NUCLEAR POWER

PLANTProf Ir Dr. Ab Saman b. Abd Kader, PEng, MIEM,MCIT

Faculty of Mechanical Engineering,UTM

 

INTRODUCTION 

Nuclear powered ships are becoming increasingly popular in advancingship technology.

Previous drawbacks for using nuclear power centred mainly around theinherent safety concerns for the crew; installation, maintenance anddisposal costs and the exceptionally high standards required forcomponent manufacturing and quality assurance.

These hurdles are slowly being overcome as more funds are beingallocated to social security and defense worldwide and as a greaterdemand is being placed on sustained performance efficiency in navalships.

Of course this does not mean to say that nuclear ships were not aroundin the earlier days.

Shown below is the picture of a nuclear ship which was taken nearly 4decades ago and shows a ship named "Otto Hahn" which was a

 

HOW DOES A NUCLEAR POWER PLANT ON

SHIP WORK

The energy generating house or propulsion plant of a nuclear powered

ship utilizes a nuclear reactor to generate heat.

The heat is generated within the nuclear reactor as a result of the

fissioning of the nuclear fuel.

Lead shields are placed around the reactor as a preventive measure

against the radiation produced from the fissioning process.

The nuclear propulsion plant operates as a pressurized water reactor

design containing both a primary and secondary system.

 

Primary sys tem :  This is where water is circulated through the reactor,

piping loops, pumps and steam generators. As the heat transferred from

the reactor to the water is done at such a high pressure, it does not boil.

Instead, the water is pumped from the steam generator back to the

reactor for re-heating.

Secondary system:  Steam which is produced at the

steam generators supply the energy required to drive the turbine

generators. The turbine generators then cause the propeller to rotate

thereby causing thrust and a forward motion to the ship.

Turbine generators are also utilized in supplying the ship with electricity. 

Once the steam has passed through the turbines, it is cooled and

condensed into water and then fed back to the steam generators by the

feed pumps.

 As can be noted, both the primary and secondary systems involve the

recirculation and renewal of water.

 

A TYPICAL NUCLEAR SHIP ARRANGEMENT 

The above mentioned theory is generic in nature and good enough togive you a broad idea what a nuclear powered ship consists of.

In this section we will take a look at a specific arrangement of a nuclearship with the help of a diagram.

 As you can see in the picture below the diagram is fairly self explanatoryand the nuclear components are shown on the left hand side of thediagram and the steam generation system which ultimately drives thepropeller shaft on the right hand side.

The nuclear reactor produces heat which is used to generate steam andthat steam in turn in used to provide motive power for turbines.

Of course this arrangement might vary in different kinds of ships but isgood enough to explain the overall idea.

 

INDEPENDENCE 

The functionality of the propulsion plant does not require oxygen thereby

allowing the ship to operate independently from any external

atmospheric requirements.

Ship maneuvering and continuosuly changing operating performance

requirements dictate highly irregular power demands.

 As can be imagined, the quality, strength and durability of component

parts are of crucial importance to ensure sustained durability under such

harsh conditions.

One should bear in mind that the internals of a nuclear reactor remain

inaccessible for inspection or replacement for an extensive period of

time.

 

FULL STEAM AHEAD FOR NUCLEAR

SHIPPING 

Preparations are under way for nuclear reactors to

make a major comeback in commercial shipping. 

 Although shipping is already highly energy efficient, pressure has

come on the industry to lower emissions.

There is the potential for market-based measures for

controlling carbon dioxide emissions, while the entry into force of

strict International Maritime Organisation controls in 2020 provides a

firm deadline against which the industry can weigh the benefits of a

range of technology enhancements and fuel options.

But with no clear technological fix to lower emissions using

traditional diesel or LPG fuels, nuclear energy is emerging as a

practical option.

 

This trend has been developing quickly in recent years and the recentannouncement of a major joint research project on the topic is the mostsignificant to date.

Marine and energy consultants BMT Group and Enterprises Shipping andTrading have joined with start-up small reactor firm Hyperion and Lloyd'sRegister to "investigate the practical maritime applications for small modularreactors."

"We will see nuclear ships on specific trade routes sooner than manypeople currently anticipate," said Lloyd's Register CEO Richard Sadler.

The organisation has been an independent service provider to the shippingindustry for 250 years.

In response to its members' interest in nuclear propulsion Lloyd's Registerhas recently rewritten its 'rules' for nuclear ships, which concern theintegration of a reactor certified by a land-based regulator with the rest ofthe ship.

 A draft of the rules was put before Lloyd's technical committee two weeksago and this represents a further step towards an international regulatoryre ime to ensure worldwide safet in a otential nuclear shi in sector.

 

Vince Jenkins of Lloyd's Register told World Nuclear News: "National

maritime regulators have little nuclear capability, so land based nuclear

regulators will be needed in support.

Since there are no internationally traded nuclear powered merchant

vessels today, our nuclear powered ship rules have suggested a

framework which may allow nuclear powered shipping to operate.

Within this suggested framework, we have developed the area where it

is felt that a ship classification society can add value and confidence to

the safety of nuclear powered vessels, the integration of the reactor

plant into the ship.“ 

 The new program of joint research is meant to produce "a concept

tanker ship design based on conventional and modular concepts," said

Lloyd's.

It noted that "Special attention will be paid to analysis of a vessel's

lifecycle cost as well as to hull-form designs and structural layout,

"

 

Nuclear power looked set for a maritime role in the 1960s thanks to early

vessels like the Savannah and Otto Hahn,

although in the end the Savannah worked for only ten years and

the Otto Hahn was repowered with diesel engines after nine years.

The Japanese-built Mutsuoperated from 1970 until 1992 but none of

these ships was a commercial success.

 A notable exception has been the icebreaker fleet that works Russia's

trade routes in the Arctic Ocean.

These vessels number only seven, but one is a cargo vessel and small

reactors of the same type are currently being fitted to the  Akademik

Lomonosov , the world's first floating nuclear power plant, set for

deployment in Russia's far east.

 

Nevertheless, there remain about 200 small reactors at sea in militaryfleets but this technology cannot easily be transferred to the civil sectordue to the requirement of using low-enriched uranium (LEU).

In the military sector of recognised nuclear weapons states, high-enriched uranium allows more compact reactor designs with weight andcontrollability benefits.

The reactor of the Hyperion system uses LEU and measures about 1.5metres by 2.5 metres.

It would produce about 70 MWt - enough for about 25 MWe forpropulsion.

Its 'battery' design simplifies refuelling to a swap-out operation every 8-10 years with the possibility of managed lease arrangements similar toaircraft engines.

 

However, incorporation of any reactor in a ship would require extensive

radiation shielding, consideration of impact protection.

 A step change in crew training would be required and there is a strong

case for crew to be supplied by reactor vendors.

Similar to nuclear power on land, the additional capital cost of nuclear

compared to fossil fuels is a significant obstacle despite the fact that

savings on fuel and potential emissions charges would make nuclear

economic in the long run.

One of the most effective ways for a diesel-powered vessel to save fuel

and emissions is to travel more slowly and avoiding this practical

constraint could make nuclear vessels particularly attractive for certain

cargoes and routes.

 

INTRODUCTION TO SHIPS POWERED BY

NUCLEAR ENERGY

The warships of many of the world’s navies are powered by nuclearenergy, normally provided by a Pressurized Water Reactor. (PWR)

The heat generated in nuclear reactors is used to raise steam to drivesteam turbines.

The turbine can either drive the ship’s shafts through a gearbox, or usethe electrical power to drive the propeller shaft.

Large submarines use nuclear energy because of its high power outputcoupled with the length of time between refueling.

These submarines are able to cruise around under the world’s oceanswithout surfacing for many months.

Other naval ships such as cruisers and aircraft carriers are also poweredby nuclear energy.

 

The use of nuclear energy to power merchant navy ships has not proven

very successful, with only a handful being built with nuclear reactors.

However there are numerous icebreakers operating in the Arctic using

nuclear energy because of the high power output required to break

through ice sometimes over a meter thick.

This is another article on the series of Nuclear Energy.

This covers the use of nuclear energy to power warships and

submarines.

We shall examine the nuclear fuel enrichment required, the operation of

the shipboard power plant, and the different methods of using this power

to drive the propeller shafts.

We begin with a quick overview of nuclear energy, moving on to its

application in powering the world’s warships and submarines.  

 

NUCLEAR ENERGY OVERVIEW

Nuclear energy relies on the reactions of enriched uranium.

In a PWR, the enriched uranium is placed in fuel rods which are bundledtogether to form the fuel assembly.

The fuel assembly is placed in the core of the reactor where the reactionstake place.

The speed and reactions are controlled by control rods and a moderator.

In the core, uranium atoms are bombarded with free neutrons producingimmense heat.

This heat is transferred by a cooling medium to a heat exchanger whereprocess water is converted to dry high pressure steam.

The steam is then used to drive a steam turbine, as in a normal steam plant.

 

SHIPBOARD NUCLEAR REACTOR

OPERATION

The operation of a nuclear reactor to raise steam outlined above is

similar to a shipboard one. The main differences being as follows,

Type of Fuel Used

 As we have seen the normal fuel used in the reactor is enriched uranium

known as UO2, however the fuel used by the PWR for ships propulsion

has a different form.

This fuel is still uranium based but alloyed with different percentages of

aluminum or zirconium.

Fuel Enrichment

The UO2 fuel is normally enriched with 3-5% of U235.

 

The uranium alloy has been enriched to contain up to 90% U235, but is

normally used at between 45-75% U235 enrichment in PWR for ships

propulsion.

Internal Neutron Shield

The internal neutron shield is designed to contain the aggressive

neutrons movement occurring within the narrow compact pressure

vessel containing the reactor core.

The neutron shield is fabricated from a high mass/density material which

provides a good source of neutron absorption.

This prolongs the reactors core life by preventing the embrittlement of

the steel of the pressure vessel, with some reactors being designed to

last between 40 and 50 years.

The Plant’s Compactness 

 

 

The shipboard nuclear reactor is very compact compared to the normal

power plant reactor.

This compact plant is normally about 4m high with a diameter of 2m.

This makes these plants particularly suitable for submarines, where

space is at a premium

Power output

The power output from ships nuclear energy plants ranges from 200MWt

for submarine propulsion to 300MWt for larger warships such as aircraft

carriers.

 

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