Nuclear Fission

Q3. A rod made from uranium-238 () is placed in the core of a nuclear reactor where it absorbs free neutrons. When a nucleus of uranium-238 absorbs a neutron it becomes unstable and decays to neptunium-239 (), which in turn decays to plutonium-239 ().

(a) Write down the nuclear equation that represents the decay of neptunium-239 into plutonium-239.

+ +

+ +

 

To gain the first mark the must be correct

The second mark is for the + or +

[2 marks]

(b) A sample of the rod is removed from the core and its radiation is monitored from time t = 0 s. The variation of the activity with time is shown below:

 

(i) Show that the decay constant of the sample is about 3.4 × 10^–6 s^–1.

T_½ = ln2 / λ

λ = ln2 / T_½

From the graph: (remember not to start at zero value for your calculation of half life!)

T_½ = (2.0 → 2.1) x 10⁵ s

λ = (3.3 → 3.5) x 10^-6 s^-1

You could substitute two points taken from the graph into A = A_oe^-λt

[2 marks]

(ii) Assume that the activity shown in the graph above comes only from the decay of neptunium.

Estimate the number of neptunium nuclei present in the sample at time t = 5.0 × 10⁵ s.

From the graph A is about 0.77 x 10 ¹² Bq at t = 5.0 × 10⁵ s.

A = Nλ

N = 0.77 x 10 ¹² / 3.4 x 10^-6

N = (2.2 → 2.4) x 10¹⁷nuclei

You could calculate it by using N_o = A_o/λ then use N = N_oe^-λt

[1 mark]

(c)

(i) A chain reaction is maintained in the core of a thermal nuclear reactor that is operating normally.

Explain what is meant by a chain reaction, naming the materials and particles involved.

A chain reaction happens when the product of a reaction is that which initialises it in the first place.

A uranium–235 nucleus captures a neutron and undergoes fission (It splits into two smaller nuclei, called fission fragments (NOT daughter nuclei!) and releases neutrons when it splits.

At least one of these neutrons produced in the original reaction go on to cause further fissions, thereby causing a chain reaction.

[2 marks]

(ii) Explain the purpose of a moderator in a thermal nuclear reactor.

A moderator slows down (reduces the kinetic energy of) neutrons produced in a fission reaction. These neutrons are travelling too quickly to be efficiently absorbed by a uranium nucleus therefore the moderator slows them down so that they can be easily absorbed by the uranium nuclei.

 

[2 marks]

(iii) Substantial shielding around the core protects nearby workers from the most hazardous radiations. Radiation from the core includes α and β particles, γ rays, X–rays, neutrons and neutrinos.

Explain why the shielding becomes radioactive.

Neutrons from the reaction vessel are absorbedby the nuclei in the shielding converting the nuclei of the shielding into unstable isotopes which emit radioactive particles.

 

[2 marks]

(Total 11 marks)