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NRC INSPECTION MANUAL DI

INSPECTION PROCEDURE 61708

ISOTHERMAL AND MODERATOR TEMPERATURE COEFFICIENT DETERMINATIONS

PROGRAM APPLICABILITY: 2515 (SUPPLEMENTAL)

61708-01 PURPOSE

This procedure verifies that the licensee's determination of the

moderator temperature coefficient (MTC) or the isothermal temperature coefficient (ITC) of core reactivity for pressurized water

reactors is technically consistent with the predicted values and

technical specification (TS) requirements.

61708-02 INSPECTION REQUIREMENTS

02.01 Ensure that the frequency of MTC (or ITC) determination

was as prescribed by the TS.

02.02 Verify the completion of the prerequisites for the ITC

measurement.

02.03 Verify licensee compliance with the ITC measurement test

precautions.

02.04 Verify that the actual plant conditions established during

the test were the same as those assumed in the analytical predictions.

02.05 Ensure that the MTC (or ITC) was calculated correctly and

was within the limits established in the TS and the limits predicted

by the vendor. If not within the TS required limits, verify the

licensee followed the applicable action statement.

61708-03 INSPECTION GUIDANCE

General Guidance

a. The TS requirements for some licensees concern ITC rather than

MTC determinations. The inspector should determine which

inspection requirements are applicable to the licensee and

utilize them as appropriate.
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Due to the specific technical expertise required to properly

perform core physics inspections, this inspection procedure

should be conducted by regional inspectors during the startup

physics testing at the beginning of core life (BOL) following

each refueling. Thereafter in core life, the
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resident inspectors should ensure compliance with these

requirements, including the end of core life (EOL) determinations.

b. Background. The MTC is defined as the change in core

reactivity per degree change in moderator temperature. The

moderator is the pressurized water of the reactor coolant

system (RCS). The MTC is calculated by measuring the ITC and

subtracting the doppler temperature coefficient (DTC) from it.

The DTC is a measure of the nonthermal neutron resonance

absorption cross-sections for U-238 and Pu-240. This value

is supplied for each particular core by the fuel manufacturer.

The ITC represents the combined effects of temperature changes

in the primary plant (moderator-MTC, fuel-DTC) on the net core

reactivity. As temperature increases, the MTC will become

more negative due to the moderator becoming less dense at an

increasing rate, thereby reflecting fewer neutrons back into

the core. The DTC will become more positive as temperature

increases, for the resonance absorption cross-sections will

flatten out and broaden, raising the likelihood of neutron

absorption into the fuel.

c. Test Method. The methods for measuring ITC are not specified,

but generally involve measuring and plotting flux level (core

reactivity) with respect to various RCS temperatures. The

slope of the plotted curve represents the measured ITC for the

specific control rod configuration. At BOL, the core

reactivity is measured using a reactivity computer. For EOL

measurements, the reactor is normally at power above the point

of adding heat (POAH). This renders interpretation of the

output from the reactivity computer impossible due to prompt

doppler feedback in response to power level changes.

Therefore, the EOL measurements should be based upon observations of the average core temperature, boron concentration,

and power level. These results should be evaluated using the

calculated values for the DTC, experimentally verified values

of boron worth, and reactor power levels determined by precise

heat balances. The change in reactivity due to the temperature changes is determined to give the ITC. The MTC is then

obtained by subtracting the DTC from the ITC.

03.01 Specific Guidance

a. Inspection Requirement 02.01

1. The regulatory basis requiring the performance of the MTC

(or ITC) determination is provided by the TS. Licensees

have also committed to the performance of these tests in

Chapter 14 of the Updated Final Safety Analysis Report.

2. Generally, the minimum frequencies required by TS for

these determinations include, but are not limited to the

following:
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(a) prior to initial operation above 5% rated thermal

power after each fuel loading.

(b) within 7 EFPD after reaching an equilibrium boron

concentration of 300 ppm.

b. Inspection Requirement 02.02. Typical ITC measurement test

prerequisites should include:

1. (a) At BOL, reactor power in the startup range, but less

than the POAH. This ensures that changes in RCS

temperature will visibly affect the net reactivity,

but that the RCS temperature will not change due to

neutron flux (i.e., the reactor will not heat up the

plant).

(b) At EOL, reactor power at any thermal power as

permitted by TS (generally above the POAH).

2. Control rod position, boron concentrations, RCS temperature, pressure and flow rate within set reference bands.

Maintenance of plant parameters within these bands is

necessary to ensure that reactivity changes due to plant

conditions differing from the reference won't result in

an erroneous MTC determination.

3. RCS heatup and cooldown rates less than the specified

maximum in order to ensure that the primary is isothermal. That is, the equilibrium plant condition where fuel

and RCS temperatures are approximately equivalent.

4. Reactor trip setpoints for the operable intermediate and

power range nuclear instruments (IRNI and PRNI) are set

less than or equal to 25% of rated thermal power.

5. A channel functional test for each IRNI and PRNI shall

have been completed satisfactorily within 12 hours prior

to initiating the physics tests or reactor startup.

c. Inspection Requirement 02.03

1. If rated thermal power exceeded the maximum allowed

setpoint for physics testing, ensure all reactor trip

breakers were opened. This power level is specified in

the TS and should also be given in the ITC measurement

test procedure.

2. Verify that the plant was stabilized before test data was

taken.

3. Rod shimming should have been avoided. If rods were

shimmed, verify that it was performed in one steady

motion and that the rod positions before and after the

shim were recorded.

d. Inspection Requirement 02.04. The closer plant conditions are

to the reference values at the time of data taking, the less
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likely that reactivity errors will be introduced which will

adversely affect the accuracy of the test results. Random

changes in these conditions would require the interpolation

of the test data to determine MTC, a process which is not

always very precise, thereby increasing the likelihood of an

inaccurate MTC determination.

e. Inspection Requirement 02.05

1. If the MTC (or ITC) is not within the required limits,

the licensee is generally required by TS to return it to

within its limits or shutdown to hot standby within six

hours.

2. It is possible for the MTC (or ITC) to meet the TS

required limits but not the vendor predicted values. In

this situation, required licensee actions are dependent

upon the commitments made in the Core Reload Topical

Analysis Report.

61708-04 REFERENCES

ANS 19.6.1, Draft 9, "Reload Startup Physics Tests for Pressurized

Water Reactors," December 1984.

END