Nuclear MASINT is one of the six major subdisciplines generally accepted to make up measurement and signature intelligence (MASINT), which covers measurement and characterization of information derived from nuclear radiation and other physical phenomena associated with nuclear weapons, nuclear reactors, processes, materials, devices, and facilities. Nuclear monitoring can be done remotely or during onsite inspections of nuclear facilities. Data exploitation results in characterization of nuclear weapons, reactors, and materials. A number of systems detect and monitor the world for nuclear explosions, as well as nuclear materials production. . Methods here are relevant to national technical means of verification for nuclear arms control. Nuclear intelligence is one of the major MASINT disciplines:
As with much of MASINT, this discipline overlaps with others, especially materials MASINT and radiofrequency MASINT, and to some extent electro-optical MASINT and geophysical MASINT. While this subdiscipline would be concerned with direct measurement of ionization, materials MASINT [Materials MASINT#Nuclear test analysis |nuclear test analysis]], on the other hand, focuses on the field or reference laboratory analysis of samples from air sampling, contaminated sites, etc. Electro-optical MASINT would examine issues such as the light flash from an explosion, and the shock wave of the blast, trasmitted through air, water, and rock, is of interest to geophysical MASINT.
In particular, there is a narrow line between nuclear MASINT and the nuclear analysis techniques in materials MASINT. The basic difference is that nuclear MASINT deals with the characteristics of real-time nuclear events, such as nuclear explosions, radioactive clouds from accidents or terrorism, and other types of radiation events. A materials MASINT analyst looking at the same phenomenon, however, will have a more micro-level view, in doing such things as analyzing fallout particles from air sampling, ground contamination, or radioactive gases released into the atmosphere.
Some nuclear MASINT techniques are placed fairly arbitrarily into this subdiscipline. For example, measurement of the brightness and opacity of a cloud from a nuclear explosion is usually considered nuclear MASINT, but the techniques used to measure those parameters are electro-optical. The arbitrary distinction here considers nuclear MASINT a more specific description than electro-optical MASINT.
Radiation survey and dosimetry
- See also: Instrumentation for radioactivity
In nuclear war, after nuclear weapons accidents, and with the contemporary threat of "dirty bomb" radiological warfare, measuring the intensity of high-intensity ionizing radiation, and the cumulative dose received by personnel, is critical safety information. The survey function measures the type of active ionizing radiation present from :
While alpha particle emitters such as those in depleted uranium(DU) (i.e., uranium 238) are not a hazard at a distance, alpha particle measurements are necessary for safe handling of projectile dust, or of damaged vehicles with DU armor.
Survey of Environments that can be Monitored by Humans
No single type of instrumentation for radioactivity meets all military requirements, even at the tactical level. Different types are needed, variously, for:
- Health risk to individuals
- Analysis of nuclear materials that emit alpha particles
- Tritium survey
The basic field survey instrument that can detect alpha particles is an scintillometer, such as the AN/PDR-77, which comes with a set of probes variously intended for alpha, beta/gamma, and low-energy X-ray radiation. The X-ray probe allows detection of plutonium and americium contamination. "Knowing the original assay and the age of the weapon, the ratio of plutonium to americium may be computed accurately and so the total plutonium contamination may be determined. 
Different instruments, such as the AN/PDR-73 or AN/PDR-74, are used for tritium survey.
There are limits to what can be determined with portable equipment. For more complex analysis, either a transportable laboratory needs to be brought to the site, or, if safety permits, to have representative samples taken to a laboratory. Analysis of radioactive trace elements, for example, can help identify the source of fuel for a given contamination incident. Some of the less portable,, but powerful instrumentation includes gamma spectroscopy, [[low background alpha and beta counting and liquid scintillation counters for extremely low energy beta emitters such as tritium.
The DoD directive makes the distinction clear that detection is harder than measurement, and the latter is necessary for MASINT:
Nuclear radiation is not easy to detect. Radiation detection is always a multistep, highly indirect process. For example, in a scintillation detector, incident radiation excites a fluorescent material that de-excites by emitting photons of light. ... the quantitative relationship between the amount of radiation actually emitted and the reading on the meter is a complex function of many factors. Since those factors may only be controlled well within a laboratory. Such a laboratory either must be moved to the field, or samples brought to a fixed laboratory.
Detectors based on semiconductors, such as germanium, have better intrinsic energy resolution than scintillators, and are preferred where feasible for gamma-ray spectrometry. Neutron detection is improved by using hydrogen-rich scintillation counters, such those using a liquid rather than a crystal scintillation source.
Surveying High-Level Radioactive Areas
Some reactor accidents have left extremely high levels, such as at the Chernobyl Disaster or the Idaho SL-1. In the case of Chernobyl, many brave rescue and mitigation workers, some knowingly and some not, doomed themselves. The very careful cleanup of the SL-1, in a remote area and where the containment retained its integrity, minimized hazards.
Since those incidents and others, remotely operated or autonomous vehicle technology has improved.
Nuclear event analysis
- See also: materials MASINT
Monitoring nuclear tests involves both chemical analysis, part of materials MASINT, and analysis of the radioactive emissions of samples, which crosses materials and nuclear MASINT. Not all nuclear MASINT involves materials analysis; see space-based radiation and EMP MASINT sensors.
Nuclear tests, including underground tests that vent into the atmosphere, produce fallout that not only indicates that a nuclear event has taken place, but, through radiochemical analysis of isotopes in the fallout, characterize the technology and source of the device. MASINT collection of fallout is most commonly done with airborne dust traps, either on manned aircraft or drones.
The Air Force Technical Applications Center (AFTAC) was the focal point for MASINT surrounding the first Chinese nuclear test on 16 October 1964. Eleven of 15 Electromagnetic pulse sensors and microbarographs detected it; correlated EMP and acoustic data gave the location and yield, and materials MASINT of airborne debris confirmed it. Under a program called TOE DANCER, 85 sorties were flown between 16 October and 5 December, with the best sample collection coming from Japan-based aircraft on the day after the explosion. While the AFTAC document was heavily censored, the sample collection involved airborne trapping on filter paper, with subsequent analysis by mass spectrometry and another method that was not declassified. 
During FY 1974, SAC missions were flown to gather information on Chinese and French tests. U-2R aircraft, in Operation OLYMPIC RACE, flew missions, near Spain, to capture actual airborne particles that meteorologists predicted would be in that airspace. Another portion of this program involved a US Navy ship, in international waters, that sent unmanned air sampling drones into the cloud. So, in 1974, both U-2R and drone aircraft captured actual airborne particles from nuclear blasts for the MASINT discipline of nuclear Materials Intelligence  .
In the current US NBC tactical monitoring vehicle, derived from the German Fuchs, radiation detection, in the US variant, is built around the the AN/VDR-2 Radioactivity, Detection, Indication, and Computation (RADIAC) set, capable of measuring beta and gamma radiation both inside and outside the vehicle. This system was first used during Operation DESERT STORM.  This vehicle can keep up with moving troops, detecting liquid and vapor hazards. Newer versions have enhanced radiation survey, meteorological, chemical, and biological sensors, as well as computer support. The newer systems are intended for both a CBR battlefield and release other than attack (ROTA) events. ROTA events include industrial accidents as well as terrorist incidents. Its computer systems, complemented by meteorological information and signature information on the CBR agents, can predict propagation and report it using tactical symbols and NBC reports NATO standards ATP45(B). In the U.S. Army, however, it is being replaced by the M1135 nuclear, biological, chemical reconnaissance vehicle
For airborne sample collection, the pattern increasingly is to use Unmanned Aerial Vehicles (UAV). Still, for long-range missions, a U-2 or reconnaissance version of the C-135 (US) or Nimrod (UK) might be used.
It is not only important to detect that a nuclear event occurred, but what produced the event. In the context of the North Korean tests, one proposed method involved measuring xenon concentrations in the air. Xenon is a by-product of different fissionable materials' reactions, so could be used to distinguish if air sampling from a North Korean test, either atmospheric testing or leakage from an underground test, could be used to determine if the bomb was nuclear, and, if so, whether the Primary was plutonium or highly-enriched uranium (HEU).
Space-based Nuclear Energy Detection
In 1959, the US started to experiment with space-based nuclear sensors, beginning with the VELA HOTEL satellites. These were originally intended to detect nuclear explosions in space, using X-ray, neutron and gamma-ray detectors. Advanced VELA satellites added electro-optical MASINT devices called bhangmeters, which could detect nuclear tests on earth by detecting a characteristic signature of nuclear bursts: a double light flash, with the flashes milliseconds apart. Using Radiofrequency MASINT sensors, satellites also could detect electromagnetic pulse (EMP) signatures from events on Earth.
Several more advanced satellites replaced the early VELAs, and the function exists today as the Integrated Operational Nuclear Detection System (IONDS), as an additional function on the MILSTAR satellites used for GPS navigation information.
Effects of Ionizing Radiation on materials
Beyond immediate biological effects, ionizing radiation has structural effects on materials.
While nuclear reactors are usually in sturdy housings, it was not immediately realized that long-term neutron bombardment can embrittle steel. When, for example, ex-Soviet submarine reactors are not given full maintenance or decommissioning, there is a cumulative hazard that steel in the containment, or piping that can reach the core, might lose strength and break. Understanding those effects as a function of radiation type and density can help predict when poorly maintained nuclear facilities might become orders of magnitude more hazardous. .
The challenge to the nuclear MASINT analyst is determining when nuclear reactors, and associated equipment, has received sufficient radiation that radiation-induced embrittlement will make the installation impossible or difficult to use safely. The most significant concern is with fast neutrons that, under high pressure and temperature, make reactor vessels and plumbing prone to breakage under operating conditions.
Because of the obvious safety implications brought about by a potential breach in the pressure vessel’s integrity, the nuclear MASINT analyst will predict the availability of nuclear power systems. For example, the power reactors on former Soviet submarines and surface ships may be predicted to make it unsafe to use that ship operationally.
Damage to Semiconductors
Ionizing radiation can destroy or reset semiconductors. There is a difference, however, in damage done by ionizing radiation and by electromagnetic pulse. Electromagnetic Pulse (EMP) MASINT is a discipline that is complementary to nuclear MASINT.
- US Army (May 2004), Chapter 9: Measurement and Signals Intelligence, Department of the Army
- Office of the Assistant to the Secretary of Defense for Nuclear and Chemical and Biological Defense Programs (February 22, 2005). Nuclear Weapon Accident Response Procedures (NARP).
- United States Department of Defense, DoD 3150.8-M, "Nuclear Weapon Accident Response Procedures (NARP)"
- Burr, William, ed. (23 March 2000), Excerpts, "History of the Air Force Technical Applications Center (AFTAC) 1 July-31 December 1964", The Chinese Nuclear Weapons Program: Problems of Intelligence Collection and Analysis, 1964-1972, vol. National Security Archive Electronic Briefing Book No. 26. p. 25
- History Division, Strategic Air Command, SAC Reconnaissance History, January 1968-June 1971. Retrieved on 2000-10-16
- Office of the Historian, Strategic Air Command, History of SAC Reconnaissance Operations, FY 1974
- Special Assistant for Gulf War Illnesses Medical Readiness, and Military Deployments (March 14, 2001). Information Paper: The Fox NBC Reconnaissance Vehicle. US Department of Defense.
- Zhang, Hui (July 2007). Off-Site Air Sampling Analysis And North Korean Nuclear Test. Institute for Nuclear Materials Management 48th Annual Meeting. Belfer Center, John F. Kennedy School of Government, Harvard University.
- Council on Ionizing Radiation Measurements and Standards (December 2004). Fourth Report on Needs in Ionizing Radiation Measurements and Standards.