ML20205D930
| ML20205D930 | |
| Person / Time | |
|---|---|
| Site: | Maine Yankee |
| Issue date: | 10/02/1985 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20205D923 | List: |
| References | |
| NUDOCS 8510170137 | |
| Download: ML20205D930 (3) | |
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION ,
MAlhE YANKEE ATOMIC POWER COMPANY REPORT YAEC-1447, ' APPLICATION OF RETRAN-02/ MOD 02 AND BIRP TO THE ANALYSIS OF THE M5LB ACCIDENT AT MYAPC"
- 1. Introduction In Cycle 6 reload safety analyses, the Maine Yankee Atomic Power Company (MYAPC) performed a postulated Main Steam Line Break (MSLB) analysis for the Main Yankee Atomic Power Station using the RETRAN 01/ MOD 03 Computer Code. The analytical methodology was approved by the staff in an SER dated December 11, 1981. The RETRAN Computer Code has undergone several revisions, and a current version of the code, RETRAN-02/ MOD 02, has been approved by the staff for use in licensing applications. The acceptance was subject to restrictions as specified in the staff SER for the generic RETRAN Computer Code. Since the RETRAN-02/ MOD 02 Code offers several advantages over the RETRAN-01/ MOD 03 Code, the MYAPC has converted the RETRAN-01/ MOD 03 MSLB models to the RETRAN-02/ MOD 02 models. The MYAPC topical report YAEC-1447 was submitted by MYAPC in a letter dated November 20, 1984, to document the MSLB analysis models using RETRAN-02/ MOD 02. The staff evaluation is addressed below.
- 2. RETRAN-02 MAIN STEAM LINE BREAK MODELS Discussion of the RETRAN-02 models of all the systems expected to function during a main steam line break accident for the Maine Yankee Atomic Power Station (MYAPS) is provided in the topical report YAEC-1447.
Heating of the reactor coolant takes place by conduction of the heat
.= , generated in the fuel pellets across the pellet to fuel cladding gap space and through the clad to the cladding surface. Reactor coolant flows along the heated clad surface and is heated by convection. The direct deposition of 2.5% of the core power in the moderator by gama radiation ,
is also modeled in the RETRAN model. During the transient, boric acid will be added to the Reactor Coolant System (RCS) fluid by mixing of the borated Emergency Core Cooling System (ECCS) injection flow. The transport of the boron concentration throughout the RCS is determined by the use of the Boron Injection RETRAN Post-Processor (BIRP) code.
The MYAPS contains three primary coolant loops. They are represented by a two-loop model containing one single loop and one lumped loop. The single loop is represented by the loop associated with a ruptured steam line. The pressurizer is attached to the single loop in the model. This provides more rapid response in the pressurizer during the steam line break. RETRAN-02 uses the nonequilibrium volume model in the pressurizer.
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l 8510170137 851002 39 DR ADOCK O
The ECCS consists of the High Pressure Safety Injection (HPSI) purrps, the Low Pressure Safety Injection (LPSI) pumps, and the Safety Injection Tanks.
During a steam line break, the system is activated. Reliance is placed on boron injected by the HPSI pumps to maintain margin to criticality after shutdown. In RETRAN-02, the third safety injection nozzle was added in the model, his new junction allows the BIRP code to simulate the boron transport more accurately.
RETRAN-02 modified the auxiliary feedwater (AFW) system operation in the model to reflect the system changes during cycle 8. In the new model, when a low steam generator level signal is received, all of the AFW goes to the faulted steam generator due to the low back pressure. Eventually, a low steam generator pressure signal closes the AFW control valves and isolation valves to the faulted steam generator. At this time, the AFW control system diverts the AFW flow to the intact steam generator. Flow continues to the intact loop until the end of transient. The physical steam generator model used in the RETRAN-02 model is unchanged from the RETRAN-01 rodel. Also no changes were made to the main steam line model in the RETRAN-01 to RETRAN-02 conversion.
The break modeled is a double-ended guillotine break at the steam generator nozzle. The use of a large bubble rise velocity in the steam generator modeling results in single-phase steam flow from the faulted steam generator. This maximizes the energy removal from the RCS and thus maximizes the RCS cooldown rate.
The main steam line break analysis uses a statistical combination of the uncertainties of the reactivity components in the computation of transient reactivity balance. This methodology has been approved by the NRC in the staff safety evaluation review of YAEC-1361, dated November 30, 1983.
- 3. RETRAN-01/RETRAN-02 COMPARISON In the topical report YAEC-1447, a comparison of the two models was performed to quantify the differences in the models' response to the sarre transient. The limiting hot full power (HFP) MSLB case reported in cycle 6
, MSLB Analysis was chosen for the comparison. For this case, the transient
- is initiated by a double-ended break of the main steam line at the steam generator nozzle.
At 10 seconds into the transient, the RETRAN-02 MSLB transient was initiated by opening the break junctions. The AFW pumps and the standby condensate pump start imediately after the break occurs. By 11.4 seconds, the reactor, turbine, and faulted loop AFW control valve trip signals are generated. Safety injection starts at 30.4 seconds when the RCS pressure signal is generated. This signal, which is coincident with the previously received signal, trips the main feater, condensate, and heater drain pumps. The RCPs are assumed to he m ped by the operator at 60.4 seconds. Blowdown of the faulted stead p ers or continues until the transient run is terminated at 300 seconds, ni4 4 01 predicted similar transient response with slightly different timing. Overall, the two RETRAN models predict very similar results. The faulted steam generator heat transfer rate, calculated internal to RETRAN-02, is slightly lower than that in RETRAN-01. The void behavior in the pressurizer and upper head are more realistic because of implementation of the nonequilibrium volume model. RCS flow and temperature response from the RETRAN-02 model
is much smoother; there are no anomalous flow reversals or sudden temperature surges. The predicted break flows are very similar between the two models. The use of the BIRP code with its improved bdron transport modeling provides more reactivity margin to shutdown during the MSLB transient.
- 4. Conclusions Basea' on the MYAPC RETRAN-02/ MOD 02 main steam line break model and the qualification comparisons discussed above, the staff concludes that MYAPC has demonstrated their capability to analyze the main steam line break accident using the RETRAN-02/ MOD 02 Computer Code. MYAPC intends to perform cycle 9 reload MSLB analysis using this newly developed RETRAN Code to replace the previously approved RETRAN-01/ MOD 3 Computer Code. We conclude that the MYAPC modified RETRAN-02/ MOD 02 MSLB model as documented in the subiect topical report is acceptable.
Date: October 2,1985 Principal Contributor:
C. Liang, DSI e
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