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Date November 2020 Marks available 1 Reference code 20N.2.SL.TZ0.6
Level Standard level Paper Paper 2 Time zone 0 - no time zone
Command term Show that Question number 6 Adapted from N/A

Question

One possible fission reaction of uranium-235 (U-235) is

U92235+n01Xe54140+Sr3894+2n01

Mass of one atom of U-235 =235u
Binding energy per nucleon for U-235 =7.59MeV
Binding energy per nucleon for Xe-140 =8.29MeV
Binding energy per nucleon for Sr-94 =8.59MeV

A nuclear power station uses U-235 as fuel. Assume that every fission reaction of U-235 gives rise to 180MeV of energy.

A sample of waste produced by the reactor contains 1.0kg of strontium-94 (Sr-94). Sr-94 is radioactive and undergoes beta-minus (β-) decay into a daughter nuclide X. The reaction for this decay is

Sr3894X+v¯e+e.

 

The graph shows the variation with time of the mass of Sr-94 remaining in the sample.

State what is meant by binding energy of a nucleus.

[1]
a(i).

Outline why quantities such as atomic mass and nuclear binding energy are often expressed in non-SI units.

[1]
a(ii).

Show that the energy released in the reaction is about 180MeV.

[1]
a(iii).

Estimate, in Jkg-1, the specific energy of U-235.

[2]
b(i).

The power station has a useful power output of 1.2GW and an efficiency of 36%. Determine the mass of U-235 that undergoes fission in one day.

[2]
b(ii).

Write down the proton number of nuclide X.

[1]
c(i).

State the half-life of Sr-94.

[1]
c(ii).

Calculate the mass of Sr-94 remaining in the sample after 10 minutes.

[2]
c(iii).

Markscheme

energy required to «completely» separate the nucleons
OR
energy released when a nucleus is formed from its constituent nucleons


Allow protons
AND neutrons.

a(i).

the values «in SI units» would be very small

a(ii).

140×8.29+94×8.59-235×7.59 OR 184«MeV» ✓

a(iii).

see «energy=»180×106×1.60×10-19 AND «mass=»235×1.66×10-27

7.4×1013«Jkg-1» ✓

b(i).

energy produced in one day=1.2×109×24×36000.36=2.9×1014«J»

mass=2.9×10147.4×1013=3.9«kg»

b(ii).

39


Do not allow X3994 unless the proton number is indicated.

c(i).

75«s»

c(ii).

ALTERNATIVE 1

10min=8t1/2 ✓

mass remaining=1.0×128=3.9×10-3«kg»

 

ALTERNATIVE 2

decay constant=«ln275=»9.24×10-3«s-1»

mass remaining=1.0×e-9.24×10-3×600=3.9×10-3«kg»

c(iii).

Examiners report

[N/A]
a(i).
[N/A]
a(ii).
[N/A]
a(iii).
[N/A]
b(i).
[N/A]
b(ii).
[N/A]
c(i).
[N/A]
c(ii).
[N/A]
c(iii).

Syllabus sections

Core » Topic 7: Atomic, nuclear and particle physics » 7.2 – Nuclear reactions
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