VM-A reactor

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The VM-A reactor was the nuclear fission reactor used in pairs to power the Soviet Navy's Project 658 and 701 (Hotel), Project 659 and 675 (Echo), and Project 627 Кит (November) first-generation submarines. It was a pressurized water reactor (PWR), using 20% enriched uranium-235 fuel to produce 70 MWt of power.

This is the reactor that powered the K-19.

The following earlier-written section describes a type of reactor (RBMK) used elsewhere, and not the VM-A, although similarities in overall design are present. But it is suggested by Reistad and Ølgaardin Russian Nuclear Power Plants for Marine Applications (p27-31) that the VM-A reactor would most likely to have have had a core height of only about 1m, and either 23 or 37 fuel groups in a fuel assembly, not 36 as in the RBMK mentioned below.

The Soviet designed RBMK is a pressurised water reactor with individual fuel channels and using ordinary water as its coolant, and graphite as its moderator. It is very different from most other power reactor designs as it was intended and used for both plutonium and power production. The combination of graphite moderator and water coolant is not found in any other power reactors. The design characteristics of the reactor were shown, in the Chernobyl accident, to cause instability when operating at low power. This was due primarily to control rod design and a positive void coefficient. A number of significant design changes have now been made to address these problems.

Pellets of enriched uranium oxide are enclosed in a zircaloy tube 3.65m long, forming a fuel rod. Two sets of 18 fuel rods are arranged cylindrically in a carriage to form a fuel assembly of about 10 m length. These fuel assemblies can be lifted into and out of the reactor mechanically, allowing fuel replenishment while the reactor is in operation.

Within the reactor each fuel assembly is positioned in its own pressure tube or channel. Each channel is individually cooled by pressurised water.

A series of graphite blocks surround, and hence separate, the pressure tubes. They act as a moderator to slow down the neutrons released during fission. This is necessary for continuous fission to be maintained. Conductance of heat between the blocks is enhanced by a mixture of helium and nitrogen gas.

Boron carbide control rods absorb neutrons to control the rate of fission. A few short rods, inserted upwards from the bottom of the core, even the distribution of power across the reactor. The main control rods are inserted from the top down and provide automatic, manual or emergency control. The automatic rods are regulated by feedback from in-core detectors. If there is a deviation from normal operating parameters (e.g. increased reactor power level), the rods can be dropped into the core to reduce or stop reactor activity. A number of rods normally remain in the core during operation.

Two separate water coolant systems, each with four pumps, circulate water through the pressure tubes. Ninety-five percent of the heat from fission is transferred to the coolant. There is also an emergency core cooling system which will come into operation if either coolant circuit is interrupted.

Steam from the heated coolant is fed to turbines to produce electricity in the generator. The steam is then condensed and fed back into the circulating coolant.

The reactor core is located in a concrete lined cavity that acts as a radiation shield. The upper shield or pile cap above the core, is made of steel and supports the fuel assemblies. The steam separators of the coolant systems are housed in their own concrete shields.