Development of In-vessel Type Control Rod Drive Mechanism for Marine Reactor

Abstract

A highly reliable control rod drive mechanism (CRDM) installed inside the reactor vessel has developed for use of an advanced marine reactor. This CRDM contributes to compactness and simplicity of the reactor system, and it can eliminate the possibility of a rod ejection accident. The CRDM works in the high temperature and high pressure water-310 • C and 12 MPa, the same atmosphere as the primary loop. Driving force is produced by a synchronous motor with the rotor of a permanent magnet, which has been developed. An innovative latch mechanism using separable ball nuts can latch the driving shaft connecting the control rod and de-latch it for scram. The rod position detector using a magnetostrictive wire type sensor on the principle of Wiedeman effect has been developed, accuracy of which is verified to have a detecting error within 1.2 mm. Ball bearings for thrust and radial supports in rotation have been developed to be capable of working under the high temperature water for a long period. Under condition of high temperature water, the performance tests on latching, de-latching, holding and vertical movement, and durability test were conducted to verify the design conditions. This CRDM can be applied to not only the marine reactor, but also to light water reactors, PWR and BWR.

In-vessel Type Control Rod Drive Mechanism Using Magnetic Force Latching for a Very Small Reactor.

“…2(a)) induces rotational torque by a driving motor, which is a built-in type synchronous motor with a permanent magnet for the rotor, the same type of motor as in the MRX. 1) The force transformation section ( Fig. 2(b)) transfers the rotational torque of the driving motor to the rectilinear motion of the shaft by a ballnut-screw, which is of a self-lubricating type which does not require oil.…”

“…For rotation, there are thrust and the radial ball bearings, which can be operated in high temperature steam. Since a ball bearing capable of operating in high temperature water has already been developed, 1) its applicability to steam conditions has now been investigated at Japan Atomic Energy Research Institute (JAERI). The latch mechanism, the most typical feature in the present concept, attracts both the motor-driven shaft and the control rod driving shaft to connect or latch them for normal operation with an electromagnetic force, or stops attracting them to separate or de-latch them for the scram.…”

“…Below the latch mechanism, the spring is set so as to help a scram to occur quickly, at any position the ship might assume, even during a hypothetical overturn, according to the design condition (4). The control rod position detector uses a magnetostrictive wire type sensor on the principle of the Wiedeman effect, the same as the MRX's one, 1) and is arranged to be parallel to the motor-driven shaft in order to avoid the magnetic effect of the latch mechanism.…”

“…If they contact without any clearance, the time lag to separation after the de-energizing of the magnet will increase due to the residual magnet flux in the shaft. This time lag, called the de-latch time, in other words, the time it takes for the latch mechanism to release the control rod shaft after receiving the scram signal, is the same as the one in the reference, 1) and should be smaller than 0.2 s as the design value for the normal ship position. In the present design, a clearance of 0.5 mm is maintained by chromium plating, which is a non-magnetic material.…”

“…An in-vessel type control rod drive mechanism (INV-CRDM), which is installed inside the reactor vessel, can help achieve a highly compact and simple reactor system, and eliminate the possibility of a rod ejection accident. The Japan Atomic Energy Research Institute (JAERI) has developed this type of CRDM, 1) driven by an electric motor, for the advanced marine reactor MRX 2) with a thermal output of 100 MW, which is mainly used for a nuclear-powered ship. The INV-CRDM of the MRX adopts several innovative components, such as a latch mechanism with separable ball nuts, a synchronous motor with the rotor of a permanent magnet, and ball bearings, all of which are capable of working in high temperatures and under high water pressure.…”