1. Egyptian Atomic Energy Authority (EAEA), Egypt
Study of switching from natural to forced convection regime during reactor operation M. GAHEEN and M. SHAAT
Egyptian Atomic Energy Authority (EAEA), Egypt

2. Switching from natural to forced convection – (1/16)
CONTENTS:
Introduction
Reactor description
Modeling
Results
Transient characteristics
Limiting conditions
Conclusions

3. Switching from natural to forced convection – (2/16)
Introduction
Switching from natural to forced convection regime during research reactor operation resulting in significant and fast decrease of the core temperature.
This incidental condition has been modelled and studied for an open pool MTR (Material Testing Reactor) using ETRR-2 data.
Results of the transient characteristics and variation of core parameters show that such a situation is not accepted from safety point of view.
The results also discussed how the operation procedure and design (safety systems, interlocks, valves,..) can prevent such incidental conditions, limit or compensate for this significant and fast decrease.
Applicable solutions are proposed in the conclusions based on the results for switching to forced convection.

4. Switching from natural to forced convection – (3/16)
Reactor description
ETRR-2 design was used as for this study.
The reactor can be operated up to 400 kW in natural convection.
The Flapper Valve is open in Natural regime operation and the heat generated is removed by natural convection at low power level (< 400 KW).
The operators are enabled to start core cooling system pump during operation in order to increase the reactor power.

5. 400 KW (natural )- 11MW (forced)
Switching from natural to forced convection – (4/16)
ETRR-2 Reactor data
400 KW (natural )- 11MW (forced)
Power level
Delay neutron fraction
75×10-6
Prompt neutrons generation time (sec)
Coolant coefficient ($/oC)
Void coefficient ($/%void)
Doppler coefficient ($/oC) 40
Coolant inlet temperature (oC)
1638T+1632×103
Fuel heat capacity (J/ok/m3) (T in oC)
1242T+2069×103
Clad heat capacity (J/ok/m3) (T in oC) 15
Fuel thermal conductivity W/(mK)
180
Clad thermal conductivity W/(mK)

6. Switching from natural to forced convection – (5/16)
Modelling
Simple model was developed to provides a coupled thermal-hydraulic and point kinetics capability specifically for use in predicting MTR core parameters behavior when switching to forced convection.
The reactor power transients have been modeled using the point reactor equations (valid especially for small cores) with continuous reactivity feedback from thermal model calculation of fuel and coolant temperatures.
The time dependent reactivity is composed of (a) the externally controlled reactivity and (b) the summation of reactivity feedback.
The Model was verified with IAEA-TECDOC-643 benchmark solutions for slow and fast loss of flow of 10 MW benchmark MTR .

7. Modelling (cont.)
where M is mass, C is specific heat capacity, P is power, UA is overall heat transfer coefficient, is average temperature, and the subscripts f and c denote respectively the fuel, and the coolant.
Variation of coolant flow rate W and/or inlet temperature Ti results in variation of reactivity and core power.

8. Model transient response Model transient response to slow loss of flow
Switching from natural to forced convection – (6/16)
Model transient response
to fast loss of flow
(Time constant = 1 sec)
Model transient response to slow loss of flow
(Time constant =25 sec)

9. Switching from natural to forced convection – (7/16)
Model verification
The model is accurate enough in predicting fuel and coolant temperatures during slow and fast loss of flow transients.
Fast loss of flow
Slow loss of flow
Parameters
PARET/ANL
Model
90.3
91.27
86.8
88.04
Max. Fuel temp (oC)
58.5
58.1
48.4
47.36
Fuel temperature (oC)
(15% of nominal flow)
60.3
59.52
58.8
58.29
Max. Coolant outlet temp (oC)
46.5
47.01
43.3
42.66
Coolant outlet temp (oC)

10. Switching from natural to forced convection – (8/16)
Pump model simulate the coolant flow transients, the equation for the time rate of the coolant flow.
The model is valid compared with ETRR-2 pump start measured data.
A time constant of 5 sec match with the measured flow

11. Switching from natural to forced convection – (9/16)
Results:
Initial conditions:
core reactivity = 0.0;
reactor is operated in steady state power of 400 kW;
Average velocity in core is 12 cm/sec;
Inlet Inlet temperature = 40 C.
Switching from natural to forced convection (Transients)
Transient A: The core flow increased in 18 sec (time constant of 5 sec) increasing the channel velocity from to 2.4 m/sec.
Transient B: same as A with cold water (20 oc) from cold leg inlet the core (i.e. inlet temperature = 20 oc)
Transient C: same as B with average velocity in core = 0.08 m/sec.

12. Switching from natural to forced convection – (10/16)
- The temperature of the fuel and coolant are decreased fast to 41 and oC respectively.
- Core reactivity increased to ~ $ in 10 sec .
- The power increased with max. of ~ 15 kW/sec. .

13. Switching from natural to forced convection – (11/16)
B: With cold water from cold leg
- The temperature of the fuel and coolant are decreased fast below 25 oC and then increased due to the significant increase of the core power.
- Core reactivity increased to ~ 0.4 $ in 5 sec.
- The power increased exponentially while the power in transient A seams constant.
C: show higher value of resulting reactivity.

14. Switching from natural to forced convection – (12/16)
A typical in-hour curve of the reactor shows that the reactivity induced could results in low doubling time (or reactor period) and SCRAM the reactor.
Such a situation is not acceptable from the point of view of safety, thus the switching of the reactor from natural to forced convection should be covered by reactor limiting conditions for safe operation.
Relation between doubling time or reactor period and reactivity

15. Switching from natural to forced convection – (13/16)
Limiting conditions:
Switching during operation is Not possible, because of fast increase of the reactivity. The reactor is automatically shutdown (SCRAM) by the safety system when switching from natural to forced convection during reactor operation due to low reactor period.
Such a situation is not acceptable from the point of view of safety, thus the reactor should be designed (safety system, interlock …) and operated (procedures, …) so that incidental situations cannot induce a significant and fast decrease of the temperature:
The reactor should be shut down first, then switched to forced convection and restarted (knowing its critical configuration).
Insertion of the control rod would compensate the increase in the reactivity resulting from fast decrease of temperature (T): only if the decrease in T is slow enough for the rod movement with limited speed to maintain the reactivity about zero. Thus, one solution to this unaccepted condition is to increases the rod insertion speed (which is not safe) or insert the control rod prior to switching process.
Increasing the opening time of the discharge motorized valve of the core cooling pump will slowly increase the core flow and control rod could compensate the reactivity in this condition.

16. Switching from natural to forced convection – (14/16)
Partial insertion of the control rod prior to switching process would compensate the increase in the reactivity resulting from fast decrease of temperatures and maintain the reactivity about zero (insertion part is equivalent to -0.4$).

17. Switching from natural to forced convection – (15/16)
Increasing of the opening time (time constant increased to 5 min) of the pump discharge motorized valve, slow the core flow and temperature decrease (compared with Transient B).
Insertion of the control rod would compensate the increase in the reactivity resulting from fast decrease of temperature (T) in this condition (only if the decrease in T is slow enough).

18. Switching from natural to forced convection – (16/16)
Conclusions
Switching from natural to forced convection during reactor operation results in situation not accepted from safety point of view.
It can be concluded that limiting conditions for safe operation should cover such incidental conditions.
The solution of switching from natural to forced convection based on the results is one of the solutions:
shutdown the reactor and re-start with the forced regime without relying on the SCRAM,
insert one of the control rods corresponding to the value > the expected resulting reactivity (e.g > 0.4 $) prior to switching from natural to forced cooling during operation, or
increase the opening time of the discharge motorized valve of the core cooling pump to decrease the temperature slowly enough to compensate with control rod .
This study is applicable to the MTR reference design and the results can be extended to other similar reactors.

19. THANK YOU