The effective (whole body) dose limit for the maximum exposed worker recommended in IAEA Safety Standards No. GSR Part 3 [2.25] is 50 mSv per year. Except for the easily controlled restricted area immediately above the core, all dose rates are low and would permit emergency operations to proceed.
2.10.8.9. HEU core removal and LEU core installation reactivity insertion accident
75 Only coolant and moderator temperatures were measured in the Slowpoke experiments [2.36]. Figure 2.34 shows calculated temperature profiles in the fuel and at the surface of the cladding for the MNSR HEU and LEU cores. The peak temperatures reached in the UAl fuel and Al cladding of the HEU core were 129°C and 124°C, respectively. This is far below the aluminium incipient melting temperature of approximately 640°C. In the LEU MNSR core, the peak temperature in the LEU UO2 fuel was calculated to be 239°C, which is far below the incipient melting temperature of 2865°C for UO2. The peak temperature at the surface of the Zircaloy-4 cladding was calculated to be 121°C, far below its incipient melting temperature of 1850°C.
FIG. 2.34. Calculated temperature profiles in the fuel and at the cladding surface in the fuel rod with peak power for rapid reactivity insertions of 6.48 mk in the generic HEU and LEU cores (Courtesy of Argonne National Laboratory, USA).
On this basis, the RELAP5 model for the generic HEU and LEU MNSRs was used to calculate additional rapid reactivity insertions of 4 mk, 8 mk, and 9 mk. The purpose was to establish an envelope of reactivity insertions which would not lead to melting of fuel rods , either in the event of an accident involving a net positive insertion of reactivity during removal of the HEU core, or installation of the LEU core. Peak fuel and cladding temperatures are shown in Fig. 2.35 for a rapid insertion of 8 mk in the HEU and LEU reactors.
FIG. 2.35. Fuel and cladding temperature profiles for rapid insertions of 8 mk in the generic HEU and LEU reactors (Courtesy of the Argonne National Laboratory, USA).
Peak reactor power, and peak fuel and cladding temperatures in the fuel rod with peak power are shown in Table 2.57 for rapid insertions of 4–9.25 mk in the HEU core and 4–11 mk in the LEU core. Again, the peak cladding temperature in the HEU core is to be compared with an incipient melting temperature of approximately 640°C for aluminium, and the peak cladding temperature in the LEU core is to be compared with an incipient melting temperature of 1850°C for Zircaloy-4. Similarly, the peak fuel temperatures are to be compared with approximately 640°C for the HEU UAl alloy fuel, and 2865°C for the LEU UO2 fuel.
TABLE 2.57. CALCULATED PEAK FUEL AND CLADDING TEMPERATURES FOR RAPID REACTIVITY INSERTIONS IN THE GENERIC HEU AND LEU MNSRS Rapid reactivity
insertion (mk) Peak power (kW) Peak clad temperature (°C) Peak fuel temperature (°C)
HEU LEU HEU LEU HEU LEU
4.0 83 77 104 101 105 148
6.48 256 221 124 121 129 239
8.0 1213 727 144 131 167 414
9.0 6028 2656 234 138 259 614
9.25 8668 — 299 — 305 —
10.0 — 9559 — 146 — 889
11.0 — 26 179 — 157 — 1111
For rapid reactivity insertions greater than 9.25 mk for the HEU core and 11 mk in the LEU core, RELAP5 predicts that the critical heat flux (CHF) or DNB will be reached and that temperatures in the fuel and the cladding will rise rapidly and could lead to melting of the fuel.
Since experiments have not been performed to verify the behaviour of an MNSR for reactivity insertions greater than approximately 4 mk, it would be prudent to consider only reactivity insertions below prompt critical (8.57 mk in the HEU core and 8.45 mk in the LEU core), for use in potential accident scenarios.
(b) Analysis of rapid reactivity insertions in the generic LEU core using the PARET/ANL version 7.5 code
Analyses of rapid reactivity insertion transients for the generic LEU core were also performed using the PARET/ANL Version 7.5 code using the same input data that was used in the calculations using the RELAP5 Version 3.3 code [2.37]. Peak power levels as a function of reactivity insertion that were computed using the two codes are in good agreement, as shown in Fig. 2.36. Peak temperatures in the fuel and cladding that are shown in Fig. 2.37 are also in good agreement.
77 (c) Analysis of reactivity insertion scenarios for removal of the HEU core
A step by step procedure for removing the current HEU core and installing a fresh LEU core was provided by the CIAE [2.35]. A description of the steps, the change in reactivity of the core, and the net excess reactivity after each step are provided in Appendix IV for the generic HEU and LEU cores.
If all steps in the procedure are performed in the specified sequence, the HEU and LEU cores will remain in a deep subcritical state, even if they were inadvertently dropped into the radial beryllium reflector to put the core into its maximum reactive state.
Three hypothetical accident scenarios for the HEU core removal operation and three hypothetical accident scenarios for the LEU core installation were evaluated in the unlikely event that the core changing procedures are not followed in the prescribed sequence.
The operations in the procedure that maintain the core in a deep subcritical state during core changing are the insertion of strings of four cadmium rabbits into the five inner irradiation positions in the beryllium reflector. In the NIRR-1 reactor in Nigeria, each string of four cadmium rabbits has a reactivity worth of –2.48 mk [2.38]. Insertion of a string of cadmium rabbits into each of the five inner irradiation positions would reduce the reactivity by 12.4 mk. It is also very important to insert the neutron and gamma detectors into the slant tubes outside the vessel to monitor changes in the neutron and gamma doses in the specified sequence. This would allow early detection of inadvertent increases in reactor power.
FIG. 2.36. Comparison of peak power calculated using RELAP5 and PARET/ANL for rapid reactivity insertions in the generic LEU core (Courtesy of Argonne National Laboratory, USA).
1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05
3 5 7 9 11
Power, kW
Reactivity Insertion, mk
___
PARET___
RELAP5FIG. 2.37. Comparison of peak fuel and cladding temperatures calculated using RELAP5 and PARET/ANL for rapid reactivity insertions in the generic LEU core (Courtesy of Argonne National Laboratory, USA).
In the first core removal accident scenario, it was assumed that the reactor staff failed to insert all five strings of cadmium rabbits as specified during removal of the HEU core, and also failed to address increases in the neutron and gamma doses as the core approached criticality and additional excess reactivity was inserted. This scenario will result in a net positive reactivity insertion of about 5.6 mk, which is less than the excess reactivity needed to reach prompt critical, which is 8.57 mk in the HEU core. The RELAP5 transient analyses described in Section 2.10.8.9 (a) indicate that melting of the fuel would not be initiated under these circumstances. However, it is very important that the five strings of cadmium rabbits are inserted in the prescribed sequence.
The maximum excess reactivity of 4 mk was assumed at the beginning of this scenario.
However, individual MNSRs may begin with a lower excess reactivity since the HEU cores will be partially burned. In addition, the reactivity worth of individual components that are removed or reinstalled in the core changing process may have smaller or larger reactivity worths in each MNSR. The conclusion is that each MNSR needs to be evaluated individually.
In the second core removal scenario, it was assumed that the reactor staff failed to insert four of the five strings of cadmium rabbits before removal of the HEU core, and also failed to address increases in the neutron and gamma doses as the core approached criticality and additional excess reactivity was inserted. Since the worth of each cadmium rabbit string is approximately –2.48 mk, the net positive reactivity insertion would be about 3.1 mk. In this case, the peak fuel and cladding temperatures would be far below their incipient melting temperatures, as described in the analyses shown in Sections 2.10.8.9 (a) and (b). There would be no damage to the fuel and no release of radioactivity,
In the third core removal scenario, it was assumed that the reactor staff failed to insert three of the five strings of four cadmium rabbits, each worth about –2.48 mk, into the five inner irradiation positions during step three of procedure for removal of the HEU core. The result is that the excess reactivity would reach 0.65 mk. The peak fuel and cladding temperatures would be far below their incipient melting temperatures, as described in the analyses shown in Sections 2.10.8.9 (a) and (b). There would be no damage to the fuel and no release of radioactivity.
0 200 400 600 800 1000 1200 1400
0 5 10 15
Fuel Max. T, °C
Reactivity Insertion, mk ___PARET
___ RELAP5
50 75 100 125 150 175 200 225 250
4.00 6.00 8.00 10.00 12.00
Temperature, C
Reactivity Insertion, mk
__
T clad max, PARET___T clad surf, PARET
___ T clad surf, RELAP
79 (d) Analysis of reactivity insertion accident scenarios for installation of the LEU core
Three similar hypothetical reactivity insertion accident scenarios were also evaluated for installation of the LEU core, assuming that the procedures for removal of the HEU core had been followed and that the HEU core had been safely removed.
In the first LEU core installation scenario, it was assumed that the reactor staff inadvertently removed the five strings of cadmium rabbits from the inner irradiation positions immediately after installation of the LEU core in the radial beryllium reflector, and before replacement of the aluminium shim tray and newly designed control rod. The staff also failed to address increases in the neutron and gamma doses as the core approached criticality and additional excess reactivity was inserted.
In this case, the net positive rapid reactivity insertion would be about 4.7 mk. The analyses shown in Sections 2.10.8.9 (a) and (b) indicate that incipient melting of the fuel would not occur. There would be no damage to the fuel and no release of radioactivity.
However, it is important to ensure that the operations specified in the LEU core installation procedures are performed in the proper sequence.
In the second installation scenario, it was assumed that the reactor staff removed four of the five strings of cadmium rabbits from the inner irradiation positions after installation of the LEU core in the radial beryllium reflector, and before replacement of the aluminium shim tray and newly designed control rod. The staff also failed to address increases in the neutron and gamma doses as the core approached criticality and additional excess reactivity was inserted.
The net positive reactivity insertion would be about 2.2 mk. In this case, the peak fuel and cladding temperatures would be far below their incipient melting temperatures, as described in Sections 2.10.8.9 (a) and (b). There would be no damage to the fuel and no release of radioactivity.
In the third installation scenario, it was assumed that the reactor staff failed to insert three of the five strings of cadmium rabbits into the inner irradiation positions in the sequence specified in the core installation procedures. In this case, the core remains subcritical throughout the installation process. There would be no damage to the fuel and no release of radioactivity.
The aforementioned analyses highlight the importance of following the removal and installation procedures in the correct sequence for HEU core removal and LEU core installation. This is particularly important for monitoring changes in the neutron and gamma doses, and the insertion and removal of the cadmium rabbits in the beryllium reflector.
81 REFERENCES TO SECTION 2
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3. CONVERSION ANALYSIS OF CHINA’S MNSRs