Following World War II, the SS bolts symbol was adopted by white supremacists and neo-Nazis worldwide. Most white supremacists use it in its Nazi form, as two bolt-like images with flattened ends. However, sometimes the symbol may have pointed bottom ends or pointed tops and bottoms. These variants of the SS bolts are most frequently associated with prison tattoos.
The SS bolts are typically used as a symbol of white supremacy but there is one context in which this is not necessarily always so. Decades ago, some outlaw biker gangs appropriated several Nazi-related symbols, including the SS bolts, essentially as shock symbols or symbols of rebellion or non-conformity. Thus SS bolts in the context of the outlaw biker subculture does not necessarily denote actual adherence to white supremacy. However, because there are a number of racists and full-blown white supremacists within the outlaw biker subculture, sometimes it actually is used as a symbol of white supremacy. Often the intended use and meaning of the SS bolts in this context is quite ambiguous and difficult to determine.
The distinction between a bolt and a screw is poorly-defined. The academic distinction, per Machinery's Handbook, is in their intended design: bolts are designed to pass through an unthreaded hole in a component and be fastened with the aid of a nut, although such a fastener can be used without a nut to tighten into a threaded component such as a nut-plate or tapped housing. Screws in contrast are used in components which contain their own thread, or to cut its own internal thread into them. This definition allows ambiguity in the description of a fastener depending on the application it is actually used for, and the terms screw and bolt are widely used by different people or in different countries to apply to the same or varying fastener.
Bolts are often used to make a bolted joint. This is a combination of the nut applying an axial clamping force and also the shank of the bolt acting as a dowel, pinning the joint against sideways shear forces. For this reason, many bolts have a plain unthreaded shank (called the .mw-parser-output .vanchor>:target.vanchor-textbackground-color:#b1d2ffgrip length) as this makes for a better, stronger dowel. The presence of the unthreaded shank has often been given as characteristic of bolts vs. screws, but this is incidental to its use, rather than defining.
Where a fastener forms its own thread in the component being fastened, it is called a screw. This is most obviously so when the thread is tapered (i.e. traditional wood screws), precluding the use of a nut, or when a sheet metal screw or other thread-forming screw is used. A screw must always be turned to assemble the joint. Many bolts are held fixed in place during assembly, either by a tool or by a design of non-rotating bolt, such as a carriage bolt, and only the corresponding nut is turned.
The first bolts had square heads, formed by forging. These are still found, although much more common today is the hexagonal head. These are held and turned by a spanner or socket, of which there are many forms. Most are held from the side, some from in-line with the bolt. Other bolts have T-heads and slotted heads. 
Many bolts use a screwdriver head fitting, rather than an external wrench. Screwdrivers are applied in-line with the fastener, rather than from the side. These are smaller than most wrench heads and cannot usually apply the same amount of torque. It is sometimes assumed that screwdriver heads imply a screw and wrenches imply a bolt, although this is incorrect. Coach screws are large square-headed screws with a tapered wood screw thread, used for attaching ironwork to timber. Head designs that overlap both bolts and screws are the Allen or Torx heads; hexagonal or splined sockets. These modern designs span a large range of sizes and can carry a considerable torque. Threaded fasteners with screwdriver-style heads are often referred to as machine screws whether they are being used with a nut or not.
The American Institute of Steel Construction (AISC) 13th Edition Steel Design Manual section 16.1 chapter J-3 specifies the requirements for bolted structural connections. Structural bolts replaced rivets due to the decreasing cost and increasing strength of structural bolts in the 20th century. Connections are formed with two types of joints: slip-critical connections and bearing connections. In slip-critical connections, movement of the connected parts is a serviceability condition and bolts are tightened to a minimum required pre-tension. Slip is prevented through friction of the "faying" surface, that is the plane of shear for the bolt and where two members make contact. Because friction is proportional to the normal force, connections must be sized with bolts numerous and large enough to provide the required load capacity. However, this greatly decreases the shear capacity of each bolt in the connection. The second (and more common type) of connection is a bearing connection. In this type of connection, the bolts carry the load through shear and are only tightened to a "snug-fit". These connections require fewer bolts than slip-critical connections and therefore are a less expensive alternative. Slip-critical connections are more common on flange plates for beam and column splices and moment critical connections. Bearing type connections are used in lightweight structures and in member connections where slip is not important and prevention of structural failure is the design constraint. Common bearing type connections include: shear tabs, beam supports, gusset plates in trusses.
Baffle-former bolts help hold together a structure inside the reactor vessel of Westinghouse pressurized water reactors (PWRs). As part of the license renewal process, licensees have committed to periodically inspect the reactor vessel internals, including the baffle-former bolts, for indications of degradation. Recent operating experience and inspections have identified more baffle-former bolts with indications of degradation than anticipated. In spring 2016, two PWRs, Indian Point Unit 2 and Salem Unit 1, identified a large number of degraded baffle-former bolts during refueling outage ultrasonic (UT) inspections. Indian Point Unit 2 and Salem Unit 1 found and reported these degraded bolt conditions in event notifications (EN): EN 51829 and EN 51902. Subsequently, two additional PWRs, D.C. Cook, Unit 2, and Indian Point, Unit 3 identified similar numbers of degraded baffle-former bolts.
Operating experience indicates that the baffle-former bolts are more susceptible to degradation in older Westinghouse four-loop reactors that have a "down-flow" configuration and have baffle-former bolts made of Type 347 stainless steel. There are seven U.S. reactors that match these characteristics: Indian Point Units 2 and 3, Salem Units 1 and 2, D.C. Cook Units 1 and 2, and Diablo Canyon Unit 1 (Diablo Canyon Unit 2 has a different configuration than Unit 1). The baffle-former bolts of all seven of these reactors have been inspected and bolts have been replaced to restore structural integrity.
The NRC's risk-informed assessment of the issue determined that degraded baffle-former bolts do not warrant the immediate shutdown of any plant. The issue does not present a significant safety concern. The industry through the Electric Power Research Institute (EPRI) has issued updated inspection guidance for baffle-former bolts which the NRC found acceptable, as documented in a staff assessment. Licensees of PWRs have been performing the inspections as recommended by the updated guidance, and performing corrective actions as needed, including replacement of degraded bolts.
There are structures located within Westinghouse reactor vessels that support and orient the reactor fuel assemblies and direct coolant flow through the core. The core baffle, one of these internal structures, is a set of vertical plates surrounding the outer rim of the reactor's fuel assemblies. The baffle provides lateral restraint to the core and directs coolant flow through the core. The vertical baffle plates are bolted to the edges of horizontal former plates, which are bolted to the inside surface of the core barrel. There are typically eight levels of former plates located at various elevations within the core barrel. The baffle-former bolts secure the baffle plates to the former plates. To cool the baffle structure, some water flowing through the reactor vessel is directed between the core barrel and the baffle plates in either a downward direction ("down-flow"), or an upward direction ("up-flow"). "Down-flow" plants place more stress on baffle-former bolts, which contributes to susceptibility of the bolts to degradation. Plants with the modified "up-flow" direction have shown little baffle-former bolt cracking as compared to the "down-flow" designs. Newer PWRs use the "up-flow" configuration and several older units have converted to the "up-flow" configuration.
Figures below: Baffle-former assembly bolts (left). This shows the three styles of baffle plate bolts: edge bolts, baffle-former bolts, and corner edge bolts. Baffle-former bolts are the bolts that have experienced the recent issues at Indian Point-2 and Salem-1. A typical core barrel baffle arrangement is shown (right).
Baffle-former bolts are subjected to significant mechanical stress and high levels of neutrons coming from the core for many years. Over time these conditions lead to degradation of the bolts, in the form of irradiation-assisted stress corrosion cracking. Significant cracking can reduce the load that a baffle-former bolt is able to support and eventually result in the detachment of the bolt head. Irradiation-assisted stress corrosion cracking is a known phenomenon and the inspection and management of this degradation mechanism is the subject of an NRC-approved Electric Power Research Institute (EPRI) Materials Reliability Program (MRP) topical report, MRP-227-A, "Materials Reliability Program: Pressurized Water Reactor Internals Inspection and Evaluation Guidelines." 041b061a72