neutron streaming analysis and shielding determination for ...neutron streaming analysis and...

Neutron Streaming Analysis and Shielding Determination for theKrsko Nuclear Power Plant

Bor Kos, Marjan Kromar, Ziga Stancar, Luka SnojJozef Stefan Institute

Jamova cesta 391000 Ljubljana, Slovenia

[email protected]

Peter KlenovsekKrsko Nuclear Power Plant

Vrbina 128270 Krsko, Slovenia

[email protected]

In this paper basic conceptual designs of shields placed at locations around the RCS (ReactorCoolant System) hot and cold leg piping entering the SG (Steam Generator) and RCP (ReactorCoolant Pump) cubicles to reduce neutrons streaming through the RCS loop pipe penetrations ispresented. In order to determine optimal position and dimensions of the shield, a thorough para-metric analysis was performed including assessing the importance of neutron streaming through thereflective insulation of the RCS piping. A conceptual and physics analysis of shielding was made inorder to enhance understanding of the neutron transport from the reactor core to the cubicle and todetermine the major and minor neutron pathways.

1 INTRODUCTION

At the Krsko Nuclear Power Plant (NEK) it was decided to evaluate possibilities of minimizingneutron dose rates from the reactor core to reasonably low levels to reduce staff radiation exposure en-tering and working in the reactor building and also to reduce long term effects on radiation-sensitiveequipment exposed to neutron radiation [1]. This action is particularly important for locations wheresignificant neutron streaming is present. The purpose of the shield determination is to reduce theneutron dose for personnel in the SG and RCP cubicles during reactor operations to a level as low asreasonable achievable.

A detailed geometrical model of the Krsko nuclear power plant for Monte Carlo neutron transportcalculation was developed based on CAD (Computer Assisted Design) models, blueprints, technicaldrawings and other available data. Monte Carlo neutron transport simulations to determine the abso-lute and relative dose rates are performed with the general purposes Monte Carlo neutron transportcode MCNP [2]. Monte Carlo calculations are coupled with deterministic [3] neutron transport codesto determine optimal variance reduction parameters, such as cell importances and weight-window pa-rameters [4].

309.1

309.2

2 GEOMETRICAL MODEL

Using a newly developed method for CAD to MCNP conversion [5] we produced a detailed modelof the entire reactor building of the Krsko NPP suitable for neutron transport simulations with specialimportance given to the modeling of the components around the reactor vessel and the RCS pipings.The concrete structures were modeled without any simplification based on the CAD models of NEK.The resulting model of the concrete structures was firstly visually compared against the CAD modeland available blueprints. No discrepancies were found in the model. A visual comparison of theoriginal CAD model without the outside walls and the resulting MCNP model is presented in Figure1. The most significant part of the geometry for the needs of determining neutron streaming andreducing it, the central concrete biological shield around the RPV shown on the left of Figure 2,was checked quantitatively. Volumes of the components calculated analytically from the CAD modelwere compared to the MCNP model [5]. The volume differences were in the order of 0.001 %.

(a) CAD model of the reactor building. (b) 3D MCNP model of the reactor building.

Figure 1: On the left is the complete concrete structure CAD model of NEK and on the right a 3Drepresentation of the newly developed detailed MCNP model without the outside walls.

The reactor vessel was modeled according to the best available data from blueprints. Specialattention was also given to modeling the reflective insulation (RI) around the RCS piping. Neutronstreaming through the RI is a major contributor to the total neutron dose in the cubicles as will beshown in section 5. Reflective insulation is modeled as a group of concentric stainless steel foils of athickness of 0.0508 mm combining to a insulation thickness of 8.89 cm. A schematic of the modelis shown on the right of Figure 2.

The reactor core is modeled based on the first fuel cycle of NEK. Results of neutron flux andneutron spectra were compared to measurements and deterministic calculations [6]. Good agreementbetween the results is observed. A detailed model of the core with a complete isotopic configurationis still under development and will be added to the model at a future date.

The detailed model will have an impact on the absolute neutron flux level, but this is not importantfor calculations of relative dose rates. Other studies have shown a detailed model of the core has anegligible effect on neutron spectra and flux inside of the cubicles [7]. The uncertainties that arisefrom the geometrical and material modeling of the reactor building and actual nuclear data are mostlikely much higher than uncertainties introduced with the simplification of the core.

Proceedings of the International Conference Nuclear Energy for New Europe, Portoroz, Slovenia, September 5-8, 2016

309.3

(a) 3D MCNP model of the centralconcrete cell

(b) Detail of the model of the RI andthe RCS penetrations.

Figure 2: On the left is the central concrete cell and on the right a detail of the RI and the RCS pipingpenetrations. Both structures play a major role in neutron transport towards the SG&RCP cubicles.

3 NEUTRON PATHWAYS

Figure 3: Contributon field in the XY-cross section ofthe Krsko NPP for a tally located in the southern cubi-cle.

To understand neutron transport fromthe reactor core of the cubicles we iden-tified the major neutron pathways. Toidentify the major neutron pathways weperformed deterministic calculations withthe DENOVO code embedded in the AD-VANTG code package. Flux and adjointflux calculations were performed for a tallylocated in the southern (03A) cubicle be-tween the cold and hot leg pipes. By multi-plying the flux and the adjoint flux we get aquantity called a contributon field shown inFigure 3. A contributon field shows us themajor neutron pathways that contribute tothe tally of interest and originate in the re-actor core. Variance reduction parameters,weight-windows, determined based on thisadjoint and forward simulation were usedfor all following simulations.

Major neutron pathways are seen pic-tured with the colors in the red part of the

color spectra in figure 3. The cold and hot leg penetrations contribute several orders of magnitudemore than other pathways. Therefore neutron shielding should be constructed in their path inside ofthe RCS piping penetrations.


309.4

4 OPTIMAL SHIELD LOCATION

Based on the information of the major neutron pathways we decided to insert neutron shields inboth of the RCS piping penetrations. The northern and southern cubicle are, for the needs of neutroncalculations, identical. All further simulations were done for the southern cubicle. Three differentlocations show on the left of Figure 4 were chosen. These locations do not take in to account theengineering limitations such as the need for cooling the surrounding concrete and not obstructingmaintenance paths. The neutron shields shown on the right side of Figure 4 fit tightly around thereflective insulation of the RCS piping and in to the central concrete structure and are 25 cm thick.This is a basic preliminary design with which a scoping study was preformed. The material used forthis preliminary design is a widely used borated neutron shielding material Shieldwerx SWX-277[8].

(a) Shield locations in the RCS penetrations. (b) Basic design of the neutronshield.

Figure 4: Three different shield locations inside of the hot and cold leg penetrations of the RCS andthe basic shield design.

(a) Relative neutron dose rate in the hot leg penetrationof the RCS.

(b) Relative neutron dose rate in the cold leg penetra-tion of the RCS.

Figure 5: Relative neutron dose rate for three different shield locations located in the hot and coldleg penetrations of the RCS.

In Figure 5 we can see the results of the MC simulation for different shield locations designated


309.5

as 1S, 2S and 3S. The value of the H*10[9] neutron dose on the y-axis is normalized to the non-shielded case. The normalization is defined with equation 1. Where D is the neutron dose rate, Φ theneutron flux and ξ the ICRP flux-to-dose conversion factors. The inverse value of the normalizationrepresents the neutron dose reduction factor. On the x-axis the distance from the reactor vesseledge along the hot leg penetration is shown. Error bars throughout this paper represent a one sigmastatistical uncertainty of the MC simulation.

Dshield

Dno−shield

=Φshield · ξ

Φno−shield · ξ(1)

The lowest value in Figure 5 around 350 cm represents the best performing neutron shield. At-tenuation in the shields is clearly seen. A slight buildup of neutrons before the shields due to neutronbackscattering is also observed. The stainless steel support structure has a negligible effect. Theapparent drop in neutron dose before and after the shield at location 1S in the hot leg is explainedby the relative position of the tally location and the shield. Shield location 2S gives slightly betterresults then the other two locations. Similar results are observed for both the hot and cold leg penetra-tions. Slightly higher neutron dose reduction factors are calculated for the cold leg. This is becauseof a bend in the geometry of the concrete surrounding the RCS cold leg penetration. Neutrons arescattered and slowed down on this bend and therefore slightly better absorption is achieved in theshield.

4.1 Neutron group energy contributions

Figure 6: Relative contribution from each energy group tothe H*10 neutron dose rate in the hot leg penetration of theRCS.

To better understand the physicsof this problem and to ensure optimalshielding neutron three group energyspectra was investigated. The upperlimit of the thermal group is 0,625 eV,the limit of the epithermal group is100 keV and 20 MeV for the fast neu-tron group limit. In Figure 6 the rel-ative contributions from each neutronenergy group to the neutron dose rateis presented for the hot leg penetrationwith a shield at the optimal position2S.

The H*10 absolute neutron doseis normalized to full reactor ther-mal power of 1994 MW. In Figure 6the predominance of the fast neutronspectra is clearly seen. To achieve op-timal shielding of fast neutrons theyhave to be moderated before they areabsorbed. To slow down more fast

neutrons a denser shield material with a higher percentage of hydrogen is needed or a thicker shield.


309.6

4.2 Neutron shield materials

Figure 7: Relative neutron dose rate calculated with differentmaterials of a 25 cm thick shield at position 2S in the hot legpenetrations of the RCS.

Several neutron shielding materi-als are available on the market. Sim-ulations were performed with threedifferent materials which meet thetemperature requirements. These arethe above mentioned Shielwerx SWX-277 and the Shieldwerx SWX-237[8]. They differ in the contents ofboron, maximum operating tempera-ture and density. The third materialused was the TRANSCO TCO-NS-262-01. Not much information besidethe isotopic information was availablefor the TRANSCO material.

Figure 7 shows that there areno significant differences between theneutron dose reduction factors of thematerials at this shield thickness. Be-cause the neutron shield materials per-

formed similarly we decided to increase the thickness of the shield in order to enlarge the neutrondose reduction factor. The results of the variation of the shield thickness are presented in chapter 5.

5 NEUTRON STREAMING THROUGH REFLECTIVE INSULATION OF RCS

Figure 8: Contributon field in the XY-cross section of theKrsko NPP for a tally located in the southern cubicle.

Increasing shield thickness shouldsignificantly increase the neutron dosereduction factor and inversely thin-ning the shield should decrease theneutron dose reduction factor. Thepreliminary results were surprising ascan be seen in Figure 8. Only slightdifferences of the neutron dose re-duction factor are observed when theshield is thinned to 12.5 cm and in-creased to 50 cm. This result canbe explained by neutron streamingthrough the stainless steel reflectiveinsulation. The model of the insula-tion is shown on the right side of Fig-ure 2. The space between the foils ofthe reflective insulation are filled withair and the neutron spectra is fast.

To test if neutron streaming is thecause of the low increase of the neutron dose reduction factor we simulated a completely theoreticalshield 50 cm thick that fits tightly around the RCS piping, as thou no RI was present. The result


309.7

is also presented in Figure 8 with the pink color. The neutron dose reduction factor is increased byseveral orders of magnitude.

6 CONCLUSION

A scoping study was performed to asses the possibilities to install neutron shielding in order toreduce neutron dose rates inside the steam generator and reactor coolant pump cubicles at NEK.Major and minor neutron pathways were identified using deterministic neutron transport techniques.Several shield locations inside the RCS hot and cold leg penetrations were tested using a basic tightlyfitting neutron shield of a thickness of 25 cm. A neutron dose reduction factor of about 10 in the hotleg penetration and 10 - 20 in the cold leg was calculated for all cases but the location 2S nestledbetween the RCS support structures and the cubicle openings performed best.

Location 2S was tested further. First of all we looked at a 3 group neutron spectra and concludedthat the fast neutrons are the major contributor to the dose rate. To ensure optimal shielding, severalshielding materials were tested. By using a 25 cm thick shield no significant differences of theneutron dose reduction factor was observed for the three tested materials.

When varying the shield thickness we noticed that the major contribution to the dose rate comesfrom fast neutrons streaming through the reflective insulation. Therefore increasing shield thicknessdoes not yield the desired result. The streaming effect was tested by simulating a theoretical shieldwhich fits tightly around the RCS piping and inside the penetrations. The dose reduction factor wasseveral orders of magnitude higher than the shield of same thickness that fit tightly around the RI.

This scoping study did not take in to account the mechanical limitations such as cooling of theconcrete and unobstructed maintenance paths. All of this limitations would reduce the neutron dosereduction factor thusly we can conclude that a factor of around 10 is an optimal theoretical valuewhich can and has served as a baseline reference for further studies.

ACKNOWLEDGMENTS

This work was performed within the scope of the contract no. 3151271 (Z-8150530) between JSIand NEK.


309.8

REFERENCES

[1] I. Mandic, V. Cindro, G. Kramberger, E. Kristof, M. Mikuz, D. Vrtacnik, ”Radiation damage inbipolar transistors caused by thermal neutrons”, IEEE, volume 1, pages 429–433. IEEE, 2003.

[2] X.-. M. C. Team. MCNP - Version 5, Vol. I: Overview and Theory. LA-UR-03-1987. URLhttps://laws.lanl.gov/vhosts/mcnp.lanl.gov/mcnp5.shtml (2003).

[3] T. M. Evans, A. S. Stafford, R. N. Slaybaugh, and K. T. Clarno, ”Denovo: A New Three-Dimensional Parallel Discrete Ordinates Code in SCALE”, Nuclear Technology, 171, 171-200(2010).

[4] S. W. Mosher et al.: ”ADVANTG - An Automated Variance Reduction Parameter Generator”,ORNL/TM 2013/416 Rev. 1, Oak Ridge National Laboratory (2015).

[5] B.Kos, L.Snoj, ”On using Grasshopper add-on for CAD to MCNP conversion”, PHYSOR, SunValley, USA, 2016

[6] B. Kos, ”Izracun doznega polja nevtronov v okolici reaktorja tlacnovodne jedrske elektrarne:magistrsko delo”, Fakulteta za matematiko in fiziko, Ljubljana, 2015

[7] M.P. Garces, ”Activation Neutronics for the Swiss Nuclear Power Plants”, ETH-Zurich, 2013.

[8] Shieldwerx, ”Radiation shielding for nuclear power plants”, 2015.

[9] ICRP, International Commission on Radiation Units and Measurements, ”ICRP Publication 74:Conversion Coefficients for Use in Radiological Protection Against External Radiation”, SAGEPublications, 1997


neutron streaming analysis and shielding determination for ...neutron streaming analysis and...

Documents