A. YAMAJI, J. SHIMAZAKI, M. OCHIAI, T. HOSHI Japan Atomic Energy Research Institute,

Tokai-mura, Naka-gun, Ibaraki-ken,

Japan

Abstract

An advanced marine reactor, MRX, has been designed to be more compact and lightweight with enhanced safety. The reactor is an integral PWR with a water — filled containment vessel, in-vessel type control rod drive mechanisms and an emergency decay heat removal system using natural convection. These are adopted to satisfy the essential requirements for the next generation of marine reactors, namely, compact, light, highly safe and easily operated. The engineered safety is accomplished through a simplified system using water in the containment vessel. The LOCA analysis shows that the core flooding is maintained even when taking into account the ship inclination. To shorten the time of the maintenance and refueling works, a one-piece removal method is proposed. This method involves removing the containment vessel with its internals and replacing it in another one whose maintenance and refueling have already been completed. The economic evaluation of nuclear ships equipped with MRXs shows that some types of nuclear container ships will hold an economically dominant position over diesel ships in the near future, because of the environmental costs of diesel ships. R&D works have been making progress in the safety study of the thermal hydraulic phenomena, in-vessel type control rod drive mechanisms, the automatic control system, the nuclear ship simulator, etc.

1. Introduction

Compared to ordinary ships, nuclear ships are capable of long and continuous periods of voyage using high power without refueling. This advantage would contribute greatly to the diversification and expansion of sea transportation and ocean development in the future, as well as contribute to the survey and research activities on a global scale, especially in the Polar region. Another advantage is that no discharge of C02, NOx and SOx occurs during navigation, which helps to prevent environmental disruption due to NOx, SOx and the greenhouse effect due to C02. Especially in regards to NOx and SOx, the discharge quantity will be severely restricted even for ships in the
future. The above mentioned characteristics strongly motivate us to develop marine reactors as an economical power source gentle to the natural environment.

On the other hand, the marine reactor should be compact and lightweight since it has to be installed in a narrow and limited space on a ship. Furthermore, a smaller marine reactor is more economical for a ship’15. It must have high safety characteristics as well as easy operation and maintenance. It should also be possible to operate automatically to a great extent, since the operation must occur in the ocean environment without aid from land.

The Japan Atomic Energy Research Institute (JAERI) is conducting a design study on an advanced marine reactor, MRX, to obtain a more compact and lightweight reactor with enhanced safety’25-’45.

The MRX is designed to use lOOMWth for a reactor plant of an icebreaker scientific observation ship, but the concept could be applied to those of general commercial nuclear ships with wide output ranges of 50 through 300MWth

Containment vessel (Inner D: 7.3m Inner H :13.0m)	Containment water cooler (4 trains) (Heat pipe type)
Control rod drive mechanism (X13) Water spray header Pressurizer heater
Perforated plate (to enhance steam condensing & water stabilizing) Emergency decay heat removal system (x3) Plate for water stabilizing (x 8)
Main coolant pump (x2) Pressure relief valve (x3) Steam generator (Once-through helical coil type: 2 trains) Reactor vessel nner D: 3.7m) Core Fuel assembly (x 19) Flow screen
Thermal insulator-
Fig.1 Concept of MRX plant

Table 1

Reactor type : Integral PWR

Thermal power (MWt) : 100

1. Core and reactivity control

Fuel/moderator material : UO2/H2O

Fuel inventory (tons of heavy metal) : 6.326

Average core power density (kW/liter) : 41

Average/maximum linear power (kW/m) : 7.626/30

Average discharge burnup (MWd/t) : 22,600

Enrichment (initial and reloaded) : 4.3/2.5%

Life of fuel assembly (year)

Refueling frequency (year)

Fraction of core withdrawn (%)

Active core height (cm)

Equivalent core diameter (cm)

Number of fuel assemblies Number of fuel rods per assembly Rod array in assembly Pitch of assemblies/fuel rods (mm)

Clad material Clad thickness (mm)

Type of control rod Number of rod clusters Number of control rods per assembly Neutron absorber material Additional shutdown system Burnable poison material : Fuel rod with Gd2(>3 and burnable

poison rod of borosilicate glass

design description

2. Reactor coolant system

(1) Coolant

Coolant medium and inventory : H2O (411)

Coolant mass flow through core (kg/s) : 1,250 Cooling mode : Forced

Operating coolant pressure(MPa) : 12

Core inlet/outlet temperature(°C) : 282.5/297.5

(2) Reactor pressure vessel

Inside diameter/Overall length (m) : 3.7/10.1

Average vessel thickness (mm) : 150

Design Pressure (MPa) : 13.7

(3) Steam generator

Number of SG : 1 (2 trains)

Type : Once-through helical coil

Configuration : Vertical

Tube material : Incoloy 800

Heat transfer surface per SG (m2) : 754

Steam/feed water temperature (°С) : 289/185

Steam/feed water pressure (MPa) : 4/5.8

(4) Main coolant pumps

Number of cooling pumps : 2

Type : Horizontal axial flow canned motor

Pump mass flow rate (kg/s) : 640

Pump design rated head (m) : 12

Pump nominal power (kW) : 145

3. Containment

Type : Water filled (simple wall)

Inner diameter/height (m) : 7.3/13

Design pressure (MPa) : 4

Design temperature (°С) : 200

depending on the type, size and velocity of ships. In addition to the icebreaker, the MRX series reactors will be favorably applied to high speed merchant ships, very large container carriers and super high-speed container ships, which need high power and long distance voyage. A view of MRX is shown in Fig. 1.

2. Design Features of the MRX

The improvement of the economy of reactor system depends strongly on the reduction of construction and operation costs. The construction cost can be reduced by means of making the compact, light and simple reactor system. The MRX adopts the following design features to be realized the above mentioned reactor system with highly passive safety : (a) Integral PWR. (b) In-vessel type control rod drive mechanisms, (c) Water-filled containment vessel, (d) Emergency decay heat removal system using natural convection. Table 1 shows the major specifications of the MRX.

(1) Integral PWR

Integral PWR could eliminate the possibility of large scale pipe rupture accidents, simplify the safety systems and reduce the dimensions of the reactor plant.

(2) Reactor core and reactor pressure vessel (RPV)

The core consists of 19 hexagonal fuel assemblies. The hexagonal assemblies, rather than rectangular ones, have been selected for reducing neutron leakage from the core and to operate with a small number of control rod clusters. The design conditions of the core specific to the marine reactor are as follows: (a) To maintain non-criticality (keff<0.99) under the

condition of normal temperature without use of a soluble poison even if one of the control rod clusters which has the largest reactivity worth is withdrawn from the core, (b) To operate the reactor with a sufficient power level for steerageway (^30% of full power) even in the case that one of the control rod clusters which has the largest worth cannot be withdrawn from the core, (c) To keep enough residual reactivity (^2%) for overriding Xe poisoning at the EOL. (d) The life time of the fuel assembly is 8 years with the plant factor of 50% (~23,OOOMWd/t). (e) The refueling frequency of 4 years with 52.6% of the fraction of the core withdrawn.

The fuel handling system is installed in land facilities. The average power density is sufficiently low (41kW/l) which shows that the core has enough margin for thermal reliability.

The RPV is relatively larger in size because of an integral PWR. This provides a larger primary water inventory with increasing the distance between the reactor core and the RPV, and reduces the neutron fluence at the RPV. The calculated value of the irradiation of fast neutrons is below 8xl0isn/cm2 (E^l. llMeV) at the inner-surface of the RPV for full power reactor operation of 20 continuous years.

(3) Control rod drive mechanisms (CRDMs)

The CRDMs are placed in the upper region inside the RPV to enhance the reactor safety with eliminating the "Rod Ejection Accident" and to achieve a compact reactor plant.

(4) Steam generator and primary circuit

The steam generator of once-through helical coil type is placed in the RPV diagonally upper region of the core. Two trains are adopted for the main steam and feed water lines. The vertical distance between the upper surface of the core and the bottom of the steam generator is selected to be 36cm to obtain a compact RPV, and an iron shield is installed between them to satisfy the design condition of the dose rate equivalent in the engine room.

The whole primary circuit is almost incorporated in the RPV except main coolant pumps, a volume control system and a residual heat removal system. The two main coolant pumps are placed in the hot leg at the upper cylindrical region of the RPV. The pressurizer is installed in the upper part of the RPV. For the maintenance and inspection, it is so designed that the reactor components in the RPV and the primary coolant pumps can be removed remotely and the steam generator tubes can be inspected from outside the RPV.