Reactor Technology: Materials, Fuel and Safety Dr. Tony Williams
Course Structure Unit 1: Reactor materials Unit 2. Reactor types Unit 3: Health physics, Dosimetry Unit 4: Reactor safety Unit 5: Nuclear fuel production Unit 6: Waste issues 2
Unit 4: Reactor Safety Reactor Safety: Measures and Assessment Transport of Nuclear Material Nuclear Safeguards 3
Beznau NPP: Protection of the population, staff and the environment from radioactive radiation - and therefore the safety of the nuclear installations - have absolute priority. The combination of technical safety barriers, redundancy of operational systems, safety culture and the permanent monitoring by authorities incorporates maximum protection against technical failures as well as external events such as fire, airplane crashes, and earthquakes. Technical safety barriers: The fuel: The fuel pellets are sintered to high chemical and mechanical stability with a melting point of 2800 C. Fuel cladding: The fuel pellets are encapsulated gas tight in tubes of zirconium alloy, which are highly resistant to corrosion. Reactor pressure vessel: The wall of the reactor surrounding the core consists of an inner layer of stainless steel and an outer layer of though, tensile carbon steel. Biological shield: The shielding concrete with a thickness of 3 m prevents radioactivity from escaping to the outside. Containment steel pressure vessel: The whole primary circuit is located inside the containment steel pressure shell composed of steel plates 30mm thick, welded together. The sub-atmospheric pressure in the annulus prevents potential radioactive gases from escaping. Reactor building outer wall: The reactor building outer wall has a wall thickness of 90cm reinforced concrete. Power Plant Safety 4
Power Plant Safety: Example Beznau Operational safety: Particular attention is paid to operational safety not only during normal operation, but especially if extraordinary events occur, such as unforeseeable defects in components of the plant. For this reason the vital components of the plant, such as control system or alarm devices, are provided in duplicate or in even higher numbers. Should any one of them fail there is always a second or third available to take over its duties. Safety culture: Basic principles of the safety culture are learning from mistakes and operating experience as well as clearly defined responsibilities. The management is responsible to ensure an optimal education and training to the employees as well as to provide enough manpower for the safe operation of the power plant. Additionally, an independent safety controller monitors and assesses the nuclear safety in Beznau NPP. He directly reports to the plant manager and the Executive Board. All these preventive measures are stipulated in the company s nuclear safety charter and are therefore binding upon the management. 5
Power Plant Safety Third party verification: The nuclear power industry is closely regulated by several laws and guidelines. National and international authorities carry out nuclear safety checks on a regular basis. The most important national authorities are the Swiss Federal Nuclear Safety Inspectorate, which monitors the abidance by the laws and the Federal Office of Public Health, which monitors permanently the radioactive radiation in the surroundings of Beznau NPP. On an international level, the operation of Beznau NPP complies with international nuclear safety standards specified by the IAEA (International Atomic Energy Agency) Safety Convention. In addition, safety in Beznau NPP is analysed and appraised by WANO (World Association of Nuclear Operators) on a regular basis. WANO is global association of nuclear power plant operators for mutual exchange of information. http://www.iaea.org/publications/factsheets/english/ines.pdf 6
Dose to third party In Switzerland, the maximum individual dose for individuals not engaged in radiation work is 1 msv per year according to the regulatory limit set in the Swiss radiation protection act (Strahlenschutzverordnung SR 814.501). In order to quantify the dose to third party arising from radioactive emissions dose to critical group is calculated. Dose to critical group is the highest dose that a hypothetical individual could possibly receive. Beznau NPP: In 2006, the release of radioactive materials into the environment via waste water and exhaust air from Beznau NPP was considerably less than the limits specified in the operating licenses. The dose to critical group was calculated by the Swiss Federal Nuclear Safety Inspectorate (HSK) to be 0.002 msv, which is less than 1 % of the annual exposure to natural radiation. The critical group are fictitious individuals that are assumed to live in the immediate vicinity of Beznau NPP and consume all the food and drinking water from the immediate vicinity of Beznau NPP. 7
Dose to third party Reprocessing: In the vicinity of the Sellafield site, the estimated critical group dose was 0.21 msv per year in 2004. In the vicinity of the LaHague site, the calculated dose to critical group was 0.0097 msv in the year 2004. Deep repositories for nuclear waste: The Swiss regulatory guideline HSK-R-21 stipulates that the release of radionuclides from a sealed repository subsequent upon processes and events reasonably expected to happen shall at no time give rise to individual doses which exceed 0.1 msv per year. The annual dose for the Reference Case of the proposed SF/HLW/ILW repository in the region of Zürcher Weinland, summed over all waste groups, is shown below. The maximum dose, which is due to 129I from spent fuel, is 4.8 10-5 msv a-1 and occurs at about one million years and is more than three orders of magnitude below the Swiss regulatory guideline and more than two orders of magnitude below the "level of insignificant dose" set at 0.01 msv a-1 by the IAEA (1996). 8
9
Power Plant Safety Risk assessment: A further important element of the safety concept is the identification and evaluation of possible operating troubles or incidents that may occur due to failure of technical systems, faulty workmanship or external events. To this end, a comprehensive risk assessment of the existing safety system is carried out periodically. Severe accidents may only occur due to the multiple malfunctions of safety systems or due to extremely unlikely events. Probabilistic safety assessment (PSA) is used as a method in the risk assessment to identify and delineate the combinations of events that may lead to a severe accident. The PSA considers a wide range of faults and takes an integrated look at the plant as a whole (system-inter-dependencies). For each combination, the expected probability of occurrence is calculated, assessed and the consequences are evaluated. In order to perform these tasks, PSA methodology integrates information about plant design, operating practices, operating history, component reliability, human behaviour, accident phenomena, and potential environmental and health effects. Internal as well as external initiating events are considered. 10
Internal event groups are: Loss of Coolant Accidents: Nuclear reactors generate heat internally; to remove this heat and convert it into useful electrical power, a coolant system is used. If this coolant flow is reduced, or fails to operate as designed, this heat can increase the fuel temperature to the point of damaging the reactor. Transient Events: The most important transient events leading to core damage are excessive steam leakage scenarios, operational disturbance (reactor trips) and failure of emergency boration. Loss of Support Systems: The most important scenarios for the loss of support systems leading to core damage are loss of main cooling water and offsite power. Internal Floods: The most important scenarios for internal floods leading to core damage are flooding of water supply pumps resulting in a loss of cooling water and flooding of the turbine building. Fires: No specific room or location dominates the fire risk at Beznau NPP. Scenarios that lead to core damage are well balanced between loss of decay heat removal scenarios and loss of coolant accidents due malfunction of the reactor coolant pumps. Turbine Missiles: Steam turbines have the potential to generate massive, energetic missiles if a turbine disc were to fail catastrophically. 11
External event groups considered in the study are: High Winds: High wind or tornado scenarios that lead to core damage include the damage of the turbine building including the switchgear in coincidence with failure of other emergency systems. Aircraft Crash: Important scenarios of aircraft crashes that lead to core damage include an aircraft crash on a building where important emergency systems are located resulting in an absolute failure of all emergency systems and spurious signals to stop all essential non-emergency equipment. Intake Plugging: In these scenarios the service water intake is plugged resulting in a loss of service water flow. External Floods: Most scenarios that lead to a core damage are scenarios without previous flood warning in combination with failure of operator actions. As a result, either a loss of a feedwater systems or the loss of coolant accident occurs. Seismic Events: Most of the scenarios leading to core damage from earthquakes include seismic damage of non-emergency systems in coincidence with random failure of emergency systems. For high acceleration levels there are also contributions from direct failure of the reactor building or from seismic damage of emergency systems. 12
PSA s can be performed at various levels, depending on the scope of the analysis For Beznau NPP the PSA includes the assessment on level 1 and level 2: A Level 1 PSA provides an assessment of plant design and operation, focusing on sequences of events that could lead to core damage and estimates the core damage frequency. It can provide major insights into design strengths and weaknesses and into ways of preventing core damage that would be a precursor to a large release of radioactive material. A Level 2 PSA identifies the ways in which radioactive releases from the plant can occur based on a core damage accident, the response of the containment to the expected loads, the transport of radioactive material from the damaged core to the environment and also estimates magnitudes and frequency of radioactive releases. This analysis provides additional insights into the relative importance of the accident prevention and mitigation measures such as the reactor containment. At each level, PSA provides the probabilities (frequencies) of adverse consequences and information on the dependence of these values on specific technical features (risk profiles). 13
The most recent PSA study for Beznau NPP was updated in 2007. As results of the level 1 PSA the core damage frequencies are shown. The results of the level 2 PSA are expressed as release frequency of radioactive material to the environment. Initiating event category Loss of Coolant Accidents Transient Events Loss of Support Systems PSA level 1 Core damage frequency [Number of events per year] 1.72e-6 2.79e-7 7.82e-7 PSA level 2 Radioactive release frequency [Number of events per year] 4.22e-8 7.78e-9 8.28e-9 Internal Floods 2.26e-7 2.77e-9 Fires 2.21e-6 2.54e-8 Turbine Missiles Total internal events 8.87e-8 5.30e-6 5.97e-10 8.71e-8 High Winds 4.26e-6 5.55e-9 Aircraft Crash 1.64e-7 2.42e-8 Intake Plugging 8.97e-7 8.80e-9 External Floods 1.93e-7 2.57e-9 Seismic Events 1.66e-5 9.28e-6* Total external events Total internal and external events 1.83e-5 2.36e-5 9.32e-6* 9.40e-6* 14
Safety 15
16
17
Transport of Nuclear Materials The IAEA Regulations for the Safe Transport of Radioactive Material set the basis for nuclear fuel cycle transport. The current version, TS-R-1 (ST-1, As amended 2003), entered into force January 2003. Radioactive material being transported should be packaged adequately to provide protection against the various hazards of the material under both normal and potential accident conditions. The prime objective is to protect people, property and the environment against the direct and indirect effects of radiation during transport. the Regulations set out several performance standards in this area. They provide for five different primary packages, (Excepted, Industrial, Type A, Type B and Type C) and set the criteria for their design according to both the activity and the physical form of the radioactive material they may contain. 18
Land transport The United Nations Economic Commission for Europe (UN/ECE) publishes the European Agreement Concerning the international carriage of dangerous goods by road (known as ADR). It contains requirements for the listing, classification, marking, labelling and packaging of dangerous goods by road. The IAEA Regulations have been adopted to apply to the transport of radioactive material under the ADR. Sea transport Since 1965, the International Maritime Organization (IMO) has published a major international instrument known as the International Maritime Dangerous Goods Code (IMDG Code). This Code is for the carriage of dangerous goods of any kind by sea. It addresses matters such as packing and container stowage, with particular reference to the segregation of incompatible substances. The IMO provisions for radioactive material are based on the IAEA Regulations. The IMDG Code offers guidance to those involved in the handling and transport of radioactive material in ports and on ships. Air transport The International Civil Aviation Organization (ICAO) has responsibility for all aspects of international civil aviation. It develops standards and recommended practices through the development of Annexes to the 1944 Convention on International Civil Aviation. In 1981, the ICAO adopted Annex 18, covering the air transport of dangerous goods and, in addition, published a set of Technical Instructions (TI) detailing the requirements for these transports. The TI contains a list of dangerous goods, as well as requirements for packing, marking, labelling and documentation fully consistent with the IAEA Regulations. 19
Type A Packages 20
Type B Packages 21
Drop Test of Mitsubishi Cask September 2004 22
Atoms for Peace On 8. December 1953, Dwight D. Eisenhower presented plans for the peaceful use of nuclear power. Implementation of a global infrastructure in the form of an international atomic energy agency (IAEA). Bilateral agreements between nuclear suppliers and their customers requiring so called "prior consent" from the supplier In order to track materials a system of labelling has been introduced in which all material falling under such agreements can be tracked throughout its life 23
Non Proliferation Treaty To prevent the spread of nuclear weapons and weapons technology, to promote co-operation in the peaceful uses of nuclear energy Opened for signature in 1968, entered into force in 1970. A total of 187 parties have joined the Treaty, including the five nuclear-weapon States. The Treaty establishes a safeguards system under the responsibility of the International Atomic Energy Agency (IAEA). 24
Isotopic Concentration (at/barn/cm 1.50E-04 Number Density for Heavy Metal Isotopes (40% void) SVEA-96/L Optima2, 4.11% U235, 14x4.0% Gd 1.30E-04 1.10E-04 9.00E-05 7.00E-05 5.00E-05 3.00E-05 U235 U236 U238 Np239 Pu238 Pu239 Pu240 Pu241 Pu242 1.00E-05-1.00E-05 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 Lattice Burnup (MWd/kgU) 25
Unit 4: Review In this unit we have adressed Which systems are typically set up to assess and improve safety The system which has been established for the Transport of Nuclear Material The concept of Nuclear Safeguards 26