[biobob21]

which of the following is a way to control a uranium-235 chain reaction?
a. add an element, such as cadmium, that absorbs neutrons
b. enclose the critical mass of uranium inside a container made of lead
c. increase the concentration of the reaction products to shift the equilibrium towards reactants
d. compress the uranium into a small volume so that most of the neutrons escape

i think its A not sure though also need help on these questions

A sample of strontium-90 is found to have decayed to 1/8 of its original amount after 97.3 years
a. 10.9 years
b. 28.1 years
c. 43.6 years
d. 174.6 years

i think its a not sure though
12. The half life of c14 is 5715 years. How long will it be until only half of the c14 in a sample remains?

13. why do positron emission and eletron capture have th same effect on a nucleus?
14. how do the structures of protons and neutrons differ with respect to quarks?
why is the constant rate of deay of radioactive nuclei so important to radioactive dating?
16. what is the relationship between mass defect and binding energy? 17. the halflife of iodine131 is 8.02 days. what percentage of iodine remains after 40.2 days

thanks n i appreciate any help you give to me 

[juanrga]

which of the following is a way to control a uranium-235 chain reaction?
a. add an element, such as cadmium, that absorbs neutrons
b. enclose the critical mass of uranium inside a container made of lead
c. increase the concentration of the reaction products to shift the equilibrium towards reactants
d. compress the uranium into a small volume so that most of the neutrons escape

i think its A not sure though also need help on these questions

Yes, strong neutron absorbers as Cadmium and Boron are the most common materials used in controlling chain reactions.

[fledarmus]

You should be able to think your way through most of these! For example, why do you put lead shielding around a reactor? What does that do? What happens when you compress U-235 into a small volume?

Your second question seems like something is missing - I don't see an actual question there.

for question 12, what does "half life" mean?

for question 13, what happens to produce a positron emission? What happens during an electron capture?

for question 15, what would happen to (for example) carbon dating if radioactive decay was not constant?

for question 17, again, what does "half life" mean?