Portal monitors can detect unshielded plutonium and HEU.

In the early 1950s, when the Soviet Union had tested a nuclear weapon but did not yet have bombers or missiles that could reach the United States, there was substantial concern, as there is today, about the possibility that a nuclear bomb could be smuggled into a U.S. city. J. Robert Oppenheimer, the scientific leader of the Manhattan Project, was asked, if presented with a large cargo package, what technology he would recommend using to determine if it contained a nuclear bomb. "A screwdriver," he replied—meaning that finding the bomb would require actually opening the package. A high-level team was put together to examine the means by which smuggled nuclear weapons could be detected, in what was known by some as the "screwdriver report."[1] The team identified several basic ideas for nuclear detection—and though technology has advanced enormously in the intervening decades, the key concepts are still the same, relying on the properties of nuclear weapons materials described above.

Potential nuclear explosive materials are radioactive; they are very dense, and therefore absorb certain types of radiation very well; and they can be fissioned. Therefore there are three basic ways of detecting them: "passive" detection of the radiation they emit; "active" detection of very dense objects in a package (such as with an X-ray machine), which then require further investigation; or "active" detection by irradiating the object and observing whether any fissions result.[2]

For the purposes of searching large areas for potential hidden nuclear weapons or materials, only passive detection is plausible: it is simply not possible to bathe large areas in X-rays or neutrons looking for nuclear bombs. Even for applications where particular objects are to be inspected, such as a vehicle or a cargo container, active interrogation by neutron or gamma-ray beams will often not be possible, for reasons of cost, complexity, and intrusiveness: in many cases, both those doing the inspection and those being inspected will object to using beams of radiation as part of the inspection.

Unfortunately, nuclear weapons and the materials to make them are quite difficult to detect at any substantial range (particularly if equipped with radiation shielding, such as a layer of lead), as plutonium and highly enriched uranium are not intensely radioactive. It must be remembered that to detect a nuclear weapon or nuclear material, a detector must not only be able to detect the radiation from this source, but also distinguish it from the natural background of radiation—placing fundamental limits on what can be detected. The decay rate—and therefore the rate of emission of radioactivity—of Pu-239, with its 24,000-year half-life, is hundreds of times less than that of 30-year half-life Cs-137. The decay rate of U-235 is 30,000 times lower than that of Pu-239. Table 2 shows the remarkably low rate of neutron and gamma emissions from weapon-grade HEU, compared with those from plutonium. In addition to having a low rate, the principal gamma ray from U-235 has a low energy as well,[3] making it easy to shield the material to avoid detection (this gamma ray will travel through lead, on average, for only a millimeter); a daughter product of U-238 emits a more penetrating gamma ray, but such a signal would only indicate the presence of an unusual amount of uranium, not the presence of highly enriched uranium. In short, HEU is quite difficult for passive detectors to find: for example, the pager-sized radiation detectors that have been distributed to customs agents in many countries would have no chance of detecting HEU with even a very small amount of shielding.[4] Plutonium is substantially easier for passive systems to detect, since it has dramatically higher neutron and gamma ray emissions.

Impurities in these materials can have a huge effect on their detectability. For example, most of the HEU in the world (including most of the U.S. and Russian HEU stockpiles) was enriched from uranium that had previously been irradiated in a reactor. As a result, the HEU is contaminated with tiny quantities of U-232 (likely in the parts-per-trillion to parts-per-billion range). One of the daughter products of U-232 emits an extremely penetrating gamma ray,[5] which is easy to detect and would penetrate many types of shielding. Hence, if this contaminant is present, HEU can be detected more easily, and shielding is more difficult. Unfortunately, however, there is a significant presence of the same gamma ray in the natural background, increasing the number of these gamma rays that would have to be detected to distinguish the signal from HEU from the natural background.[6] (Indeed, some U.S. analysts have proposed "tagging" the world's HEU stockpiles with U-232 to make smuggling of HEU easier to detect.)[7] Other impurities can cause reactions in which alpha particles from uranium or plutonium cause lighter elements to release neutrons, significantly increasing neutron count rates. In the case of plutonium, reactor-grade plutonium, with its higher Pu-240 concentration, has very much higher rates of both neutron and gamma ray emissions than weapon-grade plutonium, making it substantially easier to detect. (See Table 2.)

For obvious secrecy and security reasons, radiation measurements on real warheads are rarely published. In the so-called "Black Sea Experiment" in the late 1980s, however, Soviet authorities allowed Soviet and U.S. experts to take measurements from the nuclear warhead on a ship-based cruise missile, to assess its detectability.[8] With the hand-held and transportable gamma ray detectors the American team used in the experiment, they concluded that the warhead could only be detected about 6 meters away. Soviet scientists using a larger helicopter-borne neutron detector, however, were able to detect a neutron signal significantly above the background rate more than 70 meters above the ship.[9] That neutron signal was coming essentially entirely from the plutonium in the warhead—an all-HEU warhead would not have been detected by that means, and would have been detectable at even shorter range by the hand-held and transportable gamma detectors. In short, given the fundamental physics of the situation, finding hidden nuclear weapons at ranges of more than a few hundred meters from the detector—and much less for HEU or an HEU bomb—is simply a job that cannot be done. To find a hidden nuclear weapon, or the materials to make one, you need to know at least roughly where to look.

Imagine two important scenarios in which nuclear detectors would be expected to play key roles. In the first, either a threat is made or intelligence is gathered that suggests there is a nuclear weapon hidden in a major city, such as New York. Being prepared to deal with such a situation is the job, in the United States, of the Nuclear Emergency Search Team (NEST), who are equipped with the most advanced nuclear detectors available. Nevertheless, if the only information available was that the bomb was somewhere in a particular city, there would be very little hope of finding it—the area to be searched is simply too great, and the range at which the bomb could be detected too short. If, however, additional information was available—e.g., the bomb is probably in one of these three blocks of Manhattan—the probability of finding it would be greatly increased. If such a weapon were found, NEST is equipped with a variety of techniques to disable the bomb, designed to attempt to do so fast enough to get it disabled before any booby-trap could set it off – but given that no one would know what such a bomb's design looked like ahead of time, there would always be some risk in such an effort.

In the second scenario, the goal is to prevent nuclear weapons or the materials to make them from being smuggled across borders, or into the United States. In this case, detectors exist that will reasonably reliably detect unshielded plutonium or HEU in a car going through a border crossing, in a suitcase at an airport, and the like—in particular vehicle and pedestrian "portal monitors." It is easier to detect material that some one on foot is carrying than material in a car, especially if the car is moving at a significant speed. Tests on one typical brand of portal monitors determined that in the absence of any shielding, the pedestrian monitors could detect 10 grams of weapon-grade HEU or a third of a gram of weapon-grade plutonium, while vehicle monitors allowing the vehicles to proceed through at 8 kilometers per hour could detect 1 kilogram of HEU or as little as 10 grams of plutonium.[10] Recent tests, however, indicated that a large fraction of the equipment available on the market did not meet minimum standards for high sensitivity and low false alarm rate that had been agreed between the suppliers and the International Atomic Energy Agency—though many of the suppliers were able to upgrade their equipment to meet the agreed standards after failing the initial tests.[11] Particularly in the aftermath of the September 11 attacks, substantial research is being directed toward the development of better detectors that will be more sensitive, smaller, cheaper, and provide more detailed assessment of the material at hand.[12] Approaches to detecting nuclear material in larger packages such as cargo containers are now being tested. In many such situations—from inspecting bags at an airport to inspecting trucks at a border station—equipment is available for X-raying the contents, so that any dense material used for shielding the radiation from a nuclear bomb or nuclear material would be detected, and would then be subject to more detailed inspection.

In some of these applications, active neutron interrogation of the inspected package would be useful, to ensure that shielded HEU could be detected—but as noted above, such active systems, which emit radiation, may face strong opposition from both the inspectors and the inspected. More ambitious concepts are being developed that might allow detection of at least some types of nuclear material being brought through a city: for example, concepts have been developed for a network of detectors along a road that cars would drive by at normal speed. While each individual detector could not get a strong signal to reliably pick out nuclear materials from the surrounding background, with an intelligent signal processing system, a series of "hits" proceeding along the road at the same speed as the traffic could be detected. Of course, in any of these scenarios, if the subject of the search is intensively radioactive material that might be used in a "dirty bomb," such as cobalt-60 or cesium-137, rather than nuclear materials for an actual nuclear explosive, the probability of detection, and the difficulty of shielding, would be far higher.

Such concepts should certainly be pursued, and there is much that can and should be done to improve capabilities to interdict nuclear smuggling. But the realities of thousands of kilometers of border, millions of vehicles and cargo containers going across those borders every year, very short detection ranges for nuclear material, and intelligent attackers able to observe and avoid security measures that are put in place make the problem extraordinarily challenging. Even if every cargo container entering the United States were rigorously inspected with nuclear detectors long before it reached U.S. shores, for example, terrorists could easily put a bomb in the hold of an ocean-going yacht and sail it up the Potomac or the Hudson rivers. The greatest technical leverage on the problem of nuclear terrorism is in ensuring that nuclear weapons and their ingredients stay at the facilities where they are supposed to be, so that terrorist groups cannot gain access to them in the first place.

Protection, Control, and Accounting of Nuclear Materials and Nuclear Weapons

The security and accounting systems for nuclear weapons and nuclear material implemented domestically by states are designed to prevent theft of nuclear material by unauthorized parties—e.g., a gang of outsiders trying to sneak or shoot their way in, or insiders trying to smuggle something out. International safeguards, by contrast, are designed to detect (and thereby, one hopes, deter) diversion of nuclear material from peaceful purposes to a prohibited nuclear weapon program by the state itself. In traditional safeguards over nuclear material, this is done by examining the nuclear material accounting records prepared by the state itself, coupled with taking enough independent measurements to make a judgment as to their accuracy.[13]

Technologies for these purposes are generally divided into three basic categories:

• physical protection systems, including fences, barriers, sensors, vaults, access control systems, and armed response forces, are designed to provide detection, delay, and response: detect an insider or outsider thief early on in the course of their attempted theft, and delay the thief or thieves from getting the material long enough for protective forces to respond and defeat the attempted theft.
• material control systems, such as tags and seals on nuclear material containers, security cameras, portal monitors, "two-man-rule" procedures, and the like, are designed to ensure that any removal of nuclear material or unauthorized actions with these materials would be quickly detected.[14]
• material accounting systems are designed to measure the amounts and characteristics of nuclear material on hand, to confirm that the other two types of systems have worked and that no nuclear material has been removed (and equivalently, in the case of international safeguards, to provide assurance that all declared nuclear material is still present and accounted for, and has not been diverted).

Collectively, this set of technologies is referred to as material protection, control, and accounting (MPC&A). Any MPC&A system is only as good as the people who run it; hence, an effective program for recruiting and training capable MPC&A personnel is also an essential element of an effective system, as is a personnel reliability program that includes background checks before people are granted access, and continued monitoring to identify any behavior that might indicate a security risk. Finally, effective regulation of the entire enterprise by a regulator with good inspection capability and real enforcement power is essential.

Physical Protection

DOD-funded fencing system at Russian warhead storage site.

Physical protection systems for nuclear weapons and materials rely on the same principles and technologies used for securing other items of extremely high value. The goal is to provide maximum assurance against any theft, at minimum cost and inconvenience. Modern physical protection systems rely on a multi-layered "defense in depth," designed so that the failure of any one element of the system would not allow a theft to succeed.[15]

Typically a facility will be surrounded by an outer fence to mark off the controlled area. A more impressive fence, often a double fence with a "clear zone" equipped with intrusion detectors between the two fences, surrounds a smaller protected area within the controlled area. Commonly used intrusion detectors include microwave sensors, acoustic sensors, taut wires (designed to send a signal if bent), and the like. Once something has been detected, it has to be assessed: is it a terrorist team crossing the fence, or a rabbit? Technologies for this purpose range from human guards patrolling the perimeter (ideally with some means to communicate to a central guard station) to security cameras linked to the intrusion detectors. In the United States, such an outer perimeter with sensors is referred to as a Perimeter Intrusion-Detection and Assessment System (PIDAS). As implemented in the United States, such systems are very costly—often more than $1000 per foot.[16]

A variety of types of barriers are then used to ensure that it would take an adversary a long time to get from the point where they are first detected to the point where they can get the nuclear material and get out of the facility (or sabotage the facility, if that is what is being protected against). These delay barriers may include additional fences, vehicle barriers designed to block anyone attempting to drive in, the walls of buildings themselves, heavy locked doors, locked vaults for storing the material, and the like. Tests have shown that many barriers that look very impressive in fact offer only modest delay to a determined and well-equipped terrorist team: typical wire fences often provide only seconds of delay time, and even thick concrete walls can be blown through rapidly with shaped-charge explosives. Security cameras and alarm systems are often provided at key points within the protected area and the buildings themselves, giving those directing the response forces access to information on where the intruders are going.

Physical protection systems also include access controls, designed to ensure that only authorized personnel are allowed into sensitive buildings and areas. Typical technologies used range from human guards who check identification to automated biometric systems such as hand recognition or cornea recognition. In between, and very common, are systems where authorized personnel must swipe an ID card with a magnetic strip through a machine, and then punch in a code, under the watch of a guard, to access the facility.

Finally, in addition to detection and delay, there must be response. Under the regulations in force in the United States and many other countries, facilities with nuclear weapons or enough nuclear material to make a nuclear weapon must be protected by armed guards, in sufficient number to defeat a specified potential threat, known as the "design basis threat." The guards must have appropriate armament, training, means of communication, ability to get to the scene quickly, and protection from being killed themselves before they can intercept the attackers. Thus, for example, armed guards patrolling a perimeter may look impressive, but in many cases could readily be shot from far beyond the controlled perimeter, making them ineffective as a defense force against a well-planned attack. Typically the information from the various sensor systems and security cameras is fed to a central guard station (which itself is often duplicated in case one such station is attacked and destroyed), where those on watch can direct the response forces.

Physical protection systems can rely on complex modern technologies, from microwave sensors to biometric measurement systems—or they can be extremely simple. At the Pantex nuclear weapons site in the United States, for example, the nuclear weapons are stored in bunkers that have multi-ton concrete blocks in front of hardened steel doors. The special forklift used to lift away the blocks is kept outside the security fence, and only brought inside the fence when accompanied by an armored personnel carrier with a machine gun. Once the block is removed, the thick steel doors have two hardened locks—the key to one is held by the security force, while the key to the other is held by the site operational personnel. The security provided is effective, but simple and easy to understand.[17] In many cases, such simple, "inherently sustainable" physical protection arrangements may be more appropriate—especially for sites with limited resources for security operations—than higher-technology solutions. For some situations, rapidly deployable barriers—such as rapidly hardening foam, or piles of rubble—can be a useful complement to pre-deployed barriers, and a variety of these have been developed.

Physical protection for nuclear weapons and materials being transported from one place to another poses a particular problem, as during transportation it is impossible to maintain the extensive set of barriers and delays that can be put in place at a fixed site. Hence, reliance must be placed on technologies that can be brought along during the transport, and on immediate response to an attack by armed guard forces. In the United States, special Safe Secure Trailers (SSTs), equipped with a variety of devices to protect the drivers and escorts from attack and to make life extremely difficult for anyone who attempts to open the trailer and remove its contents without authorization, are used to carry nuclear weapons and materials.[18]

Fundamental to designing an effective physical protection system is characterizing what is to be protected (e.g., materials that could be stolen, vital areas that could be sabotaged), and against what threat. The threat the system is designed to defeat is known as the "design basis threat": in the U.S. system, and in many other countries, the whole process of designing and evaluating physical protection systems is based on one simple question: can they reliably defeat the specified design basis threat? U.S. nuclear power plants, for example, are required by regulation to have security systems in place that can protect them from sabotage by one small team of well-armed and well-trained outside attackers, one insider, or both working together.

The process of analyzing the possible paths by which outsiders might break into a facility, or insiders might smuggle nuclear material out, is known as vulnerability assessment. A wide range of techniques, from tabletop exercises to elaborate computer models of facilities, have been developed for this purpose.[19] Once vulnerabilities have been identified, different approaches to fixing them can be compared on the basis of cost and effectiveness, using similar techniques.

Realistic performance testing is a key part of this process. The reality is that many physical protection systems that look good on paper fail miserably when confronted with a capable and clever attacker—particularly one with insider information on their weaknesses. Thus, in the United States, for example, for facilities with nuclear weapons or substantial quantities of weapons-usable nuclear materials, regular performance tests—where outsider teams try to break in, in a mock battle, or insiders try to smuggle material out—are required. Vulnerabilities identified in these tests are then fixed, and the process begins again. Regular testing of the performance of individual components of the system—does the alarm really work, does the response force actually arrive in the amount of time assumed, and the like—is also important.

Material Control

A wide range of technologies are used to ensure that nuclear material remains under control and that any tampering or attempt at removal is rapidly detected. A number of elements of the physical protection system—such as locked vaults, access control systems, and the like—are also key elements of material control. Security cameras and other systems to monitor the areas where nuclear material is stored or processed, and other key strategic points, are also a key element of a nuclear material control system.

Tags and seals are a fundamental part of a material control system, and are in wide use around the world. Tags are used to identify a particular item—such as an individual warhead, or a particular canister of nuclear material—so that another similar one (such as a dummy warhead) cannot be substituted for it without detection. Tags that have been developed for nuclear arms control applications include intrinsic surface tags—in which an image of a microscopic area of the surface of an object, such as a missile launcher, is taken and can be compared against that same item when inspected later—and the "reflective particle tag," in which a strip of material containing reflective particles in random orientations is applied to the object's surface, and an image is taken recording the particular arrangement of the particles on that strip.[20] Both of these approaches, and others that have been developed, can be made rather difficult to duplicate.

Electronic tags that can be queried and send a signal to a receiver, describing their position and status, have also been developed. Some electronic tags require a power supply of some kind, such as a battery, but others are powered by the radio waves from the query sent to them, and require no power supply of their own. Thus, all of the containers in a nuclear material storage area can be frequently queried to ensure they have not been removed. Indeed, systems that provide real-time reports on the position of nuclear material in transit, uploaded to satellites and then downloaded to central monitoring stations, are now in use.

Seals (sometimes referred to as tamper-indicating devices, or TIDs) are used to indicate whether something has been tampered with—e.g., material removed from a container, or a door opened. For example, fiber-optic seals are widely used for nuclear material control. In one common fiber-optic system, the fiber is looped over the sealed item so that the item cannot be opened without breaking the fiber, and then crimped off in a way that breaks several of the individual fibers within the strand in a unique pattern. An image of that pattern is taken when the seal is applied, which can then be compared to images taken in later inspections. Some types of such seals can be remotely queried, and others are able to report any tampering as soon as it occurs (though that requires a continuous power supply for the seal).

Tags and seals of various types are in wide use in non-nuclear commercial industry. Commercial industry has for some time been on a quest for the "penny tag"—an electronic tag that would cost only one cent—so that the technology could be applied to inventory control of virtually everything, down to cereal boxes.[21] As that example suggests, however, in most of commercial industry the technology is being driven toward low cost and ease of use, more than to very high levels of security, which must be the focus for nuclear weapon and nuclear material applications.

A range of other technologies can be incorporated into material control systems as well. For example, in a number of cases the shelves holding nuclear material containers have been rigged to sense the weight of the containers: if anything is removed from the container, an alarm is sent immediately.

Portal monitors designed to detect any attempt to carry nuclear material out of the facility are a fundamental part of any good MPC&A system. As noted earlier, a variety of specific monitors are available, and they are able to detect both plutonium and HEU. Metal detectors used in conjunction with the portal monitors can detect any shielding that might be used to help smuggle out nuclear material. In addition to monitoring individual people going into and out of key buildings and areas, portal monitors can also be used to monitor vehicles, though in that case the sensitivity is not as impressive.

Material Accounting
If nuclear material is in the form of individual items—such as nuclear warheads, or fuel elements—these items can simply be counted, and as long the items themselves have not been tampered with, it is easy to confirm that nothing is missing. Thus, while considerable emphasis is placed on keeping an accurate and up-to-date count of nuclear warheads, actually counting the number is not very difficult—putting the main technological emphasis in the case of warheads on physical protection (as well as on use control devices incorporated within the weapons themselves, designed to make it very difficult to set the weapons off without authorization).

When nuclear material is handled and processed in bulk form, such as in powders or solutions, the accounting problem becomes much more complex. Then, the amount of material must be measured, and all the available measurements have at least some uncertainty. Determining whether there are 19 or 20 nuclear warheads in a bunker is not very difficult, but confirming that there are 10 tons of plutonium at a plant and not 9.99 tons—a potentially crucial difference, since the remaining 0.1% represents 10 kilograms of plutonium, more than enough for a bomb—can be a tremendous challenge.

The theft of HEU that took place at the Luch Production Association in Podolsk, Russia, in 1992 illustrates the importance of an accurate accounting system. An insider at the plant, knowing that the system was designed so that if output was within 3% of input the difference was considered normal losses to waste, stole small amounts of HEU at a time, eventually accumulating 1.5 kilograms of stolen HEU without being detected. A more effective accounting system—or a more effective material control system—would have detected the theft in its early stages.[22]

In general, for both domestic and international safeguards, facilities handling nuclear material are divided into separate "material balance areas," and accounting measurements are taken at regular intervals to try to balance the books for each. In principle, the inventory at the end of a particular accounting period should equal the inventory at the beginning, plus any material brought into that area during the period, minus any material shipped to another area during that period. Since all the measurements have small uncertainties, however, these inventories rarely match exactly, leaving some material unaccounted for (MUF), or inventory difference (ID):

MUF = beginning inventory + inputs - ending inventory - outputs

A large MUF could be the result of material having been stolen or diverted—or it could be the result of measurement uncertainties. (Of course, a MUF resulting from measurement uncertainty could be either positive or negative—that is, it could appear that there was more nuclear material present than there should be.) Various statistical tests are used to judge whether the MUF is large enough by comparison to the measurement uncertainties to justify the conclusion that material has actually been removed or diverted. In essence, if the MUF is substantially larger than the uncertainty in the measurement, the hypothesis that it results from a random measurement problem is rejected, and further investigation of the discrepancy is called for. Thus, as important as the absolute size of the MUF is the variance of the MUF from one measure to the next—the key measure of the uncertainty in the accounting system. In current practice in international safeguards, to keep the rate of false alarms low, there is not considered to be a discrepancy requiring further investigation unless MUF is greater than three times the variance of MUF.

How much can these measurement uncertainties be reduced? This depends on the specific design and characteristics of individual facilities. Particular measurements can in some cases be made with accuracies of one part in a thousand, or even better[23]—but the overall accounting system for a facility is not so precise. In general, for example, the uncertainties in measuring throughput of plutonium through a large reprocessing plant are generally thought to be somewhat below 1%, but not dramatically below that figure. Traditionally, such nuclear material balances were measured by cleaning out the process lines of a facility, and physically measuring all of the material on hand. At a reprocessing plant, this might be done once a year. At a large reprocessing plant or other plutonium bulk-handling plant, however, such an approach is neither fast enough nor sensitive enough: with balances only taken once a year, it might take a year to detect that material had gone missing, and the accounting system would be completely unable to detect the loss of enough material for a bomb. (If the plant is processing 8 tons of plutonium per year, for example, as some of the modern plants do, with a measurement uncertainty (generously) estimated at 0.5%, and a detection threshold set at three times the uncertainty, only the removal of more than 120 kilograms of plutonium would be detected.) It is too expensive to clean out the plant much more frequently than this.

For this reason, various approaches to "near real time accountancy" (NRTA) have been developed, in which data is collected frequently at points throughout the process in the plant, and in-process material that cannot be measured is estimated with computer models. If a comparable measurement precision could be achieved 30 times a year, then the detection threshold for abrupt theft or diversion would be 4 kilograms of plutonium—a dramatic improvement. But if the theft or diversion were protracted—removing small amounts of material at a time, as in the case of the Luch theft in 1992—the improvement from NRTA would not be as impressive, and is harder to calculate. NRTA systems use a variety of statistical tests to analyze the large number of different MUF measurements to detect both abrupt and protracted removals of nuclear material. While the best tests are complex, their basic concept is simple: they examine the slope of the line represented by the many MUF measurements—if it is significantly different from zero, that is, if the MUF is increasing over time, that suggests a removal of material may be taking place.[24] By such means, both the operators of large plutonium processing plants and the International Atomic Energy Agency (IAEA) believe that accounting systems can be made accurate enough to detect any significant diversion—though critics continue to disagree.[25]

Nuclear measurement technologies make use of all of the characteristics of nuclear materials described earlier—their mass, their radioactivity, the heat they emit, and more.[26] In some cases, samples of the nuclear material are taken for chemical analysis in a laboratory—so-called "destructive analysis." In many other cases, however, reasonable accuracy can be achieved with non-destructive assay (NDA) of the material. A few examples may suffice:

• Weight. If the type of nuclear material in a particular object or container is already well known, then how much of it is present can be assessed simply by weighing it. Hence, highly accurate scales are a key part of nuclear material accounting systems.
• Heat. Similarly, measurements of the heat output from a sample can be used to measure how much plutonium is present with surprising accuracy, if the isotopic mix is known. Unlike a weight measurement, a heat measurement is not affected by non-radioactive material mixed in with the plutonium.
• Gamma Emissions. Each type of nuclear material emits gamma rays at characteristic energies. Hence, using instruments known as gamma spectrometers, the spectrum of gamma rays emitted from a sample can be measured, and the concentration of different isotopes in the sample can be assessed.
• Passive Neutron Emissions. HEU does not emit enough spontaneous neutrons to be very useful in measuring HEU quantities—but plutonium does. A neutron well counter, for example, can count the total neutron rate from a sample of material. This total count approach, however, has the disadvantage that it includes not only neutrons from spontaneous fission taking place in the sample, but also neutrons from the room background, and neutrons from interactions of the alpha particles emitted by the sample with lighter-element impurities. Hence, a complementary approach is known as neutron coincidence counting—which counts only those neutrons where two or more neutrons are detected at once (as would occur from fission), excluding the other neutrons.
• Active Neutron Emissions. Both HEU and plutonium will fission if struck by a neutron beam. Hence, neutron counting can be coupled with bombarding the sample with a neutron beam, to count the neutrons from the induced fissions. While counting passive neutrons effectively only counts the amount of Pu-240 (since its neutrons usually dominate all other spontaneous fission neutrons in the sample), an active approach can also measure U-235 and Pu-239. Active neutron well coincidence counters are available commercially, and are quite accurate.
• Absorptive Properties. Plutonium and uranium, as atoms with high atomic numbers, absorb energy at distinctive energies associated with the energies needed to dislodge electrons from their outer electron shells. This property is exploited in the "k-edge densitometer," which measures the absorption of particular X-ray energies passing through the input solution at a reprocessing plant, to assess the nuclear material concentration in the solution.

In each of these cases, accurate measurement depends crucially on effective calibration of the measuring equipment. This requires a regular measurement control program, laboratory standards to check measurements against, and related measures.

Physical protection, material control, and material accounting systems can be highly effective in preventing theft of nuclear warheads and nuclear material—but only if all the elements of the system are effective and working together, and all of the human participants give security and accounting for th