Occupational Radiation Sources Sources of Contamination
Objectives • List the origins for sources in a nuclear power plant. • Identify the classification of radionuclides produced in the fission process and where they are produced. • Provide an explanation of the fission process and its products. • Recognize a fission process with fission fragments and subsequent decay fission products.
Objectives • Define a ternary fission process event and its functions. • Provide a description of fuel rod cladding and its function. • List the types and origins of radiation emitted from the reactor core. • Identify the origins of radiation emitted from the reactor coolant. • Identify the importance of fission products produced.
Objectives • List the types of fission products. • Differentiate the types of fission products by their properties, isotopes, and removal process. • Name the origins of activation products • Name the origins of activation products. • Distinguish activated corrosion products by their origin, properties, isotopes, and removal process. • Define “crud” and describe its affect on a PWR and BWR system.
Objectives • Describe activation water oxygen products by the isotopes and radiological hazards. • Describe activation water air and impurities products by the isotopes and radiological hazards. • Describe activation water chemistry chemical products by the isotopes and radiological hazards. • Identify the mechanisms for tritium production, its half-life and radiological hazards.
Objectives • Compare sources of radiation outside the reactor core and coolant. • Define “Hot Spot” and identify potential areas for its occurrence • List the sources of radiation produced outside the plant and brought into the plant environment. • Define contamination and explain its sources.
Objectives • Identify “ hot particles” by definition and sources. • Explain the types of contamination, their potential for exposure and precautions utilized to limit the potential. • Contrast individual occupational dose and collective occupational dose and the reduction of each.
Origins of Sources of Radiation • Produced within the plant • Produced outside plant and received into plant
Types of Radionuclide Products FISSION PRODUCTS ACTIVATION PRODUCTS
Fission Process • Releases ~200 MeV Energy • Energy heats water • Initiates the production of Power
Binary Fission Process Produces2 heavy nuclei - Fission Fragment Fission Fragment
Fission Fragments • Light fragment – • 72-100 • Heavy fragment - • 110-162 • ~80 different fragments
Fission Process & Fission Fragments 14156Ba 9236Kr 23592U + 10n -> 14156Ba + 9236Kr + 3 10n + γ
Fission Process & Fission Fragments 23592U + 10n -> 14054Xe + 9438Sr + 2 10n + γ
Fission Process & Fission Fragments 23592U + 10n -> 14256Ba + 9236Kr + 2 10n + γ
Fission Process & Fission Fragments 13853I 23592U 9539Y 23592U + 10n -> 13853I + 9539Y + 3 10n + γ
Fission Process & Fission Fragments 23592U + 10n -> 14456Ba + 8936Kr + 3 10n + γ
Fission Products Fission fragments and their decay products ~250 different isotopes are known as fission products
Fission Process & Fission Products neutron 13552Te 13554Xe 13553I 13555Cs 23592U b g 13556Ba g b 9742Mo 9740Zr 9741Nb
Fuel Rod • Contains 235U Fuel Pellets • Made of thin metal sheath – • Cladding • Provide mechanical support • Uniform heat transfer • Protect fuel • Contain products
Radiation in the Core n– Fission Process γ- Fission Process γ γ – Fission Product Decay n γ – Activation Product Creation γ– Activation Product Decay
Radiation from the Coolant γ- Release of fission product γ n γ γ γ n n n γ γ
Fission Product Release Rate • Chemical nature of fission product • Pressure across cladding • Fuel temperature • Size of cladding crack • thermal stresses • corrosive action by coolant • mechanical forces • internal gas pressures
Radiation from the Coolant γ- Release of fission product nγ – fission product from tramp uranium outside cladding γ nγ – fission product from tramp uranium in cladding material n γ γ γ n γ γ– Activation of corrosion n n γ γ
Reactor Coolant Loop Structure Material • Stainless steel • Zircaloy • Inconel • Carbon steel • Steel & Copper Alloys nickel chromium cobalt
Radiation from the Coolant γ- Release of fission product nγ – fission product from tramp uranium outside cladding γ nγ – fission product from tramp uranium in cladding material n γ γ γ n γ γ– Activation of corrosion n n γ γ γ– Activation of coolant & impurities γ-Transuranic elements
Fission Products • Oxygen availability • Different volume • Increase pressure • Thermal conductivity • Melting point • Radiation source • Chemical properties • Physical properties • Radiological properties • Chemical change • Physical change • Radiological change
Fission Process – Fission Products Noble gases Very volatile – disperse Insoluble – build pressure, diffuse quickly Normally short half-lives Kr-85, Kr-88, Xe-133, Xe-135 PWR – waste decay system BWR – air ejector, gland seal system Noble gases Particulates Chemical state – nuclide dependent Soluble – degree nuclide dependent Aerosol – volatile Various half-lives most <2 months Diffuse slowly Removed demineralizer Halogens- Iodines Volatility form dependent Isotopes – I-131, I-133, I-135 Removal form dependent Halogens Particulates
Fuel Defect Operation • A reactivity maneuver restriction was imposed, limiting power changes to 3%/hr between 80-100%. • Chemistry verified that increased coolant Xenon activity was not caused by cross-contamination between units. • Letdown purification flow was raised and additional sampling for fission product trends was started. • A second Xenon/Iodine spike occurred 72 days into the operating cycle. • A significant increase in Iodine 131 occurred 400 days into the cycle, indicating that the cladding crack had opened up.
Fuel Defect Operation • Off gas activity increased from 2 micro curies/second to 480 micro curies/second and peaked at 1000 micro curies/second. • Reactor coolant Iodine levels increased by more than a factor of 10. • End cap weld failure can result in hydriding and cladding perforation. • Chemistry samples confirmed that a fuel defect was present. • The station increased sampling of the off gas release. • Conservative limits were placed on power ramp rates to mitigate additional cladding damage. • A control rod was fully inserted to suppress local power around the suspected fuel rod.
Activation Products • Corrosion Products • Chemicals • Air • Water • Impurities
Activation Corrosion Products Cr-51 Mn-54 Corrosion from Core & Coolant System Mn-56 • Coolant System • Nickel • Cobalt • Iron • Manganese Ni-63 Fe-59 Zirconium – Cladding Copper – Condenser Silicon/organic material – Water Purification Co-60 Fe-55 Zn-65 Co-58
Activation Corrosion Products Iron 58Fe + 10n -> 59Fe Stainless steel Inconel x Stellite Ni58(n,p)Co58 Co59(n,g)60Co
Corrosion Products Forms of Soluble cationic Soluble anionic Insoluble Co58 Mn54 Cr51 Co60 Fe55 Fe59
CRUD Insoluble voluminous colloid-like corrosion products Leads to cladding defects Blocks cooling canals Poor thermal conductivity
CRUD -BWR Main “crud” – Co60 from stellite Deposits on bottom of fuel Accumulates in vessel – requires special cleaning circuit
CRUD - PWR Main “crud” – Co58 from Inconel Throughout coolant system - removal purification system • “Crud” mobile • Transport affect – • Coolant pH • Hydrogen Concentration
CRUD Chemistry control Removal from system Creates serious radiation hazard Filtrate to radwaste Proper pH Corrosion inhibitors use Develop & select corrosion-resistant material Mechanical cleaning of chemical washing
Crud Bursts During Station Outages Important Points • A crud burst was in progress at the time the vessel head was removed because hydrogen peroxide addition was delayed for two hours. • Letdown flow was maintained at too low a value to effectively clean up the reactor coolant system before the head lift began • The recirculation pumps were tested with their discharge valves fully open. • The crud in solution following the shutdown plated out in the reactor coolant system because of the prolonged time the reactor coolant system was maintained at 340 degrees Fahrenheit.
Crud Bursts During Station Outages Contributors • Operations personnel were mot responsive to chemistry requests to increase letdown flow rate. • Chemistry procedures did not incorporate EPRI guidance on the concentration of soluble Cobalt 58 that would have minimized radiological hazards. • Station personnel did not recognize the radiological implications of starting the recirculation pumps. • There were no restrictions on the number of recirculation pumps that could be started at the same time. • The intermediate range compensating voltage should have been adjusted within 20 to 60 minutes following shutdown. • Upper and lower limits on source range count rate were not established to ensure the intermediate range detectors were adjusted during periods of low gamma radiation levels.
Activation Water Products 2H(n,g)3H x 18O(n,g)19O 16O(n,g)17N 26.8 sec 4.14 sec
Activation of Oxygen in Water Nitrogen Produced n β Major source in steam lines 7.2 sec half-life – so source on shutdown Shielding requirements due to gamma PWR reactor coolant BWR reactor coolant and steam lines 16O 16O 167N p 16O(n,p)16N γ 167N -> 166C + β + γ 16N 166C
Activation of Oxygen in Water Nitrogen Produced p β+ 16O 16O BWR masked by N-13 PWR minimal significance 9.9 min Half-life BWR discharge as effluent N-13 also produced by – 14N(n,2n)13N 137N a 16O(p,a)13N 137N -> 136C + β+ 13N 136C
Activation of Oxygen in Water Fluorine Produced n β+ 18O 18O Very soluble 110 min half-life PWR liquid activity BWR feedwater activity 189F p 18O(n,p)18F 189F -> 188O + β+ 18F 188O
Activation of Air - Argon Produced Ar-41 half-life 1.38 hrs High Ar-41 content indicates air in coolant 40Ar(n,g)41Ar 40Ar 41Ar 41K n b g ~1% air Argon Air impurity in water Deaerated water low Ar-40 content 4118Ar -> 4119K + b + γ
Activation of Impurities 34S 35Cl 35S 13C 14C Half-life 2730 yrs 14N Ternary fission
Activation of Chemicals Tritium produced by: Fission process Reaction on lithium & boron Activation water
Activation of Chemicals Boric Acid Chemical shim Burnable poison Control reactor level Lithium and Boron Additive Neutron absorbers Impurities Boron Activation Lithium Activation PWR’s 105B 63Li 73Li 31H n Fast neutron a Thermal neutron Lithium and Boron Contribute to H3 tritium production Lithium pH control B-10 activation Low or high energy neutrons
Activation of Chemicals Tritium Half-life 12.3 yrs Decays by beta only Hard to detect Part of coolant T2 or HT exchanges with Hydrogen in H2O HT = H2O -> HTO + H2 Difficult to separate Discharged to environment in condenser water Major source of activity in effluents