C N S Presentation T E A M B

Malfunction A #1 (Drop of all control rods in CBA)

Drop of control rod Generally, control rods fall down for shutdown. Scram : loss of electricity → demagnetized → control rod assembly falls down by its weight in reactor → shutdown

Drop of all control rods in CBA CBA One control rod assembly consist 24 rods Total rod assemblies divide four of control bank and shutdown bank Control rod assembly misalignment Drop of control rod assembly Drop of control bank

A model of rod cluster control assembly

Cases kori 3.: control rod-systematic obstacle happen, one control rod falls down inner reactor, then shutdown occurs. yeonggwang 4.: linked line was cutted, in control rod drive- power switch, one control rod dropped.

Control-rod Accidents Uncontrolled rod withdrawal from a subcritical condition Uncontrolled rod withdrawal at power control-rod ejection(PWR only)

Control-rod Insertion Problem -expectation- Method of analysis Assumption - Reactor at 102% of rated power with the rod control system on manual. a. Maximum moderator reactivity feedback effect. b. minimum doppler coefficient. - LOFTRAN code is used.

Control-rod Insertion Problem -expectation- Core heat flux

Control-rod Insertion Problem -expectation- Average Coolant Temperature

Simulation analysis codes The transient react states about drop accident of Rod Assembly, Rod Bank are analyzed LOFTRAN codes. That calculate the variables including temperature, pressure and the level of power.

Number of steps in the control rod bank A are withdrawn

Net Reactivity

Relative prompt power

Concentration of delayed neutrons

Core coolant temperature

Average fuel temperature

“Departure from Nucleate Boiling” marginal for core

Prediction from FSAR Usual inserted reactivity (accident of one rod assembly drop) = - 1,200pcm DNBR > 1.3 (accident of one rod assembly drop) There is no damage of core!!

Results and Discussions At first on 100% Power operation Reactivity=(k-1)/k=1 (k=0; critical) Relative prompt power=1 (100%) Concentration of delayed neutron=119000

Results and Discussions After drop of all control rods in CBA The negative reactivity inserted. Relative prompt power and Concentration of delay neutron fell down rapidly but saturate at low later.

Results and Discussions The Control Rods absorb the thermal neutrons -> Prompt Drop In large variation of reactivity the prompt neutrons have a more effect on a change in transient power than the delayed neutron, but we could not get the data from CNS.

Results and Discussions Core coolant temperature decreased about 12 degree and average fuel temperature are fell down and increased later. DNBR has increased to 2.29 at maximum and fell down and saturate. The system returns a new equilibrium because of moderator reactivity feedback effect. In general the simulation of malfunction coincides with FSAR.

Malfunction F #39 (Make up water blocked)

Scenario Organization Tank, regenerative heat exchanger, outflow heat exchanger, orifice, demineralizer, volume control tank(VCT), Charging Pump

Coolant outflow regenerative heat exchanger Temperature down

Orifice Outflow heat exchanger Temperature down Pressure down Demineralizer

Mix Ion Demineralizer : remove radioactive material Positive ion Demineralizer : remove Positive ion, Especially Cs isotope

Volume Control Tank (VCT) regenerative heat exchanger Temperature up Return Coolant filter Charging Pump Makeup water Blocked blocked

Data Analysis

Boron Concentration at CVCS (ppm)

VCT Boron Concentration (ppm)

Volume Control Tank Level

Total Charging Pump Flow

Discussion Make up water blocked Pump is out of condition (supplying pure water and boron) Sudden power loss of CVCS

Discussion Flow of boron water into CVCS is stopped Water level of tank decrease Boron concentration decrease

Discussion CVCS system action Water level goes down, and when level shows “low-low”, system will be shut down Charging flow decrease

Make up water blocked Inflowing to CVCS is stoped Water level of tank drcrease CVCS system shut down Entire boron concentration decrease Radioactive material increase

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The End