10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition

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3.2.4 Degraded or Unanalyzed Condition

§ 50.72(b)(3)(ii) "Any event or condition that results in:

(A) The condition of the nuclear power plant, including its principal safety barriers, being seriously degraded; or
(B) The nuclear power plant being in an unanalyzed condition that significantly degrades plant safety."

§ 50.73(a)(2)(ii) "(a)(2)(ii) Any event or condition that resulted in:

(A) The condition of the nuclear power plant, including its principal safety barriers, being seriously degraded;
(B) The nuclear power plant being in an unanalyzed condition that significantly degraded plant safety." In a condition that is outside the design basis of the plant; or In a condition not covered by the plant’s operating and emergency procedures." In a condition that was outside the design basis of the plant; or In a condition not covered by the plant's operating and emergency procedures.

An LER is required for a seriously degraded principal safety barrier or an unanalyzed condition that significantly degrades plant safety. If not reported under § 50.72(a), (b)(1), or (b)(2) an ENS notification is required under § 50.72(b)(3) [an 8-hour report].

Combined Page: 10 CFR 50.72(b)(3)(ii), Degraded or Unanalyzed Condition

Discussion

(A) Nuclear Power Plant, Including Its Principal Safety Barriers, Being Seriously Degraded

(A) The condition of the nuclear power plant, including its principal safety barriers, being seriously degraded.

(B) Unanalyzed Condition that Significantly Affects Plant Safety

The 1983 Statements of Consideration for 10 CFR 50.72 and 50.73 indicated the following with regard to an unanalyzed condition that significantly compromises plant safety:

The Commission recognizes that the licensee may use engineering judgment and experience to determine whether an unanalyzed condition existed. It is not intended that this paragraph apply to minor variations in individual parameters, or to problems concerning single pieces of equipment. For example, at any time, one or more safety-related components may be out of service due to testing, maintenance, or a fault that has not yet been repaired. Any trivial single failure or minor error in performing surveillance tests could produce a situation in which two or more often unrelated, safety-grade components are out-of-service.

Technically, this is an unanalyzed condition. However, these events should be reported only if they involve functionally related components or if they significantly compromise plant safety.[1]

When licensees are applying engineering judgment, and there is a doubt regarding whether or not to report, the Commission’s policy is that licensees should make the report.[2] For example, small voids in systems designed to remove heat from the reactor core that have been previously shown through analysis not to be safety significant need not be reported.

However, the accumulation of voids that could inhibit the ability to adequately remove heat from the reactor core, particularly under natural circulation conditions, would constitute an unanalyzed condition and would be reportable.[3]

In addition, voiding in instrument lines that results in an erroneous indication causing the operator to misunderstand the true condition of the plant is also an unanalyzed condition and should be reported.[4] The level of significance of these cases generally corresponds to the inability to perform a required safety function. For instance, accumulation of voids that could inhibit the ability to adequately remove heat from the reactor core, particularly under natural circulation conditions, has an effect similar to a condition that could prevent the fulfillment of the safety function of the AFW system.

Beyond the examples given in 1983, an example of an event reportable as an unanalyzed condition that significantly degraded plant safety would be the discovery that a system required to meet the single failure criterion does not do so.

In another example, if fire barriers are found to be missing, such that the required degree of separation for redundant safe shutdown trains is lacking, the event would be reportable as an unanalyzed condition that significantly degraded plant safety. On the other hand, if a fire wrap, to which the licensee has committed, is missing from a safe shutdown train but another safe shutdown train is available in a different fire area, protected such that the required separation for safe shutdown trains is still provided, the event would not be reportable.

Examples

(1) Failures of Reactor Fuel Rod Cladding Identified during Testing of Fuel Assemblies Radiochemistry data for a particular PWR indicated that a number of fuel rods had failed during the first few months of operation. Projections ranged from 6 to 12 failed rods. The end-of-cycle RCS iodine-131 activity averaged 0.025 microcuries per milliliter. Following the end-of-cycle shutdown, iodine-131 spiked to 11.45 microcuries per milliliter. The cause was a significant number of failed fuel rods. Inspections revealed that 136 of the total 157 fuel assemblies contained failed fuel (approximately 300 fuel rods had through-wall penetrations), far exceeding the anticipated number of failures.

The defects were generally pinhole sized. The fuel cladding failures were caused by long-term fretting from debris that became lodged between the lower fuel assembly nozzle and the first spacer grid, resulting in penetration of the stainless-steel fuel cladding. The source of the debris was apparently a machining byproduct from the thermal shield support system repairs during the previous refueling outage.

The event is reportable because the cladding failures exceed expected values and are unique or widespread.

(2) Reactor Coolant System Pressure Boundary Degradation Due to Corrosion of a Control Rod Drive Mechanism Flange While the plant was in hot shutdown, a total of six control rod drive mechanism (CRDM) reactor vessel nozzle flanges were identified as leaking. Subsequently, one of the flanges was found to be eroded and pitted. While removing the nut ring from beneath the flange, it was discovered that approximately 50 percent of one of the nut ring halves had corroded away and that two of the four bolt holes in the corroded nut ring half were degraded to the point that there was no bolt–thread engagement.

An inspection of the flanges and spiral wound gaskets, which were removed from between the flanges, revealed that the cause of the leaks was the gradual deterioration of the gaskets from age. A replacement CRDM was installed and the gaskets on all six CRDMs were replaced with new-design graphite-type gaskets.

The event is reportable because there is a material defect in the primary coolant system that cannot be found acceptable under ASME Section XI.

(3) Degradation of Reactor Fuel Rod Cladding Identified during Fuel Sipping Operations With the plant in cold shutdown, fuel sipping operations appeared to indicate that a significant portion of cycle 2 fuel, type “LYP,” had failed; i.e., 4 confirmed and 12 potential fuel leakers. The potential fuel leakers had only been sipped once before the ENS notification was made. The licensee contacted the fuel vendor for assistance onsite in evaluating this problem.

An ENS notification was made because the fuel cladding degradation was thought to be widespread. However, additional sipping operations and a subsequent evaluation by the licensee’s reactor engineering department with vendor assistance concluded that no additional fuel failures had occurred; i.e., the abnormal readings associated with the potential fuel leakers was attributed to fission products trapped in the crud layer. Based on the results of the evaluation, the licensee concluded that the fuel cladding was not seriously degraded and that the event was not reportable. Consequently, after discussion with the regional office, the licensee appropriately retracted this event.

References

  1. 48 FR 39042, August 29, 1983, and 48 FR 33856, July 26, 1983.
  2. 48 FR 39042, August 29, 1983.
  3. 48 FR 39042, August 29, 1983, and 48 FR 33856, July 26, 1983.
  4. 48 FR 39042, August 29, 1983, and 48 FR 33856, July 26, 1983.