Radiation Heating of Thermocouple above Fuel Assembly

17th Symposium of AER, of Sept, 2007, Yalta, Ukraine Jávor Erika,NPP Paks, Hungary Task: determination of rise in temperature from radiation-heating in thermocouple above the fuels radiation-heating in thermocouple – MCNP calculation rise in temperature from heating – heat transfer calculation Divided into two parts:

17th Symposium of AER, of Sept, 2007, Yalta, Ukraine Jávor Erika,NPP Paks, Hungary main sources of radiation-heating in thermocouple (TC) ? - prompt neutron and γ energy deposition - γ of neutron capture - γ from decay of isotopes depends on ? - boron acid concentration ? - radial: how many neighbouring fuels ? - position of thermocouple ? - type of TC ? - axial: how many noduses ? NO calculations like this: many questions ?

17th Symposium of AER, of Sept, 2007, Yalta, Ukraine Jávor Erika,NPP Paks, Hungary neutron and γ heating calculation: MCNP fuel assembly key diameter: TC measuring thread diameter = 500 : 1; fuel assy volume : TC measuring thread volume = : 1 modell: important parts only MCNP variance reduction methods: - larger radial TC volume (self-shielding: smaller density) - importance – MCNP technics - different geometry modells constant density of water referred to outlet temperature of fuel at average power (1375MW/349) : 297 o C

17th Symposium of AER, of Sept, 2007, Yalta, Ukraine Jávor Erika,NPP Paks, Hungary 1. modell: fuel pins, fuel head, TC and its surroundings ( protective pipe-block bottom grid plane ) boundary planes lower: half fuel (mirror boundary condition) upper: TC +20 cm (γ reflected from neutron capture in structural parts ) 1 central fuel + 1/3 of neighbouring fuels 1 central fuel + 2/5 rings of neighbouring fuels quick, simple modell – but correct result ? relative dependence of γ heating in TC on - boron acid concetration - neighbouring fuels - type of TC - noduses A modell B modell

17th Symposium of AER, of Sept, 2007, Yalta, Ukraine Jávor Erika,NPP Paks, Hungary 2. modell: whole core (30 o symmetry) ( fuel pins, fuel head, TC and its surroundings: protective pipe-block bottom grid plane) boundary planes lower: half fuel (mirror boundary condition) upper: TC +20 cm (γ reflected from neutron capture in structural parts ) real core: Unit 4 cycle 1 (fresh fuel at BOC, burned up fuel at EOC) correct result – but slow calculation ! to determine absolute energy deposition in TC

17th Symposium of AER, of Sept, 2007, Yalta, Ukraine Jávor Erika,NPP Paks, Hungary MCNP calculations: 2 ways criticality (CC) calculation neutron,photon, electron derived from fission -> distribution by place,E, Ω -> collected in file as source for next (fix source-FS) calculation energy deposition by place k eff etc. FS calculation: input - a fix particles source from file energy deposition by place etc.

17th Symposium of AER, of Sept, 2007, Yalta, Ukraine Jávor Erika,NPP Paks, Hungary main sources of radiation-heating in TC : a) prompt γ from fission CC writes fix source: prompt γ -> γ FS: γ energy collection in TC b) γ from isotopes decay MCNP: γ of fission products are NOT calculated c) neutron direct energy deposition CC writes fix source: fission neutron -> neutron FS: neutron energy collection in TC d) γ from neutron capture (water, structural parts ) CC writes fix source: fission neutron -> neutron FS: γ energy collection in TC 1. modell A B

17th Symposium of AER, of Sept, 2007, Yalta, Ukraine Jávor Erika,NPP Paks, Hungary main sources of radiation-heating in TC : b) γ from isotopes decay: ORIGEN: number(/second) and spectrum of γ from decay of fission products in fuel at average power - number of prompt γ from fission / one 235 U fission: from literature - prompt γ from fission spektrum: CC calculation (energy deposition in fuel, energy groups as in ORIGEN) - number of fisson in fuel at average power / sec: 1375 MW / 349 fuels / 180 MeV / fisson - γ from isotops decay compared to prompt γ: numbers/sec SAME, BUT their spectrums different: 2 FS calculations (A modell): γ source: fuel spectrum: prompt γ resp. isotops decay γ energy (MeV)

17th Symposium of AER, of Sept, 2007, Yalta, Ukraine Jávor Erika,NPP Paks, Hungary TC heating depends on: boron acid concentration: – 3 – 0 gr/kg, A modell: 1 -> 1,35 radial: neighbouring fuels: B modell, CC - FS calc: energy deposition/fuel (%): central fuel12,4 (1 fuel)12,4 ( /1 fuel) 1st neighbouring fuels40,3 (6 fuels) 6,7 ( /1 fuel) 2nd neighbouring fuels39,4 (12 fuels) 3,3 ( /1 fuel) 3rd neighbouring fuels7,9 (18 fuels) 0,4 ( /1 fuel) TC:high position surrounding of TC lower shielding for neutrons position of TC : A modell 4 mm uncertainty in position of TC: 4,4 % uncertanty in heating

17th Symposium of AER, of Sept, 2007, Yalta, Ukraine Jávor Erika,NPP Paks, Hungary TC heating depends on: axial: 20 noduses: A modell, neutron: Watt spectrum, γ: prompt fisson γ spectrum energy deposition/fuel (%): photonneutron top nodus88,870,2 2nd nodus 9,920,9 3rd neighbouring fuels 1,2 8,9 upper two noduses: 90 % of total heating in TC nodus power depends on: fuel power, burnup, position (inner- peripherial, control rod), cycle (BOC-EOC) not independent: statistical analysis – top nodus power: neutron: Watt spectrum, γ: prompt fisson γ spectrum 1-year2-year3-year4-yeartotal fuels BOC 0,290,40,360,170,31±0,01 middle0,370,490,440,220,39±0,01 EOC0,460,570,510,280,46±0,01 during cycle: ~50 % change in power -> ~62 % change in heating

17th Symposium of AER, of Sept, 2007, Yalta, Ukraine Jávor Erika,NPP Paks, Hungary heating in TC: a) prompt γ from fission 30.5 % b) γ from isotops decay 4.8 % c) neutron direct energy deposition 0.5 % d) γ from neutron capture (water, structural parts ) 64.2 % housing2.59 mW/g hermetic case2.6 mW/g TC pipe2.58 mW/g MgO powder2.21 mW/g TC measuring thread 2.76 mW/g 10 4 Gy/h gamma-dose power in TC

17th Symposium of AER, of Sept, 2007, Yalta, Ukraine Jávor Erika,NPP Paks, Hungary Statistic analysis measured TC during cycle: increase ? γ-radiation in TC s depends on: - boron acid – decreases -> γ increases - pin powers in upper noduses (because of burnup) – increase -> γ increases Unit 1 cycle 14: 1,5 o C rise in TC average temperature but : TC in 2 position decrese core power increase 0,3 % dT increase

17th Symposium of AER, of Sept, 2007, Yalta, Ukraine Jávor Erika,NPP Paks, Hungary rise in temperature from heating – heat transfer calculation modell: as in MCNP calculations measured temperature: temperature at welding point of TC this rise in temperature depend on: - value of radiation-heating - heat transfer (thermal resistance) in TC heat transfer calculations: - heat transfer at housing, hermetic case is good - critical point: MgO around welding point of TC imporosity and density of MgO is unknown: thermal conductivity of MgO around welding point of TC ?

17th Symposium of AER, of Sept, 2007, Yalta, Ukraine Jávor Erika,NPP Paks, Hungary Result rise in temperature from heating: ~0 - ~2 o C, depending on parameters radiation-heating: calculated at acceptable accuracy thermal conductivity of MgO around welding point of TC: uncertain head of TC needs to know in details more kind of TC ?