Fission and Fukushima part 1

Gordon J. Aubrecht, II

SOS/AAPT presentation, 30 April 2011

“Fukushima Dai-ichi”

Fukushima means “Blessed island”

Dai-ichi means No. 1Dai-ni means No. 2

Dai-san means No. 3Dai-yon means No. 4

etc.

What happened on 11 March?

Friday, March 11, 2011 at 05:46:23 UTCFriday, March 11, 2011 at 02:46:23 PM at epicenter

at a depth of 32 km

The earthquake of magnitude 9 (energy release ~ 2 EJ = 2 x 1018 J)

was followed by a tsunami that rose as high as 14 m!

Site of the earthquake

Date - UTC Time Latitude Longitude Magnitude Fatalities Region5/22/60 19:11 -38.29 -73.05 9.5 1655 Chile

3/28/64 3:36 61.02 -147.65 9.2 125 Prince William Sound, Alaska12/26/04 0:58 3.295 95.982 9.1 227898 off the west coast of N. Sumatra11/4/52 16:58 52.76 160.06 9 Kamchatka, Russia

3/11/11 5:46 38.322 142.369 9 28050 Near the E. Coast of Honshu, Japan

How many magnitude 9 or higher earthquakes do we know of?

(the magnitude scale was first devised in 1934)

“Japan faces power struggle,” Jeff

Tollefson, Nature 472, 143-144 (2011),

14 April 2011.

The tsunami hit the BWRs at Fukushima, and made the backup

generators inoperable.

This initiated catastrophic events.

To understand what happened, we need to understand something about the operation of reactors.

To do that, we need to get some background in how energy can be

liberated by combining or breaking up nuclei.

Fusion — energy liberated by combining nucleons into larger

nuclei.

Fission — energy liberated by breaking up nuclei into pieces.

Both happen because of binding energy. This is the energy that the

nucleons (constituents of the nucleus) give up to become part

of the nucleus.

It is the difference between the sum of the masses of protons,

neutrons, and electrons in an atom and the actual atomic mass of that

atom.

The first nucleons in the nucleus fall farthest in. The next don’t fall quite as far, and they continue to

fall a lesser distance in as the number of nucleons increases.

This occurs because of the Pauli Exclusion Principle, the same

reason that not all electrons in an atom are in the same shell.

It is the average binding energy per nucleon that counts.

On the left side of this curve, the nucleons on average give up more energy as A increases up to about

iron-56.

After iron-56, the nucleons on average give up less energy as A

increases.

If we’re forming nuclei on the left side of this curve, fusing

nucleons, energy is released—the binding energy.

Fusion holds promise on Earth as well, but there are still no energy-

producing fusion reactors operating.

In normal stars, elements up to iron are made (depending on size; our Sun will make only carbon in

its normal operation) by fusion inside the star. This is because

temperatures in the Sun’s center are millions of kelvin.

To get binding energy to be released on Earth, we need to go to the other end of the curve of

the binding energy.

Splitting something with A = 235,

we can produce, say, nuclei with A = 100 and 135 or other

combinations.

The nuclei with the smaller A are deeper on average than for A =

235, so energy, binding energy, is released when the bigger nucleus

breaks up.

The energy released is about 200 MeV per fission.

This energy must be captured. Typically, in a nuclear reactor,

water is used to capture much of the energy released.

There are two main types of reactor utilized: BWRs and

PWRs.

The six reactors at Fukushima are BWRs.

The GE BWR schematic. http://www.gereports.com/the-mark-i-containment-system-in-bwr-reacto

In general, for any harm caused by something:

Dose = Potency * Exposureand

Health risk = Dose * Exposed population

[= Potency * Exposure * Exposed population]

For exposure to radiation,Dose = Q * Exposure,

where Q is a quality factor establishing “the value of

absorbed dose of any radiation that engenders the same

risk as a given absorbed dose of reference radiation.”

The gray [Gy] is now the internationally accepted measure of absorbed energy from any type

of radiation: it is the exposure from radiation losing 1 J/kg of

material (such as tissue).This is actually exposure, but is

known as “absorbed dose.”

The sievert [Sv] is the internationally accepted measure of dose: it is the dose consequent

to an exposure to ionizing radiation undergoing an energy

loss of 1 J/kg of bodytissue [i.e., 1 gray].

This chart is in millisieverts

per hour.

Converted to mrem (1 Sv = 100 rem)

Now that we have some idea of reactor type and health risk, I will

turn to the details of the Fukushima disaster in part 2.