SL-1: Designed for Remote Power and Heat

SL-1’s mission was to provide power to radar stations along the northern perimeter of North America; a series of such stations was known as the DEW (Defense Early Warning) Line.

The Army’s designation , SL-1, tells us that the plant was a stationary, low power reactor, and that it was the first of its kind. The design work was done by Argonne National Laboratory in 1955-1956 under the name Argonne Low Power Reactor (ALPR).

SL-1’s mission was to provide power to radar stations along the northern perimeter of North America; a series of such stations was known as the DEW (Defense Early Warning) Line.

These radar stations were designed to provide a means of detecting and tracking intercontinental ballistic missiles (ICBMs) originating from the Soviet Union across the North Pole. At the time that the SL-1 power plant was proposed, the construction of this series of radar stations was a high national priority.

In order to provide the most timely warning possible, the DEW line radars were to be built in cold and remote places, but they required substantial quantities of electricity and heat. The system designers concluded that the best available power source would be a nuclear combined heat and power plant designed to operate for several years without new fuel.

Light Vessel

Because of the need to transport the plant to a remote area, weight was a primary design consideration. This criteria led to the selection of a boiling water reactor (BWR). In this type of reactor, steam is generated directly in the reactor vessel with no intermediate heat exchangers needed.

Since overall pressures are lower in a BWR than in a pressurized water reactor (PWR), the mass of the required pressure vessel can be reduced. For the SL-1, the pressure vessel’s design pressure was only 27 bar (400 psi). It was 4.5 m (14.5 ft) tall, not including the head, and 1.35 m (4.5 ft) in diameter. The shell thickness was just 1.91 cm (0.75 in).

This low pressure design resulted in an easily transportable pressure vessel with a mass of approximately 5 metric tons, not including reactor internal parts or the vessel head, which could be separately packaged and shipped.

Compact Core

The SL-1 reactor core filled only a small portion of the pressure vessel. The majority of the volume of the vessel was a chimney that allowed for steam to be separated from water. The dimensions of the active portion of the core were approximately 68 cm (27 inches) high by 86 cm (34 in) across.

To achieve a self-sustaining nuclear reaction in such a small volume – under the technical constraints existing at the time – it was necessary to use highly enriched uranium as the fuel material.

To protect the uranium from interaction with water, it was alloyed with aluminum and then clad with a layer of aluminum alloy. This material choice was thought to provide good corrosion resistance and a low affinity for neutrons, with the known disadvantage of having a relatively low melting point. Since the SL-1 was designed for low pressure operation, the temperature limits on the aluminum cladding were considered acceptable.

Long Core Life

The remoteness of the sites for the DEW line radars encouraged the system designers to specify cores that could operate for up to three years without refueling. The sites were designed for minimum staffing levels and maximum availability.

To meet the challenging core life goal, the designers specified the use of materials called burnable poisons. What this kind of material does is to dampen the reactivity of the core when it is new and the fuel is fresh. As the fuel is consumed, so is the burnable poison, ideally leading to a core that can be controlled when it is new but which can last longer than one without the burnable poisons.

In the late 1950s, when the SL-1 core was being designed and built, however, the use of burnable poisons was a new idea that was not well refined. The poison alloys that were available were not able to be fully integrated into fuel plates, so they were tack welded in strips to the side plates of selected fuel assemblies. The strips were made of aluminum alloyed with 0.6 per cent (by weight) boron-10.

The designers were not satisfied with this arrangement, and plans were made for improved fuel designs in later cores. Because of the Cold War nature of the program, there was a considerable sense of urgency to continue with the plant operation, testing, and operator training, even if the core was not quite fully developed.

Because of the time constraints imposed on the program, there was little testing of the boron aluminum strips, and no testing of their behavior under the high temperature, high neutron flux conditions in an operating core.

Control Rods

The reactor was controlled by five cruciform-shaped control rods. These rods were fabricated by mechanically covering thin plates of cadmium with the same aluminum alloy used on the fuel plates. At intervals, the control rods were pierced with small holes with the aluminum cladding spot welded to form a seal around the cadmium.

The small number of rods simplified plant construction by reducing the number of control rod drive mechanisms needed.

The low number of rods made each rod’s contribution to the core response greater than is normally the case. (Note: Under current design requirements, nuclear reactors must be able to remain shutdown under all conditions even if one control rod is stuck in the fully withdrawn position. This “one stuck rod” requirement did not exist at the time that SL-1 was designed.)

No Containment

The SL-1 and its planned successors did not include a conventional reactor containment structure. Since they were designed for deployment only in remote areas, it was not thought to be necessary.

Instead, the reactor building was a simple steel cylinder with 0.65 cm (0.25 in) thick walls. It was 14.5 m (48 ft) high and 11.5 m (38.5 ft) in diameter. No special provisions were made to ensure that the building would be air tight.