Physics 180

Nuclear Weapons, Nuclear War, and Arms Control

Frederick K. Lamb

Final Examination and Answers

Spring 1996

 

Name _________________________________________

ID No. _________________________

 

 

 

• This is a closed book examination of 2 hours duration.

• Answer all 10 questions. All count equally (20 points each).

• Write your answers in the space provided on these pages.

If you need more room, write on the back of the page.

For full credit on definitions, give numbers where appropriate.

 

 

 

1. ___20___ 6. ___20___

2. ___20___ 7. ___20___

3. ___20___ 8. ___20___

4. ___20___ 9. ___20___

5. ___20___ 10. ___20___

 

 

TOTAL SCORE ___200____

 

1. Nuclear Physics

Define 'fissionable nuclide' and name two. [5 points]

A fissionable nuclide can be caused to split apart (fission) by a neutron. All fissile nuclides are fissionable, by definition (for three examples, see the answer to the next part of the question). Two fissionable but non-fissile nuclides relevant to the course are U-238 and Th-232.

 

 

Define 'fissile nuclide' and name two. [5 points]

A fissile nuclide has some probability of being fissioned by a neutron of any energy. [A non-fissile but fissionable nuclide can only be caused to fission by neutrons with energies above the fission threshold energy.] Three fissile nuclides relevant to the course are U-233, U-235, and Pu-239.

 

 

Define 'critical mass'. What is the critical mass of a bare sphere of 100% Pu-239? About how large would a sphere of Pu-239 with this mass be? [5 points]

A critical mass is the smallest mass of an element that can sustain a chain reaction. The critical mass of a bare sphere of Pu-239 is about 10 kg (22 lb). Such a sphere is 5.2 cm in radius, or about the size of a baseball.

 

 

What is the difference between reactor-grade and weapon-grade plutonium? Can a weapon made of reactor-grade plutonium be made to explode? [5 points]

Reactor-grade [high burn-up] plutonium contains more of the isotopes Pu-240, Pu-241, and Pu-242 than does weapon-grade plutonium. [Full credit given for citing Pu-240 alone.]

Yes, reactor-grade plutonium can be made to explode.

2. Thermonuclear Weapons

a) Shown here is a schematic diagram of a standard thermonuclear weapon. Indicate on the diagram the locations of the following key components:

[10 points]

• 6LiD fusion packet

• U-238 case

• Fission trigger

• Tamper

• Styrofoam and reflectors

 

 

b) Describe briefly the roles of each of these five components. [10 points]

6LiD fusion packet: The thermonuclear 'fuel' of the weapon. When exposed to neutrons, the 6Li acts as a 'catalyst', providing more neutrons and and tritium to fuse with the deuterium (D), releasing energy.

U-238 case: Has two purposes: holds the weapon together, increasing the yield of the fusion packet; fissions when exposed to the intense neutron flux from the fusion reactions, adding to the fission yield of the weapon.

Fission trigger: Provides the X- and g-rays that compress and heat the fusion packet, causing the thermonuclear reaction to go.

Tamper: Slows propagation of the debris and shock wave from the fission trigger into the fusion packet, allowing the fusion reaction to proceed for some time before the fusion packet is disrupted.

Styrofoam and reflectors: Styrofoam holds the fusion packet in place and, when exposed to the X- and g-rays from the fission trigger, creates a high-pressure plasma that helps to compress the fusion packet. Reflectors direct X- and g-rays onto the surface of the fusion packet.

3. Weapon Effects

a) Give brief (one- or two-sentence) definitions of the following terms. [12 points]

• Ionizing radiation—Radiation (UV, X-rays, and g-rays) that is energetic enough to strip electrons from atoms and molecules.

 

• Physical dose—The energy deposited in the target tissue or material by the incident radiation. Measured in rads.

 

• Relative biological effectiveness (RBE)—The factor that describes the relative effectiveness of the particular type of radiation in causing the type of biological damage of interest; converts a dose in rads into a dose in rems.

 

• Chronic exposure—Exposure to radiation lasting more than 24 hours. [To be distinguished from acute exposure, which is exposure lasting less than 24 hours.]

 

• Half-life—The time after which there is a 50% probability that a given radionuclide has decayed. [For a macroscopic quantity of a given radionuclide, the time after which the amount still remaining has fallen to half the original amount. Note that the half-life can be defined only for a single radionuclide, since a mixture of radionuclides will have a mixture of half-lifes.]

 

• Neutron bomb—Technically called an enhanced radiation weapon (ERW), a "neutron bomb" is a thermonuclear nuclear weapon that has been designed to increase as much as possible the total number of neutrons produced when it explodes while decreasing as much as possible the strength of the blast. [This is done by minimizing the amount of material in the fission trigger and eliminating material, such as the fission blanket that is present in a "conventional' three-stage thermonuclear weapon, that would absorb neutrons, while increasing the relative amount of the fusion fuel.]

 

 

b) Answer briefly the following questions about nuclear winter. [8 points]

• Explain qualitatively what causes the effect called nuclear winter and how nuclear weapons may produce it. [3 points]

Fires ignited by nuclear explosions create soot, which is lofted into the stratosphere. The soot reduces the amount of solar energy that reaches the earth's surface and hence the temperature in the atmosphere below the stratosphere falls. The stability of the stratosphere and the lack of 'weather' there allow the soot to remain airborne for a long time (months to years).

 

 

• Three-dimensional (3D) calculations were found to give results quantitatively different from one-dimensional (1D) calculations. Describe the key differences. [2 points]


The temperature is reduced less, and for a shorter period of time. Freezing is not prevalent, making the result more of a "nuclear fall" than a "nuclear winter".

 

 

• List two important features or processes that cannot be included in 1D calculations but can be included in 3D calculations. Explain their importance. [3 points]


(1) Differences between land and ocean. The temperature of most interest is the temperatue over land, where people live and crops are grown, but most of the earth's surface is ocean. Oceans retain much greater amounts of heat than land, ameliorating the effects of lack of sunlight.

(2) Spreading of smoke in E-W and N-S directions, effects of winds and weather, including 'washout' of smoke. Smoke may take some time to spread over a whole continent, so that its effects are localized for some time. Winds may cause smoke to cover some areas but not others. Rains, including rains triggered by the smoke, may wash some of it out of the troposphere before it reaches high altitudes, where it would remain for a much longer time.

(3) Transport between the Northern and Southern hemispheres. This transport may take months, or may not ever be significant, in which case the areas not covered by smoke will not be directly affected.

4. ICBMs

a) List the four phases in the flight of a MIRVed ICBM and indicate the approximate duration of each in minutes. [8 points]

Phase Duration (Minutes)

 

i) Boost 1-5

 

ii) Post-boost [or bus] 5

 

iii) Midcourse 20

 

iv) Terminal 0.5-1

 

 

 

b) What is the approximate flight time in minutes for U.S. ICBMs to typical targets in the Soviet Union? If one was launched in error, could the U.S. do anything to prevent its warheads from exploding? [4 points]

30 minutes. No.

 

 

 

c) Define the terms circular error probable (CEP) and bias as used in discussions of ICBM warhead accuracy. [8 points]

 

CEP: The radius of a circle that encloses half (and only half) of the impact points of the RVs.

 

 

Bias: The distance between the aimpoint and the center of the distribution of RV impact points.

 

 

 

5. Nuclear Policies

Give short (one or two sentence) definitions of each of the following nuclear policies and give the approximate time interval, if any, during which it was declared U.S. policy (you may specify the interval by giving a range of years or presidential administrations).
[4 points each]

 

• Counterforce—A policy of emphasizing the ability to quickly launch devastating attacks on the leadership and nuclear forces of an adversary. Major elements of this policy include a large arsenal of prompt, hard-target killer weapons (powerful and accurate), a large menu of pre-programed targets, quick and effective early warning, communications, command, and control, and post-attack assessment capabilities, to permit shoot-look-shoot tactics to assure complete destruction of the adversary's forces. This has been operational U.S. policy since the mid-1960s and declared U.S. policy since the early 1970s.

 

• Assured destruction—A policy of deploying secure retaliatory nuclear forces of sufficient size to destroy any adversary, even after a first strike is absorbed. The emphasis is on the security of the forces and command and control, and their sufficiency for deterrence. This was declared U.S. strategic policy during 1963–1973.

 

• First use—A policy of attacking with nuclear weapons first, if one is losing a conventional war. This has been declared U.S. and NATO policy since the early 1950's. The term "first use" was introduced to avoid having to use the politically more unpalatable term "first strike" to describe this policy.

 

• Minimum deterrence—A policy of deterring nuclear attack by deploying only enough nuclear forces to cause unacceptable damage to the population and industry of any attacker. The emphasis of this policy is on avoiding an arms race while still deterring nuclear attack. Defense Secretary Robert McNamara reportedly considered 400 deliverable EMTs sufficient to deter the Soviet Union from attacking the U.S. This has never been U.S. policy.

 

• Flexible response—A policy of responding to any attack by choosing among a variety of options (weapons, targets, timing), including use of nuclear weapons to counter a conventional attack. This has been declared U.S. and NATO policy since 1965.

6. Nuclear Arms Race

In the early 1970s, the United States decided to deploy MIRVs on its long-range ballistic missiles.

 

• Describe two important technological developments that made this deployment possible. [5 points]

Technological developments that made possible deployment of MIRVs include (1) development of rockets able to place heavier payloads in earth orbit, (2) development of smaller and more accurate guidance systems that could fit inside a post-boost vehicle (PBV), and (3) development of PBVs to launch several satellites with a single rocket.

 

• What was the sequence of rationales used to justify the decision? Why did the rationale evolve? [5 points]

First justified as a penetration aid to defeat the crude Soviet ABM system being deployed around Moscow; dropped when ABMs were banned by the ABMT. Next justified as a bargaining chip to get the Soviets to negotiate seriously about banning MIRVs; dropped when Soviet began to seriously discuss banning MIRVs in the SALT I talks. Finally justified because the U.S. had a (temporary) technological lead.

 

• Name two important institutions or special interest groups that benefited from the decision in the short run? How did they benefit? [5 points]

Institutions or groups that benefited included (1) the Air Force, which got a new long-range nuclear weapon program and a commitment to a policy of counterforce and war-fighting weapons for the future; (2) the nuclear weapons laboratories, which got to design, develop, and test a whole series of new nuclear warheads; (3) the nuclear weapons industry, which got the money to build many new and more expensive nuclear weapons and delivery systems.

 

• Describe three important ways that this decision reduced U.S. security in the long run. [5 points]

This decision was disastrous for U.S. security in the long run. Important negative effects include (1) the acceleration of the arms race that followed the creation of thousands of new and "lucrative" targets and the perceived need to develop, test, and deploy new, mobile ICBMs in order to counter the theoretical vulnerability of silo-based ICBMs; (2) increased crisis instability caused by the fact that both sides now had strong theoretical incentives to strike first; (3) major new impediments to arms limits or reductions caused by the decision to build large numbers of new, more accurate, and more powerful weapons, by the perceived need to develop, test, and deploy mobile ICBMs, and by the verification difficulties created by the deployment of mobile missiles.

7.  Limitations on Nuclear Testing

Three treaties that limit nuclear testing are listed below. For each treaty—

• Give the year the agreement was signed

• Name the parties to the agreement

• List the key provisions of the agreement

• Describe one way compliance with the agreement can be monitored

 

 

• Limited Test Ban Treaty (LTBT) [6 points]

Year: Signed August 5, 1963; went into effect October 10, 1983.

Parties: U.S., U.K., and S.U. were original signatories; more than 100 nations have since signed.

Key provisions: Prohibits testing of nuclear weapons in space, underwater, and in the atmosphere; of unlimited duration (with usual escape clause).

One method of monitoring: Air-sampling of radionuclides/satellite detection of double optical flash. [either answer is acceptable]

 

 

• Threshold Test Ban Treaty (TTBT) [8 points]

Year: Signed July 3, 1974.

Parties: U.S. and S.U.; U.S. refused to ratify it until new verification protocols were signed in 1989. However, both parties made political commitments to abide by the Treaty.

Key provisions: Limits underground nuclear tests to yields no greater than 150 kt after March 1976. The parties agreed that one or two slight, unintended breaches per year would be a cause for concern but would not be considered a violation of the Treaty. A Protocol details data that would have been exchanged to asist verification and restricts testing to designated test sites (DTSs).

One method of monitoring: Remote seismic monitoring of explosion-induced ground motions; on-site inspections; hydrodynamic yield estimation. [any one answer is acceptable]

 

• Peaceful Nuclear Explosions Treaty (PNET) [6 points]

Year: Signed May 28, 1976

Parties: U.S. and S.U.; U.S. refused to ratify it until new verification protocols were signed in 1989. However, both parties made political commitments to abide by the Treaty.

Key provisions: Limits underground nuclear explosions outside DTSs to planned yields no greater than 150 kt after March 1976; limited the planned aggregate yield of multiple explosions to no more than 1500 kt. A detailed Protocol specified monitoring procedures, including advance notification and on-site hydrodynamic monitoring for multiple explosions with large aggregate yields. Established a Joint Consultative Commisssion to discuss any compliance issues.

One method of monitoring: Seismic monitoring of explosion-induced ground motions; on-site inspections; hydrodynamic yield estimation. [any one answer is acceptable]

 

 

 

 

8. Acronyms

Translate the following acronyms [2 points each]:

• START—Strategic Arms Reduction Talks

• SCC—Standing Consultative Commission

• SALT—Strategic Arms Limitation Talks

• SLBM—Submarine-launched baliistic missile

• ACDA—Arms Control and Disarmament Agency

• C3I—Communications, command, control, and intelligence

• EMP—Electromagnetic pulse

• SIOP—Single Integrated Operational Plan

• EMT—Equivalent megatonnage

• CTBT—Comprehensive Test Ban Treaty

9.  Nuclear Proliferation

List the five nations that openly possess nuclear weapons (declared nuclear weapons states). Give the year in which each first exploded a nuclear device. [10 points]

United States (1945), Soviet Union (1949), Great Britain (1952), France (1960), People's Republic of China (1964).

 

List the three nations that are thought to have nuclear weapons or near-nuclear capability, even though they do not acknowledge having a nuclear weapons programs (de facto nuclear weapon states). [3 points]

India, Pakistan, and Israel.

 

Which de facto nuclear weapon state recently gave up its nuclear weapons and joined the Non-Proliferation Treaty regime? [1 point]

South Africa.

 

Give short (two or three sentence) definitions of the following terms. [6 points]

• Enrichment—Physical processing of fissionable material by means such as gaseous diffusion, centrifugal separation, and laser separation, to increase the fraction of the material that is fissile. An example is enrichment of natural uranium (mostly U-238) to increase the fraction of nuclei in it that are fissile (U-235).

 

 

• Reprocessing—Chemical processing of fissionable material to separate desired nuclides from undesired nuclides. An example is processing of partially-used reactor fuel to separate Pu-239 from the fuel, for use in making nuclear weapons.

 

 

• Horizontal proliferation——Spread of nuclear weapons to formerly non-nuclear-weapon states.

 

10.  Non-Proliferation Treaty (NPT)

The Non-Proliferation Treaty divides treaty signatories into nuclear-weapon states and non-nuclear-weapon states.

What are the major restrictions on the nuclear-weapon states, under the terms of the treaty? [4 points]

NWS must not transfer nuclear weapons or control over nuclear weapons to any NNWS directly, or indirectly. [Article I]

NWS must not assist, encourage, or induce any NNWS to manufacture or otherwise acquire nuclear weapons or other nuclear explosive devices, or control over such weapons or explosive devices. [Article I]

 

What are the major obligations of the nuclear-weapon states, under the terms of the treaty? [4 points]

NWS agree to help NNWS with "potential benefits from any peaceful applications of nuclear explosions". [Article V]

NWS agree to pursue negotiations in good faith to end the nuclear arms race at an early date and to move toward nuclear disarmament. [Article VI]

 

What are the major restrictions on the non-nuclear-weapon states, under the terms of the treaty? [4 points]

NNWS must not receive nuclear weapons or other nuclear explosive devices, or control over such weapons or devices directly, or indirectly. [Article II]

NNWS must not manufacture or otherwise acquire nuclear weapons; and must not seek or receive any assistance in the manufacture of nuclear weapons or other nuclear explosive devices. [Article II]

NNWS must accept IAEA safeguards to prevent diversion of nuclear energy from peaceful uses to nuclear weapons or other nuclear explosive devices. [Article III]

 

Under the terms of the treaty, what conditions, if any, are there on the transfer or sale of nuclear reactors and related peaceful nuclear technologies to non-nuclear-weapon states? [4 points]

None. [Article IV]

 

Under the terms of the treaty, after 25 years a conference must be held at which the treaty must be extended indefinitely, extended for another definite period, or cancelled. What is the status of this conference? [4 points]

It was held in 1995 and extended the NPT indefinitely.