ACT science practice test 10

Directions: Each passage is followed by several questions. After reading a passage, choose the best answer to each question and fill in the corresponding oval on your answer document. You may refer to the passages as often as necessary.

You are NOT permitted to use a calculator on this test.

There are four planets in our solar system called gas giants: Jupiter, Saturn, Uranus, and Neptune. They are so named because they are composed largely of gases rather than solids. Figure 1 shows how temperatures of the atmospheres of Jupiter, Neptune, and Saturn vary with altitude above the cloud tops. Table 1 gives the composition of the planets in both relative abundance of gases and the altitude at which those gases are most abundant. Table 2 gives what the temperature at the cloud tops would be without greenhouse warming.

Figure 1

Table 2
PlanetTemperature at cloud tops without greenhouse warming (K)

1. According to Figure 1, the temperature of Neptune remains the same as altitude above the highest cloud tops increases from:

F. -250 km to -200 km.
G. -150 km to -50 km.
H. 0 km to 100 km.
J. 150 km to 200 km.

2. According to Figure 1, the temperature of Jupiter changes the most between:

A. -150 km and -50 km.
B. -50 km and 50 km.
C. 50 km and 100 km.
D. 100 km and 200 km.

3. Considering only the gases listed in Table 1, which gas is more abundant in the atmosphere of Jupiter than in the atmosphere of either Neptune or Saturn?

F. H
G. CH3
H. NH3
J. He

4. Based on Table 2, the average temperature at Saturn's cloud tops without greenhouse warming is how many degrees cooler than the temperature given in Figure 1?

A. 5 K
B. 25 K
C. 75 K
D. 150 K

5. Which of the following statements about H and He in the atmospheres of the 3 planets is supported by the data in Table 1?

F. Both Saturn and Neptune have a higher relative abundance of He than of H.
G. Both Saturn and Jupiter have a higher relative abundance of He than of H.
H. Both Jupiter and Neptune have an equivalent relative abundance of He and H.
J. Both Saturn and Neptune have a lower relative abundance of He than of H.

Nuclear fission occurs when the nucleus (central core) of an atom splits into multiple parts. This splitting is accompanied by the release of a large amount of energy, as in nuclear weapons and nuclear power plants.

A chemical element is said to be radioactive if it is prone to fission. Fission is often the result of the nucleus of a radioactive atom absorbing a free neutron (an uncharged nuclear particle). When a fission event occurs, the nucleus often splits into two new nuclei and produces free neutrons. This process generates the possibility of a chain reaction. If, on average, a fission event produces one neutron and that neutron causes another nucleus to fission, the reaction is said to be critical; that is, it will sustain itself, but not increase in magnitude. If one fission event releases more free neutrons than are required to initiate another fission event, the reaction is said to be supercritical; that is, it will sustain and increase in magnitude. If more neutrons are required to initiate a fission event than are released in fission, the reaction is said to be subcritical: the reaction will not sustain itself.

Many factors affect how many neutrons from each fission event will trigger another fission event. The most important factor is the mass (m) of the substance. The criticality of a substance also depends on the substance's purity, shape, density, temperature, and whether or not it is surrounded by a material that reflects neutrons.

In a nuclear weapon, a radioactive substance is made highly supercritical. One of the primary challenges in building a nuclear weapon is keeping the radioactive material subcritical prior to detonation, then upon detonation, keeping it supercritical for a long enough period of time for all of the material to fission before it is blown apart by the energy of the blast. A fizzle occurs when a nuclear weapon achieves supercriticality but is blown apart before all of the radioactive material fissions.

The first nuclear weapons were made of enriched uranium, or U-235. The density (ρ) of U-235 under normal conditions is 19.1 g/cm3. For U-235 to attain a supercritical state, the product of its mass and density must exceed 106 g2/cm3. If it is assembled over too long a time (t), it will achieve slight supercriticality and then fizzle. Therefore, the speed of assembly (measured as t divided by ρ), must be less than 10-5 sec × cm3/g (Michelson's Criterion).

Two schemes for the assembly of a supercritical amount of U-235 that avoid fizzle are discussed below.

Gun-Type Weapon

At one end of a tube, similar to a gun barrel, is a hollow, subcritical cylinder of U-235 with a mass of 48 kg; on the other end is a subcritical pellet of U-235 with a mass of 12 kg. The pellet is propelled by a small explosion down the tube and into the cylinder of U-235. The combined mass of the two pieces of U-235 is great enough to induce a supercritical state. Since the combined cylinder of U-235 is at or near normal density, the assembly process must be completed in less than 2 × 10-4sec to meet Michelson's Criterion.

Implosion-Type Weapon

A 15-kg sphere of U-235 is surrounded by explosives. When the explosives are simultaneously detonated, the U-235 is compressed in order to achieve supercriticality. The explosives are designed to compress the U-235 to a density of approximately 70 g/cm3 in less than 10-7 sec.

6. For both types of weapon, avoiding fizzle is difficult because:

A. the mass of U-235 must be large.
B. 2 separate pieces of U-235 must be brought together.
C. U-235 is highly unstable.
D. of the speed with which the U-235 must be assembled.

7. Comparing the mass of uranium used in the two types of weapons reveals that:

F. the mass of U-235 used in the implosion-type weapon is less than the mass of U-235 used in the gun type weapon.
G. the mass of U-235 used in the implosion-type weapon is greater than the mass of U-235 used in the gun type weapon.
H. the mass of U-235 used in the implosion-type weapon is greater in some cases and less in some cases than the mass used in the gun-type weapon.
J. the mass of U-235 used in both weapons is approximately the same.

8. Both types of weapons use explosives in order to:

A. increase the heat of the U-235.
B. release the nuclear energy of the weapon from the confinement of the bomb's casing.
C. achieve supercriticality of U-235.
D. generate neutrons to start the chain reaction.

9. For an implosion-type weapon, when U-235 has reached supercriticality, to which of the following is the value of ρ closest?

F. 10-3 g/cm3
G. 0.1 g/cm3
H. 100 g/cm3
J. 106 g/cm3

10. In the implosion-type weapon, the explosives are used to:

A. trigger the first fission events.
B. heat the U-235 so it will become supercritical.
C. increase the density of U-235.
D. produce additional damage.

11. In order to achieve a supercritical state just before detonation, both methods:

F. increase the product of the mass and density of the U-235.
G. decrease the product of the mass and density of the U-235.
H. increase the amount of U-235 in the weapon.
J. decrease the time necessary for all the U-235 to fission.

12. Scientists are trying to build a bomb using only 8 kg of U-235. Presently they can achieve a ρ of 150 g/cm3 with t = 10-2 sec. Which of the following changes would be the most likely to get the weapon to meet Michelson's Criterion?

A. Decrease both t and ρ.
B. Decrease t and leave ρ the same.
C. Increase t and decrease ρ.
D. Increase t and leave ρ the same.