3. Strontium-90 is present in spent nuclear fuel and in radioactive waste from n
ID: 1032209 • Letter: 3
Question
3. Strontium-90 is present in spent nuclear fuel and in radioactive waste from nuclear reactors and also as fallout from nuclear tests. It is a beta-emitter. The following graph shows the decay of 100 g of strontium-90 over a period of 144 years. 100 75 50 25 144 Time (years) (a) Deduce the nuclear equation for the decay of strontium-90 (b) Explain how the graph demonstrates that the radioactive decay of strontium-90 is kinetically a first order process. (c) Calculate the mass of strontium-90 remaining after 200 years. (d) Suggest why strontium-90 is a particularly hazardous radioactive isotope to humans.Explanation / Answer
a) 90Sr undergoes radioactive decay by emitting electrons and the beta decay equation is given as:
3890Sr 3990Y + -10e
The -10e denotes a negative (-ve) beta particle or an electron. As 90Sr decays, it forms 90Y or Yttrium-90 which has a half life of 64 hours. The 90Y decay results into a stable substance called Zirconium (Zr).
b) A first order reaction is a chemical reaction in which the rate of reaction is directly proportional to the concentration of the reacting substance. Looking at the given graph we can see that the concentration of the reactants decreasing with time. In radioactive decay, the number of atoms decaying in unit time is proportional to the total number of radioactive atoms present at that time.
N = - ( decay constant) N t
(N/t) = - (decay constant) N
We can see that radioactive decay is first order kinetics because the decay rate is proportional to the first power of the radioactive atoms present
c) The time taken by a decaying radioactive substance to reduce to half of its original amount is referred to as the half life of that substance.
90Sr has the half life of 28.8 years.This means that it takes 28.8 years to decrease the amount of 90Sr by half of its initial amount.
If the original amount is x and the half life is 28.8, then after 200 years
200/28.8=6.94 ; This means that it will go through approximately 7 half lives.
xg----1st half life-----x/2--------2nd half life-----x/4------3rd half life----x/8-------4th half life--------x/16----5th half life------x/32------------6th half life--------x/64------------7th half life---------x/128
Therefore after 200 years, x/128g will be left behind.
d) Strontium-90 is considered very harmful for humans especially if it is inhaled or ingested accidently. If it enters the human body through the food or water, because of its similarity to calcium, it gets absorbed by the bones and teeth. Because of its long half life of 28.8 years it damages the body for a very long time. Bone cancers and damage to bone marrow is commonly seen.
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