LBLOCA

ATHLET 1.2 A system code LBLOCA,SBLOCA experimental facility... 

DRASYS performed continuously PHARE 2.13, Bubble condenser qualification LBLOCA... 

During 1988, new data was developed for the RVSS materials at Oak Ridge National Laboratory (ORNL) (References 2 and 3). This data indicated that low neutron flux at low temperatures caused greater embrittlement of the materials used in RVSS than previously anticipated. This increased material embrittlement or "upward shift" in NDTT reduces the fracture toughness of these materials and, under certain specific and conservative transient conditions such as an earthquake or Large-Break Loss-Of-Coolant Accident (LBLOCA), could conceivably result in the failure of the supports thus permitting the reactor vessel to move. 

The SMART contains major primary components such as a core, twelve Steam Generators, a Pressurizer, four Main Coolant Pumps, and forty-nine Control Element Drive Mechanisms (CEDMs) in a single PRV. The integral arrangement of the primary system removes large size pipe connections between major components and thus, fundamentally eliminates the possibility of LBLOCA. 

Integral reactor No large primary piping Large LOC A Physically eliminated LBLOCA... 

LBLOCA Large-Break Loss of Coolant Accident... 

Application of Standard Engineering Correlations to LBLOCA Code Calculations... 

Discrepancies between code-set power levels for a very complicated problem like the LBLOCA are not anticipated to be excessive considering both code families are benchmarked to the same set of conditions and the accident transient is not significantly different from the benchmarking. In the case of the FI phase, the rapidity of the transient and timing remain critical. 

Relatively high temperatures are projected for the tank wall and top plate, which could present difficulty with rapid tank refill and resultant steam production. The tank draindown and subsequent gamma heating phase for the LBLOCA occur with the displacement of the heavy water by light water through ECS addition. With the light water and the tank drained down, there is no concern for recriticality. (For an intermediate LOCA, this situation probably does not apply as the tank can drain down without activation of the ECS.)... 

As previously described, the RELAP5 and modified RELAP5 models and codes set the boundary conditions that are critical to the FLOWTRAN-TF assembly model. These codes have been subjected to an extensive benchmarking effort based on full-scale primary system hydraulic tests. While these tests do not fully simulate the transient nature of the LBLOCA accident, and no heating is... 

Other DBEs, such as the loss of septifoil cooling, will be reanalyzed integral to the SAR Chapter 15 upgrade process using improved calculational procedures and codes. It is not anticipated that limits from these DBEs will eclipse the gamma heating DEGB-LBLOCA power limit, which currently dominates restart. 

While this discussion focuses on the FI DEGB-LBLOCA analysis, because of the formative nature of the gamma heating and ECS analyses, the single-phase conditions in the primary system make the resultant methodologies and models developed for the FI phase of the LOCA applicable to the analyses of other DBEs. More specifically, as previously indicated, this technology has been applied to the LOPA, PSB, and GRW accidents. 

Analysis of the LOPA, pump shaft break (PSB), and gang rod withdrawal (GRW) accidents (Reference 19) DBAs, which traditionally defined the SRS reactors maximum power limit envelope, were analyzed using similar T/H criteria, modeling, and computational techniques (TRAC, FLOWTRAN-FI and physics code suites) as those developed for the LBLOCA FI phase analysis. Preliminary results are shown in Table 1. 

DOE has provided WSRC guidance for the application of single failure criteria (Reference 12). For DBEs that are likely to occur, single failure vulnerabilities must be corrected. For those DBEs that are not likely to occur, a Justification for Continued Operation (JCO) must be submitted for DOE approval along with a corrective action plan. Thus, the technical basis demonstrating that the single failures selected by WSRC for each phase of the LBLOCA are credible and limiting needs to be documented. This is an open item and is a requirement of Section 11.1 of this SER. 

In developing the FI and ECS LBLOCA scenarios, limiting credible single failure criteria were considered as opposed to worst possible failures. 

It remains to be documented that there are no heat flux dependent local surface phenomena that can initiate OTE. ECS separate effects experiments have thus far been performed in channels with a single heater wall at heat fluxes twice the values expected in the core for the ECS phase of a LBLOCA initiated from 50 percent power (Restart Criterion 4,d). This is an open item. 

For the LBLOCA, the acceptable power level limit that has been proposed is that which results in a reasonable probability (84 percent or better on a core-wide basis) that no fuel assembly is damaged. This probability applies to the FI phase of the LOCA. This probability has not been determined for the ECS phase. This is an open item. 

Historically, for the SRS reactors, a DBA core damage fraction of 1 percent has been associated with the LBLOCA, which is consistent with the capability of the airborne activity confinement system. 

Currently, there are three DBAs that appear enveloping and could potentially limit reactor power the large break loss-of-coolant accident (LBLOCA) and the loss of pumping accident (LOPA), which are judged to very low probability events and the loss of septifoil cooling accident, which has occurred twice during the life of the SRS reactors. 

On November 30, 1989, based on preliminary calculations of power level (References 3 through 5), WSRC recommended power limits of 53 percent and 61 percent full core power for the FI and ECS phases of the LBLOCA, respectively. While there has been no written confirmation or documentation provided, recent estimates of acceptable power limits have varied widely and none of these "power limits" have been subjected to the scrutinity of a stringent external technical review and/or independent confirmatory calculations, as were the 1989 tentative FI and ECS power limits. 

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