Past exam questions Flashcards
Binding energy is often given as…
…a positive number.
Explain the advantage of using the K:Ar ratio instead of the K:Ca ratio for dating rocks.
Ca is naturally present in most rocks, whereas Ar is not. Ar is therefore a much more reliable isotope to look for because it is a more accurate measure of the amount of K that has decayed.
Which activity of the material would have led the researchers to an age of 30,000 years?
Use A = A0e^-kt equation with 30000 years plugged in to get activity
State the main assumptions for the method of carbon dating.
Dead matter is NOT exchanging with the atmosphere, thus 14C activity decays
There is an equal global distribution of 14C
All living matter is in equilibrium with atmospheric 14C and has the same activity
Atmospheric carbon has a known activity at the time to be dated
Describe the major energy producing process in stars.
Hydrogen burning - occurs at > 10 million K and very high pressures. This leads to very slow fusion of hydrogen
4 1H –> 4He + 2 beta+
Describe the important difficulties for using fusion for energy production here on earth.
> > 10 million K temps are required, and fusion is still slow even at these temps
Also need containment of the high temperature at high pressure for long enough time. These containment fields and the heating cost a lot of energy so it is difficult to produce more energy than the process costs.
What can be used as an alternative fuel to H fusion?
2D + 3T –> 4He + n (this reaction has the lowest ignition T)
D = from water, T = from n capture (n + 6Li = 3T + 4He)
What can be used as a ‘catalyst’ for fusion?
12C (carbon cycle occurring in the sun)
Myons = ‘heavy’ electrons, bring D and T nuclei closer together. Ignition temp 1000 K.
How can 238U be used to produce energy?
238U + n –> 239Pu + 2 beta.
239Pu is fissile (similar to 235U - can ‘take up’ neutrons and undergo fission to give fission products and more neutrons for the chain reaction)
239Pu can be obtained from the above nuclear transformation
The 239Pu produced can be separated from U in the THORP process
Why do all radioactive fission products emit beta radiation but not alpha radiation?
All fission products have an n/p ratio that is too high (should be closer to 1:1 for elements smaller than U)
Beta- radiation reduces the n/p ratio but alpha radiation (4He) increases the n/p ratio (because n/p>1)
Explain why the radioactive isotopes of light elements are not alpha emitters.
Stable light elements have an n/p ratio of 1:1 (or close to it)
Alpha decay reduces both the p and n number by 2
Alpha decay for isotopes with n/p>1 would increase the n/p ratio further, and alpha decay for isotopes with n/p<1 would decrease the ratio further
This is moving away from the ideal 1:1 ratio so is unfavourable
Everything stored in the same room as uranium will become radioactively contaminated even without direct contact. Explain why this is.
The noble gas Rn is formed in the U decay series
The gas decays into solid radioactive products that will settle in the atmosphere surrounding the uranium – thus making it radioactive
What is the advantage of uranium oxides over the metal in the elemental form as nuclear fuel? Which oxide is preferred?
Oxides are chemically much more stable than the metal
Oxygen can also act as a moderator
UO2 is the preferred oxide
