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The collective dose to the Japanese population has been estimated from a hypothetical accident of a nuclear submarine if it sinks in an offshore region around Japan. A computer code system DSOCEAN has been used for assessing the collective dose due to radionuclides released to the ocean from a sunken nuclear submarine. The estimated collective effective dose commitment from all of the radionuclides released after the break of the fuel pellets is estimated to be 2.5 x104 man-Sv. The contribution of 241Am to the total collective effective dose commitment is the highest, followed by 137Cs, 238Pu, 240Pu, 239Pu and 241 Pu. The maximum of the estimated collective effective dose by the annual intake of marine products after radionuclide releases for one year is approximately 0.3% of the annual average dose by the natural radiation that is reported by UNSCEAR.

Compactness and light weight are of importance for a reactor plant used for ship propulsion. An advanced marine reactor, MRX, which has been designed by Japan Atomic Energy Research Institute, has accomplished a passively safe, compact and light weight design by adoption of an integral PWR and a water filled containment vessel. Dose equivalent rates in all areas inside and outside a ship are required to be reduced below design criteria without any bulk shield outside the containment vessel. In this paper, a shielding design for installation of the steam generator in the vicinity of the reactor core to achieve a compact plant by reducing fast neutrons in the steam generator and to enable the engine room, where pipes of the secondary loop are installed, to be classified as surveillance area, not controlled area, is described. The main purpose to classify the engine room as the surveillance area is the improvement of working efficiency of crew. To accomplish the design goal, a relatively thick core barrel an...

Japan Atomic Energy Research Institute has been conducting a design study of an advanced marine reactor MRX. MRX employs a water-filled containment vessel. In a case of loss of coolant accident (LOCA), the discharge of water from the reactor vessel almost stops when the pressures of reactor vessel and containment vessel become equal. The core flooding can be maintained passively. The MRX, therefore, does not adopt a safety injection system which the conventional reactors have.The purpose of this study is to investigate the thermal hydraulic behavior in the water-filled containment vessel during the LOCA. This report describes the fundamental experimental study focused on pressure increase of the containment vessel. The experiment was carried out by using a small scale apparatus. The pressures and temperatures in the vessels were measured. According to our observation, the vapor bubbles at nozzle exit contracted or expanded caused by flashing due to pressure reduction or condensation in the containment water. The pressure increase in the containment vessel was revealed to be the sum of compressed nitrogen gas pressure and saturated steam pressure. One-volume thermal equivalent model can predict well the pressure increase in the containment vessel in this experiment.

This paper describes the experiments of the first Japanese nuclear ship ‘Mutsu’, to investigate the effects of sea wave on the thermal hydraulics of marine reactor system while cruising through various sea conditions. The experimental data were analyzed in time-domain by RETRAN-02/GRAV code. This code was modified so as to simulate the ship motion effect on reactor thermal hydraulics. The data were also analyzed in frequency domain by Blackman Turkey method for the calculation of the spectrum and response function. The experiments involving ship maneuvering were performed by cruising on different wave heights, as well as wave directions in the northern Pacific ocean. From the experiments, vertical acceleration due to ship motion was found to induce direct variation of water levels in the SGs and the pressurizer. The water level variations were largest in the head wave, but smallest in the following wave. On the other hand, the following wave caused greater variation of the reactor power when the feed back...

An experiment on transient single phase natural circulation in a closed loop was conducted to study the core cooling behaviour by natural circulation in the loop-type PWR for marine application. The system employed a loop that had two U-shaped, and three inverse U-shaped flow paths, and a small elevation difference between the heater and the cooler. The experiment was conducted at atmospheric pressure with the heater input held constant ( Q = 0.5 kW to 3.0 kW ), and the cooling water flow rate of the cooler constant during the transient following tripping of the pump. From the experiment, a flow stability map was obtained which correlated heater input versus elevation differences. Comparison between experimental and numerical results obtained by RETRAN-02 analyses yielded general agreement.

A number of operational transient analyses of the nuclear-powered ship Mutsu have been performed in response to Japanese nuclear safety regulatory concerns. The RETRAN and COBRA-IV computer codes w...