Chernobyl and Beyond: Nuclear Power Renaissance and

Naveed Maqbul1Research Paper No.52

April 2011Chernobyl and Beyond: Nuclear Power Renaissance and Apprehensions

In an energy starved and carbon constrained world with increasing global awareness of

the risks of climatic changes, nuclear power is always held up as a clean and efficient way to meet the economic development objectives. Keeping in view its vital importance in socio-economic development, the Government of Pakistan envisions a 20 fold increase in the existing nuclear generation capacity by the year 2030.

The three significant accidents in over 50 years of civil nuclear power although hampered the worldwide quest for nuclear power, considering the current energy crisis and a steady advancement in nuclear technology, nuclear power would continue to play an important role in fulfilling the world’s future energy demands.

This article aims to discuss the technological advancements in the nuclear industry following the two major accidents of TMI and Chernobyl and the lessons learned from provisional assessments of recent Fukushima nuclear disaster that may be utilized to regain public confidence on nuclear option as a safe and viable source of electricity production.

1. NUCLEAR REACTOR TECHNOLOGY

A nuclear reactor is a device to initiate and control a nuclear chain reaction for the generation of electrical energy. When a heavy fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron, it may undergo nuclear fission i.e. splitting into two or more lighter nuclei which are called fission products accompanied by release of more neutrons and a large amount of energy that is several million times more per reaction than any chemical reaction. The neutrons so generated initiate another fission and so on thus leading to a self sustaining chain reaction.

The reactor core is enclosed in a thick stainless steel container called the Reactor Pressure Vessel (RPV).The power output of a reactor is adjusted by control rods that are used to absorb neutrons and control the power generated. Most of the reactors employ a cooling system that is physically separated from the water that is boiled to produce pressurized steam like the Pressurized Water Reactor (PWR). In such reactors, the heat generated into the reactor core is removed by a coolant which passes through a heat exchanger called steam generator to produce high pressure steam which in turn, drives the turbine to generate electricity. The primary system pressure is controlled in a PWR by a component called Pressurizer which utilizes pressure relief and safety valves in case of an increase

Chernobyl and Beyond: Nuclear Power Renaissance and Apprehensions

ByNaveed Maqbul

ABSTRACT

2Research Paper No.52 Naveed Maqbul


and heaters to control a decrease in primary pressure deviation from the normal limit.

In some reactors called the Boiling Water Reactors (BWR), the water is boiled directly in the reactor core to produce steam and the pressure is controlled through the flow of feedwater. As such there is no steam generator or Pressurizer components in BWRs. Thermal reactors which are most commonly operating in the world require a moderator to slow down the neutrons in order to sustain the chain reaction. Light water, heavy water and graphite are normally used as the moderating material. The main pressure retaining components of the reactor containing highly radioactive fluid are placed in a steel lined concrete building called the reactor containment. The figure below shows different components of a typical PWR [1].

The safety of the nuclear power plants is based on the concept of Defence in depth by virtue of which hierarchical physical barriers are deployed in the

designs such that if one barrier fails, the other barriers do not consequentially fail to prevent the release of radioactivity into the environment. These physical barriers typically include the ceramic fuel pellets, metallic fuel cladding, primary pressure retaining system and the reactor containment made up of steel reinforced concrete. The barriers are further protected by a series of levels of defence using operating procedures and engineered safety systems to prevent the occurrence and mitigate the consequences of all the postulated accidents. The safety systems providing protection from accident conditions include the Reactor Protection System (RPS), Emergency Core Cooling System (ECCS), containment spray system etc.

Nuclear power reactors can be categorized on the basis of their design classification as generation-I, II, III and

generation-IV reactors. Generation-I were early prototype designs which started operation in mid 50s and were the first full scale reactors used



for peaceful purposes of electricity generation. Generation-II refers to commercial reactors built up to the end of the 1990s and include the Pressurized Water Reactors (PWRs), Boiling Water Reactors (BWRs), CANDU (Canadian Deuterium Uranium reactor) which is a Canadian-invented Pressurized Heavy Water Reactor (PHWR), Advanced Gas Cooled Reactors (AGR), and Russian designed PWRs called VVERs. Most of the reactors currently operating in the world belong to this generation. At present there are about 440 nuclear power plants operating in the world out of which the pre-dominant ones are PWRs (60%) and BWRs (20%).

A generation-III reactor is a development of the generation-II design incorporating evolutionary

improvements such as improved fuel technology, superior thermal efficiency, standardized design for reduced maintenance and capital costs and longer operational life (60 years of operation, extendable to 80+ years) compared to 40 years of currently used generation-II reactors. Some examples

of Generation-III reactors are Advanced CANDU Reactor (ACR-1000), AP600, Advanced Boiling Water Reactor (ABWR), and Advanced Pressurized Water Reactor-1400. The figure below [2] shows different generations of nuclear power plants, their types and significant features of advance reactors in a time scale.

Generation III+ designs offer significant improvements in safety and economics over Generation III advanced reactor designs which are under construction or planned for near future deployment. Some examples of Generation-III+ reactors are AP1000, European (or Evolutionary) Pressurized Reactor (EPR), Advanced Pressurized Water Reactor-1400, and VVER-1200. The most prominent feature of

generation-III technology is the use of passive safety systems to control the accident progression. The function of these systems is based upon simple laws of physics e.g. gravity, condensation and evaporation and as such the systems do not require any external power source or manual action to initiate and perform



their intended function. These systems can be used to cool down the reactor core and remove heat from the fuel even in case of a Loss of Coolant Accident (LOCA) accompanied by complete failure of electrical power, considered to be a dangerous condition for a nuclear power plant. Examples of such systems are passive Emergency Core Cooling System (ECCS) deployed in AP-1000 and passive core flooding system in ABWRs.

Generation-IV reactors (or Gen-IV) are a set of theoretical nuclear reactor designs currently being researched. Most of these designs are generally not expected to be available for commercial construction before 2030. Examples are Very High Temperature (molten salt or gas cooled) Reactors (VHTR), Super Critical Water Cooled Reactor (SCWR), Gas cooled or sodium cooled Fast Breeder Reactors.

2. CHERNOBYL AND TMI ACCIDENTS

The Chernobyl Accident which occurred at Ukraine on 26th of April, 1986, considered to be the worst nuclear disaster in the history of nuclear power, shocked the world and instigated concerns in the nuclear industry about the safety of Soviet nuclear power plants as well as use of nuclear power, in general, throughout the world. The accident occurred in unit No. 4 operating an early designed reactor called “RBMK”, a graphite moderated light water cooled reactor which was regarded as a Russian (BWR).

The accident initiated during a simulation of loss of off-site power test and involved a sudden surge in power which led to a series of explosions and rupture of top part of the reactor container. Since there was no containment building unlike the European and US reactors, rather the reactor was housed in an ordinary confinement building, there was a large release of radioactive material into the environment. The accident caused death of 30 operators and firemen within a period of three months and several further deaths later due to acute radiation dose and resulted in widespread contamination extending to neighboring countries including Sweden and Finland. The accident was rated at level-7 (having widespread health and environmental consequences), the highest level, on the IAEA International Nuclear Event Scale (INES). Design flaws such as high positive void coefficient1 of reactivity, operator errors, faulty procedures and a poor safety culture environment were attributed to be the causes of the accident. However, Chernobyl was an older version of the RBMK reactors which experienced significant modifications in design following the accident. At present, only 11 RBMK reactors are operating in Russia after several safety upgrades including reduction of positive void coefficient, inclusion of more absorber rods and reduction of scram (rapid shutdown) time and improved containment design. The picture below shows an aerial view of the Chernobyl nuclear plant damage from an explosion and fire that sent large amounts of radioactive material

1. The feature leads to a rapid increase in reactor power in case of core heat up and formation of voids in the core. Current reactors have a negative void coefficient which decreases the power on void formation, an inherent safety characteristic.



into the atmosphere [3].

The next generation of Soviet designed reactors called VVER were Russian version of PWRs which are in operation in Russia and several

countries of the eastern bloc of soviet era including Czech Republic, Slovak Republic, Bulgaria and Hungary. These reactors also underwent significant design modifications under the auspices of the IAEA and funding from EU which brought the safety of current VVERs almost at par with the PWRs. As a result, these reactors possess a full-fledged emergency core cooling system,2 a robust reactor pressure vessel and reactor containment in the true

sense. With these modifications and an increased generation capacity, Russia exported two VVERs to China; six others units are planned to be exported, two each to Iran, India and Czech Republic. In addition, several units are

in operation in Russian Federation itself and Ukraine.

In spite of the gravity of Chernobyl accident, it was another accident occurring seven years earlier in 1979 at the Three Mile Island (TMI) Pennsylvania, USA which had a profound influence on the nuclear industry. Rated at level-5 on the INES, two scales below Chernobyl, the accident occurred in a reactor having a robust design based on well proven technology with a

2. A modern emergency core cooling system is a multi stage system comprising of high pressure, medium pressure and low pressure systems injecting cold borated water in the core in case of loss of system pressure and core heat up unlike early reactors which had a single stage low pressure system.



commendable safety record. The accident initiated while the reactor was operating at full power with the loss of secondary side feedwater which removes the heat from the primary coolant. The resulting heat up and pressurization of primary side led to opening of the pressure relief valve installed at the Pressurizer. The valve got stuck open and remained unnoticed by the operators due to a faulty instrument indicator resulting in a small Loss of Coolant Accident (LOCA). As a result, the Emergency Core Cooling System (ECCS) was automatically actuated but terminated by the operators who misjudged the core water level. The loss of water from the primary system resulted in core uncovery, increase in fuel temperature and a consequent fuel meltdown.

Despite melting of about one-third of the core, the Reactor pressure Vessel (RPV) maintained its integrity and contained the damaged fuel. The defense in depth approach worked resulting in containment of the radioactivity in the containment building and enabling the operators to eventually cool the damaged core and regain control of the system. The release of radioactivity into the environment remained within the acceptable limits and did not pose any health hazard. There were however, significant health affects due to the psychological stress on the individuals living in the area. The

figure below [4] shows molten fuel and structural material inside the reactor pressure vessel of TMI nuclear power plant.

3. ADVANCEMENT IN NUCLEAR TECHNOLOGY

The accidents initiated a worldwide safety research leading to improved nuclear power plant designs, operating procedures and more elaborate and stringent safety requirements by the start

of the 21st century. As a result, the concept of events beyond design basis and severe accidents emerged and gained importance. According to this concept, the nuclear power plants though designed to cope with accidents of very low probability3 of the order of 10-4, are still susceptible to sequence of events due to multiple equipment failures and operator errors leading to severe core degradation and large scale release of radioactive material into the environment. The current safety requirements

envisage an increased use of Probabilistic Safety Assessment (PSA) complementing the deterministic safety analysis traditionally used in design evaluation and safety analysis and use of best estimate or realistic approach instead of a conservative4 design approach. Consequently nuclear power plants were gradually equipped with additional

3. Chances of once in 10,000 years of reactor operation4. Conservative design includes a large, often un-quantified safety margin which although increases the cost

heavily, does not always contribute to reduction in risk during an accident.



safety features to prevent and mitigate the consequences of BDBAs5 including Severe Accidents involving significant core degradation.

On the basis of the safety research and advancement in safety standards, reactors having modernized designs were developed by incorporating additional features to cope with severe accidents. Some examples are the use of passive r e c o m b i n e r s to control the hydrogen concentrations to avoid its d e t o n a t i o n , installation of pilot operated relief valves to prevent high pressure melt ejection and external cooling of the reactor pressure vessel u n d e r g o i n g core melt to avoid its failure.

4. FUKUSHIMA NUCLEAR DISASTER

On March 11, 2011, an earthquake of magnitude 9.0 followed by a Tsunami suspected to be as high as 14 meters struck the eastern coast of Japan housing the Fukushima-I (Dai-ichi or number 1) and Fukushima-II (Dai-ini or no. 2) nuclear power stations. Four of the six reactor units of Fukushima-I were the most badly hit by the event creating a disaster which will have long term health and environmental consequences

and might hamper the perception about the nuclear energy as a safe and reliable source of electricity generation.

The affected reactor units contained the earliest version of BWRs with Mark-1 containment designed in late 60’s by the General Electric of USA. In such designs, the reactor core is contained in the cylindrical steel Reactor Pressure Vessel (RPV) in which the steam

is produced directly through boiling of reactor coolant. The RPV is enclosed in a b u l b - s h a p e d p r i m a r y c o n t a i n m e n t made up of r e i n f o r c e d concrete also known as the dry well. This dry well has an atmosphere

inerted by nitrogen in order to avoid hydrogen explosion and maintain its integrity. Below the dry well is the torus shaped wet well which comprises of a large rounded pressure suppression pool. In case of increase in the RPV pressure the excess steam is piped into the pool through relief valves and quenched under water to reduce system pressure. If the wet well pressure exceeds the dry well pressure it is vented into the rectangular reactor building surrounding the primary containment. The reactor building or the secondary containment of a BWR is not as robust as PWR containment and has a smaller

5. Beyond Design Basis Accidents are outside the design envelope of a nuclear power plant and have a potential of severe core damage and meltdown, if not mitigated.



free volume that is more susceptible to hydrogen explosions. A typical BWR with Mark-I containment [5] is depicted in the picture below:

The accident resulted in partial core meltdown at units 1, 2 and 3 and hydrogen explosions at unit 1, 3 and 4 destroying the upper part of the reactor buildings containing the reactors. The unit 2 experienced an explosion in the pressure suppression chamber threatening the integrity of its primary

containment. Unlike other units, unit 3 contained MOX fuel which was a mixture of Uranium and Plutonium. The Plutonium is potentially more dangerous in the event of a release to environ due to its toxicity and very long half life. The unit 4, defueled in the spent fuel pool in November, 2010 for maintenance, had relatively hot fuel (with higher decay heat and radioactivity) stored in the spent fuel pool. The water in the spent fuel pool of unit 4 boiled off due to loss of cooling and heated fuel produced large amounts of hydrogen due to oxidation of Zirconium (cladding material) which

exploded resulting in multiple fires in the reactor building of unit-4. Boiling of pool water virtually occurred in all the four units exposing and damaging the spent fuel. The sequence of events at unit 1-4 resulted in release of a substantial amount of radioactivity outside the plant which contaminated the sea water and the environment and led to evacuation of general public within a radius of 20 km. Units 5 and 6 of Fukushima-I were in safe (cold) shutdown condition and were not affected by the accident due

to recovery of onsite power earlier as compared to units 1- 4. Fukushima-II containing another four reactor units which were operating at the time of the accident were tripped automatically and brought to safe shutdown state.

What actually happened at Fukushima-I was that first the earthquake caused loss of off-site electrical power followed by an intense flooding due to tsunami which resulted in failure of all diesel generators providing on-site power, a failure that is termed as Station Blackout in the nuclear industry.



Although the units were shutdown automatically following the earthquake, there was appreciable decay heat which needed to be removed. The power failure first degraded and ultimately stopped the cooling of the reactor core. The water level in the core dropped rapidly exposing the fuel rods. As a result, the temperature of fuel, cladding and structural material gradually increased. When the temperature of cladding material (Zirconium) reaches around 1000° Celsius, it reacts with water and steam to produce zirconium oxide and generate highly inflammable hydrogen gas. The reaction is exothermic which further raises the core temperature. The hydrogen first escaped into the primary containment and then vented into the surrounding reactor building by the operators as a bid to relieve pressure in the containment. Uncontrollable high concentrations of hydrogen led to explosion which blew apart the top of reactor buildings and released the radioactivity into the environment. The damaged reactor buildings of units I-IV are shown in the photograph below [6].

Another important aspect of the accident was the heat up of spent fuel rods which are normally immersed in water in a storage pool in the reactor building. Due to loss of cooling and ultimate boiling of all the water in the pool, the spent fuel heated up and hydrogen was produced. Hydrogen accumulating above the pond exploded and breached the reactor building resulting in radioactive release outside.

On April 1, highly contaminated water which was stored in the basement of the turbine building after cooling

the reactor core of unit 2 was found to be leaking into the sea. The leakage was, however, stopped in the next five days. The accident has minimal transboundary implications as according to a press release of the Heads of the European Radiological Protection Competent Authorities (HERCA), air masses from the dispersion of the releases from the Fukushima accident although have reached as far as Europe, the radioactivity level is very low and will not have any human health or environmental consequences [7].

Rescue operations are being fervently undertaken by the Japanese authorities. Tokyo Electric Power Company (TEPCO) which is the operator and in-charge of Fukushima rescue operations has announced a roadmap towards restoration from the accident comprising of immediate actions that are divided into three groups; namely, cooling, monitoring and decontamination [8]. The main objective would be to ensure cooling of the reactor core to avoid further fuel meltdown which may breach the primary containment integrity and cooling of the spent fuel in all the affected units so as to prevent further release of radioactivity into the atmosphere, sea and soil. Efforts are also in progress to prevent spreading of radioactive substances contained in the dust, debris and water vapor by spraying synthetic material on the ground. TEPCO officials are planning on fitting the three damaged reactor buildings with fabric covers and filters to limit radiation release [9].

The rating of the event initially fixed at level-4 on the INES was later enhanced to level-5. Experts, however,



debated the Japanese classification of event on the INES and French and Finnish regulatory authorities according to their estimation rated the event at level-6 (higher than TMI) owing to damage at multiple reactor units and higher radiological consequences. On April 12, the Japanese Nuclear and Industrial Safety Agency (NISA) provisionally rated the event at level-7 on INES (major accident with wide spread environmental consequences) considering the accidents that occurred at Units 1, 2 and 3 as a single event and using estimated total release to the atmosphere as a justification.

According to the current status of Fukushima-I [10], 55% of fuel in reactor core of unit-1, 35% of unit 2 and 30% of unit 3 is suspected to be damaged including meltdown and the fuel still remained uncovered with water. The structural integrity of the Reactor Pressure Vessel (RPV) housing the reactor core in unit’s 1-4 is unknown due to high radiation fields in the area. The integrity of primary containment vessel containing RPV is also suspected to be damaged at unit 2. The reactor building has been reported to be severely damaged at units 1, 3 and 4, while slightly damaged at unit 2. The fuel integrity in the spent fuel pools of units 1 and 2 remains unknown while it is suspected to be damaged at units 3 and 4.

5. PRELIMINARY LESSONS LEARNED FROM FUKUSHIMA

Though the overall safety record of the type of BWRs which experienced

the event was satisfactory worldwide, the events far exceeded the design capacity of the plants which were designed to withstand an earthquake of magnitude 7.8 and a Tsunami of 5.5 meters. Since all the technical details of the accident are still not available, it is too early to draw all the lessons learned. The Western European Nuclear Regulatory Association (WENRA) which is a network of chief regulators of EU countries has proposed “stress tests” [11] including targeted re-assessment of the existing safety margins in the light of the events which occurred in Fukushima. The scope of re-assessment mainly includes:

• Extremenaturaleventsexceedingthe design basis such as earthquakes, flooding and other external conditions challenging a specific site leading to loss of safety functions.6

• Prolonged complete loss ofelectrical power and ultimate heat sink.

• Degraded conditions in thespent fuel storage.

• Accidentmanagementstrategiesto prevent and mitigate severe core damage and consequential hydrogen accumulation.

• Possibilityofseveralunitsbeingaffected at the same time.

6. THE FUTURE OF NUCLEAR POWER IN PAKISTAN AND THE WORLD

6. The fundamental safety functions to be adequately performed at all times during reactor operation are control of reactivity, cooling of fuel and confinement of radioactive fission products.



A number of countries are pushing to develop nuclear power generation capacity, with Pakistan among them. In a carbon constrained world, with increasing global awareness of the risks of climatic changes, nuclear power is held up as a clean and efficient way to meet economic development objectives while limiting carbon emissions. In addition to the large scale development in existing infra structure in countries like India and China, several other countries like United Arab Emirates, Turkey, Egypt, and Vietnam, among others, have expressed a desire to embark on nuclear power generation.

Pakistan’s current electricity mix is dominated by natural gas, hydro, and oil/diesel generation. The total generation capacity of Pakistan is 19.5 GW and consists of approximately 50% from natural gas, 30% from hydro power, and 16% from oil/diesel. Nuclear power’s current contribution to the mix of electricity generation is 2.4%. Pakistan’s current electricity generation capacity does not meet the demand creating significant shortfalls which result in power deficit as high as 5 GW during peak demand period [12]. Pakistan has two nuclear reactors in operation having a combined generation capacity of 425 MWe (KANUPP, a 125 MWe CANDU type NPP operating at Karachi and Chashma unit-1, 300 MWe PWR in operation near Mianwali). A third nuclear reactor C-2 (generation II+) having a net capacity of 325 MWe will commence operation in the current year. In addition, two more units which are replica of C-2 will be constructed at the Chashma site. Pakistan considers nuclear technology an important tool for its socioeconomic development. The

Government of Pakistan has adopted an Energy Security Plan 2030, which envisions utilization of all types of energy resources and enhancement of the role of nuclear power generation in the next few decades. Under this plan, the nuclear generation capacity is to be increased to 8,800 MW by 2030.

Undoubtedly, the Fukushima accident is an extraordinary event which is going to change the world’s perception about nuclear power and bring a temporary halt to the nuclear renaissance owing to re-consideration of the nuclear power programmes of existing and aspiring countries. However, considering the current energy crisis worldwide and the phenomenal growth taking place especially in the developing nations, nuclear power would continue to play an important role in fulfilling the future demands of electricity. However, a thorough re-assessment of the operating nuclear power plants especially those of older design of Fukushima vintage and further strengthening of the safety requirements in the light of Fukushima experience are essential to revive public confidence on nuclear as a safer and cleaner source of electricity production.

7. REFERENCES

1. Animated Diagram of a Pressurized Water Reactor: www.nrc.gov

2. Generation IV roadmap from Argonne National Laboratory: www.ne.anl.gov

3. Chernobyl Disaster Aftermath: www.en.wikipedia.org.



4. Graphic TMI-2 Core End-State Configuration: www.nrc.gov

5. BWR Mark I Containment sketch: www.commons.wikimedia.org

6. The crippled Fukushima Dai-ichi nuclear power plant an aerial photo from top to bottom, Unit 1 through Unit 4 taken by small unmanned drone: Air Photo Service Co. Ltd., Japan.

7. 4/20/2011- HERCA Press release on the Fukushima accident: www.herca.org

8. TEPCO’s Press Release (April 17, 2011) “Roadmap towards Restoration from the Accident at Fukushima Daiichi Nuclear

Power Station”: www.tepco.co.jp

9. Meltdown fears at Fukushima, radiation spreads; ABC News. 30 March 2011: www.abc.net.au

10. Status of nuclear power plants in Fukushima as of April 28th, 2011. Japan Atomic Industrial Forum ( JAIF) :www.jaif.or.jp

11. First proposal about European “stress tests” on nuclear power plants: www.wenra.org

12. Is Nuclear Power Pakistan’s Best Energy Investment? Assessing Pakistan’s Electricity Situation; a paper by John Stephenson and Peter Tynan Dalberg, Global Development Advisors.

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Chernobyl and Beyond: Nuclear Power Renaissance and