of procedures to be used in a major accident.

To prevent automatic safety systems from interrupting the test, they disabled them. To simulate an emergency shutdown, they decided to lower the reactor power, but not completely shut down the reactor, because coming back to full power from a complete shutdown is a lengthy procedure. They wanted to reduce the power from the normal 3000 MW to 500 MW, but they overshot, reducing the power instead to 30 MW. At these low power levels, the reactor output is unstable. Xenon-135 created by uranium fission poisoned the reaction. At normal power levels the high neutron flux transmutes the Xe-135 to other nuclides that don't absorb neutrons as easily. At these low power levels, though, the xenon builds up, further quenching the reaction. Trying to get the power up to 500 MW, the operators gradually pulled out all of the control rods.
Suddenly the power level shot up. Why isn't clear, but the consequences were severe.
The operators' ignorance and casual attitude were certainly the immediate cause of the accident. Nevertheless some blame can also be shared by the designers of the RBMK reactors, who made a reactor unforgiving of error, as well as the plant authorities, who promoted the opinion within the community and the plant itself that reactor accidents were virtually impossible.

Representatives of the western nuclear power industries were quick to point out...
Although only 32 died as a direct result of the explosion and fire, Ukrainian Health Minister Andrei Serdyuk said, in April of 1995, that the total death toll from the Chernobyl accident had reached 125,000, and disease rates have soared. That number is probably an exageration, because Ukraine is trying to get Western aid. Playing up the devastation may increase the amount of aid that will be granted. Nevertheless, clearly the toll has been much greater than 32. Children, pregnant women, and the 432,000 Chernobyl rescue workers have been especially hard-hit. For instance, there had been 533 cases of thyroid cancer among children since the accident, compared to the 30 to 40 that would have been expected in the same time span, had the accident not occurred. Numbers of blood diseases (such as leukemia) and diseases of the respiratory, digestive, and nervous systems have also soared. Approximately 160,000 people, who formerly lived within thirty kilometers of the plant, have since been resettled elsewhere.

Effects have been felt far outside the borders of Ukraine. For instance, there was much fallout of Cesium-137 in eastern Turkey, bordering the Black Sea where much of Turkey's tea is grown. Shortly after the accident, tea picked up radioactivity of up to 25,000 Bq per kilogram of dried tea leaves. By 1992, that activity had dropped to 200 Bq/kg. Whole-body exposure in 1986 would, for a typical tea drinker (which in Turkey means drinking a lot), amount to 0.66 mSv.