Natural Circulation Reactor
A natural circulation reactor generates a thermosiphon flow to provides natural circulation of coolant through a core without an external circulation force during normal operation of the reactor. Natural circulation arises because of the fluid density difference between the heat source (core) and the elevated heat sink (helical coil heat exchanger). The primary coolant may be circulated naturally, due to natural circulation set up by heating due of the primary coolant in the vicinity of the operating nuclear reactor core. Alternatively, an external pump can be employed, for example having an external stator and a rotor coupled with the pressure vessel volume via a suitable conduit or tube. The coolant pumps provide assistance to natural circulation driving the primary coolant flow.
Vice Admiral Hyman G. Rickover briefed senior naval officers on 2 July 1962, urging them to include a natural circulation reactor submarine in the fiscal year 1964 shipbuilding program. He told them that, in some respects, he considered the project's drive for simplicity a return to earlier engineering concepts. If the machinery was less efficient then, it had the compensating virtues of ruggedness, reliability, simplicity, and easy maintenance. These qualities were vanishing as the navy was installing complicated high speed machinery, often beyond the abilities of officers and men to maintain, in order to squeeze the most energy from every ounce of fuel oil.
Rickover's strategy for reducing noise was to get rid of equipment; if that was impossible, to turn it off during quiet operation; to slow down the component; or to redesign the particular equipment to get rid of rotating components. Naval Reactors was analyzing the design of the fluid systems in the propulsion plant and scrutinizing every valve to see if it could be eliminated.
During the 1960's, Rickover had successfully developed the natural circulation reactor (NCR) which, by operating without the use of noisy pumps at low speeds and requiring less use of pumps at higher speeds, provided a significant improvement in quietness. Electric Boat laid the keel of the Narwhal (SSN 671), authorized by Congress in the 1964 program, on 17 January 1966. Launching came on 9 September 1967 and commissioning on 12 July 1969. Although the natural-circulation reactor was successful, the navy built no more ships of that class; that step was not necessary to incorporate the advances into future submarines.
The natural circulation reactor can operate without inherently noisy pumps only up tb a power output corresponding to approximately ten knots in speed. Current submarines routinely operate below this speed.
Even if detected, the missile submarine's commander will resort to a variety of tactics rather than to move to high speed. These tactics include evasive maneuvering, launching decoy torpedoes, or calling in other naval craft to help throw the pursuer off track. A submarine running in the range of twenty to twenty-five knots creates such a large amount of noise that it can be detected from a great distance and hence over a large ocean area, thus allowing attackers to converge. Some in the Navy have argued that a pursuing attack submarine (or possibly a destroyer) goes "blind" when a speed of about twenty-five knots is reached, i.e. its sonar is so hampered by its own flow noise that it becomes harder to trail an SSBN than at lower speeds.
As atomic-powered ship appliances, unitary type pressurized water reactors (referred to as PWR) wherein a steam generator and a pump are housed in adjacent to the nuclear reactor. The reactor core is located in the lower part of a large pressure vessel. Under the core there is an opening, designated density lock, between the circulating water system which cools the core and the above-mentioned pool. In this opening, which consists of a number of parallel vertical tubes, hot cooling water is stably stratified over colder pool water. Above the core, the hot water coming from the core flows upwards through a riser. At a certain distance above the core (which varies between the variants), the riser is in communication with the pool via an additional opening, designated upper density lock, which operates, in principle, in the same way as the lower one.
With the aid of a control system, the flow through the core is maintained at such a level that no flow through the density locks takes place during normal operation. The hot water of the riser will flow out, in natural circulation, into the reactor pool through the upper density lock, and the lost water is replaced by pool water which flows into the primary circuit through the lower density lock.
The primary advantage of a natural circulation system is simplicity. The elimination of active power supplies and pumps can greatly simplify the construction, operation and maintenance of the system. Furthermore, elimination of the pumps and connecting piping also eliminates accident scenarios associated with loss of pump flow, pump seal rupture accidents and loop seal manometer effects during Small Break Loss- of- Coolant- Accidents (SBLOCAs). Another advantage is that the flow distribution in parallel channel cores is much more uniform in a natural circulation system. In addition, the two-phase fluid flow characteristics as a function of power are also better in a natural circulation system. That is, th e flow increases with power, whereas in a forced circulation two-phase fluid system, the flow decreases with an increase in power. The primary disadvantage of a natural circulation system is that the driving head is low. To increase the flow rate at a fixed power would require either an increase in the loop height or a decrease in the loop resistance, either of which might increase costs.
In general, the mass flux through a natural circulation cooled core is low. As a result, the allowable maximum channel power is lower leading to a larger core volume compared to a forced circulation system of the same rating. Furthermore, large core volumes can result in zonal control problems and stability. While instability is common to both forced and natural circulation systems, the latter is inherently less stable than forced circulation systems. This is attributable to the nonlinear nature of the natural circulation phenomenon, where any change in the driving force affects the flow which in turn affects the driving force that may lead to an oscillatory behavior.
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