Related Rule
United States of America
Practice Relating to Rule 42. Works and Installations Containing Dangerous Forces
Section A. Attacks against works and installations containing dangerous forces and against military objectives located in their vicinity
The US Air Force Pamphlet (1976) states:
In view of the general immunity of the civilian population and civilian objects and the requirement of precautions to minimize injury or damage to them, many states have urged a rule absolutely prohibiting attacks upon works and installations containing “dangerous forces”, such as water held by a dam or radioactive material from a nuclear generating station, if the attack would release such dangerous forces. The United States has not accepted that such a rule, prohibiting attacks on works and installations containing dangerous forces, exists absolutely if, under the circumstances at the time, they are lawful military objectives. Of course their destruction must not cause excessive injury to civilians or civilian objects. Under some circumstances attacks on objects such as dams, dykes and nuclear electrical generating stations may result in distinct and substantial military advantage depending upon the military uses of such objects. Injury to civilians may be nonexistent or at least not excessive in relation to the military advantage anticipated. However, there are clearly special concerns that destruction of such objects may unleash forces causing widespread havoc and injury far beyond any military advantage secured or anticipated. Target selection of such objects is accordingly a matter of national decision at appropriate high policy levels. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 5-3(d).
The US Air Force Commander’s Handbook (1980) states:
Protocol I to the 1949 Geneva Conventions restricts attack against dams, dikes, and nuclear power stations, if “severe” civilian losses might result from flooding or radioactivity. While the United States is not yet a party to this protocol, such attacks may be politically sensitive. Consult the Staff Judge Advocate for the exact status and provisions of Protocol I and the exceptions to its rules (see also paragraph 3-8 [collateral damage] …). 
United States, Air Force Pamphlet 110-34, Commander’s Handbook on the Law of Armed Conflict, Judge Advocate General, US Department of the Air Force, 25 July 1980, § 2-3(c).
The US Naval Handbook (1995) states:
Dams, dikes, levees, and other installations, which if breached or destroyed would release flood waters or other forces dangerous to the civilian population, should not be bombarded if the potential for harm to noncombatants would be excessive in relation to the military advantage to be gained by bombardment. Conversely, installations containing such dangerous forces that are used by belligerents to shield or support military activities are not so protected. 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-2.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Transportation, US Coast Guard, October 1995 (formerly NWP 9 (Rev. A)/FMFM 1-10, October 1989), § 8.5.1.7; see also § 8.1.2.
The Annotated Supplement to the US Naval Handbook (1997) states:
Attacks on [works and installations containing dangerous forces] are, of course, subject to the rule of proportionality … The practice of nations has previously indicated great restraint in the attacks of dams and dikes, the breach of which would cause such severe civilian losses … See, however, the U.K. destruction of the Ruhr dams during WW II … For an example of U.S. application of this principle in the Vietnam conflict, see President Nixon’s news conference of 27 July 1972. 
United States, Annotated Supplement to the Commander’s Handbook on the Law of Naval Operations, prepared by the Oceans Law and Policy Department, Center for Naval Warfare Studies, Naval War College, Newport, Rhode Island, November 1997, § 8.5.1.7, footnote 125.
The US Naval Handbook (2007) states:
Dams, dikes, levees, and other installations, which if breached or destroyed would release flood waters or other forces dangerous to the civilian population, should not be bombarded if the anticipated harm to civilians would be excessive in relation to the anticipated military advantage to be gained by bombardment. Conversely, installations containing such dangerous forces that are used by belligerents to shield or support military activities are not so protected. 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-12.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Homeland Security, US Coast Guard, July 2007, § 8.9.1.7.
It is reported that during the Korean War, the US air force regularly targeted dams in order to flood transport routes and other communications lines. 
Robert F. Futrell, The United States Air Force in Korea 1950–1953, Office of Air Force History, United States Air Force, Washington, D.C., Revised edition, 1983, pp. 668–669.
It is reported that during the Vietnam War in 1972, the United States planned to attack a hydroelectric plant at Lang Chi, which was estimated to supply up to 75 per cent of Hanoi’s industrial and defence needs. If the dam at the site were breached, as many as 23,000 civilians could have died in the resultant flooding. The US President’s military advisers estimated that if laser-guided bombs were used, there was a 90 per cent chance of the mission being accomplished without breaching the dam. On that basis, the US President authorized the attack, which destroyed the electricity generating plant without breaching the dam. 
W. Hays Parks, “Air War and the Law of War”, The Air Force Law Review, Vol. 32, 1990, pp. 168–169.
In 1987, the Deputy Legal Adviser of the US Department of State stated: “We do not support the provisions of Article 56 of the 1977 Additional Protocol I, concerning dams, dykes, and nuclear power stations … nor do we consider them to be customary law.” 
United States, Remarks of Michael J. Matheson, Deputy Legal Adviser, US Department of State, The Sixth Annual American Red Cross-Washington College of Law Conference on International Humanitarian Law: A Workshop on Customary International Law and the 1977 Protocols Additional to the 1949 Geneva Conventions, American University Journal of International Law and Policy, Vol. 2, 1987, p. 427.
With respect to the apparent inconsistency between the US rejection of the provisions in Article 56 of the 1977 Additional Protocol I and the simultaneous acceptance of Article 15 of the 1977 Additional Protocol II, he stated:
The United States military based its objections on a pragmatic, real-world estimation of the difference between the two situations. The military perceives that in international conflicts, many situations may arise where it is important to attack and destroy parts of an electric power grid, such as a nuclear or hydroelectric generating station. In internal conflicts, on the other hand, such a significant real-world need will not exist. Preserving the military option in international conflicts where such facilities are more likely to become an object of military attack, therefore, is very important. 
United States, Remarks of Michael J. Matheson, Deputy Legal Adviser, US Department of State, The Sixth Annual American Red Cross-Washington College of Law Conference on International Humanitarian Law: A Workshop on Customary International Law and the 1977 Protocols Additional to the 1949 Geneva Conventions, American University Journal of International Law and Policy, Vol. 2, 1987, p. 434.
The Deputy Legal Adviser further stressed:
All other rules of war designed for the protection of civilian populations, such as the rule of proportionality and the rule of reasonable precautions and advanced warning, govern these attacks [against works and installations containing dangerous forces]. The United States maintains the position that it cannot accept the almost total prohibition on such attacks contained in article 56. In any case, in situations where the United States military targets a part of the power grid connected to a hydroelectric or nuclear facility, the United States would have to consider the possible effects on the civilian population and strive to obtain its military objective in ways that would not inflict drastic effects on that population. 
United States, Remarks of Michael J. Matheson, Deputy Legal Adviser, US Department of State, The Sixth Annual American Red Cross-Washington College of Law Conference on International Humanitarian Law: A Workshop on Customary International Law and the 1977 Protocols Additional to the 1949 Geneva Conventions, American University Journal of International Law and Policy, Vol. 2, 1987, p. 434.
In 1987, the Legal Adviser of the US Department of State stated:
Article 56 of Protocol I is designed to protect dams, dikes, and nuclear power plants against attacks that could result in “severe” civilian losses. As its negotiating history indicates, this article would protect objects that would be considered legitimate military objectives under customary international law.
Attacks on such military objectives would be prohibited if “severe” civilian casualties might result from flooding or release of radiation. The negotiating history throws little light on what level of civilian losses would be “severe”. It is clear, however, that under this article, civilian losses are not to be balanced against the military value of the target. If severe losses would result, then the attack is forbidden, no matter how important the target. It also appears that article 56 forbids any attack that raises the possibility of severe civilian losses, even though considerable care is take to avoid them.
Paragraph 2 of article 56 provides for the termination of protection, but only in limited circumstances. If it is once conceded that a particular dam, dike, or nuclear power station is entitled to protection under article 56, that protection can only end if it is used “in regular, significant, and direct support of military operations”. In the case of nuclear power plants, this support must be in the form of “electric power”. The negotiating history refers to electric power for “production of arms, ammunition, and military equipment” as removing a power plant’s protection, but not “production of civilian goods which may also be used by the armed forces”. The Diplomatic Conference thus neglected the nature of modern integrated power grids, where it is impossible to say that electricity from a particular plant goes to a particular customer. It is also unreasonable for article 56 to terminate the protection of nuclear power plants only on the basis of the use of their electric power. Under this provision, a nuclear power plant that is being used to produce plutonium for nuclear weapons purposes would not lose its protection. 
United States, Remarks of Judge Abraham D. Sofaer, Legal Adviser, US Department of State, The Sixth Annual American Red Cross-Washington College of Law Conference on International Humanitarian Law: A Workshop on Customary Law and the 1977 Protocols Additional to the 1949 Geneva Conventions, American University Journal of International Law and Policy, Vol. 2, 1987, pp. 468–469.
In 1991, in response to an ICRC memorandum on the applicability of IHL in the Gulf region, the US Department of the Army stated:
While the U.S. shares the concern expressed in Article 56 of Protocol I regarding carrying out an attack against a target that may result in release of ‘dangerous forces’, targeting decisions regarding the attack of such facilities are policy decisions that must be made based upon all relevant factors. … The U.S. does not recognize a protected status for enemy air and ground defenses placed in proximity to structures containing such ‘dangerous forces’. 
United States, Letter from the Department of the Army to the legal adviser of the US Army forces deployed in the Gulf region, 11 January 1991, § 8(Q), Report on US Practice, 1997, Chapter 1.9.
In 1991, in a report submitted to the UN Security Council on operations in the Gulf War, the United States stated that nuclear storage facilities were considered to be a legitimate military target. 
United States, Letter dated 8 February 1991 to the President of the UN Security Council, UN Doc. S/22216, 13 February 1991, p. 1.
During the Gulf War, the US air force struck research reactors that were under International Atomic Energy Agency (IAEA) safeguards. US officials declared that the United States was not bound by any obligation prohibiting attacks on nuclear research facilities. 
Report on US Practice, 1997, Chapter 1.9.
A press release referred to official statements recalling that the United States had signed but not ratified the 1977 Additional Protocol I and, as a result, had made no commitments not to attack nuclear facilities. 
Report on US Practice, 1997, Chapter 1.9.
According to the Report on US Practice, the United States does not apply special restrictions on attacks against works or installations containing dangerous forces. The report states:
It is the opinio juris of the United States that attacks are governed by the same legal criteria as attacks against any other military targets. In non-international armed conflicts, the United States regards Article 15 of Additional Protocol II as establishing an appropriate standard. 
Report on US Practice, 1997, Chapter 1.9.