Reactivity Control

It has been shown that meeting the MTC constraint in a RMPWR while maintaining acceptable performance is a key challenge in core design. Typical neutron absorbers used for reactivity hold- down are predominantly thermal absorbers and thus make the MTC worse. It is therefore essential to control the reactor while minimizing the increase in MTC. Reactivity control is now investigated for the 11.5 mm pin diameter homogeneous assembly design.

Soluble boron is one of the main ways reactivity is controlled in PWRs. It does, however, make the MTC significantly more positive. As the coolant expands, the boron absorption cross-section decreases and the boron content of the core reduces. It is therefore essential to minimize the soluble boron concentration. In addition, the soluble boron worth is significantly reduced, from ~–6.0 pcm/ppm (unmodified 4.4 wt% LEU PWR SOC calculated for 1000 ppm) to ~–0.6 pcm/ppm, due to reduced coolant volume and the harder neutron spectrum. The MTC gets worse by ~0.007 pcm/K /(pcm controlled). So, for every 1000 pcm of excess reactivity to be suppressed, ~1780 ppm of unenriched boron is required, and this makes the MTC worse by 7 pcm/K. For comparison, the MTC gets worse by ~0.004 pcm/K /(pcm controlled) in the unmodified LEU PWR. There are also operational and safety issues associated with high boron content and enrichment.

Stationary BPs (used here to describe also solid fixed poisons in the core including Gd mixed in the fuel, integral fuel burnable absorbers (IFBAs) and fixed burnable absorbers) are also very commonly used for power shaping and reactivity control. Black absorbers (which absorb a large proportion of incident neutrons and cause a large flux depression) or grey absorbers (which absorb

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some incident neutrons) can be placed in some or all of the pins. The RMPWR has a harder neutron spectrum than a PWR which makes BPs less effective. They absorb fewer neutrons initially and also burn out more slowly, making it difficult to design them such that they burn out by the end of the first cycle.

Evenly distributing the BPs in the fuel increases the rate at which they burn out, but also makes their overall effect on the MTC worse. However, a localized heavy-worth BP (e.g. within a fuel pin or guide tube) will create a local flux dip, which mitigates the MTC by making the number of neutrons absorbed less sensitive to changes in the BP absorption cross-section following spectral changes.

The use of 167Er to improve the MTC by acting as a resonance absorber was proposed for similar configurations in (Rahman et al., 2012). This was found not to be effective as the 0.4 eV 167Er resonance was at too low an energy. The normalized flux at 0.4 eV increases with coolant temperature for a conventional LWR, but decreases for an RMPWR. In addition, due to the relatively low cross-section of 167Er and the presence of other mild absorbers in its depletion chain, it does not burn out sufficiently fast to make it an effective BP (Fig. 2.8).

Dispersed BPs can be implemented by mixing Gd2O3 or Er2O3 with the fuel or by applying a thin

coating of ZrB2 (IFBA) respectively. IFBA coatings are typically 1.5 mg/inch, and 3–4.5 mg/inch

may be possible. Distributed Er and Gd loadings can be selected based on the initial required reactivity worth of the poison, but must be configured such that they burn out within the first cycle. For the poison to burn out sufficiently quickly, only Gd appears to have a sufficiently high absorption cross-section. This absorber would need to be added to a large number of pins in the RM designs to ensure burn-out within one cycle (Fig. 2.8). This is in contrast to UO2 fuel where Gd is

only placed in a minority of pins to prevent burn-out over too short a time. Putting Gd in a small number of pins or concentrating 10B in burnable absorbers (i.e. fixed absorbers in the guide tubes) is not effective.

Gd2O3 is most effective in this spectrum as the capture cross-sections of 155Gd and 157Gd (~37 and

98 barns in this spectrum) are higher than the capture cross-section of 10B (~28 barns) and 167Er (~26 barns) leading to sufficiently rapid burn-out. It is potentially easier to mix Gd or Er into a radioactive pellet than to apply an IFBA coating to a radioactive rod. One issue is that Gd and Er are lanthanides, which makes them tend to stay with minor actinides (MAs) during reprocessing. However, in Section 2.4.4, a heterogeneous assembly is shown to be neutronically favourable, which does not require mixing of Gd or Er with MAs.

Gd distributed over a large number of pins deteriorates the MTC by ~0.004 pcm/K /(pcm controlled) which is just over half the figure for soluble boron.

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2.4.2.1. Control Rods

One advantage of the Th-TRU fuelled RMPWR (or indeed a U-TRU fuelled RMPWR) is the relatively low reactivity swing over the cycle at equilibrium. Due to the high MA content of the feed, this low reactivity swing may also be true of transition cycles, but to determine this requires further analysis. From the linear reactivity model, it is estimated that without control rods or BPs, this could be as little as 2500 pcm for the homogeneous case and less than 5000 pcm for heterogeneous fuel assembly designs (see Section 4.4). Provided there is sufficient rod worth, this is sufficiently low to make mechanical shim an option for short- and long-term reactivity control in place of some or all of the soluble boron.

Highly enriched (95 at% 10B in the B considered here) B4C rods are necessary for the RMPWR. The

hard neutron spectrum necessitates 10B enriched B4C rods to provide a sufficient shutdown margin.

The same rod poison is also assumed for the RBWR, which is consistent with previous studies (Downar et al., 2012).He production and depletion of 10B will limit the rod life and result in a reduced control worth.

In contrast to other reactivity control mechanisms, the control rods improve the MTC. This is because the reduction in 10B absorption cross-section ensuing from the reduction in moderator density is outweighed by the increase in neutron flux on the control rod, so the overall number of captures increases. Essentially, if the control element is large enough, then it behaves more like a black absorber and the increase in flux (due to the decrease in absorption cross-section in the rest of the fuel assembly) becomes more significant than the decrease in the rod poison absorption cross- section.

A rod is a sufficiently high worth control element to improve the MTC. When the rod worth is reduced (by artificially reducing the B4C density without changing the volume of displaced water in

this case), the beneficial effect reduces. In this manner, it is possible to quantify how big a control element needs to be to improve the MTC (Fig. 2.9), and it is apparent that while a control rod has sufficient worth to make the MTC more negative, integral burnable absorbers do not.

While wet annular burnable absorbers (WABAs) have already been ruled out as they do not deplete sufficiently rapidly, the WABA worth is expected to be substantially less than –0.2, which from Fig. 2.9 is insufficient to improve the MTC. It is worth noting that replacing the coolant in the guide tubes by void (i.e. the act of displacing the coolant) makes the MTC worse by 2–3 pcm. There is therefore a significant MTC advantage in using mechanical shim to control reactivity.

49 -25 -20 -15 -10 -5 0 5 10 15 -0.4 -0.3 -0.2 -0.1 Rod worth 0 M T C ( p c m /K )

Reduced worth rod Full worth rod

Fig. 2.9. Rod worth vs MTC (for a case with 0.0 pcm/K MTC when unrodded).