ML19260C350
| ML19260C350 | |
| Person / Time | |
|---|---|
| Site: | Oregon State University |
| Issue date: | 11/19/1979 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19260C344 | List: |
| References | |
| NUDOCS 7912260233 | |
| Download: ML19260C350 (6) | |
Text
Enclosure SAFETY EVALUATION OF THE OREGON STATE UNIVERSITY PROPOSED MODIFICATIONS TO THE TRIGA REACTOR CONSOLE ELECTRONICS (TAC 11681)
I.
INTRODUCTION By letter dated April 16,1979 (Reference 1), Oregon State University requested sixteen (16) changes to the Technical Specifications for their TRIGA reactor, License No. R-106.
Four (4) of these changes involve modifications to the TRIGA console electronics. The present Oregon State TRIGA Reactor (OSTR) console was installed in 1967 and contains many printed circuit boards which are no longer available.
The licensee does not have a complete set of spare boards and is concerned that the reactor may be shutdown for an unreasonable length of time if problems with the console electronics are exprienced. Therefore, the licensee has decided to purchase from General Atomic, the console manufacturer, new electronic packages to upgrade their TRIGA console. These packages are standard instrumentation on new TRIGA consoles.
II.
DISCUSSION The proposed Technical Specifi :ation changes to Sections 3.5.2,
" Reactor Control System" and 3.5.3, " Reactor Safety System" are needed to reflect the addition of this new instrumentation. The new instrumentation whica utilizes all solid state modular construc-tion with integrated circuitry, should provide increased reliability over the existing instrumentation, and, therefore, upgrade the console electronics.
The new instrumentation package includes:
(a) A 9.5 - decade multirange linear channel (Model NML-2)
(b) A 10 - decade log power channel (Model NLW-2)
(c) A period circuit (Model NR-4)
(d) A linear safety channel (Model NP-5)
(e) A preamplifier (Model PA-5)
(f)
Pulsing logic (g) Calibration circuits for linear and log power and period, and (h)
Power supplies, including a high voltage supply (Model HV-6).
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. The relationship between the present instrumentation and the new instrumentation is shown in Figum 1 and listed below.
(a) The present multirange linear channel using an ion chamber will be replaced by the new 9.5-decade linear channel driven by a fission chamber.
(b) The present log channel with a period circuit using an ion chamber will be replaced by the 10-decade log channel, also with a period circuit, driven by a fission chamber. The same fission chamber drives the new 9.5-decade linear channel.
(c) The present count-rate (startup) channel using a fission chamber will be removed.
Its function will be taken over by the new 10-decade log and 9.5-decade linear channels.
(d) The present percent power channel using an ion chamber will remain unchanged, and (e) A new linear " safety power level" channel with scram capability, driven by an ion chamber will be added.
This channel will be identical (except for new electronics) to the present percent power channel.
III. EVALUATION The proposed modifications to the console instrumentation consists of:
(1)
Replacing the present multirange linear channel using an ion chamber with the new 9.5 decade linear channel driven by a fission chamber. This same fission chamber is also used to drive the new 10-decade log channel. We view this arrangement as a single " linear-log" channel, as failure of the single detector (fission chamber) means the loss of both linear and log information to the operator.
Since this new fission chamber is physically larger than the existing detector, it cannot be located in the same position.
The new fission chamber will be placed into the existing log ion chamber shroud, which will accept the physical size of the new chamber. We have reviewed this arrangement and find that the new location will basically provide the same source-fuel-detector geometry as the existing detector, and does not constitute a problem with detector shadowing. We in conjunction with the Reactor Safety Branch (RSB) find this change in detector location acceptable.
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. The present linear channel performs both safety (scram) and control functions. The linear portion of the new " linear-log" channel will provide a signal to the servo system for auto-matic power level adjustment. The new safety power level channel will provide the scram function.
(2) Replacing the present multirange log channel with a period circuit using an ion chamber with a new 10-decade log channel, also with a period circuit, driven by a fission chamber. This new instrumentation is considered to be the log portion of the new " linear-log" channel. This channel will provide the startup interlock function (preventing control rod withdrawal at a count rate of less than 2 cps) currently parformed by the count-rate (startup) channel. The count rate channel will be no longer required.
The new calibration circuits for the log and linear power and period circuits are similar to the existing calibration circuits in that they generate test signals to the channel electronics for checking proper circuit alignment. Six different calibra-tion signals are provided for calibration of both of the log and linear circuits. Two separate period calibration signals are used. The new calibrate switch (period / log test switch) is not spring-loaded as are the existing switches.
To preclude leaving the calibrate switch in a calibrate position, the switch dill be connected to the source and lKw interlocks. We find this arrangement to be acceptable.
(3) Adding a new " safety power level" channel, with scram capa-bility, driven by an ion chamber.
This channel will be identical (except for new electronics) to the present percent power channel.
The two channels will use separate ion chambers. This safety power level channel will provide scram capability. This channel, however, will trip only at 110% of full power (i.e.,
1.1 MW) whereas the present linear power channel trips at 110% of each range. Block diagrams of the present and proposed design are shown in Figure 1.
The use of wide-range " log-linear" channel wi.th the automatic servo presents a different situation regarding the separation of safety and control instrumentation.
The existing instrumentation cannot cause the loss of both automatic power level adjustment and period protection via a single detector failure. Upon loss of the control signal to the servo system, period protection is still provided for a reactor trip, in addition to the fuel element temperature trip and a 110% neutron level trip. However, a period-limiting circuit in the existing design limits the regulating rod speed so that the period never gets shorter than about 12-15 seconds, thus period scram protection (period <3 sec.) is not utilized 1617 063
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, when in the automatic mode. The licensee has stated (Reference
- 2) that the maximum increase in fuel temperatsee, before scram via a level trip, would be about 240*C from ambient or about 260*C. This temperature is well below the limitin system setting (LSSS) for fuel temperature (510 C)g safety
, which in itself incorporates a large safety margin before the fuel temperature safety limit (ll50*C) is reached.
Failure of the fission chamber detector in the proposed system causes the loss of both period protection and the control signal to the servo system for automatic power level adjust-ment. Protection would still be provided by two neutron level trips at 110% of full power and a fuel element temperature trip at 510 C.
The licensee has stated (Reference 3) that the maximum increase in fuel temperature resulting from this detector failure before a reactor trip (via one of the two level trips) would be about 40*C from ambient, or approximately 60*C.
Therefore, even though period protection would be lost due to failure of the fission chamber in the new design, the incmase in fuel temperature would actually be less than that for a similar detector failure in the present system.
In addition, the reactivity insertion rates postulated above are not nearly as rapid as during a routine pulse for which an acceptable safety analysis has been documented.
For the above reasons, we find that the deletion of level trips at 110% of each range and the period / control circuitry configur-ations to be acceptable.
(4) Removing the count-rate (startup) channel.
Its interlock function, which is to prevent control rod withdrawal at count rates less than 2 cps, will be taken over by the new " linear-log" channel.
The licensee has determined that overpower cinditions will not produce saturation or fold-over in any of the proposed new instrumentation channels. All minimum reactor safety channel functions, interlock functions, and operable measuring channels required by the current Technical Specifications will remain unchanged.
The Plant Systems Branch has reviewed the proposed modifications to the OSTR console instrumentation described above, and finds these equipment and design modifications acceptable. We have also reviewed the other console electronics included in the proposed package (i.e., pulsing logic, calibration circuits, preamplifier, and power supplies), and have found this instrumentation to be acceptable.
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s IV. TECHNICAL SPECIFICATIONS The proposed changes to the OSTR Technical Specifications are:
1.
Change Number 1:
In Section 3.5.2 (Reactor Control System), in the table listed in the Specification:
Add " Safety Power Level" as a measuring channel, effective in the steady-state (s.s.) and square-wave (s.w.) modes.
2.
Change Number 2:
In Section 3.5.3 (Reactor Safety System),
Table I:
Change " Log Power Level" in column 1 to read " Wide-Range Log Power Level."
3.
Change Number 3:
In Section 3.5.3 (Reactor Safety System),
Table I:
Change " Linear Power Level" in column 1 to read " Safety Power Level."
4.
Change Number 4:
In Section 3.5.3 (Reactor Safety System),
Table II:
Change " Count-rate Channel" in column 1 to read " Wide-Range Log Power Level Channel."
We find these proposed modifications to Sections 3.5.2 and 3.5.3 of the Technical Specifications adequately reflect the proposed changes in console electronics for the OSTR, and are correct, and, therefore, are acceptable.
V.
CONCLUSION Based on our review of the licensee's submittal, we conclude that the proposed modifications to the OSTR console electronics and the associated Technical Specification changes are acceptable.
VI.
REFERENCES 1.
OSU letter, C. Wang and C. Smith to R. Reid, April 16, 1979.
2.
OSU letter, C. Wang to R. Reid, October 10, 1979.
3.
OSU letter, C. Wang and C. Smith to R. Reid, August 17, 1979.
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