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Evaluation of the status of the irradiated n_TOF spallation target CERN

Evaluation of the status of the irradiated n_TOF spallation target CERN M. Brugger , P. Cennini , A. Ferrari, Y. Kadi , E. Lebbos , J.Lettry , F. Saldaña , S. Sgobba , V. Vlachoudis and CIEMAT

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Evaluation of the status of the irradiated n_TOF spallation target CERN

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  1. Evaluation of the status of the irradiated n_TOFspallation target CERN M. Brugger, P. Cennini, A. Ferrari, Y. Kadi, E. Lebbos, J.Lettry, F. Saldaña, S. Sgobba, V. Vlachoudis and CIEMAT D. Cano-Ott, E. González, C. Guerrero, F. Martín-Fuertes, E. Mendoza, G. Piña, J. Quiñones

  2. After 4 years in water, with some drying process with pressured air in between, and 3 years in air in a humid atmosphere, the Pb shows evident signs of oxidation and corrosion. Such a situation was not totally unexpected since there has been little or no chemical control of the water chemistry during the four years of irradiation and the target was left unattended in air anohter 3 years (CIEMAT’s presentation during the n_TOFpannel review meeting). • The analysis of the available data (photographs, videos, TDR and n_TOF performance report) has revealed some deficiencies in the old Pb target design: • Insufficient cooling, in particular at the hottest spots. • Insufficient mechanical stability, enhanced with the temperature. • Lack of chemical stabilisation of the water. • All these aspects have lead to a corrosion due to water chemistry, enhanced by the temperature effects both mechanical and chemical. The degradation of the Pb has taken place (presumably) over the irradiation period and was a continuous rather than discrete process.

  3. 1. Old target cooling The water cooling was not sufficient at the hottest spots: Impact point of the proton beam: the water was at very high temperature and most likely boiling. Face next to the neutron pipe: at high temperature or boiling. Between the Pb blocks: no water circulation, high temperature. cold water in 1 2 3 hot water out

  4. The proton beam deposits its energy in a very local way. According to MCNPX Monte Carlo simulations performed, energy/proton = 12.1 GeV. 80% of this energy is deposited inside a cylinder of a few cm radius around the beam axis.

  5. CIEMAT has performed an analysis of the thermalhydaulic behaviour of the n_TOF target under steady state conditions. The following general assumptions have been used: • The Pb Target is a solid block, dimensions 0.8*0.8*0.6 m • The total deposited thermal power is 4 kW (7 kW was theoretical power, but only ~60% is absorbed as own MCNPX calculations indicate). • A realistic axial power profile. • The analysis consists in three complementary parts: • Hand calculations. Estimate the wall heat transfer regime and the heat transfer coefficients. • MELCOR model calculations for determining the heat transfer coefficients (see CIEMAT’s report 1). • 3D heat transport calculations: ANSYS-CFX and an alternative code.

  6. The relation between the non-dimensional Reynolds and Grashof numbers determines the flow regime: • If Re2 < Gr then: Natural convection regime • If Re2 > 10*Gr then: Forced convection regime • Re = (1000*0.06*0.05)/6E-4 ~ 5000; Gr=(9.8*0.003*50*0.033*10002)/0.0012 ~40·106 • Re’ = (1000*0.06*0.2)/6E-4 ~ 20000; Gr’=(9.8*0.003*50*0.23*10002)/0.0012 ~1.2·1010 • Proton beam entrance window: • Nu =HTC*L/k= 0.046(Gr*Pr)0.33 → HTC ~ 470 W/m2/K • A power removal breakdown is now assumed, as follows: 4 kW ~ 1 + 1 + 2, meaning that 1 kW is evacuated through front side, 1 through rear side and 2 through lateral sides, as a conservative breakdown. • Assumed evacuation area: R=0.1 m; flow area = 0.03 m2 → q” ~ 33 kW/m2 • ΔT = q”/HTC → ΔT ~ 70 ºC • Twater = 30 ºC (continuous renewal) → Twall ~ 100 ºC • The HTC have been calculated with the MELCOR code (used for reactor safety calculations) and confirms this values. The power removal breakdown has been estimated in the ANSYS-CFX 3D calculation, providing similar, but not exactly the same figures.

  7. Old Pb target. The water at the proton beam entrance point is boiling!

  8. Understanding the heat transfer in Pb • The thermocouples did show values below the boiling point during the runs. • Accurate positions are not documented but there are rough indications. • A detailed heat transfer analysis with a 3D numerical model. • The geometry of the Pb target was slightly simplified to a parallelepiped of 60x80x80 cm3. • Only the heat conduction inside the lead was computed in detail. • Educated guesses, based on MELCOR simulations, had to be made for the lead-water heat transfer coefficients. • The bulk water temperature was assumed to be 300K (27 ºC).

  9. Tmax inside the Pb target = 187ºC (or 169ºC) at 10 cm after the proton beam entrance. Temperature profile along various axes.

  10. T @ proton beam entrance point >100ºC BOILING! T @ proton beam exit point = 60ºC T @ exit face centre = 40ºC T @ +6 cm vertical from the proton beam entrance point = 60ºC Conclusion: the temperature gradients in Pb are very high. 6 cm away from the boiling point the temperature drops 70ºC. This explains why the thermocouples where indicating “apparent” safe values when it was not the case.

  11. 2. Mechanical stability As a consequence of the temperature, the mechanical stability of the target was affected. According to the values found in the literature, Pb is capable of supporting 1 m of Pb on top of it at room temperature: yield stress and creep stress. Both vary with the temperature (the values can be found in technical journals). The temperature excess inside the Pb (due to the lack of cooling) has caused a deformation of the central part of the Pb target.

  12. The weakening of the Pb because of the temperature has caused various deformations.

  13. 3. Water chemistry There are very clear signs of a strong pitting corrosion at the entrance of the proton beam. Such effects are very well known in nuclear power plants (cracks in the fuel cladding): the very hot (boiling) water carries more oxygen, thus allowing the Pb to change its oxidation state to higher values: Pb Pb2+ + 2e- Pb Pb4+ + 2e- Hydroxides are formed and a very acid local medium which attacks the metal is produced: Pb2+ + 2H2O  Pb(OH)2+ 2H+ Pb4+ + 4H2O  Pb(OH)4+ 4H+ Plume due to the boiling water Boling water H+ H+ H+ H+ Proton beam

  14. The whole Pb target surface is oxidised. It is very clear that Pb oxide deposits a the exit face of the target follow the temperature distribution pattern. Moreover, a tentative identification of the oxide type by its colour indicates that higher oxidation states are reached at the hottest places. The higher the temperature, the higher is the oxidation state and the solubility. The insufficient cooling inhibits the passivation and stabilisation of the Pb oxide layer. Plumes produced by water/steam at the Pb block junctions are also visible in the vicinity of regions where temperature inside the target is expected to be higher. (*) The Pb oxides are ordered by increasing oxidation state

  15. The corrosion has been modelled by a CIEMAT chemist. • High oxidation conditions • Tª  Vapour phase • pH dependence • Eh (redox potential) • Oxidation process • Pb(II) • Pb(IV) • Pitting corrosion Pb Pb+2 Pb+4 Pb+4 H+ H+ H+ Pb+2 Electron flow Pb+2

  16. Conclusion It is possible to control the chemistry of the water, passivate the Pb and thus minimise and control the corrosion!

  17. Possible minor galvanic corrosion has been identified as well: transfer of Fe to the Pb. Additional Pb oxide deposits due to the convection of the water around the proton beam impact point. The same kind of deposits on the bottom of the pool.

  18. Status of the Pb target pool (pending) The possibility of re-using the Pb target pool needs to be clarified from the technical point of view: Structures on the proton beam entrance window: holes? shadows? glue? Structures on the target pool walls. Holes Conclusion: inspection with a high resolution camera.

  19. Realistic and feasible improvements to the Pb target design • Huge experience (not ONLY bad) has been acquired with the Pb target: • The status of the target looks worse than it really is after 3 years in air. • Good knowledge of its behaviour after 4 years of irradiation. • Water is a good coolant. • Requirements for the new target concept: • Improve the cooling and lower the temperature of the Pb target: increase the speed of the water (efficiency of the heat transfer) and the water flow (amount of heat removed). • Improve the mechanical design and minimise the deformation: reduce the number of interfaces, particularly at the hottest spots, and reduce the mechanical stress. • Control the chemistry, thus eliminating the pitting and reducing the overall corrosion.

  20. New n_TOF target concept 1. Cooling Simplify the water flow paths. Go from natural to forced convection. Increase the speed of the water at the hot spots on the Pb surface and increase the water flow for enhancing the heat removal. 2. Structural design Reduce the mechanical stress: A reduced target mass. Re-design the Pb blocks: make them larger and following the 10º proton beam angle. Use of improved pilings. Insertion of horizontal stainless steel plates. Reduction of the number of interfaces. 3. Chemistry Add carbonates (CO3-) to the water for a proper Pbpassivation. Add a sacrifice anode. Control of the PH.

  21. MELCOR thermal analysis of the new target concept Old pump A new pump would increase the water velocity to the necessary value If: water velocity through the pump pipes increased to some 10 m/s Then: Water velocity through the channel at the proton impact point would increase to 1 m/s (instead of 0.06 m/s)

  22. Water flowing at 1 m/s in front of the proton beam impact point allows to reach the forced convection regime. HTC=1100 W/m2/K. A reduction of 25% in the power (4kW to 3 kW) allows to reach safer margins. Hot but no longer boiling.

  23. Summary and conclusions (i) • A huge experience (not ONLY bad) has been acquired with the actual Pb target. • In particular, a valuable amount of data about its behaviour after 4 years of irradiation. • Pb cooled with water can be considered as a safe option and perhaps the ONLY option following or very tight time schedule! • The data available have been evaluated carefully and a consistent and coherent description of the target degradation mechanism has been identified. • Most of the effects observed, oxidation, (pitting) corrosion and mechanical instability, can be attributed to the temperature excess produced by an insufficient cooling at the hottest places. • In particular, the release of spallation products to the water can be explained by the chemistry of the Pb, due to the increased solubility of high oxidation Pb states..

  24. Summary and conclusions (ii) There is a concept (but not designs) new Pbspallation target that eliminates or minimises the effects shown in the previous one but does not force to make a totally new and UNKNOWN design: A realistic modification of the existing cooling circuit will improve the cooling efficiency and cooling power, thus lowering the temperature of the Pb surface down to 60º C at its hottest point (proton beam impact). In addition to the lower temperatures, an optimised mechanical design will improve significantly the mechanical stability, minimising the thermal stress and avoiding its consequences. A control of the chemistry will allow to eliminate the pitting corrosion at the proton beam impact point and stabilise the Pb target surface by passivation.

  25. Summary and conclusions (iii) • The important combined effort has been made during the last months for evaluating the status of the old Pb target. • Its status is well understood and a consensus has been reached between CERN and the collaboration. • CERN and the collaboration are preparing an action plan which allows to have a new target ready for running in 2008. • A complete and detailed engineering study is needed urgently. We should keep our minds open, IMPOSE ALL the REAL CONSTRAINTS and let the engineers do the design according to our specifications. The engineers will have to evaluate in detail all the constraints and find the optimal solution considering: • The operation experience • The status of the actual installation (re-utilisation of the actual pool, proton beam window, cooling circuit…) • Our requests: physics, performance, safety, future upgrades… • The cost • The time scale! We need to run in 2008

  26. We have to transform the impossible into POSSIBLE!

  27. Understanding the thermocouple values The thermocouples did show values below the boiling point during the runs. Accurate positions are not documented but there are rough indications TC2 TC1 TC3

  28. Pb target parametric thermal analysis: point 2 (i) Conditions for the flow regime: If (Re**2) < Gr then: Natural convection regime If (Re**2) > 10*Gr then: Forced convection regime Water velocity ~ 0.06 m/s (estimated from the specifications in the TDR and the water flow 6 l/s) Re = (1000*0.06*0.05)/6·10-4 ~ 5000; Gr=(9.8*0.003*50*0.03**3*1000**2)/0.001**2 ~40·106 Re’ = (1000*0.06*0.2)/6·10-4 ~ 20000; Gr’=(9.8*0.003*50*0.2**3*1000**2)/0.001**2 ~1.2·1010 There is a natural convection regime: Nu =HTC*L/k= 0.23*(Gr*Pr)**0.226 → HTC ~ 330 W/m2/K Assumed deposited power 4 kW ~ 1 + 1 + 2 (evacuated as 1 through front side + 1 through rear side + 2 through lateral sides: conservative breakdown, other possibilities lead to worse conditions) Assumed evacuation area: R=0.1 m; flow area = 0.03 m2→ q” ~ 33 kW/m2 ΔT = q”/HTC → ΔT ~ 100 ºC Twater = 30 ºC (continuous renewal) → Twall > 100 ºC Conclusion: boiling is not excluded (it depends on the assumed power breakdown)

  29. Pb target parametric thermal analysis: point 1 at the proton beam side (ii) Point 1 (Proton beam side): Assumed evacuation area: R=0.1 m; flow area = 0.03 m2 → q” ~ 33 kW/m2 The tortuous path around proton beam impact point suggests that the water is nearly stagnant (no fresh water renewal) → Recirculation is expected → Twater > 30 ºC Twall > 100 ºC Conclusion: the boiling is almost certain!

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